August 1987 Bulletin 870 (technical)
of Organic Soils
G. H. Snyder, Editor
Agricultural Experiment Station
Institute of Food and Agricultural Sciences
University of Florida, Gainesville
J. M. Davidson, Dean for Research
Flooding drastically alters the chemistry and biology of organic
soils, primarily because of greatly reduced availability of oxygen in
the soil for chemical and biological processes. Flooding decreases
microbial oxidation of soil organic matter and reduces associated
conversion of organically bound nitrogen to plant available forms.
Flooding increases solubility, and therefore plant availability, of
phosphorus, iron, and manganese. Soil pH generally increases as a
result of flooding, and remains elevated for some time after flooding is
completed. When the flood is removed, availability of phosphorus and
of certain micronutrients such as iron, manganese, and zinc, may be
reduced by the elevated soil pH.
Populations of a number of soil insects are reduced by properly
timed flooding of suitable duration. Flooding is routinely used to
control nematodes, particularly in vegetable production systems.
Some plant pathogens, mainly fungi, are controlled by flooding, espe-
cially during warm periods. Growth of most terrestrial weeds is
suppressed by flooding, although many weeds reestablish soon after
the flood is removed.
By properly understanding the effect of flooding on properties of
organic soils, flooding can be a valuable management tool for crop
ERRATA: On pages 15, 18, and
20, ha yr should read ha/yr,
pounds acre should read
pounds/acre, and m2 day
should read m2/day.
TABLE OF CONTENTS
Introduction: Flooding as a management practice in the
Everglades Agricultural Area. .......................... 1
G. H. Snyder
The effect of flooding on physical, chemical, and
microbiological properties of Histosols ...................... 7
K. R. Reddy
The effect of flooding on Histosol fertility management ........ 23
G. H. Snyder
The effect of flooding on insect populations ................... 27
R. H. Cherry
The effect of flooding on nematode populations ................ 35
J. M. Good
The effect of flooding on plant pathogen populations ........... 41
J. O. Strandberg
The effect of flooding on weed populations .................... 57
J. A. Dusky
G. H. Snyder, Ph.D.
University of Florida (IFAS)
Everglades Research and Education Center
P.O. Drawer A
Belle Glade, FL 33430
Ronald H. Cherry, Ph.D. J. A. Dusky, Ph.D.
Associate Professor Associate Professor
University of Florida (IFAS) University of Florida (IFAS)
Everglades Research and Everglades Research and
Education Center Education Center
P.O. Drawer A P.O. Drawer A
Belle Glade, FL 33430 Belle Glade, FL 33430
J. M. Good, Ph.D. K. R. Reddy, Ph.D.
Former Professor and Professor
Center Director University of Florida (IFAS)
University of Florida (IFAS) Agricultural Research and
Everglades Research and Education Center
Education Center P.O. Box 909
P.O. Drawer A Sanford, FL 32771
Belle Glade, FL 33430
J. O. Strandberg, Ph.D.
University of Florida (IFAS)
Agricultural Research and Education Center
P.O. Box 909
Sanford, FL 32771
Flooding as a Management Practice in the
Everglades Agricultural Area
G. H. Snyder
The Everglades Agricultural Area (EAA) encompasses 260,000
hectares (650,000 acres) of the upper Everglades, extending from
Lake Okeechobee south to the Broward County line (Fig. 1.1). Most of
the soils in the EAA are organic (Histosols), generally containing
85% or more organic matter by weight (Snyder et al., 1978). The area
is intensively farmed with annual cash receipts currently averaging
near 500 million dollars. The principal crops are sugarcane and
winter vegetables. Sod, rice, and cow-calf ranching account for most
of the remaining agricultural production.
The EAA formerly was a broad freshwater swamp. It now contains
a vast network of public and private canals for drainage and irriga-
tion (Fig. 1.2). During the normally dry winter season irrigation
water is drawn from Lake Okeechobee and distributed throughout
the EAA by the canal network. The same canal system is used for
drainage during the normally rainy summer period. Excess drainage
water from the EAA can be routed into the Lake, into several water
conservation areas bordering the EAA to the east and south, and into
the Atlantic Ocean or Gulf of Mexico. Virtually all crops are irri-
gated, and almost all irrigation is accomplished by maintaining
desired water tables through the use of field ditches, water control
structures, and pumps. All farms in the EAA are well equipped for
precise water control.
There are other smaller, but still economically important, organic
soil deposits in Florida. Examples include the vegetable production
area on the north shore of Lake Apopka, the caladium bulb produc-
tion area (Sunvale) south of Lake Istokpoga in Highlands county, and
the Fellsmere-Blue Cypress Lake area west of Vero Beach. Water
management practices vary somewhat in these areas, but in general
mirror those of the EAA to the extent that is practical.
Flooding is a common, though not universal, cultural practice in
the EAA. Growers flood fallow fields for a number of reasons, among
which are disease and insect control, nematode control, improvement
of soil tilth and reduction of soil loss from biological oxidation. Rice is
the only crop for which flooding is maintained during the crop produc-
tion period. Most flooding is conducted during the summer rainy
S. CYPRESS -'.. THE >
F NFLORIDA- e
Area, and surrounding areas.
season when water is abundant. To facilitate flooding, fields, or
groups of fields, are surrounded by temporary muck dikes (Fig. 1.3).
Often the soil surface within a field is lower than that of limestone
rock roads adjoining the field, so the road itself may serve as a dike
(Fig. 1.4). Temporary pumps may be installed to lift water from
canals into the diked fields (Fig. 1.5). Vegetable growers, however,
frequently have permanently installed pumping systems for flooding
and drainage, since the same fields are flooded each summer (Fig.
GULo es lead0Mes 1
1.6). Most Histosols in the EAA have fairly high percolation rates
season when water is abundant. To facilitate flooding, fields, or
groups of fields, are surrounded by temporary muck dikes (Fig. 1.3).
Often the soil surface within974), a field is lower than that of the pumpedstone
rock roads adjoining the field, so the road itself may serve as a dike
(Fig. 1.4). Temporary pumps may be installed to lift water from
canals into the diked fields (Fig. 1.5). Vegetable growers, however,
frequently have permanently installed pumping systems for flooding
and drainage, since the same fields are flooded each summer (Fig.
1.6). Most Histosols in the EAA have fairly high percolation rates
(Zelazny and Carlisle, 1974), and no doubt much of the pumped
i PRIVATELY BACKPUMPED LANDS
STATE OWNED LANDS
wsmnan.um LEVEE (L)
Q BASIN BOUNDARY CONTROL STRUCTURE (S)
OKEECHOBEE ,9 L-8
4 4 HOOVER DIKE
'_j| 1| ^BELLE GLADE
L- 3 BAS IN
TRACT \ 1 FA RM
LL 7 L-6
L-.4 e L-'
Figure 1.2. The Everglades Agricultural Area, showing some of the
major water management facilities.
floodwater cycles back to canals surrounding the fields. Thus, pump-
ing rates can exceed open pan evaporation by severalfold (Craig,
1953). Most EAA Histosols are underlain by limestone bedrock and
other nearly impervious layers, so there is virtually no recharge of
aquifers below the EAA (Healy, 1975). Because of the pumping costs,
the flood is held no longer than is considered necessary for the pur-
pose intended. A common practice for fallow fields is to maintain
three weeks of flooding, followed by one or two weeks of drainage and
another three-week flood period.
Figure 1.3. Top: A bulldozer may be used to create and destroy tem-
porary muck dikes. Bottom: A temporary dike was created for flood-
ing by pushing soil from right to left.
Flooding drastically alters the soil environment, principally
through reduction of available oxygen. Flooding also influences the
activity of virtually all soil-borne organisms. It can be used to man-
age certain plant pests and to achieve other plant culture objectives.
Figure 1.4. The rock road (left) serves as a dike between a flooded
vegetable field (right) and a sugarcane field (far left).
Figure 1.5. A temporary pump being used to flood a rice field.
Figure 1.6. Permanent pump installation for vegetable fields that are
flooded each summer. The pump operating on the left is used for
flooding. The pump on the right is used to drain the fields.
Information about the effects of flooding on organic soil properties,
if available at all, generally is scattered among a variety of publica-
tions extending over a number of decades. This bulletin attempts to
bring together into one document current knowledge on the effect of
flooding on various chemical, physical, and biological processes in
organic soils, and to relate this knowledge, as much as possible, to
problems of crop production.
Craig, A. L. 1953. Hydrology, Florida Agricultural Experiment Stations
Annual Report, 1953. p. 260.
Healy, H. G. 1975. Potentiometric surface and areas of artesian flow of the
Florida aquifer in Florida. Florida Dept. Natural Resources, Bur. Geology
Map Series 73.
Snyder, G. H., H. W. Burdine, J. R. Crocket, G. J. Gascho, D. S. Harrison,
G. Kidder, J. W. Mishoe, D. L. Myhre, F. M. Pate, and S. F. Shih. 1978.
Water table management for organic soil conservation and crop production
in the Florida Everglades. Univ. Florida, IFAS, Agric. Exp. Sta. Bull. 801,
Zelazny, L. W. and W. H. Carlisle. 1974. Physical, chemical, elemental and
oxygen-containing functional group analysis of selected Florida Histosols.
In Histosols, Chap. 6, Soil Sci. Soc. Am. Special Publ. No. 6, Madison, WI.
The Effect of Flooding on Physical, Chemical,
and Microbiological Properties of Histosols
K. R. Reddy
Flooding has marked effects on the physical, chemical, and bio-
logical properties of organic soil, thus affecting the availability of
several essential plant nutrients. The availability of some plant
nutrients is increased as a result of flooding, while the availability of
other nutrients is decreased. Thus, nutritional disorders may be
created in the plant tissue. Flooding an organic soil creates an ox-
ygen-free environment in the soil atmosphere. Dissolved oxygen (02)
is rapidly consumed by aerobic microorganisms, creating anaerobic
conditions. Changes in the 02 status of the soil activate several
biological oxidation-reduction processes. Temporary anaerobic con-
ditions in organic soil can occur as a result of short-term flooding due
to heavy rainfall or application of irrigation water. If the soil mois-
ture is increased above a critical value, small pores between soil voids
will be filled with water, restricting gaseous exchange. Organic soils
used for rice cultivation are flooded throughout much of the rice
growing season, thus creating anaerobic conditions in the soil profile.
Alternate flooding and draining is also used as a management techni-
que in some rice paddies, but this practice may be detrimental with
respect to utilization of both fertilizer and native-soil N and P. Sever-
al reviews have recently been published on the effects of flooding or
poor aeration on physical, chemical, and biological properties of
mineral soils (Ponnamperuma, 1972; Gambrell and Patrick, 1978;
Rowell, 1981; Reddy and Patrick, 1983). In this chapter the effects of
flooding on the physicochemical and biochemical properties of orga-
nic soils in relation to their agronomic and environmental signi-
ficance will be examined.
In well-drained soils most of the gaseous exchange occurs through
the soil voids, but in flooded soils normal gaseous exchange is re-
stricted because water blocks the entry of gases. Upon flooding, 02
diffusion into a saturated or a flooded soil will depend on consumption
of 02 at the soil surface as well as depth of the overlying floodwater.
Oxygen diffusion in these systems is about 10,000 times slower than
diffusion in gas-filled pores. In a well-drained organic soil, 02 con-
sumption rates are usually lower than the rate of potential 02 diffu-
sion from the atmosphere. As a result, soil is maintained in aerobic or
oxidized states. Since many organic soils have a tendency to retain
water, 02 diffusion through the soil profile is restricted, resulting in
the formation of anaerobic microsites. In a flooded organic soil, 02
diffusion is extremely slow, and the slow rate of 02 supply through
floodwater compared to the demand at the soil surface results in the
depletion of 02 within a few hours after flooding.
Turner and Patrick (1968) reported 02 depletion in 36 hours after
flooding of a mineral soil. The greater potential consumption of 02 by
a flooded organic soil, compared to the supply rate through the
floodwater results in the development of two distinctly different soil
layers: 1) an aerobic or oxidized surface soil layer where 02 is present;
and 2) an underlying anaerobic or reduced soil layer where no free 02
is present. The thickness of the aerobic layer is determined by the 02
concentration of the floodwater (Howeler and Bouldin, 1971), the rate
of 02 consumption by the underlying soil (Engler and Patrick, 1974),
and by the rate of water percolation through the soil profile. Oxygen
consumption rates of organic soils are usually higher than those of
mineral soils because of the high organic matter content of the
former. As a result, the aerobic layer in flooded organic soil may be
very thin (only a few mm) compared to considerably thicker layers in
Restriction of 02 supply through the floodwater into the soil also
alters the metabolic activity of soil microorganisms. Rapid disap.
pearance of 02 from the soil is accompanied by an increase of other
gases produced through the metabolic activity of the microorgan-
isms. Carbon dioxide (CO2), nitrogen (N2), nitrous oxide (N20),
methane (CH4), hydrogen (H2), ethylene (C2H4) and hydrogen sulfide
(H2S) accumulate in flooded soils. The composition and concentra-
tion of these gases vary with the intensity of anaerobic conditions,
amount of organic matter, and environmental conditions.
Incorporation of organic wastes (crop residues, animal wastes, and
municipal wastes) into a nonflooded soil can increase the 02 demand,
thus decreasing 02 concentration of the soil profile. As a result, a
portion of the soil profile can be anaerobic with the volume of soil
under anaerobic conditions depending on the rate of 02 diffusion
through the profile and on the 02 consumption rate. Because of high
water-holding capacity of organic soils, a significant portion of the
soil can become anaerobic.
The three most important physicochemical properties of soil that
are affected by flooding are pH, oxidation-reduction potential or
redox potential (Eh), and ionic strength or salt concentration.
The pH of most soils tends to approach neutrality after flooding;
acid soils increase and alkaline soils decrease in pH. The equilibrium
pH for flooded mineral soils is between 6.5 and 7.5, but data for
flooded organic soils are not available. Among the more likely com-
pounds involved in buffering the pH of flooded soils are carbonates,
carbonic acids, and iron and manganese hydroxides. Because of low
concentrations of Fe and Mn in organic soils, carbonates and carbonic
acids most likely regulate pH.
Oxidation reduction potential (Eh) is a measure of electron avail-
ability. This parameter is used to differentiate between flooded and
well-drained soil. It characterizes the intensity of reduction and
identifies likely forms of redox couples in flooded soils. An inert
electrode, usually platinum, is used to measure the redox potential of
a soil. Well-drained organic soils usually have characteristic Eh of
> 300 mV and, as the soil 02 decreases, soil Eh decreases. In flooded
organic soils, this value can be as low as 350mV (Fig. 2.1). Changes
in redox couples as a function ofEh in mineral soils are shown in Fig.
2.2. Oxygen disappeared at Eh values of about 300mV. Nitrate is
removed at Eh values between 200 and 300 mV followed by the
reduction of manganese (Mn) and iron (Fe), and sulfate. Detailed
discussions of the stability of redox couples in flooded mineral soil
systems are presented by Ponnamperuma (1972), and by Gambrell
and Patrick (1978).
Flooding an organic soil also causes an increase in the amount of
ions present in soil solution. In slightly acid soils, the reduction of
Mn and Fe increases the cation concentration in the solution, while
in slightly alkaline soils calcium (Ca) and magnesium (Mg) contri-
bute to a similar increase in ionic strength. The cations that accumu-
late in flooded organic soil are ammonium (NH4), Ca, Mg, Fe, and Mn,
which tend to occupy the majority of the exchange complex. The
cations formed during the reduction process (particularly NH4, Fe
and Mn) displace other cations from the cation exchange complex into
the soil solution, thus increasing the ionic strength.
Flooding an organic soil excludes atmospheric gases from entering
the soil. Within a few hours, dissolved 02 is depleted by the aerobic
microflora. Under 02-free conditions, aerobic microflora die or be-
come dormant, while facultative and obligate anaerobic microflora
predominate. Some microbially mediated transformations are elim-
inated while the rates of others decline and new processes predomin-
ate. Flooding also suppresses the fungal population. In fact, flooding
is used as a common management practice to reduce populations of
soil-borne pathogens. Overlying floodwater of a flooded organic soil
also supports photosynthetic microflora such as algae. Since the
S\ WELL-DRAINED ORGANIC SOIL
O I FLOODED ORGANIC SOIL
cc 200- /
-400 1 I 1 I I
0 5 10 15 20 25
Figure 2.1. Changes in redox potential with time of a well-drained
and flooded organic soil.
- 100 Fe2x- l400
0 Mn +2
- 75 300 '
N N3- N
z S-2 S a)
0 25 02 -100
0 \ I I I 0
500 300 200 100 0 -100 -200 -300
REDOX POTENTIAL, mV
Figure 2.2. Stability of various redox couples shown as a function
of redox potential (data represent mineral soils only).
photosynthetic process liberates 02 and consumes CO2, this process
may have special significance in flooded organic soils. Photosynthesis
can elevate 02 levels of the floodwater beyond normal air-saturation
and this can reduce the degree of anaerobiosis at the aerobic/anaero-
bic soil interface.
The reactions by which most microflora satisfy their metabolic
needs involve biological oxidation. During this process, organic and
inorganic compounds are used as an energy source, function as elec-
tron donors, and subsequently are oxidized. Since each oxidation is
accompanied by a corresponding reduction, there must be suitable
electron acceptors present to receive the electrons released during
oxidation of substrate. Depending on the redox status of the flooded
organic soil, two general types of microbial metabolism are found:
1) processes utilizing inorganic electron acceptors (02; nitrogen ox-
ides such as NO3, NO2, NO, and N20; manganic compounds; ferric
hydroxide compounds; sulfate (SO4); CO2; and H20; and 2) fermenta-
tive processes in which organic molecules (e.g. succinate) are used as
electron acceptors (Yoshida, 1975). Under substrate nonlimiting con-
ditions and in the absence of competition among electron acceptors,
these types of microbial metabolism can occur simultaneously in
different zones of the same soil. For example, in a typically well-
drained soil, 02 can be used as an electron acceptor during respira-
tion by aerobic bacteria, while nitrate is used by facultative
anaerobes in anaerobic sites of the same soil. The latter condition
probably is more predominant in organic soils because of their high
waterholding capacity. In a flooded organic soil, aerobic respiration
occurs in the floodwater and at the floodwater-soil interface, while
facultative anaerobic respiration and anaerobic respiration occur in
the deeper soil zones (Fig. 2.3). These processes occur simultaneously.
Aerobic processes liberate much more energy than anaerobic pro-
cesses. Utilization of substrate carbon (C) under these conditions is
relatively high, ranging from 20 to 24%, depending on the microflora.
Consequently, greater release of energy allows more efficient syn-
thesis of cell biomass per unit of substrate C oxidized. A detailed
discussion of microbial oxidation of organic matter in drained organic
soils is presented by Tate (1980).
When the 02 supply is cut off, obligate aerobes can no longer
function and the microbial community shifts to facultative anaerobic
bacteria. These bacteria are capable of utilizing nitrate (NO3) and
oxidized manganic compounds and ferric compounds as electron
acceptors during the oxidation of substrate C. Under drained condi-
tions, organic soils tend to accumulate NO3, which is one of the end
WATER Oxygen Reduction Zone
SOIL I Oxygen Reduction Zone Aerobic
Eh = 300 my Respiration
Nitrate Reduction Zone
Mn4' Reduction Zone
Eh = 100 to 300 my Facultative
Fe3' Reduction Zone
Eh = 100 to 100 my
IV Sulfate Reduction Zone Anaerobic
Eh -200 to -100 my Respiration
V Methane Formation Zone
Eh = -200 my
Figure 2.3. Schematic presentation of a flooded organic soil showing
the zones with different microbial metabolism.
products of mineralization (microbiological conversion of an element
from organic to inorganic form) of organic N by microflora. Upon
flooding, this NO3 can be rapidly utilized by facultative anaerobic
bacteria, a process called NO3 respiration or dissimilatory NO3 re-
duction. Dissimilatory NO3 reduction to gaseous end products is
commonly known as denitrification. The intermediate N oxides of
this process can also potentially be used as electron acceptors. Deni-
trification is more predominant immediately after flooding an orga-
nic soil, since organic soils usually have a high concentration of NOs
under drained conditions. However, for continuously flooded organic
soils, dissimilatory reduction of NO3 to NH4 probably plays a more
significant role during anaerobic respiration. Nitrate reduction to N2
occurs at Eh values of 200 to 300 mV, while NO reduction to NH4
occurs atEh values of less than 100 mV (Patrick, 1960; Buresh and
As the demand for electron acceptors increases, facultative anaero-
bic microorganisms can also utilize manganic compounds as electron
acceptors for their respiratory activities. During this process, Mn4
is reduced to Mn2 +. Manganic reduction occurs in the same Eh range
as for NO3 respiration. Once available, NO3 and Mn4 + are consumed,
and facultative anaerobes next reduce Fe3 to Fe2 +. This reduction
occurs in an Eh range of 100 to 100 mV. In mineral soil, Fe3+
reduction has been shown to play a significant role in organic matter
decomposition. The significance of both Fe and Mn as sources of
electron acceptors in flooded organic soils is still unknown.
When the Eh of a flooded organic soil falls below 100 mV, the
activity of facultative anaerobic bacteria is inhibited and obligate
anaerobes become active. These bacteria utilize SO4, CO2, and some
organic acids as electron acceptors during the oxidation of organic
matter. In flooded organic soils, SO4 reduction probably plays a
significant role in organic matter decomposition because of the rel-
atively high concentration of SO4 in the soil. This process involves
the oxidation of substrate C (electron donor) and the transfer of
electron to SO4 respiration. An extensive review of SO4 respiration in
anaerobic systems was presented by Krouse and McCready (1979),
and Thauer and Badziong (1980). Sulfate reduction is inhibited by
the presence of 02, N03, Mn4 and Fe3+.
Under extremely anaerobic conditions, strict anaerobes obtain
their energy during the oxidation of a limited range of substrate C
compounds while reducing CO2 to CH4. In flooded organic soils this
process can play a significant role only if the soils are flooded for a
The behavior of N in organic soils is markedly affected by flooding.
Flooding the soil results in the accumulation of NH4, instability of
NOs, and a lowered N requirement by bacteria during organic matter
decomposition. The most important forms of inorganic N are NH4,
NO3, NO2, N2, and N20. These compounds are the end products of
specific biological reactions. The biochemical processes functioning
organic N -> NH4
NH4 -- NO2 -- NO3
NO3 -* NO2 -- N2
Dissimilatory NO3 reduction:
NO3 -* NH4
N2 -* organic N
NH4 -* NH3
Some of these processes are schematically presented in Fig. 2.4.
N, N20 NH3
2 AIR decomposition
WATER NH4 N --- HNO, --- HNO,
S OXIDIZED NH4 N -- HNO2 ---m-HNO
nset ZONE diffusion
NH/4 N- diffusion
\ fixation JI
fi ation Organic N j
N NOrganic N denitrification HN
N2-N20 leaching HNO-
Nitrogen transformations diffunctioning in the aerobic floodwater
tion and identification are the dominant processes. These processes
Figure 2.4. Nitrogen cycle in flooded organic soil planted with rice.
Nitrogen transformations functioning in the aerobic floodwater
and at the aerobic mud-water interface include mineralization, ni-
trification, and volatilization. In the anaerobic soil layer, mineraliza-
tion and denitrification are the dominant processes. These processes
dictate the amount ofN loss from flooded soil. Similar N transforma-
tions can potentially occur in the rhizosphere of the rice plant, due to
oxygen transport to the roots via the plant's porous stem.
During the mineralization process, organically bound N is liber-
ated in the form of NH4 N. The reverse transformation of inorganic
N compounds into organic forms is defined as immobilization.
Flooded soils differ greatly from drained soils in terms of mineraliza-
tion and immobilization reactions of N. One of the major differences
is the rate of organic matter decomposition, since under drained
conditions organic matter breakdown is influenced by a wide range of
bacteria and fungi. In flooded soils, however, absence of 02 will alter
the microflora and the pathways of organic matter breakdown. The
nitrogenous compounds present in the soil organic fraction persist for
long periods in nature, which is reflected by the small proportion of
the N reservoir that is mineralized each growing season. Mineral-
ization rates of N in flooded organic soils were in the range of 260
to 500 kg N/ha yr (230 to 445 pounds acre), as compared to
1000 to 1500 kg N/ha yr (890 to 1335 pounds acre) under drained
conditions (Terry, 1980; Reddy and Rao, 1983). The quantity of N
mineralized by flooded Histosols cannot be accounted for by crop
uptake alone, indicating the possibility of other loss mechanisms in
the system. However, rice growth can be totally supported by the
amounts of N mineralized from flooded organic soils.
Nitrogen in flooded organic soil can be lost via 1) nitrification-
denitrification; 2) NH3 volatilization; and 3) leaching. Detailed re-
views on these topics are presented by Buresh et al. (1980), Savant
and DeDatta (1982), Sahrawat (1983), and Reddy and Patrick (1984).
Nitrification-denitrification reactions are known to occur simul-
taneously in flooded organic soils where both aerobic and anaerobic
zones coexist, such as would be the case in a flooded organic soil
containing an aerobic layer over an anaerobic layer or in the aerobic
rhizosphere of rice. The nitrification reaction occurs in the aerobic
zone with the denitrification reaction occurring in the anaerobic
zone. The major supply of NH4 to the aerobic soil layer or overlying
floodwater comes from 1) mineralization of organic N in the aerobic
layer or the floodwater and 2) diffusion of NH4 from the underlying
anaerobic layer. The majority of the NO3 feedstock for the denitrifica-
tion is derived as a result of nitrification of NH4 in the floodwater and
in the aerobic layer. The NOs formed in the aerobic layer is continual-
ly diffused to the anaerobic layer in response to the concentration
gradient, where it undergoes reduction to gaseous end products. For a
flooded organic soil in the laboratory with no percolation, mineraliza-
tion of N in the anaerobic layer was found to occur at a rate of 0.11 g
N/m2 day (1 pound acre), and NH4 diffusion from the anaerobic to
the aerobic layer and floodwater was found to be 0.045 g N/m2 day
(0.4 pounds acre) (Reddy and Rao, 1983). The sequential N processes
functioning in a flooded organic soil are: 1) mineralization in the
anaerobic layer, 2) upward diffusion of NH4 from the anaerobic layer
to the aerobic layer or to the floodwater, 3) nitrification in the
floodwater-soil interface, 4) downward diffusion of NO3 into the
anaerobic layer, and 5) denitrification in the anaerobic layer (Terry
and Tate, 1980). Losses due to these processes accounted for about
35% of the ammonium mineralized in the anaerobic layer (Reddy and
Rao, 1983). Losses under field conditions in flooded soils planted with
rice will be lower because of plant assimilation of N. Under field
conditions, however, other loss mechanisms such as ammonia vola-
tilization and leaching may also play a significant role in removing N
from flooded organic soil.
Ammonia volatilization occurs under specialized conditions. It pri-
marily occurs in the floodwater, with the rate of the process depend-
ing on pH, temperature, and NH4 concentration of the floodwater. A
slow rate of NH4 diffusion in flooded organic soil, and active nitrifica-
tion at the floodwater-soil interface, decrease the NH4 concentration
of the floodwater. This in turn decreases N loss due to NH3 volatiliza-
In flooded organic soils, loss of N due to leaching can be serious.
Flooding of the soils significantly increases soluble organic N in the
leachate compared to drained conditions (Reddy, 1982). Similarly,
flooding also increases soluble NH4 in the drainage effluent. When
rice is cultivated on these soils, soluble NH4 and organic N concentra-
tion of the leachate can both be significantly reduced.
Nitrogen fixation in the flooded organic soil potentially occurs in
the floodwater and in the aerobic surface soil layer. In the anaerobic
layer, N2 fixation can be significantly inhibited by high ammonium N
concentration of the soil solution (Buresh et al., 1980).
Many studies have shown inconsistent responses of lowland rice to
phosphorus (P) fertilization, even though upland crops on the same
soils respond markedly to P application (Patrick and Mahapatra,
1968). The unique soil conditions created by flooding influence the
transformation and availability of both soil and fertilizer P. Flooded
organic soils show higher availability of P compared to drained
organic soils. The mechanism of high P availability in flooded organic
soils is not well understood, whereas the mechanisms controlling P
release in flooded mineral soils are well established. Redman and
Patrick (1965) reported that extractable P in 26 Louisiana mineral
soils increased by an average of 21% due to flooding. Khalid et al.
(1977) reported an increase in soluble P for 13 of 20 soils when
incubated under anaerobic conditions as compared to incubation
under aerobic conditions. Under flooded conditions, the amount of
total P released from flooded organic soils was about 4 to 6 times
higher than the release from drained organic soils (Fig. 2.5).
O 10 -EPC 0.67 pg/ml
u, EPC 2.25 Vg/ml
0 0 -- -- --
18 I I I I I I I
0.3 1 2 3 4
PHOSPHORUS IN SOLUTION, jLg/ml
Figure 2.5. Phosphorus adsorption isotherm of organic soil under
aerobic (drained) and anaerobic (flooded) conditions.
Increase in P solubility in flooded mineral soil usually has been
attributed to the reduction of ferric oxyhydroxide and ferric phos-
phate compounds to more soluble Fe2+ forms and to the hydrolysis of
P compounds. Increased P release in flooded organic soils is probably
due to the solubilization of organic matter during anaerobic decom-
position and to increased release of inorganic P as a result of solu-
bilization of Fe, aluminum (Al), and Ca-phosphates. In alkaline min-
eral soils, increase in available P was not due to soil reduction of
ferric phosphate, but rather to increased soil water content. Reasons
for not detecting an increase in available P as the alkaline soils were
reduced were due to 1) lack of reductant soluble P and 2) control of
soluble P by the calcium system. Although these mechanisms have
been documented for mineral flooded soils, no data are available for
flooded organic soils.
Flooding an organic soil increases release of the soluble P into the
soil solution and decreases the adsorptive capacity of the soil. Phos-
phorus adsorption isotherms measured under aerobic and anaerobic
conditions are shown in Fig. 2.5 for an organic soil collected from the
Zellwood area. Adsorptive capacity of organic soil is significantly
higher under aerobic conditions than under anaerobic conditions.
Phosphorus flux from a flooded organic soil into the overlying water
column was found to be 9.8 mg P/m2 day (Reddy and Rao, 1983).
Potassium (K) is not involved in oxidation-reduction reactions and
is therefore less affected by flooding than either P or N. However,
flooding increases the availability ofK in the soil solution. In flooded
organic soil, this effect is due to the accumulation of large amounts of
ammonium on the exchange complex, resulting in the displacement
of K. A large fraction of K in the soil solution as a result of flooding
should increase the availability of K to rice, but it probably also
results in greater leaching losses of K. Most of these observations
were noted in mineral flooded soils (Patrick and Mikkelsen, 1971).
Calcium and Magnesium Processes
Flooding also increases the concentration of Ca and Mg in the soil
solution. This increase is due to displacement by other cations which
occupy the exchange complex. Accumulation of dissolved CO2 and
carbonic acids under flooded conditions can also result in the dissolu-
tion of CaCO3 and MgCO3 (Ponnamperuma, 1972).
Sulfur(s) is involved in oxidation reactions and is present in soils in
different valence states. The most common valences are +6 (sul-
fates), 0 (zero) (elemental sulfur), and -2 sulfidess). The valence
state of S is governed by the 02 status of the soil. In a drained soil, the
principal S transformations are 1) oxidation of elemental S, thiosul-
fates, and sulfides; 2) mineralization of organic S; and 3) incorpora-
tion of S and SO4 into microbial tissue, and uptake by plants (Thauer
and Badziong, 1980).
Under flooded conditions, SO4 reducing obligate anaerobic bacte-
ria use SO4 as an electron acceptor, resulting in the formation of S2 -.
Sulfate reduction is inhibited by the presence of NO3. For example, S
oxidizing bacteria such as Thiobacillus denitrificans can reduce ni-
trate to gaseous products while oxidizing S to SO4. Sulfide formation
occurs in flooded organic soils only after all of the NO3 is lost from the
system. Connell and Patrick (1968) reported S2- formation in
anaerobic soil with Eh of < 150 mV. Optimum pH for S2- produc-
tion was found to be between 6.5 and 8.5. Sulfide concentrations of an
anoxic soil can be decreased as a result of precipitation with metallic
cations such as Fe, Mn, copper (Cu), zinc (Zn), lead (Pb), mercury
(Hg), and cadmium (Cd) (Engler and Patrick, 1975). In organic soils,
low concentrations of metallic cations can increase the possibility of
sulfide toxicity to plants. Although some of the sulfur in reduced form
sulfidess) may accumulate in the soil as metal sulfides and elemental
S, most is eventually oxidized to sulfates. Microorganisms play a
significant role in this process. Sulfides, once formed, will diffuse into
the surface soil layer where they undergo oxidation to elemental S
and subsequently to SO4. Sulfate thus formed tend to diffuse down-
ward into the anaerobic soil layer, where they undergo reduction to
sulfide again. This cycle will be continuously repeated if an adequate
quantity of substrate carbon is available and if the desired microflora
Micronutrient and Heavy Metal Processes
Flooding an organic soil affects the availability, solubility, and
oxidation-reduction state of several micronutrients and heavy met-
als. In flooded organic soils, humic substances have the ability to
interact with metal ions, metal oxides, and hydroxides. The forma-
tion of metal-organic complexes of widely differing chemical and
biological stabilities and characteristics is a well-known fact, though
relatively little is known about the chemistry of metal-humic in-
teraction products. The ability of inorganic surfaces to catalyze or-
ganic reactions may be important in the synthesis, alteration, and
degradation of humic materials (Schnitzer, 1978). Conversely, the
continuous changes during decomposition will influence the forma-
tion of metal-organic associations. Humic material in flooded soils is
usually characterized by large molecular weights and greater
structural complexity, thus resulting in increased metal retention
capacity (Gambrell et al., 1977).
Organic soils are usually low in Fe and Mn oxides, so these systems
play a small role in the reduction process. In mineral flooded soils, Fe
and Mn reduction processes play significant roles during microbial
respiration. Manganese oxides are reduced between + 200 and + 300
mV at pH levels between 6.0 and 7.0 and at -100mV at pH 8.0
(Gotoh and Patrick, 1972, 1974). Reduction of Fe and Mn has an
indirect impact on the chemistry of the flooded soil, such as 1) an
increase in water soluble Fe and Mn; 2) pH increases; 3) displacement
of other cations from the exchange complex into the soil solution; 4)
increases in the solubility of P and silicon (Si); and 5) formation of
new minerals (Ponnamperuma, 1972).
Micronutrients such as Zn, Cu, molybdenum (Mo), cobalt (Co), and
boron (B) are not as readily affected by flooding as Fe and Mn but
their availability, especially those metals specifically adsorbed onto
Fe3+ and Mn4 oxides and hydrous oxides, may be increased in-
directly by flooding. Uptake of Zn by rice seedlings was found to be
greater for aerobic than for anaerobic soils. Solubility of these metal
cations is also affected by precipitation with sulfides, thus decreasing
their toxic concentrations.
AGRONOMIC AND ENVIRONMENTAL SIGNIFICANCE
The agronomic significance of varying soil redox conditions is well
documented with respect to plant nutrition. In well-drained soils
where aerobic respiration predominates, soil organic matter decom-
position can potentially convert about 10 to 20% of the N contained in
organic matter to inorganic N which can be used by crops. Organic
soils (Histosols) can release 410 to 1250 kg N/ha yr (365 to 1112
pounds acre) and 38 to 185 kg P/ha yr (34 to 165 pounds acre) as a
result of aerobic decomposition of soil organic matter. Shifting from
aerobic to facultative anaerobic or obligate anaerobic metabolism
significantly alters soil nutrient availability. Low nutrient require-
ments of anaerobic bacteria will have beneficial effects on the avail-
able plant nutrient accumulations in O2-deficient soils. In flooded
soils, inorganic redox systems such as NO3 and Mn4' oxides and the
Fe3 + oxyhydroxides support organic matter decomposition. This pro-
cess is similar to the decomposition supported by O2, but at lower
efficiency. However, reduction of Mn4 and Fe3 to soluble forms
increases plant available Mn2+ and Fe2+. Reduction of NO3 to
gaseous end products during facultative anaerobic respiration is not
a beneficial process with respect to N economy of agricultural soils.
As the metabolic activity shifts to obligate anaerobes, decomposition
of organic matter can result in the accumulation of organic acids,
aldehydes, sulfides, and organic sulfur compounds which, under cer-
tain conditions, are toxic to plants.
In organic soils, decomposition of organic matter contributes a
discharge of 12 to 56 kg N/ha yr (10 to 50 pounds acre) into drainage
effluent. During aerobic respiration of organic soils, NO3 in the
drainage effluent is a serious problem, since these soils are often
drained to keep the water table down and excess drainage water is
being discharged into adjacent water bodies. As a result of continuous
organic matter decomposition, Florida's organic soils are subsiding
at a rate of 2.5 cm (1 inch) per year. To reduce soil loss, it is recom-
mended that a portion of the soil profile be returned to its original
flooded state. Although increasing the water table or complete flood-
ing shifts the metabolic activities to facultative anaerobes or obligate
anaerobes, these anaerobic processes solubilize organic matter, thus
increasing the soluble organic C, soluble organic N, and soluble P in
the drainage effluents as compared to aerobic conditions. Discharge
of these effluents into adjacent water bodies can enhance the eu-
trophication process. Rice cultivation in flooded organic soils can
significantly reduce soluble N and P of the drainage effluent.
Buresh, R. J., and W. H. Patrick, Jr. 1981. Nitrate reduction to ammonium
and organic nitrogen in an estuarine sediment. Soil Biol. Biochem. 13:279-
Buresh, R. J., M. E. Casselman, and W. H. Patrick, Jr. 1980. Nitrogen
fixation in flooded soil systems. A review. Advan. Agron. 33:149-192.
Connell, W. E. and W. H. Patrick, Jr. 1968. Reduction of sulfate to sulfide in
waterlogged soil. Soil Sci. Soc. Am. Proc. 33:711-715.
Engler, R. M. and W. H. Patrick, Jr. 1974. Nitrate removal from floodwater
overlying flooded soils and sediments. J. Environ. Qual. 3:409-413.
Engler, R. M. and W. H. Patrick, Jr. 1975. Stability of sulfides of manganese,
iron, zinc, copper, and mercury in flooded and non-flooded soil. Soil Sci.
Gambrell, R. P., and W. H. Patrick, Jr. 1978. Chemical and microbiological
properties of anaerobic soils and sediments. In Plant Life in Anaerobic
Environments. D. D. Hook and R. M. M. Crawford (eds.). Ann Arbor Sci.,
Ann Arbor, pp. 375-423.
Gambrell, R. P., R. A. Khalid, M. G. Verloo, and W. H. Patrick, Jr. 1977.
Transformation of heavy metals and plant nutrients in dredged sediments
as affected by-oxidation-reduction potential and pH. II. Materials and
Methods, Results and Discussion. Rept. DACW-39-74-C-0076. Office of
Dredged Material Research. U.S. Army Engineer Waterway Experiment
Gotoh, S., and W. H. Patrick, Jr. 1972. Transformation of manganese in a
waterlogged soil as affected by redox potential and pH. Soil Sci. Soc. Am.
Gotoh, S., and W. H. Patrick, Jr. 1974. Transformation of iron in a water-
logged soil as affected by redox potential and pH. Soil Sci. Soc. Am. Proc.
Howeler, R. J. and D. R. Bouldin. 1971. The diffusion and consumption of
oxygen in submerged soils. Soil Sci. Soc. Am. Proc. 36:202-208.
Khalid, R. A., W. H. Patrick, Jr., and R. D. DeLaune. 1977. Phosphorus
sorption characteristic of flooded soils. Soil Sci. Soc. Am. J. 41:305.
Krouse, H. R. and R. G. L. McCready. 1979. Biogeochemical cycling of sulfur.
In Biogeochemical Cycling of Mineral Forming Elements. P. A. Trudinger
and D. J. Swaine (eds.) Elsevier Publ., New York, NY. pp. 401-430.
Patrick, Jr., W. H. 1960. Nitrate reduction rates in a submerged soil as
affected by redox potential. 7th Int. Congress of Soil Sci., Madison, WI.
Patrick, Jr., W. H. and I. C. Mahapatra. 1968. Transformation and availabil-
ity to rice of nitrogen and phosphorus in waterlogged soils. Adv. in Agron.
20:323-359. Acad. Press Inc., New York, NY.
Patrick, Jr., W. H. and D. S. Mikkelsen. 1971. Plant nutrient behavior in
flooded soil. In Fertilizer Technology and Use. Soil Sci. Soc. Am. Proc.
Madison, WI. pp. 187-215.
Ponnamperuma, F. N. 1972. The chemistry of submerged soils. Adv. Agron.
Reddy, K. R., 1982. Mineralization of nitrogen in organic soils. Soil Sci. Soc.
Am. J. 46:561-566.
Reddy, K. R., and W. H. Patrick, Jr. 1983. Effects of aeration on reactivity
and mobility of soil constituents. In Chemical Mobility and Reactivity in
Soil Systems. Soil Sci. Soc. Am. Special Publ. No. 11:11-33.
Reddy, K. R. and P. S. C. Rao. 1983. Nitrogen and phosphorus fluxes from
flooded organic soil. Soil Sci. 136:300-307.
Reddy, K. R., and W. H. Patrick, Jr. 1984. Nitrogen transformations and loss
in flooded soils and sediments. CRC Critical Reviews in Environ. Control.
Redman, F. H., and W. H. Patrick, Jr. 1965. Effect of submergence on several
biological and chemical soil properties. Bull. no., 592. Agric. Expt. Stat.
Louisiana State University, Baton Rouge, LA.
Rowell, D. L. 1981. Oxidation and reduction. In The Chemistry of Soil
Processes. D. J. Greenland and M. H. B. Hayes (eds.). John Wiley & Sons,
New York. pp. 401-461.
Sahrawat, K. L. 1983. Nitrogen availability indexes for submerged rice soils.
Adv. Agron. 36:415-451.
Savant, N. K., and S. K. DeDatta. 1982. Nitrogen transformation in wetland
rice soils. Adv. Agron. 35:241-302.
Schnitzer, M. 1978. Reactions of humic substances with minerals in the soil
environment. In Environmental Biogeochemistry and Geomicrobiology.
Vol 2. W. E. Krumbein (ed.). Ann Arbor Sci. Ann Arbor, MI.
Tate, R. L. 1980. Microbial oxidation of organic matter of Histosols. Adv.
Microbiol. Ecol. 4:169-201.
Terry, R. E. 1980. Nitrogen mineralization in Florida Histosols. Soil Sci. Soc.
Am. J. 44:747-750.
Terry, R. E., and R. L. Tate, III. 1980. Denitrification as a pathway for nitrate
removal from organic soils. Soil Sci. 129:162-166.
Thauer, R. K. and W. Badziong. 1980. Respiration with sulfate as electron
acceptor. In Diversity of Bacterial Respiratory Systems. Vol. 2. C. J.
Knowles (ed.). CRC Press, Inc., Boca Raton, FL. pp. 66-85.
Turner, F. T. and W. H. Patrick, Jr. 1968. Chemical changes in water-logged
soils as a result of oxygen depletion. Trans. 9th Int. Cong. Soil Sci. 4:53-65.
Yoshida, T. 1975. Microbial metabolism of flooded soils. In Soil Biochemistry
E. A. Paul and A. D. McLaren (eds.). Marcel Dekker, Inc., New York. pp.
The Effect of Flooding on Histosol Fertility Management
G. H. Snyder
Flooding so drastically affects the chemical and microbiological
properties of organic soils that it is not surprising that adjustments in
fertility practices may be needed when flooding is practiced. In the
Everglades some of these adjustments are fairly well recognized and
are routinely used by local growers. But, as flooding is more widely
used in the area, it is likely that additional consequences of flooding
on soil fertility will become evident. Certainly additional research on
this topic is warranted. Two aspects of the effect of flooding on soil
fertility need to be considered: 1) how does flooding affect fertility
management of the crop being grown with flooding, and 2) how does
flooding affect fertility management of the crop grown after the flood
FERTILITY MANAGEMENT OF A FLOODED CROP
Chapter 2 of this bulletin offers an excellent discussion of the
chemical properties of flooded organic soils, so these properties will be
discussed only briefly here as they relate to field problems in soil
fertility. Since flooding decreases the soil redox potential, several
plant nutrients normally present in the soil in an oxidized state are
reduced. This change in oxidation state can greatly alter nutrient
availability, sometimes increasing and sometimes decreasing it, de-
pending on the specific nutrient.
Most nitrogen (N) in well-drained soils occurs as the nitrate
(NO3 N) form. In flooded soils NO3 N is rapidly reduced to forms
that are, by and large, gaseous and therefore lost from the system
(denitrification). Furthermore, in the case of drained Histosols, N
release from decomposing organic matter mineralizationn) generally
is more than adequate for crop growth, so fertilizer N is seldom
needed. However, flooding greatly inhibits N mineralization. Thus,
because of both denitrification and reduced N mineralization, the N
fertility of organic soils is greatly altered by flooding. After a period of
prolonged flooding (several months or more), crop production may be
increased by N fertilization (Snyder and Jones, 1983). In most cases
reduced forms of N, such as urea or ammonium N (NH4-N), are
preferred to NO3 N for fertilization of flooded crops in order to
minimize denitrification losses. However, percolation rates in the
Everglades are sufficiently great that NO3 N may be a suitable
source if applied to a crop with an established root system.
Phosphorus (P) solubility in Histosols has been shown to increase
with increased water table height (Terry et al., 1980), and presum-
ably is maximized by flooding. Therefore, the need for P fertilization
is probably less for flooded crops than for most upland crops. Never-
theless, flood-cultured taro (Colocasia esculenta) responded to pre-
plant P fertilization in two trials conducted on Pahokee muck with
pH near neutrality (Snyder, 1984; Snyder, 1985a). However, rice
(Oryza sativa) yield was not increased by P fertilization of a muck soil
with pH 6.2 and pre-plant water extractable soil P of only 2.7 kg/ha
(2.4 pounds/acre) (Snyder, 1985b).
Potassium (K) is not naturally abundant in most Histosols (Lucas,
1982), and it is fairly mobile in these soils as well. Therefore, it would
appear that flooded crops growing on organic soils should benefit from
K fertilization. In tests conducted in the Everglades in 1982 and
1984, however, rice failed to respond to pre-plant K fertilization even
though soil samples taken prior to flooding were not high in K
(Snyder and Jones, 1983; Snyder, 1985b). These results demonstrate
that, in spite of theory and speculation about P and K status of flooded
organic soils, field trials are needed to accurately assess the fertility
requirements of various crops.
Flooding greatly increases the availability of iron (Fe) and man-
ganese (Mn). In several instances the author has observed these
deficiencies on drill-seeded rice prior to flooding, but the deficiencies
have been corrected by flooding. pH-induced Mn deficiency of rice
appears to be corrected especially rapidly upon soil flooding.
FERTILITY MANAGEMENT ADJUSTMENTS REQUIRED
AFTER FLOODING IS ENDED
Flooding generally increases the pH of acid soils. In the Ever-
glades, this probably occurs in part because the canal water used for
irrigation has a high pH (Volk and Sartain, 1976). However, organic
soils from Wisconsin also increased in pH when saturated with de-
mineralized water in the laboratory (Isirimah and Keeney, 1973).
These pH changes persist for some time after the flood is removed
(Table 3.1). The availability of several nutrients is decreased with
increasing soil pH. In order to improve micronutrient (especially Mn)
availability for sugarcane, recommendations of the Everglades Re-
search and Education Center Soil Test Lab (Kidder, 1980) specify the
use of sulfur (S) in the planting furrow when soil pH is 6.5 or greater.
Flooding may increase soil pH sufficiently that this recommendation
Table 3.1. The pH of selected Everglades Agricultural Area organic
soils before and after summer flooding.
flooding After flooding
Soil Soil Soil Soil
Location Date pH Date pH Date pH Date pH
M/Y M/Y M/Y M/Y
EREC 6/79 7.2 9/79 7.5 12/79 7.8 3/80 7.6
Resmondo 3/80 5.1 9/80 5.9 3/84 5.6 -
Baker 3/80 5.7 11/80 6.4 4/84 6.2 -
Shawano 2/81 4.9 10/81 5.8 -
Data provided by Dr. R. L. Lucas, Mr. Manuel Porro, and G. H. Snyder.
Soil-test P is determined by the Everglades Lab in water extracts of
organic soils (Thomas, 1970). In general, water solubility of P is
decreased by elevated soil pH, so soil-test P probably will be reduced
by flooding because of increased soil pH. In addition, Fe released from
insoluble compounds by flooding will be more reactive with P, there-
by reducing P water solubility (Sah and Mikkelsen, 1986). Thus,
higher rates of P fertilization will be specified for fields that have
been flooded than for those that have not (Snyder et al., 1986). It is
uncertain at this time whether such an increase is warranted. Is the
lower soil-test P an artifact of the soil test procedure (water extrac-
tion), or has P availability been permanently altered by flooding,
perhaps because of removal in the leachate?Unfortunately, insuf-
ficient data are available at this time to provide an answer to this
question. The conservative suggestion at this time is to accept the soil
It is fairly clear that K is quite mobile in organic soils, and that K in
fallow soils is lost from the root-zone by flooding (Lucas, 1982). Thus,
soil-test K is lower in soils after flooding, and increased K fertiliza-
tion will be recommended (Snyder et al., 1986). If a crop is grown
during the flooding period, considerable K may be absorbed by the
crop and returned to the soil in crop residues, even though this K may
not be accounted for in the soil test analysis. This may be a good way
of minimizing K losses during flooding.
Isirimah, N. 0. and D. R. Keney, 1973. Nitrogen transformations in aerobic
and waterlogged Histosols. Soil Sci. 115:123-129.
Kidder, G. 1980. Computerized soil test results and fertilizer recommenda-
tions from AREC Belle Glade, Belle Glade AREC Research Report EV-
Lucas, R. E. 1982. Organic Soils (Histosols). Research Report 435, Michigan
State University Agricultural Experiment Station and Cooperative Ex-
tension Service, East Lansing MI, and University of Florida (IFAS) Agri-
cultural Experiment Stations, Gainesville, FL. p. 78.
Sah, R. N. and D. S. Mikkelsen. 1986. Effects of anaerobic decomposition of
organic matter on sorption and transformations of phosphate in drained
soils: 2. Effect of amorphous iron content and phosphate transformation.
Soil Sci. 142: 346-351.
Snyder, G. H. and D. B. Jones. 1983. Rice fertility trials. Sixth Annual Rice
Field Day Report, Belle Glade AREC Research Report EV-1983-6. pp. 1-5.
Snyder, G. H. 1984. Root and herbaceous crops production for biomass.
Methane from Biomass and Waste, 1st. Quarterly Report, 1984. Univer-
sity of Florida (IFAS) Center for Biomass Energy Systems, G038 McCarty
Hall, Gainesville, FL 32611. p. 39.
Snyder, G. H. 1985a. Root and herbaceous crops production for biomass.
Methane from Biomass and Waste, 1st. Quarterly Report, 1985. Univer-
sity of Florida (IFAS) Center for Biomass Energy Systems, G038 McCarty
Hall, Gainesville, FL 32611. p. 50.
Snyder, G. H. 1985b. Rice fertility research. Eighth Annual Rice Field Day
Report, Belle Glade EREC Research Report EV-1985-7. p. 21-30.
Snyder, G. H., R. H. Caruthers, J. Alvarez, and D. B. Jones. 1986. Sugarcane
production in the Everglades following rice. Journal American Society of
Sugar Cane Technologists 6.
Terry, R. E., G. J. Gascho, and S. F. Shih. 1980. Effect of depth to water table
on the quality of water in the Everglades Agricultural Area. Proceedings of
the 6th International Peat Congress. Duluth, MN. pp. 700-704.
Thomas, F. H. 1970. Sampling and methods used for analysis of soils in the
soil testing laboratory of the Everglades Experiment Station. Everglades
Station Mimeo Report EES65-18, Reprinted July, 1970. University of
Florida (IFAS) Everglades Research and Education Center, P.O. Drawer
A, Belle Glade, FL 33430.
Volk, B. G. and J. B. Sartain. 1976. Elemental concentrations of drainage
water from Everglades Histosols as affected by cropping systems. Proceed-
ings Florida Soil and Crop Science Society 35:177-183.
The Effect of Flooding on Insect Populations
Ronald H. Cherry
Probably no aspect of soil insect management has been as ne-
glected as flooding (Genung, 1976). A few scattered references to
flooding for insect control outside of Florida are found in entomologi-
cal literature. These include wireworm control in Pennsylvania
(Thomas, 1930) and grub control in India (Avasthy, 1967). However,
a review of the subject shows that probably more flooding research to
control insects has been done in the Florida Everglades Agricultural
Area (EAA) than anywhere else in the world. Reasons for the exten-
sive flooding for insect control (Table 4.1) in the EAA are water
availability, level land, and difficulty with chemical control of soil
insect pests in the highly organic muck soils. The following is a brief
review of research conducted in the EAA on flooding for insect con-
Cutworms: Subterranean cutworms (Family Noctuidae; Order
Lepidoptera) are a major insect pest in the Everglades. Injurious
infestations occur annually, damaging practically all types of vege-
table and field crops grown in the area. The black cutworm Agrotis
ipsilon (Rott.) and the granulate cutworm Agrotis subterranea (Fab.)
probably constitute 98% of the subterranean cutworm population on
these organic soils (Genung, 1959). Observations at Belle Glade
indicated that heavy rains that produced standing water for several
hours drowned cutworms, but that many cutworms survived in mere-
ly sodden or saturated soil by congregating on the higher spots or
crawling onto vegetation or debris. This suggests that under certain
conditions, artificial flooding might be a feasible means of cutworm
control (Genung, 1959). In simulated flooding experiments at 2, 4, 6,
8, 16, 24, and 48 hours of flooding, no recovery or survival of black or
granulate cutworms occurred after the 16-hour treatment (Genung
Grubs: Since 1971, several species of grubs (Family Scar-
abaeidae: Order Coleoptera) have been noted causing damage to
sugarcane in South Florida. Of these pests, the grub Ligyrus subtro-
picus Blatchley is the species of primary economic importance (Gor-
don and Anderson, 1981). Genung (1976) conducted flooding experi-
ments on L. subtropicus under laboratory conditions but did not
Table 4.1. Crops flooded or water table managed for pest control in
the Everglades (Genung, 1976).
Crop % Flooded Basis Pest Managed
cane only) 36.0 Ann. Wireworms
Radishes 80.0 Ann. Wireworms, cutworms
Lettuce 75.0 Ann. Sclerotinia, wireworms,
cutworm, & nematodes
Leaf crops 75.0 Ann. Wireworms, cutworms,
Celery 100.00 Ann. Sclerotinia, nematodes,
Sweet corn 50.0 Ann. Wireworms
Potatoes 50.0 Ann. Wireworms, nematodes
Carrots 80.0 Ann. Nematodes, wireworms
Pasture" 50.0 Ann. to White grubs, chinch bugs
"aFlooding intermittent for white grubs, high water table (25-30 cm) maintained to
prevent or deter bug build-up.
report the insect stages) tested. Summers (1977) stated that the most
effective control for L. subtropicus was flooding the standing cane for
5 to 7 days during the months of August to November, or flooding the
cut stubble after harvest prior to February. Flooding standing cane
for 5 to 7 days did not reduce sucrose and gave satisfactory control of
L. subtropicus. Summers also noted that some grub species are more
difficult to control. Prewitt and Summers (1981) concluded that L.
subtropicus larvae can be killed by flooding for a period of 8 days,
although this presents problems in standing cane and is customarily
used to kill infestations prior to planting. In contrast, Watve et al.
(1981) obtained data under laboratory conditions which indicated
that 100% mortality of both L. subtropicus and Cyclocephala paral-
lela grubs could be obtained with only 5 days of flooding. Part of the
confusion generated by the previous studies concerning flood dura-
tion required to control grubs was due to the lack of detailed data
within the studies. Cherry (1984) conducted an extensive study to
more fully determine the effect of flooding as a control method for L.
subtropicus and C. parallel grubs. Flooding experiments were con-
ducted under simulated and actual field conditions to determine
flooding mortality of grubs in Florida sugarcane (Fig. 4.1). The se-
quence of flood tolerance of L. subtropicus was adults greater than
eggs, and eggs greater than larvae and pupae (Table 4.2). Increased
water temperature significantly increased flooding mortality of L.
subtropicus. C. parallel was also shown to be more flood tolerant
than L. subtropicus. In field studies, the 5-day flood significantly
Figure 4.1. Grubs (Ligyrus subtropicus) can be controlled in stand-
ing sugarcane by proper flooding techniques (pictured: Dr. R. H.
Table 4.2. Percent mortality" of different stages of L. subtropicus
under different flood durations (Cherry, 1984).
Flood Duration (days)
Stages Ib 5 10
Egg 9.8 b 40.0 b 75.3 b
Larvae-1st instar 26.0 c 100.0 c 100.0 c
Larvae-2nd instar 24.3 c 100.0 c 100.0 c
Larvae-3rd' instar 24.7 c 85.6 c 100.0 c
Pupae 21.8 c 100.0 c 100.0 c
Adults 0 a 0 a 0 a
"aAdjusted for natural mortality by Abbott's formula.
"bMeans in the column followed by the same letter are not significantly different at the
0.05 confidence level as determined by Duncan's multiple range test.
"Dependent upon seasonal temperature (see text).
reduced L. subtropicus populations in sugarcane fields during
September 1982 (Table 4.3). Mortality was reduced on extreme field
edges because of incomplete water coverage on the higher ground.
The effect of flooding on sugarcane yield is also discussed by Cherry
in the study.
Sugarcane Borer: The sugarcane borer, Diatraea saccharalis
(Fab.) (Family Pyralidae: Order Lepidoptera) is the most destructive
insect pest of the above-ground portion of sugarcane in Florida (Gif-
ford, 1964). Ingram et al. (1938) reported that a sugarcane grower,
Fellsmere Plantation, practiced flooding the stubble fields with water
following harvest to kill borers in the stubble and in the trash left on
the fields. Prior to the adoption of this control measure, heavy losses
were suffered from borer injury, but since its adoption losses had been
markedly reduced. Examinations to determine the effectiveness of
the above flooding in killing borer larvae were made on January 27
and 28, 1936. Fields were continuously flooded for 85 hours beginning
on January 10. The trash was burned prior to flooding. A mortality of
95% was obtained. From these data, Ingram et al. concluded that the
flooding of stubble fields to control borers was a very profitable
practice at Fellsmere, especially since so little cost was involved
Table 4.3. Mortality of third instar grubs of L. subtropicus in flooded
sugarcane fields in Florida (Cherry, 1984).
S SE grubs Flood-
lodged stool, induced
Field-Treatmentb Pre-flood Post-flood' (%)d
Control-periphery 12.1 t 2.51 10.3 1.64 0
Control-interior 6.1 0.67 5.4 1.36 0
5-day flood-periphery 8.5 0.94 0.63 0.28 91.2
5-day flood-interior 1.3 0.28 0 0 100
10-day flood-periphery 5.5 1.02 0.75 0.41 84.5
10-day flood-interior 2.5 0.53 0 0 100
"40 lodged sugarcane stools (20 pre- and 20 post-flood) sampled per field.
"bNumber of fields sampled for control, 5-day flood, and 10-day flood were 1, 4, and 2,
"Significantly (t-tests, p < 0.05) fewer grubs in all areas except nonflooded controls
dMortality adjusted for natural mortality from control by Abbott's formula.
because of gravity flooding and drainage. The authors also concluded
that this practice had apparently not affected the stand of cane
produced from flooded fields. Ingram et al. (1951) felt that flooding
was of little use against wireworms but again noted that flooding was
beneficial for sugarcane borer control.
Wireworms: Wireworms (Family Elateridae: Order Coleoptera)
are the hard, shiny, cylindrical larvae of click beetles and are pests of
a wide variety of crops in South Florida. The wireworm, Melanotus
communis (Gyl.), is the most destructive soil insect of sugarcane in
Florida (Gifford, 1964). Ingram et al. (1938) concluded that flooding
was ineffective for control of wireworms in the Everglades. Ingram et
al. (1951) still indicated flooding was of little use against wireworms
but that it was beneficial in sugarcane borer control. Wilson (1947)
considered flooding beneficial in wireworm control only as a preven-
tive to oviposition, since adults could not oviposit on land covered
with water. Wilson and Hayslip (1951) also held this view. Genung
(1970) determined that either continuous or alternate flooding
appears effective for control of the wireworms M. communis and
Conoderus sp. The minimal effective continuous flooding period in
these tests was 6 weeks (Table 4.4). Since a 4-2-4 weekly alternation
of flooding to drying repeatedly showed perfect control and since 2-2-2
alternations produced 71 to 87% mortality, Genung also thought that
a 3-2-3 alternation should normally be effective. Later field tests by
Genung (1976) also showed prolonged field flooding caused reduc-
tions of wireworm populations.
Table 4.4. Mortality percentages obtained with Melanotus communis
under different periods of sustained flooding during June
and July 1966 (Genung, 1970).
Initial Delayed Total
Time Average Average Average
Increment % Mortality % Mortality % Mortality
3 days 0 0 0
7 days 3.3 10.0 13.3
14 days 3.3 10.0 13.3
21 days 23.3 10.0 33.3
28 days 33.3 43.3 76.6
35 days 63.3 13.3 76.6
Check 10.0 0 10.0
This group of animals include those species other than the pest
species that was the target of the flooding practice. Flooding is un-
doubtedly beneficial to many bird species feeding on various insects
in flooded fields as noted by Genung (1970). Genung reported that
soon after flooding many semi-aquatic and wading birds moved into
the flooded areas. It was believed that the surfacing soil organisms,
including wireworms, were the major attractions to birds in these
impoundments. Under natural flooding in pastures, earthworms,
white grubs, cutworms, and wireworms were observed at the surface
where many birds gathered. Birds observed at wireworm flooding
sites included mottled duck, king rail, killdeer, snipe, blacknecked
stilt, gulls, American egret, little blue heron, white ibis, glossy ibis,
limpkin, and Weston's boat tailed grackle. Genung also noted that
flooding is not an unmixed blessing since populations of carabid
larvae and other predators in and on the soil were reduced, as indi-
cated by the sampling of flooded and unflooded areas. To more fully
determine the effect of flooding on certain beneficial insects, Genung
(1976) conducted additional experiments. Data from these tests are
shown in Table 4.5. Flooding caused reductions of populations of all of
the organisms including such predatory forms as Carabidae, Formic-
idae, and Chilopoda.
Flooding has been and will continue to be used for insect control in
the EAA. The key to effective insect control with flooding lies in the
intelligent use of the technique. This includes noting flood suscepti-
bility of the pest species, stage of the insect, duration of flooding, and
water temperature. Flood studies in this review in conjunction with
available weather data (Casselman, 1970) should be useful in select-
ing the most effective methods for control of various Everglades
Table 4.5. Percent of soil cores containing insects and other arthropods under flooding, clean fallow, and no
treatment, August 1972 (Genung, 1976).
tation Treatment' CWW SPWW W.Gbs. CW Cyd. Ten. Cbd. Dmpa. Cent. Ants Sowbug
Crops, Alt. 0 0 0 0 0 0 0 0 0 0 0
Crops, Clean 2 1 0 1 1 0 0 0 0 2 0
Grasses Untreated 8 5 7 6 4 0 6 0 8 12 1
"100 samples taken in each treatment.
bCWW = corn wireworm, SPWW = southern potato wireworm, W.Gbs. = white grubs, CW = cutworms, Cyd. = Cydnidae,
Ten. = Tenebrionidae, Cbd. = Carabrdae, Dmpa. = Dermaptera, Cent. = Centipeder.
Avasthy, P. N. 1967. The problem of white grubs of sugarcane in India. Proc.
12th Int. Soc. Sugar Cane Tech. 1321-33.
Casselman, T. W. 1970. The climate of the Belle Glade area. Fla. Agr. Exp.
Sta. Circ. S-205, 17 pp.
Cherry, R. H. 1984. Flooding to control the grub, Ligyrus subtropicus (Co-
leoptera: Scarabaeidae), in Florida Sugarcane. Jour. Econ. Entomol.
Genung, W. G. 1959. Ecological and cultural factors affecting control of
subterranean cutworms in the Everglades. Fla. State Hort. Soc. Proc.
Genung, W. G. 1964. Cutworm control studies. Fla. Agr. Exp. Station Annu.
Rep. 1964: p. 279.
Genung, W. G. 1970. Flooding experiments for control of wireworms attack-
ing vegetable crops in the Everglades. Fla. Entomol. 53:55-63.
Genung, W. G. 1976. Flooding in Everglades soil pest management. Proc.
Tall Timbers Conf. 6:165-72.
Gifford, J. R. 1964. A brief review of sugarcane insect research in Florida
1960-64. Soil and Crop Soc. of Fla. Proc. 24:444-53.
Gordon, R. D., and D. M. Anderson. 1981. The species of Scarabaeidae
(Coleoptera) associated with sugarcane in south Florida. Fla. Entomol.
Ingram, J. W., H. A. Jaynes, and R. N. Lobdell. 1938. Sugarcane pests in
Florida. Proc. 6th Int. Cong. Soc. Sug. Cane Tech. 89-98.
Ingram, J. W., E. K. Bynum, R. Mathers, W. E. Haley and L. J. Charpentier.
1951. Pests of sugarcane and their control. USDA. Circ. 878.
Prewitt, J. C., and T. E. Summers. 1981. White grubs of sugarcane in south
Florida. Second Inter-American Sugar Cane Seminar Miami, Florida.
Summers, T. E. 1977. Flooding for the control of the white grub, Bothynus
subtropicus in Florida. Amer. Soc. Sugar Cane Tech. 7:128.
Thomas, C. A. 1930. A review of research on the control of wireworms. Pa.
Agr. Exp. Sta. Bull. 259.
Watve, C. M., J. D. Miller, M. G. Bell, and K. D. Shuler. 1981. A summary of
research activities on white grubs injurious to Florida sugarcane. Second
Inter-American Sugar Cane Seminar. Miami, Florida 51-60.
Wilson, J. W. 1947. Present status of the wireworm problem in Florida. Proc.
Florida State Hort. Soc. L1X:103-6.
Wilson, J. W. and N. C. Hayslip. 1951. Insects attacking celery in Florida.
Fla. Agr. Exp. Sta. Bull. No. 486.
The Effect of Flooding on Nematode Populations
J. M. Good
Flooding has been recognized as a viable means of controlling plant
parasitic nematodes for more than 70 years. As early as 1907, Ernst
Bessey (1911) observed control of root-knot nematodes (Meloidogyne
spp.) on vegetables in flooded muck fields on islands (now submerged
and not farmed) in Lake Okeechobee. In 1921, J. R. Watson reported
that flooding of sandy soils on the lower East Coast of Florida for part
of the summer eliminated root-knot. At about the same time, re-
search at the University of California (Brown, 1933; Tyler, 1933)
established that flooding peat soils in the Sacremento/San Joaquin
Delta for 4 months killed root-knot larvae, but at least 12 months of
submergence were required to rid the soil of nematode eggs. In 1933,
flooding also was reported to control root-knot on tomatoes in the
Philippine Islands (Fajardo et al., 1933). Many years later, re-
searchers in New Jersey (Bird and Jenkins, 1965) found that up to
one year of flooding was required to achieve high level control
of some types of nematodes in cranberry peat bogs. These studies
showed that awl (Dolichodorus sp.), ring (Criconemoides spp.),
sheath (Hemicycliophora spp.), and spiral (Helicotylenchus spp.)
nematodes can tolerate high water tables and can not be effectively
controlled by flooding. On mineral soils in Louisiana root-knot and
stunt (Tylenchorhynchus sp.) nematodes, which also commonly occur
in Florida, were more effectively controlled by flooding than by fumi-
gating rice fields prior to planting (Hollis and Johnson, 1957).
NEMATODES IN HISTOSOLS
In the early 1950's flooding was tested at the Everglades Experi-
ment Station as a method for controlling root-knot nematodes (Fig.
5.1) in a rice-vegetable rotation on muck soils (Thames and Stoner,
1953). Celery and bean crops, which followed a rice crop kept flooded
for 66 days, were either free of galling or only slightly galled, while
the same crops were moderately to severely galled following rice
grown without flooding. Research at the Central Florida Experiment
Station (Christie, 1959) showed that when celery was grown in plots
with soil moisture held above the optimum needed for celery growth
for a period of 4 months, root-knot was not evident on cowpeas that
subsequently were grown in the plots. In the 1960's additional stud-
ies were done on Everglades muck soils. Fisher and Winchester
Figure 5.1. Celery plants (bottom) affected by root-knot nematodes,
and a normal celery plant, (top). All plants are the same age. (photo
by VL. Guzman).
(1964) found that a 2-week sequence of flooding-drying-flooding was
more effective in controlling root-knot than continuous flooding for 1,
3 or 5 months. Twelve months of continuous flooding was required to
rid the soil of all root-knot nematodes. In 1961, Rhoades (1964)
showed that dry fallowing and flooding gave about the same degree of
root-knot control on muck soil in Central Florida. Since dry summer
fallowing is destructive to soil fertility and increases the rate of
subsidence, it was concluded that flooding for 10 to 12 weeks is the
more practical way of reducing root-knot populations as well as
conserving muck soils.
PRINCIPLES OF CONTROL
The control principles involved in flooding are not completely
understood. Presumably flooding eliminates host plants, and the
nematodes starve. In addition, flooding decreases the oxygen content
of soil, which may kill some nematode species by asphyxiation.
Chemicals lethal to nematodes, such as butyric and propionic acids,
hydrogen sulfide, and perhaps others usually develop in flooded soils
containing high amounts of organic matter (Mai et al., 1968). Re-
search in Louisiana established that flooded rice fields had enough
hydrogen sulfide, which is produced by sulfur bacteria, to kill 100% of
the stunt nematodes in 10 days (Rodriguez-Kabana et al., 1965).
Based on the above research and much experience, the Everglades
Research and Education Center recommends a cycle of 4 weeks
flooding plus 2 weeks drying, and an additional 4 weeks flooding for
control of nematodes and other pests (Guzman et al., 1973). Of course,
it has been difficult for growers to follow this program exactly be-
cause of planting and harvesting schedules and local weather condi-
Figure 5.2. Celery seed beds are flooded for nematode control (top),
prior to shading, bed formation, and fumigation (bottom).
Successful control of nematodes by flooding requires good land
preparation to assure that weeds are killed and roots of the previous
crop are well decomposed. Flooded fields should be as level as possible
to assure a uniform flood of 5 to 10 cm (2 to 4 inches) required to
produce anaerobic conditions in the soil.
For seedbeds, flooding should extend several feet beyond the bor-
ders of the bed, and high spots along post lines and at edges of the field
should be leveled and kept flooded; otherwise, nematodes can be
reintroduced into the growing area during planting and cultivation
operations. Usually soil fumigation follows flooding of vegetable
seedbeds to assure that transplants are totally free ofnematodes (Fig.
5.2). Nematode-infected transplants should never be set in fields that
have been flooded or fumigated for nematode control.
Flooding of Everglades muck appears effective for controlling root-
knot and stunt nematodes on vegetables and sugarcane. Little is
known about nematode damage to seedling rice before the flood is
applied, but flooding young rice should protect the rice plants
through harvest and should extend nematode protection to subse-
quent crops of vegetables and sugarcane.
On muck and sandy soils of South Florida, flooding controls root-
knot and stunt nematodes. However, for some yet unknown reason,
sugarcane and vegetables are severely injured by a greater number of
nematode types on sandy soils than on muck. Nematode species such
as awl, sheath, spiral and sting (Belonolaimus sp.) nematodes com-
monly occur in sandy soils and may not be effectively controlled by
flooding. Additional research is needed to evaluate managing these
and other nematodes by flooding.
Bessey, E. A. 1911. Root-knot and its control. USDA, Bur. P. Ind. Bull. No.
Bird, G. W. and W. R. Jenkins. 1965. Effect of cranberry bog flooding and low
dissolved oxygen concentration on nematode populations. Plant Dis. Rept.
Christie, J. R. 1959. Plant nematodes, their bionomics and control. Fla. Agr.
Exp. Sta. p. 256.
Brown, F. S. 1933. Flooding to control root-knot nematodes. Jour. Agr. Res.
Fajardo, T. G., and M. A. Palo. 1933. The root-knot nematode, Heterodera
radicicola (Greef) Muller, of tomato and other plants in the Philippine
Islands. Philippine J. Sci. 51:457-481.
Fisher, D. W., and J. A. Winchester. 1964. The effects of flooding on root-knot
nematodes in organic soil. Soil and Crop Sci. Soc. Fla. 24:150-154.
Guzman, V. L., H. W. Burdine, E. O. Harris, Jr., J. R. Orsenigo, R. K.
Showalter, P. J. Thayer, J. A. Winchester, E. A. Wolf, R. D. Burger, W. G.
Genung, and T. A. Zitter. 1973. Celery production on organic soils of south
Florida. Univ. of Fla. Agric. Exp. Sta. Bull. 757. p 79.
Hollis, J. P., and T. Johnson. 1957. Microbiological reduction of nematode
populations in water saturated soils. Phytopath. 47:16.
Mai, W. F., E. J. Caians, L. R. Krusberg, B. F. Lownsbery, E. W. McBeth, D. J.
Raski, J. N. Sasser, and I. J. Thomason. 1968. Ch. 9. Reduction of nematode
populations through land-management and cultural practices. In Control
of plant-parasitic nematodes. Nat. Acad. Sciences Pub. 1696.
Rhoades, H. L. 1964. Effect of fallowing and flooding on root-knot in peat soil.
USDA Plant Dis. Reptr. 48 (4):303-306.
Rodriguez-Kabana, R., S. Jordan, J. Walfredo, and J. P. Hollis. 1965. Nema-
todes: Biological Control in rice fields: role of hydrogen sulfide. Science
Thames, W. H., Jr. and W. N. Stoner. 1953. A preliminary trial of low land
culture rice in rotation with vegetable crops as a means of reducing root-
knot infestation in the Everglades. Plant Disease Reptr. 37:178-192.
Tyler, J. 1933. The root-knot nematode. Calif. Agr. Exp. Sta. Cir. 330.
Watson, J. R. 1921. Control of root-knot. II. Fla. Agr. Exp. Sta. Bull. 159:30-
The Effect of Flooding on Plant Pathogen Populations
J. O. Strandberg
Fallow flooding is an example of plant disease management using
cultural practices. Flooding for disease control is both environmen-
tally acceptable and effective; it can reduce populations of soil-borne
pests including plant pathogens. Flooding for plant disease control is
not confined to Histosols but is used there most often because the
nature and origin of these soils often place them in low-lying areas
near an available, easily manageable source of water. On other soils
and especially in some regions, constraints such as the lack of an
inexpensive and easily manipulated water supply may limit the
feasibility of flooding. Climate may impose severe restrictions as
well. Flooding may also be limited because it uses large amounts of
water, and drainage water from flooded fields may contribute to large
amounts of nutrient runoff.
IMPACT OF FLOODING ON PLANT PATHOGENS
Flooding the soil affects plant pathogens in many ways, not all of
which are detrimental. What might be considered excess soil mois-
ture from the viewpoint of growing a crop is often beneficial or even
essential to the life systems of many fungal plant pathogens. For
example, abundant soil moisture is essential for the reproduction,
dissemination, and establishment of disease by fungi such as species
of Pythium and Phytopthora. Moreover, floodwater, like irrigation
water, can physically spread many pathogens. Water sources often
introduce the primary inoculum from which plant disease epidemics
arise. Prolonged flooding can be detrimental to both crops and plant
pathogens, but flooding cropland during a fallow period (if weather is
suitable) can offer many opportunities for better disease manage-
ment. This chapter is concerned with fallow flooding for disease
management and the use of aquatic crops such as rice, which may
serve the same purpose.
Unfortunately, little is known about the control of pests and dis-
eases by flooding. The restricted, highly regional use of flooding has
limited research to a few specific crops and pathogens. Nevertheless,
it is useful to examine these studies in order to formulate some
general principles. The literature on flooding for disease control has
been reviewed by Stover (1979).
EARLY FLORIDA STUDIES
Early information on flooding was based mostly upon observations
and limited to only one or two pathogens. The earliest work on
flooding for disease control was done in Florida. Flooding for disease
control in Florida was first recommended by Brooks (1942), who
apparently summarized several grower reports and personal ob-
servations on the utility of flooding and deep plowing to control
Sclerotinia sclerotiorum, the fungus that incites pink rot of celery.
Brooks observed that 4 weeks of summer flooding of celery fields was
adequate to kill a large percentage of sclerotia (the overseasoning
stage of S. sclerotiorum) that persisted in the soil of affected fields.
Brooks emphasized that fields had to be well-harrowed and then
completely covered with water. A few years later Moore (1949), also
working in Florida, employed both field and laboratory tests to deter-
mine that sclerotia of the pink rot fungus decayed completely within
23 to 45 days under flooded conditions. He found that alternate
flooding at 3-day intervals was less effective than continuous flood-
ing, and that neither green organic matter applied to the soil surface
before flooding nor the depth that sclerotia were buried in soil greatly
influenced the efficacy of the flooding treatment. However, sclerotia
buried in non-flooded soil under ambient field conditions decomposed
more slowly than those held under flooded field conditions. Moore
used sand, marl, and muck soil, and his results were similar for all
three soil types. Brooks, and later Moore, established a firm basis for
recommending flooding as a disease control measure for Sclerotinia
Stoner and Moore (1953) demonstrated that the cultural practices
employed in lowland rice production, particularly flooding, were
quite compatible with the goal of reducing populations of the pink rot
fungus in soils of Florida's Everglades region. In their plots, decom-
position of sclerotia was often complete after only 20 days in fields
where constantly moving water was supplied for lowland (paddy) rice
culture. This was about half the time required for an equivalent
effect under static (non-flowing) flooding conditions. Stoner and
Moore further demonstrated that sclerotia rotted, or partially rotted,
by the flooding treatments were not viable. They concluded that
summer rice culture was highly compatible with the disease-control
objective of summer flooding to control Sclerotinia sclerotiorum and
that the rice crop could partially pay for the cost of flooding. These
early reports firmly established the practice of flooding to control the
pink rot fungus in the Everglades region and the Central and West
Florida celery production areas. From Sumatra, Van Schreven (1948)
reported on the use of flooding to control black shank of tobacco
initiated by Phytophthora parasitica var. nicotiana.
MORE SPECIFIC DETAILS OF THE FLOODING PROCESS
About the same time (1950's), fallow flooding was tried in Hon-
duras to control banana wilt disease caused by the soil fungus Fusar-
ium oxysporum f. cubense. This procedure employed flooding with 60
to 150 cm (2 to 5 feet) of water for periods of up to 18 months. The
treatment was often effective. Stover et al. (1953, 1961) and Stover
(1953a, 1953b, 1954, 1955) studied some of the more basic processes
involved in controlling Fusarium spp. These papers provide much of
the detailed information available on the flooding process. Stover et
al. (1953) first studied changes in populations of algae, bacteria, and
fungi occurring during the flooding period. They found that the
general fungal populations in the soil, including Fusarium spp.,
decreased dramatically during the first 30 to 56 days after flooding
and remained low, but constant, for the remainder of the flooding
period. About 30 to 60 days after draining (at 180 days), fungal
populations, including Fusarium spp., increased rapidly. In this
system, fungal populations were rapidly and severely reduced in the
flooded soil, but more fungi, including Fusarium spp., were constant-
ly being introduced by the inward-flowing river and canal water
needed to maintain flooded conditions (Stover et al. 1953a, 1961). The
rapid increase in fungal populations observed 30 to 60 days after
draining flooded fields was attributed to fungi that persisted in flood-
water above the soil or at the well-oxygenated soil-water interface.
Apparently, these survivors were deposited on the newly drained soil
and recolonized it. Further investigations by Stover (1953a) demon-
strated that, although overall fungal populations were observed to
decline rapidly in flooded soils, low numbers ofFusarium oxysporum
f. cubense could persist for up to 165 days. Sand, clay loam, or organic
soil types had no consistent effects on this persistence under flooded
conditions. Another important finding by Stover (1953b) was that
temperature greatly affected the persistence of F. oxysporum f.
cubense; the survival rate at 13C (550F) was 10 to 20 times greater
and fungi persisted 90 days longer than at 24 to 34C (75 to 93F).
Stover presented good evidence to show that the lack of oxygen (02)
was a primary factor in the flooding process and the persistence of the
banana wilt pathogen.
Stover (1954, 1955) confirmed in laboratory experiments the points
mentioned above. He concluded that lack of oxygen was the primary
factor affecting the persistence of Fusarium sp. in the soil. Stover
indicated that, under the conditions encountered in banana produc-
tion, the population that survived flooding by existing in floodwater
or in the soil-water interface of the flooded soil found its way into
deeper layers of soil (where Fusarium spp. had been eradicated dur-
ing flooding) after the soil was dry and cracked following the draining
process. The cracks provided a route for fungi to be washed into the
soil by rain. Stover's findings for Fusarium spp. are applicable to
flooding in Florida vegetable production. Not only are Fusarium spp.
important components of the general complex of soil-borne diseases
in Florida, but Stover's observations on the survival and recoloniza-
tion of soil by Fusarium spp. may also apply to many other filamen-
tous fungi. If vegetable pathogens survive flooding in a manner
similar to that ofF. oxysporum f. cubense, they could easily be redis-
persed into the soil by the cultivation required to plant the crops.
Later studies have sought to explain the more general effects of
flooding on microorganisms including plant pathogens. Mitchell and
Alexander (1962) studied microbiological changes in flooded soils and
the decline in populations of three Fusarium spp. in 12 soils collected
in Central America. They found no influences due to soil pH or clay
content, but did find that soil organic matter content influenced the
rate and extent of the decline of Fusarium spp. populations; faster
rates of decline were associated with larger organic matter content.
Fewer numbers of plant pathogenic fungi survived when more or-
ganic matter was present. They suggested that products toxic to fungi
were formed when soils with large organic matter content were
flooded. They also showed that holding moist soil under controlled
anaerobic conditions did not fully substitute for controlled anaerobic
conditions produced by flooding to reduce populations of soil fungi.
Several recent studies involve other plant pathogenic fungi.
Although these studies frequently report specific phenomena which
may apply only to the pathogen studied, the results do not conflict
with general principles which might be gleaned from early flooding
research. It is therefore helpful to examine these studies to reinforce
any general principles which may have emerged from the work with
species of Sclerotinia and Fusarium.
Ioannou et al. (1977) found that flooding tomato field soils in
California reduced the production rate of microsclerotia of Verticil-
lium dahliae. However, they concluded that flooding treatments were
not very effective in controlling Verticillium wilt of tomatoes planted
following flooding. Ioannou et al. investigated in detail the rela-
tionship of soil gases to the inhibitory effects on microsclerotia pro-
duction. Levels of 02 and carbon dioxide (CO2), but not ethylene,
under both flooded and unflooded conditions were the primary factors
affecting V. dahliae in these studies; 02 was greatly reduced in their
flooded treatments. However, researchers flooded soil for only short
periods (20 to 60 days), so this may have accounted for the relative
ineffectiveness of short-term flooding in reducing tomato wilt in their
tests. Their results are important because information was obtained
about ethylene, which has been frequently suggested as a compound
that affects soil fungal populations especially because it is produced
under anaerobic conditions (Smith, 1976). However, there is little
evidence to date that ethylene is an important factor in the flooding
Verticillium dahliae has received much attention in the area of
cultural controls because this important disease of cotton, tomatoes
and other crops is not controlled economically by chemicals. But-
terfield et al. (1978) found that among a series of possible crop rota-
tions with cotton in California, only paddy rice culture, which in-
volved a period of continuous flooding, eradicated V. dahliae from
most but not all of their test plots; other rotations were much less
effective. However, Pullman and DeVay (1981) were more successful
in controlling Verticillium dahliae by summer flooding of cotton
fields with or without rice culture. They observed that neither flood-
ing during the cool fall months, nor irrigated, but unflooded, rice
culture were as effective in reducing subsequent damage by V.
dahliae to cotton planted on these soils. Measured populations of
microsclerotia declined much more slowly during cool-weather flood-
ing or irrigation, and increased more rapidly after draining the field
than those which underwent warm-temperature flooding or summer
paddy rice culture. Both greenhouse and field experiments showed
that flooding periods in excess of 6 to 8 weeks were required even
under summer conditions. Rice culture increased the effectiveness of
flooding; longer flooding periods were required when rice plants were
not present. Beneficial results as measured by cotton lint yields were
observed into the third year following effective flooding treatments.
Rice culture has also been effective in greatly reducing damage to
tobacco by the black shank disease in Sumatra (Van Schreven, 1948).
Examples thus far have included only soil-borne diseases. Flooding
is also useful in controlling foliar disease-producing organisms which
may overseason or persist between crops in soil or plant debris.
Seedling rust of safflower can be controlled by short-term flooding
(Klisiewicz, 1977). Disease incidence presumably resulting from the
teliospores ofPuccinia carthami, which persisted in the soil between
safflower crops, was greatly reduced by flooding fields for 7 days
during high temperature periods. Controlled environment experi-
ments showed that flooding at temperatures above 36'C (970F) was
adequate to eradicate the pathogen from infested soil within 4 to 7
days. Cooler temperatures or alternating day and night temperature
regimes employing night temperatures below 26C (78F) were not
effective, and reduction of viable teliospores required longer flooding
periods at cooler temperatures.
Fallow flooding can also reduce populations of fungi that benefit or
flourish during conditions of excess soil moisture, because moisture
levels present during the flooding period are greatly in excess of the
"high moisture" conditions sometimes required by these fungi. Spe-
cies of Pythium are good examples. They cause damping-off of young
seedlings and reduce root systems of several vegetable crops by
attacking and killing the small roots. The root system is greatly
reduced and plant growth is slowed. Damage by Pythium spp. is
commonly associated with excess soil moisture and, indeed, Pythium
species and related fungi apparently need a film of water on soil
particles or water-filled soil capillaries for the swimming stage (zoo-
spores) to disperse and spread. High soil moisture may also increase
the likelihood of attack and damage of plant roots. Still, populations
of Pythium spp. are effectively reduced by fallow flooding. Such
flooding is common practice for Florida celery and carrot production
on organic soils.
Strandberg (1985) showed that soil populations of Pythium spp.
that attack carrots and other vegetables rapidly decreased during a
4- to 6-week flooding period. Like the Fusarium spp. studied by
Stover et al. (1953), soil populations of species of Pythium declined
rapidly to very low levels during the flooding process, but increased
after draining and cultivation of the fields (Fig. 6.1). In flooded carrot
fields on organic soil, the soil beneath the soil-water interface is very
low in oxygen and Pythium spp. populations there are greatly re-
duced or eliminated. However, it is likely that Pythium spp. survive
in the floodwater or at the soil water interface much as Stover found
for Fusarium spp. They can then recolonize the field following flood-
ing and draining. Land preparation for planting carrots would redis-
tribute surviving fungal propagules in the soil.
Temperature can also be an important factor in Pythium spp.
control by flooding. In controlled environment experiments, popula-
tions ofPythium spp. declined very slowly during flooding at temper-
atures below 20C (680F) but declined at rates similar to those
observed in field experiments at temperatures of 24 to 280C (75 or
83F). In controlled environment studies, neither alternating periods
(weekly) of flooding and draining, nor alternating or continuous
saturation of soils were nearly as effective as continuous flooding in
reducing Pythium spp. population (author, unpublished). When soil
was flooded for long periods in large containers in the greenhouse,
surviving populations of Pythium spp. did not increase after drain-
ing. In carrot culture, overall populations ofPythium spp. are greatly
reduced in flooded soils and remain low enough for a short period
after draining (30 to 90 days) so that the vulnerable tap roots of young
carrots can develop with a lower probability of damage from Pythium
spp. Fallow flooding is very effective for this purpose and is routinely
i i oV
, 1 --0--FLOODED
i u II I I I I
10 20 30
WEEKS AFTER HARVEST
Figure 6.1. Populations of Pythium spp. in two vegetable production
fields at Zellwood, Florida. Following the harvest of a sweetcorn crop,
one of the fields was flooded for 6 weeks, then drained, cultivated,
and planted to carrots. The non-flooded field was covered with weed
growth and was intermittently disked prior to planting carrots.
Pythium spp. populations were reduced to low enough levels in the
flooded field to allow carrot tap roots to develop with very low disease
damage from Pythium spp. (from Strandberg, 1985).
GENERAL EFFECTS OF EXCESSIVE SOIL MOISTURE
ON PLANT PATHOGENS
Although flooding has not been extensively tested to control very
many diseases, both the good and bad effects of excess soil moisture
have been examined for many fungi and diverse results have been
reported. Frequently, high levels of soil moisture are beneficial to
fungi in some way; this will be discussed later. Often, however, high
levels of soil moisture (excess soil water with or without high temper-
atures) have been found to be detrimental to the survival of soil fungi.
Verticillium dahliae has received much attention in this regard.
Usually, a water-saturated soil coupled with high temperature have
adversely affected the survival of microsclerotia of V. dahliae (Green,
1980) compared to their survival in drier soils. Keim and Webster
(1974) obtained similar results for Sclerotium oryzae, a fungus that
incites the stem rot disease of rice. Sclerotia in wet soil incubated at
24C (750F) for 2 to 10 weeks had a lower percentage of viability than
those incubated at 1C (34F), those maintained in dry soil, those
recovered from dry soils and then incubated at 24C, or sclerotia that
received alternate wetting and drying. These results are similar to
those observed for Verticillium dahliae and may also apply to other
sclerotia-forming fungi such as Sclerotinia sclerotiorum. However, it
has been reported that up to 24% of the sclerotia of Sclerotium oryzae
recovered from rice soils after 4 months of flooded rice culture were
still viable (Krause and Webster, 1972). This indicates that Sclero-
tium spp. may be quite resistant to flooded conditions. A related
species, Sclerotium rolfsii, which incites the southern blight disease
on several crops, may well be more resistant to flooding than other
pathogens because it continues to be a problem in Florida carrot
production fields where flooding is routinely practiced. In contrast,
Sclerotinia sclerotiorum, which was widespread before flooding was
common, is now seldom found in routinely flooded fields (author,
BENEFICIAL EFFECTS OF FLOODING ON
Flooding may also have beneficial effects upon plant pathogenic
fungi. Soil moisture levels considered excessive for crop production
are required by many fungi to complete some of their life processes
such as reproduction, dissemination, and the colonization of host
plants. Excess moisture may also predispose plants to disease. A
large and varied literature associated with this topic has been re-
cently and extensively reviewed by Stolzy and Sojka (1984). General-
ly, the effects due to flooding that could be considered beneficial to
plant pathogens are created by short-term flooding, i.e. a few hours or
a few days. Such periods of excess water may be deleterious to crop
plants as well (Stolzy and Sojka, 1984), but some crops such as celery
are noted for their tolerance of water-saturated or flooded soils. With
tolerant crops, there may be opportunities for insect pest control by
short-term flooding during crop production. However, this may some-
times be impractical because of the favorable conditions created for
plant pathogenic fungi or for establishment of disease by short-term
flooding. For example, Stanghellini and Burr (1973) found that short
periods of wet soil conditions favored Pythium aphanidermatum by
increasing nutrient availability for oospore germination, while the
water-filled soil pores were helpful in zoospore dissemination (see
also Duniway, 1979). Ferrin (1985) correlated periods of disease
increase with periods of high soil moisture following rainfall for
Phytophtora parasitica var. nicotiana, the incitant of black shank of
tobacco. Numerous studies involving many fungi demonstrate simi-
lar beneficial effects on pathogens resulting from short-term flooding
or soil saturation (Stolzy and Sojka, 1984; Cook and Baker, 1983).
WHY DOES FLOODING WORK?
It is not clear why or how fallow flooding works to reduce soil
populations of fungi. As mentioned, most of our knowledge comes
from narrowly focused experiments with specific fungi. Yet it is
obvious that all pests inhabiting the soil are affected by long-term
flooding. There is no direct evidence to assume that the same factors,
for example, lack of oxygen, act to kill or inhibit all pest species.
Reddy and Patrick (1983) recently reviewed the literature on the
chemistry of flooded soils (see also Reddy, this bulletin; Howeler and
Bouldin, 1971; and Ponnamperuma, 1972). In flooded Histosols, 02 is
rapidly depleted. Concurrently, fairly large concentrations of CO2
are produced and there is potential for the production of methane
(CH4), ethylene (C2H4), hydrogen sulfide (H2S), and perhaps other
unknown compounds. Some of these compounds have been impli-
cated in nematode control in flooded soils. Ethylene is often men-
tioned as a possible compound produced in the soil that greatly affects
soil biology (Smith, 1976). Lack of 2 is often cited as a possible cause
of population declines in flooded soil, yet it may not be a direct cause
so much as a result; there are alternative explanations. For reducing
populations of Fusarium spp., Stover (1955) found that anaerobic
conditions at normal soil moisture levels were not as effective as
anaerobic conditions with flooding. Pythium spp. propagules exposed
to soil gases, but not the soil solution in a flooded organic soil,
survived much longer than propagules that were in contact with the
soil solution (Strandberg, unpublished). Thus, anaerobic conditions
coupled with flooding itself seem to be the most effective treatment
for reducing some soil fungi.
One explanation for this fact is that organisms that flourish in
anaerobic and water-saturated environments are the primary factors
in reducing fungal populations. Taylor and Guy (1981), working in
New Zealand with the decline and replanting problems of fruit and
forest trees, noted that the general problem of decline was not evident
in poorly drained soils. They studied flooding as a possible control. In
laboratory experiments, flooding was found to be effective in eradi-
cating eight types of wood-decaying fungi growing on wood samples
buried and then flooded for 12 weeks. However, individual samples of
all eight fungi were recovered from treatments receiving irrigation
as well as those receiving no irrigation. In more extensive laboratory
experiments, the researchers flooded samples of several soil types.
The test fungus (Peniophora sacrata, a basidiomycete) was not killed
by flooding sterile soil containing wood colonized by this species. The
attribute of the active soils that eradicated P. sacrata during flooding
could be transferred to sterile soil by adding as little as 0.5% of the
active soil sample to the sterile soil. This is good evidence for a
biologically based effect in this system. Taylor and Guy thought this
transferable activity was due to bacteria that flourish during anaero-
bic conditions. Menzies (1962) reported that sclerotia of Verticillium
dahliae were killed by open flooding within 6 weeks. Under flooding
plus nitrogen conditions (to exclude 02), sclerotia were killed in 3
weeks. He also found substantial effects due to organic soil amend-
ments with flooding; sclerotia were killed and the total fungus
population was much reduced after 5 days when organic matter was
added. Menzies concluded that a fungicidal compound was produced,
with effects that lasted several days after the soil was drained.
Other biotic effects have also been reported. Sneh et al. (1977),
found that in flooded soils, oospores of species of Phytopthora,
Pythium, and Aphanomyces euteiches were parasitized by chytrids,
oomycetes, an actinomycete, and bacteria. The oospore parasites
were quite different in those soils with moisture levels below water
holding capacity. These experiments were short term (up to 10 days
of flooding), but help to demonstrate possible biotic factors that may
be involved in the flooding process.
DISEASE MANAGEMENT PROBLEMS CAUSED
The possibility of detrimental effects on crop production due to
flooding was mentioned previously in this article. The direct, delete-
rious effects of flooding on sensitive crop plants easily damaged by
water-saturated soils are apparent. Less obvious, but also det-
rimental, are the beneficial effects of excess soil moisture on the life
systems of pathogens and the predisposition of host plants to disease.
These subjects have been extensively reviewed (Stolzy and Sojka,
1984; Duniway, 1979). Since most applications of flooding for disease
control are applied during fallow periods, it is unlikely that a serious
attempt at flooding for disease control will interact unfavorably with
crop plants or favorably influence life processes of most fungi and
bacteria; however, our knowledge is rather scant on this point.
A more obvious and well-documented problem is the introduction
and physical spread of plant pathogens by water used for flooding.
Flowing water is an effective, well-known dispersal agent for many
disease-producing organisms. This problem may often represent a
severe constraint to flooding for disease control. Thus, the introduc-
tion of pathogenic fungi in water used for flooding should be an
important consideration in any decision to flood. We have already
discussed the findings of Stover (1953) concerning the introduction of
Fusarium spp. by river and canal water. Similar sources of plant
pathogens are more important than once believed; plant pathogens
have been detected in irrigation water, according to several reports.
Shokes and McCarter (1979) detected species of Rhizoctonia, Fusar-
ium, Pythium, and Phytopthora in irrigation water sources in Geor-
gia, and others have documented similar situations. Gill (1970)
demonstrated that pathogenic species of Pythium persisted in irriga-
tion ponds, and mentioned numerous similar studies that reinforce
this point. It must be concluded that pathogenic fungi can survive in
water sources used for flooding as well as in floodwater itself, in the
water-soil interface of flooded fields, and in floating or submerged
plant debris as Stover (1953) has shown for Fusarium oxysporum f.
PRACTICAL ASPECTS AND OPPORTUNITIES FOR
FLOODING IN DISEASE MANAGEMENT
Research results discussed thus far demonstrate our knowledge of
flooding to control plant disease is fragmentary; effects on only a few
specific fungi have been studied. However, Stover (1979) mentions
that plant pathogenic bacteria may also be controlled by flooding. In
spite of this, flooding is extensively practiced in crop production
systems on Florida's organic soils. Multiple benefits are realized from
flooding, so the grower is likely to receive some return. Another
reason for flooding is that there are no reasonably effective chemical
alternatives for the control of soil-borne disease-producing orga-
nisms and other pests. Flooding is worth trying. As a result, diseases
and damage caused by Sclerotinia sclerotiorum (pink rot of celery,
lettuce drop, etc.) Sclerotium rolfsii (southern blight), root damage by
Pythium sp., and wireworms have decreased markedly in the past 15
years in crops grown on organic soils. Carrot root damage caused by
Pythium spp. has not been eliminated but has been drastically re-
duced from levels once sustained by growers, and it is well below
levels now seen where flooding is not employed. Rice culture seems to
offer disease control as an added benefit.
Flooding for disease management can and does work. However,
water is a valuable resource; it should be used in an efficient and
conservative way. Studies on effects of flooding on three fungal spe-
cies, which are themselves important or representative of plant
pathogens important in Florida vegetable production (Sclerotinia
sclerotiorum, Fusarium spp. Pythium spp.), provide some idea of
the type of flooding treatment necessary. Studies of other plant
pathogens' responses to flooding, as well as other uses of flooding, can
help formulate more effective, efficient flooding treatments.
Consideration of practical aspects of flooding for disease manage-
ment must include what is known about the basic processes involved
in flooding. The most demanding requirements, such as sufficiently
warm soil-water temperatures and long enough flooding periods,
should be thought out and planned for far in advance when estab-
lishing goals. Proposed flooding regimes must also include crop and
soil management as well as general pest management practices. In
most cases, disease control uses can be fully compatible with other
IMPORTANT CONSIDERATIONS FOR FLOODING
Using present knowledge of flooding's impact on only a few plant
pathogens, some guidelines can be established that are likely to
improve the effectiveness of flooding for general disease manage-
1. Time or season of flooding: In general, the higher the am-
bient air and soil temperature, the more effective flooding will be
during the time interval it is applied. This places some seasonal and
geographical constraints on the use of flooding. An estimate of the
potential application of flooding for disease control may be obtained
from the average weekly air temperatures for the two major Histosol-
based vegetable production areas in Florida. For these locations,
average monthly air temperatures were well correlated with average
soil temperatures (r 0.98). A better indication of optimum flooding
temperatures are the degree days accumulated above 7C (45F).
These data indicate that flooding periods should be increased during
December through February or March (Fig. 6.2).
Experience with species of Pythium, Fusarium, Verticillium
dahlia and other fungi indicates that temperature during flooding is
a very important factor. Thus, the flooding period must be extended
when temperatures average below approximately 20'C (68F); there
is some indication that any flooding at low temperatures may be
2. Appropriate flooding interval: All things considered, the
longer the flood period the better; but pumping costs and alternative
land uses must be evaluated. For general purpose disease control in
0 BELLE GLADE
JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC
Figure 6.2. Degree days accumulated above a 7C (450F) threshold
temperature for the Everglades and Central Florida Histosols. During
months with low degree day accumulation, flooding periods should be
increased (data from National Oceanic and Atmospheric Administra-
tion covering 1950 to 1981).
Florida during warm weather, flooding periods should probably last
from 4 to 6 weeks, whereas in cool seasons 6 to 10 weeks may be
necessary to obtain the same effects.
3. Alternate flooding and disking: The work of Stover et al.
(1961) with Fusarium spp. indicates that pathogen populations that
survive flooding by persisting in the soil-water interface or on the soil
surface during draining might be more vulnerable to eradication if
disked deep into the soil and reflooded. This makes sense, but there
are two problems. Work with most plant pathogens to date indicates
that several weeks of flooding are needed for each flooding interval.
The commonly observed practice of two weeks of flooding alternated
with draining and disking does not seem consistent with reported
research results. Two weeks is insufficient time to reduce pathogen
populations to a low level. Added to this are the problems of re-
introduction of pathogens with water used to reflood the area and the
increased nutrient runoff resulting from multiple flooding. Clearly,
the time and expense involved in alternate flooding and disking
requires more research to jutify this type of flooding regime. Work in
controlled environment tests with Pythium spp. (Strandberg 1985)
suggests that this approach is not very effective. Alternate flooding
and disking programs may have originated in attempts to combine
disease control with control of the root-knot nematode. These uses
may not be compatible.
4. Water depth: Most fallow flooding in Florida involves flooding
to a depth of 10 cm (4 inches) to perhaps 40 cm (16 inches). The high
oxygen demand of Histosols depletes oxygen very rapidly below the
soil water interface regardless of the water depth. Apparently 10 to
40 cm of water is adequate. Controlled environment work (done in
several studies) has demonstrated that even 5 (2 inches) to 10 cm (4
inches) of water is effective, but merely saturating the soil was not
effective in the case of Pythium spp. The reasons for this are not
known, but apparently enough oxygen is available at the soil-water
interface for large populations of pathogens to survive. However,
more work will be needed to clarify this point. Maintaining a satu-
rated soil would conserve more water and would lessen the nutrient
runoff when fields are drained, if it could be found to be effective.
5. Field condition before flooding: Early reports stressed
that fields must be free of green plant material. This requirement is
unclear; the effectiveness of paddy rice culture as a substitute for
fallow flooding indicates that the absence of plants may not be essen-
tial. However, many pathogens can survive in plant debris and large
amounts of plant debris persisting in water as floating detritus seems
counter-productive to the intent of flooding. Thus, fields should prob-
ably be well-disked to incorporate all crop residue prior to flooding.
6. Rice culture: This culture seems to be fully compatible with
flooding for disease control, presenting an attractive alternative to
fallow flooding and offering opportunities for more efficient land and
Brooks, A. N. 1942. Control of celery pink rot. Fla. Agr. Exp. Sta., Press Bull.
Butterfield, E. J., J. E. DeVay, and R. H. Garber. 1978. The influence of
several crop sequences on the incidence of Verticillium wilt of cotton and
on the population of Verticillium dahliae in soil. Phytopathology 68:1217-
Cook, R. J., and K. F. Baker. 1983. The nature and practice of biological
control of plant pathogens. Am. Phytopathol. Soc., St. Paul, MN 539 pp.
Duniway, J. M. 1979. Water relations of water molds. Annu. Rev. Phyto-
Ferrin, D. 1985. Spatial and environmental considerations in the epidemiol-
ogy of black shank of tobacco. Ph.D. Thesis, University of Florida, Gaines-
Gill, D. L. 1970. Pathogenic Pythium from irrigation ponds. Plant Dis. Rept.
Green, R. J. 1980. Soil factors affecting survival of microsclerotia of Verticil-
lium dahliae. Phytopathology 70:353-355.
Howeler, R. H., and A. R. Bouldin. 1971. The diffusion and consumption of
oxygen in submerged soils. Soil Sci. Soc. Am. Proc. 35:202-208.
Ioannou, N., R. W. Schneider, and R. G. Grogan. 1977. Effect of flooding on
the soil gas composition and the production of microsclerotia by Verticil-
lium dahliae in the field. Phytopathology 67:637-644.
Keim, R., and R. K. Webster. 1974. Effect of soil moisture and temperature on
viability of sclerotia of Sclerotium oryzae. Phytopathology 64:1499-1502.
Klisiewicz, J. M. 1977. Effect of flooding and temperature on incidence and
severity of safflower seedling rust and viability ofPuccinia carthami tel-
iospores. Phytopathology 67:787-790.
Krause, R. A. and R. K. Webster. 1972. Sclerotial production, variability
determination, and quantitative recovery of Sclerotium oryzae from soil.
Menzies, J. D. 1962. Effect of anaerobic fermentation in soil on survival of
sclerotia of Verticillium dahliae. (Abstr.). Phytopathology 52:743.
Mitchell, R., and M. Alexander. 1962. Microbiological changes in flooded
soils. Soil Sci. 93:413-419.
Moore, W. D. 1949. Flooding as means of destroying the sclerotia of Scleroti-
nia sclerotiorum. Phytopathology 39:920-927.
Ponnamperuma, F. N. 1972. The chemistry of submerged soils. Ad. in Agron.
Pullman, G. S., and J. E. DeVay. 1981. Effect of soil flooding and paddy rice
culture on the survival of Verticillium dahliae and incidence of Verticil-
lium wilt of cotton. Phytopathology 71:1285-1289.
Reddy, K. R., and W. H. Patrick, Jr. 1983. Effects of aeration on reactivity
and mobility of soil constituents. In Chemical mobility and reactivity in
soil systems. Soil Science Society of America, Madison, WI pp. 11-33.
Shokes, F. M., and S. M. McCarter. 1979. Occurence, dissemination, and
survival of plant pathogens in surface irrigation ponds in southern Geor-
gia. Phytopathology 69:510-516.
Smith, A. M. 1976. Ethylene in soil biology. Annu. Rev. Phytopathology
Sneh, B., S. J. Humble and J. L. Lockwood. 1977. Parisitism of oospores of
Phytophthora megasperma var. sojae, P. cactorum, Pythium sp. and Apha-
nomyces euteiches in soils by oomycetes, chitridiomycetes, Hypomycetes,
Actinomycetes and bacteria. Phytopathology 67:622-628.
Stanghellini, M. E., and T. J. Burr. 1973. Effect of soil water potential on
disease incidence and oospore germination of Pythium aphanidermatum.
Stolzy, L. H., and R. E. Sojka. 1984. Effects of flooding on plant disease. T. T.
Kozlowski. (Ed.) Flooding and plant growth. Academic Press, New York,
NY. 575 pp.
Stoner, W. N., and W. D. Moore. 1953. Lowland rice farming, a possible
cultural control for Sclerotinia sclerotiorum in the Everglades. Plant Dis.
Stover, R. H. 1953a. Effect of soil moisture onFusarium oxysporum f. cubense
in the laboratory. Phytopathology 43:499-504.
Stover, R. H. 1953b. Effect of soil moisture on Fusarium spp. Can. J. Bot.
Stover, R. H., N. C. Thornton and V. C. Dunlap. 1953. Flood fallowing for
eradication ofFusarium oxygsporum f. sp. Cubense. I. Effect of flooding on
fungal flora of clay loam soils in Viva Valley, Honduras. Soil Sci. 76:225-
Stover, R. H. 1954. Flood fallowing for eradication ofFusarium oxysporum f.
cubense. II. Some factors involved in fungus survival. Soil Sci. 77:401-414.
Stover, R. H. 1955. Flood fallowing for eradication ofFusarium oxysporum f.
cubense. III. Effect of oxygen on fungus survival. Soil Sci. 80:397-412.
Stover, R. H. 1979. Flooding of soil for disease control. In Soil Disinfestation:
Developments in Agricultural and Managed Forest Ecology. Vol. 6.
D. Mulder (Ed.) Elsevier Scientific Publishing Co., New York 368 p.
Stover, R. H., R. C. Hildreth and N. C. Thornton. 1961. Studies on Fusarium
wilt of bananas. VII. Field Control. Can. J. Bot. 39:197-206.
Strandberg, J. 0. 1985. Flooding organic soils to control Pythium sp. which
attack carrot and other vegetables. Proc. Florida State Hort. Soc. 97:164-
Taylor, J. B., and Esme M. Guy. 1981. Biological control of root infecting
Basidiomycetes by species of Bacillus and Clostridium. New Phytol.
Van Schreven, D. A. 1948. Investigations on certain pests and diseases of
Vorstenlanden tobacco. Tijdschr. Plantenziekten 54:149-174.
The Effect of Flooding on Weed Populations
J. A. Dusky
Weeds compete for space, light, moisture, and nutrients. If uncon-
trolled, weeds reduce crop yield and quality and increase the cost of
harvesting. Secondary effects of weed infestations on crops are often
unrecognized. Some weed root exudates or leachates and weed res-
idues can affect crop growth. Weeds are also hosts for diseases,
insects, nematodes and viruses that attack crops. It has been esti-
mated that weeds cause an annual loss of 10% in agricultural produc-
tion in the United States (Shaw, 1978).
Weeds multiply and reproduce by both sexual and vegetative
(asexual) means. Weed seeds generally have no method of active
movement, yet the spread of weed seeds, plus their ability to remain
viable in the soil for many years, poses one of the most complex
problems of weed control. Weed seeds may be transported by wind,
water, animals (including man), machinery, crop seed, grain feed,
hay, and straw. Weed species are also able to produce many viable
seeds per acre. Areas of Louisiana heavily infested withjohnsongrass
(Sorghum halepense (L.) Pers.) had an average number of 4,092,000
viable johnsongrass seeds per hectare (1,657,000 seeds per acre)
(Phillips and Chilton, 1949). Numerous studies have been conducted
to determine weed seed production. Stevens (1932) reported that
barnyard grass (Echinochloa crusgalli (L.) Beauv.) produced 7,160
seeds per plant, lambsquarter (Chenopodium album L.) 72,450 seeds
per plant, and redroot pigweed (Amaranthus retroflexus L.) 117,400
seeds per plant. One witchweed (Striga sp.) plant may produce as
many as one-half million seeds (Barnes, 1960). Some weed seeds
remain viable for many years while others may die within a few
weeks in an unsuitable germination environment. The persistence of
a weed is dependent upon its ability to reinfest the soil and then
Temperature, moisture, oxygen, light, and the presence of inhibi-
tors affect seed dormancy as well as mechanical factors directly
related to the seed. Germination is associated with uptake of water
and oxygen, use of stored food, and, generally, a release of carbon
dioxide. Specific requirements for seed germination differ among
weed species. Under optimum conditions, some seed may not germi-
nate because of imposed or innate dormancy. Dormancy may deter-
mine the time of year a seed germinates or it may delay germination
for years, thereby guaranteeing the viability of weed seeds for years
to come. Once a weed has germinated, its ability to grow and repro-
duce is dictated by environmental conditions.
For years, the high organic matter (muck) soils in Florida have
been flooded during the summer months as a fallow land manage-
ment practice for control of diseases and insects and the reduction of
soil subsidence. It has also been thought to be a process that controls
weeds (Guzman, et al., 1973). The following discussion is somewhat
speculative, since only limited data are available. Needed research
will also be discussed.
Germination is dependent upon oxygen, and the amount of soil
oxygen depends upon soil porosity, soil depth, and soil organisms that
utilize oxygen. Soil oxygen content is inversely proportional to car-
bon dioxide content. Soil carbon dioxide content increases and oxygen
content decreases with depth, and differences can be marked during
wet periods or in flooded soils (Ponnamperuma, 1976). Soil oxygen
necessary for seed germination varies among species. Optimum ger-
mination usually occurs at 20% soil oxygen, which is similar to the
oxygen content of air (21%). Most weed seed germinate in the upper
2.5 to 5.0 cm (1 to 2 inches) of soil where the soil atmosphere composi-
tion is close to that of the air. Cultivation increases soil aeration, thus
increasing the oxygen content (Bibbey, 1948). Flooding produces an
oxygen-deficit and the oxygen content of flooded rice land may be less
than 1% (Erygin, 1936). Generally, increasing 02 concentrations
stimulates germination, whereas increasing CO2 concentrations in-
hibits germination. Rare exceptions are species like cattails (Typha
sp.), which are adapted to heavy soils with poor aeration. It appears
that flooding of soils would reduce weed seed germination and de-
crease weed infestations. Excess water in the soil reduces or delays
seed germination of most plant species and this response is attributed
to lowered soil oxygen rather than excess water.
Light also affects weed germination. Several hundred species have
been studied and about half require light for maximum germination.
Day length and light quality also influence seed germination (Mayer
et al, 1975). Generally, weed germination, especially for small-seeded
species, is promoted by light. Large-seeded species tend to show an
indifference to light, but this response has no absolute value, is
related to quality, intensity, and duration, and is influenced by
temperature. The red portion of the light spectrum may play an
important role in germination. In a flooded cultural system, the
intensity and quality of light reaching weed seeds in the upper
surface layers would be drastically reduced, which could reduce weed
Optimum temperatures for seed germination vary with weed spe-
cies. Three critical temperatures are relevant. Most species exhibit a
minimum and a maximum temperature below or above which no
germination takes place, and an optimum temperature where seeds
will germinate the quickest. These temperatures are fairly specific,
but they may vary with the seed origin, seed age, storage conditions,
light, etc. With some species it is even more complex, for optimum
germination occurs with alternating temperatures. Redroot pigweed
(A. retroflexus) seed in a germinator set at 200C remained dormant for
6 years (Crocker and Barton, 1953). Seed could be induced to germi-
nate at any time by raising the temperature, by hand-rubbing the
seeds and replacing them in a germinator at 200C, by partial dessica-
tion, or by alternating temperatures. Temperature itself, although
extremely important, does not explain why seeds do or do not germi-
nate. Often a seed will have another form of dormancy that is re-
sponsible, e.g., some Bromus sp. have a primary dormancy of 4 to 5
weeks after maturity, which can be broken only when subjected to
low temperatures (Steinbauer and Grigsby, 1957). Seeds stored for 4
to 5 weeks will germinate at 20 to 26 C. In a flooded situation the
temperature at the soil surface would be altered and lower tempera-
tures would affect germination.
The presence of an inhibitor also affects seed germination. There
are two types of inhibitors: one present in the seed itself which must
be removed before the seed will germinate, and one which is present
in the soil due to residues from other species. The phenomenon of
inhibitors is not well understood, but it is known that the inhibitor
must be removed before germination can take place.
The effects of flooding upon previously noted processes can only be
speculated upon, for little information exists. A number of things
may happen when a weed-free field containing a large soil seed
reserve is flooded. Soil-atmosphere oxygen content is drastically re-
duced and carbon dioxide content is increased (Ponnamperuma,
1972), the temperature at the soil surface may decrease, and the
amount of light reaching the soil surface would be decreased, with a
change in the relative proportion of specific wavelengths as well.
Each of these conditions alone could reduce germination, if not stop it
altogether. A combination of all these factors would likely prevent
germination of most weed seed, except those specifically adapted for
such conditions. Most researchers believe that reduced germination
would be a result of low oxygen content. However, the depth of the
flood would affect the influence each of these factors has on germina-
What becomes of that seed? There are two possibilities. The seed
could die because of a harsh environment for germination or it could
become dormant but remain viable. In this author's opinion the seed
under flooded conditions would become dormant. Beal in 1879 buried
20 different weed species. Eleven species were still viable 20 years
after they were buried and nine were still alive 40 years after they
were buried. In 1960, 80 years after their burial, moth mullein
(Yerbascum blattaria L.) still had a germination between 70 and 80%
(Darlington and Steinbauer, 1961). In another study, Toole (1946)
reported that 38 years after burying, 91% of jimsonweed (Datura
stramonium L.) seed and 7% oflambsquarter (C. album) germinated.
Of the 107 species buried, 36 species still germinated after 38 years.
Seeds buried deep in the soil do not germinate but may lie dormant
for years; when brought to the surface they germinate promptly.
Aeration involving increased oxygen supply is probably responsible.
Conditions that exist when seeds are buried are similar to those in a
periodically flooded field: low oxygen content in the soil atmosphere,
reduced light, and reduced temperatures. From personal observation
it appears that few, if any, seeds germinate in flooded fields and
seedling establishment is poor if they do so. Only species specifically
adapted to such a situation would survive. However, Brums and
Rasmussen (1953, 1957, 1958) have found great variation in the
length of time that seeds remain viable in fresh water. There was
little or no variation in germination between depths, but there were
considerable differences in viability among species. Some species still
germinated 3 to 5 years after storage in fresh water. In some cases
storage in water tended to break dormancy and increased the per cent
germination, especially after 2 to 4 months in the water.
A "flush" of weeds is noticed when water is removed from flooded
fields in the Everglades Agricultural Area. Oxygen content of the soil
is increased as is the soil temperature and amount of light reaching
the soil surface. After the field is disked or plowed even more weed
seeds germinate, probably because of increases in 02 and tempera-
ture. By cultivating the soil, buried seeds are brought to the surface
and to a suitable environment. Table 7.1 lists the most common weed
species of the Everglades Agricultural Area observed in fields after
floods have been removed.
What happens to an established seedling when it is flooded? The
environmental conditions that play an important role in seed ger-
mination also play important roles in seedling growth, vegetative
growth and, ultimately, reproductive growth. Flooded soils, with low
soil oxygen, reduce growth of most crops although differences exist
among species (Hoveland and Mikkelsen, 1967; Williamson, 1964).
In contrast, dry matter accumulation by species such as rice is fav-
ored by low soil oxygen (Luxmoore and Stolzy, 1969). Hoveland and
Buchanan (1972) reported that fall panicum (Panicum dichoto-
miflorum (L.) Michx.) was more tolerant of flooded soil than Texas
panicum (Panicum texanum Buck L.). Root development of Texas
panicum was reduced by 50% under a regime of flooding for 6 days,
followed by 4 days of drainage, repeated three times for one month.
Orsenigo (1969) compared goosegrass weed weights under four soil
moisture regimes: air dry, field capacity, flooded completely (2.5 to
5.0 cm [1 to 2 inches]), and an alternate flooding, drying and flooding
regime of two weeks each. Flooding produced one-third and alternate
flooding, drying and flooding produced one-half the weed weights
recorded for the containers maintained at field capacity. The toler-
ance of a weed species once established and then flooded would be
dependent upon the species, length of flooding period, depth of flood,
the growth stage at which it was flooded, and other environmental
conditions. Weed species such as goosegrass (Eleusine indica (L.)
Gaertn) and spiny amaranth (Amaranthus spinosus L.) will die when
submerged by a flood (Table 7.1). Fall panicum (P. Dichotomiflorum
(L.) Michx.), nutsedge (Cyperus sp.), redstem (Ammannia sp.), and
dayflower (Commelina sp.) seem to survive a flood even if initially
covered by water (Table 7.1). The effects of flooding upon a weed's
competitive ability with a crop such as rice is unknown.
What happens to the weed seed present in the water and soil after a
flood is removed? If the seed in the soil or in the water was viable, the
drained environment will determine the germination of that seed.
Table 7.1. The effects of flooding on weed populations in the Ever-
glades Agricultural Area.
Weed species Weed species which
Weed species which controlled by either germinate or
germinate after a a properly survive during a
flood is removed timed flood properly timed flood
spiny amaranth spiny amaranth dayflower
(Amaranthus spinosus) (Amaranthus spinosus) (Commelina sp.)
goosegrass goosegrass nutsedge
(Eleusine indica) (Eleusine indica) (Cyperus sp.)
crabgrass crabgrass primrose willow
(Digitaria sp.) (Digitaria sp.) (Jussiaea sp.)
purslane purslane fall panicum
(Portulaca oleracea) (Portulaca oleracea) (Panicum dichoto-
nutsedge barnyard grass Caperonia palustris
(Cyperus sp.) (Echinochloa sp.)
fall panicum barnyard grass
(Panicum dichoto- (Echinochloa sp.)
(Setaria sp.) (Colocascia esculenta)
Some viable seeds will find environmental conditions favorable for
germination and seedling establishment, necessitating some type of
weed-control measure. The remaining seed can remain dormant un-
til more favorable conditions exist. Seed present in the water may
also migrate into drainage canals and be transported to another field.
Thus, floodwater can serve as a vehicle for disseminating weed seed.
Will flooding control weed populations? In some cases, flooding will
reduce weed seed germination. However, the question remains as to
whether seed reserves remain viable and under what type of flooding
regimes. Species differ in tolerance to flooding if already established.
Some weeds can be controlled by flooding whereas others cannot.
Flooding as a weed management tool would have to be incorporated
into present weed management practices as Smith (1981) proposes
for red rice control in rice. Combinations of flooding regimes and
herbicide treatments provided the most efficacious control of red rice.
The situation is complex and more research is needed.
Barnes, J. E. 1960. Weeds and weed seeds. Warrex Corp. Chicago. 96 pp.
Bibbey, R. 0. 1948. Physiological studies of weed seed germination. Plant
Bruns, V. F. and L. W. Rasmussen. 1953. The effects of fresh water storage on
the germination of certain weed seeds. I. White top, Russian knapweed.
Canada thistle, morning glory and poverty weed. Weeds 2:138-147.
Bruns, V. F. and L. W. Rasmussen. 1957. The effects of fresh water storage on
the germination of certain weed seeds. II. White top, Russian knapweed,
Canada thistle, morning glory and poverty weed. Weeds 5:20-24.
Bruns, V. F. and L. W. Rasmussen. 1958. The effects of fresh water storage on
the germination of certain weed seeds. III. Quackgrass, green bristlegrass,
watergrass, pigweed, and halogeton. Weeds 6:42-48.
Crocker, W. and L. V. Barton. 1953. Physiology of Seed. Chronica Botanica
Darlington, H. T. and G. P. Steinbauer. 1961. The eighty-year period for Dr.
Beal's seed viability experiment. Amer. J. Bot. 48:321-325.
Erygin, P. S. 1936. Changes of enzymes, soluble carbohydrates, and intensity
of respiration of rice seeds. Plant Physiol. 11:821-832.
Guzman, V. L., H. W. Burdine, E. D. Harris, Jr., J. R. Orsenigo, R. K.
Schowalter, P. L. Thayer, J. A. Winchester, E. A. Wolf, R. D. Berger, W. G.
Genung and T. Z. Zitter. 1973. Celery production on organic soils of south
Florida. University of Florida Exp. Sta. Bull. 757. 79 pp.
Hoveland, C. S. and E. E. Mikkelson. 1967. Flooding tolerance of ladino,
white, intermediate white, persian and strawberry clovers. Agron. J.
Hoveland, C. S. and G. A. Buchanan. 1972. Flooding tolerance of fall pani-
cum and Texas panicum. Weed Science 20:1-3.
Luxmoore, R. J. and L. H. Stolzy. 1969. Root porosity and growth responses of
rice and maize to oxygen supply. Agron. J. 61:202-204.
Mayer, A. M. and A. Polkahoff-Mayher. 1975. The Germination of Seeds. 2nd
edition. Pergamon Press, Oxford. 8:35.
Orsenigo, J. R. 1967. Simulated Flooding for Weed Control. Ann. Res. Rept.
of The Inst. of Food and Agric. Sci., Univ. of Florida, p. 297.
Phillips, R. P. and S. J. P. Chilton. 1949. Seed populations ofjohnsongrass
in Louisiana cane areas. Proc. South. Weed Science Soc. 2:59-60.
Ponnamperuma, F. N. 1972. The chemistry of submerged soils. Adm. Agron.
Ponnamperuma, F. N. 1976. Temperature and the chemical kinetics of flood-
ed soils. In Climate and Rice. Int. Rice Res. Inst. Los Banos, Phillipines. pp.
Shaw, W. C. 1978. Herbicides: The cost/benefit ratio-the public view. Proc.
South. Weed Science Soc. 31:28-47.
Smith, R. J. 1981. Control of red rice (Oryza sativa) in water-seeded rice
(0. sativa). Weed Science. 29:663-666.
Steinbauer, G. P. and B. H. Grigsby. 1957. Field and laboratory studies on the
dormancy and germination of the seeds of Chess (Bromus secalinus L.) and
downy bromegrass (Bromus tectorum L.). Weeds 5:1-4.
Stevens, O. A. 1932. The number and weight of seeds produced by weeds.
Amer. J. Bot. 19:784-794.
Toole, E. H. 1946. Final results of the Duvel buried seed experiment. J. Agric.
Williamson, R. E. 1964. The effect of root aeration for plant growth. Proc. Soil
Sci. Soc. Amer. 28:86-90.
UNIVERSITY OF FLORIDA
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flooding on the properties of organic soils.
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