Title Page
 Board of control and staff
 Table of Contents
 Importance of organic matter in...
 Rate of loss of organic matter...
 Maintenance of organic matter in...
 Literature cited

Group Title: Bulletin University of Florida. Agricultural Experiment Station
Title: Organic matter in Florida soils
Full Citation
Permanent Link: http://ufdc.ufl.edu/UF00015109/00001
 Material Information
Title: Organic matter in Florida soils
Series Title: Bulletin University of Florida. Agricultural Experiment Station
Physical Description: 15 p. : ; 23 cm.
Language: English
Creator: Thompson, L. G ( Leonard Garnett ), 1903-
Smith, F. B ( Frederick Burean )
Publisher: University of Florida Agricultural Experiment Station
Place of Publication: Gainesville Fla
Publication Date: 1947
Subject: Humus -- Florida   ( lcsh )
Soils -- Florida   ( lcsh )
Genre: government publication (state, provincial, terriorial, dependent)   ( marcgt )
bibliography   ( marcgt )
non-fiction   ( marcgt )
Bibliography: Includes bibliographical references (p. 15).
Statement of Responsibility: L.G. Thompson, Jr., and F.B. Smith.
General Note: Cover title.
 Record Information
Bibliographic ID: UF00015109
Volume ID: VID00001
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: ltqf - AAA7561
ltuf - AEN6159
oclc - 18253738
alephbibnum - 000925508

Table of Contents
    Title Page
        Page 1
    Board of control and staff
        Page 2
        Page 3
    Table of Contents
        Page 4
    Importance of organic matter in soils
        Page 5
        Page 6
        Page 7
        Page 8
    Rate of loss of organic matter in soils
        Page 9
        Page 10
        Page 11
    Maintenance of organic matter in Florida soils
        Page 12
        Page 13
        Page 14
    Literature cited
        Page 15
Full Text

Bulletin 433





Single copies free to Florida residents upon request to

July, 1947


J. Thos. Gurney, Chairman, Orlando
N. B. Jordan, Quincy
Thos. W. Bryant, Lakeland
M. L. Mershon, Miami
J. Henson Markham, Jacksonville
J. T. Diamond, Secretary, Tallahassee


John J. Tigert, M.A., LL.D., President of the
H. Harold Hume, D.Sc., Provost for Agricul-
Harold Mowry, M.S.A., Director
L. O. Gratz, Ph.D., Asst. Dir., Research
W. M. Fifield, M.S., Asst. Dir., Admin.
J. Francis Cooper, M.S.A., Editors
Clyde Beale, A.B.J., Associate Editors
Jefferson Thomas, Assistant Editors
Ida Keeling Cresap, Librarian
Ruby Newhall, Administrative Managers
K. H. Graham, LL.D., Business Managers
Claranelle Alderman, Accountants



W. E. Stokes, M.S., Agronomist'
Fred H. Hull, Ph.D., Agronomist
G. E. Ritchey, M.S., Agronomists
G. B. Killinger, Ph.D., Agronomist
H. C. Harris, Ph.D., Agronomist
W. A. Carver, Ph.D., Associate
Fred A. Clark, B.S., Assistant


A. L. Shealy, D.V.M., An. Industrialist'1
R. B. Becker, Ph.D., Dairy Husbandman'
E. L. Fonts, Ph.D., Dairy Technologists
D. A. Sanders, D.V.M., Veterinarian
M. W. Emmel, D.V.M., Veterinarian'
L. E. Swanson, D.V.M., Parasitologist
N. R. Mehrhof, M.Agr., Poultry Husb.h
G. K. Davis, Ph.D., Animal Nutritionist
R. S. Glasscock, Ph.D., An. Husbandman
P. T. Dix Arnold, M.S.A., Asst. Dairy Husb.'
C. L. Comar, Ph.D., Asso. Biochemist
L. E. Mull, M.S., Asst. in Dairy Tech.6
Katherine Boney, B.S., Asst. Chenx.
J. C. Driggers, B.S.A., Asst. Poultry Husb.
Glenn Van Ness, D.V.M., Asso. Poultry
S. John Folks, B.S.A., Asst. An. Husb.
W. A. Krienke, M.S., Asso. in Dairy Mfs.


C. V. Noble, Ph.D., Agri. Economist'1
Zach Savage, M.S.A., Associates
A. H. Spurlock, M.S.A., Associate
I. E. Alleger, M.S., Associate
D. L. Brooke, M.S.A., Associate

Orlando, Florida (Cooperative USDA)
G. Norman Rose, B.S., Asso. Agr. Economist
J. C. Townsend, Jr., B.S.A., Agr. Statistician'
J. B. Owens, B.S.A., Agr. Statisticians
W. S. Rowan, M.S., Asst. Agr. Statisticians

Ouida D. Abbott, Ph.D., Home Econ.'
R. B. French, Ph.D., Biochemist

A. N. Tissot, Ph.D., Entomologist'
H. E. Bratley, M.S.A., Assistant

G. H. Blackmon, M.S.A., Horticulturist1
F. S. Jamison, Ph.D., Truck Hort.
Byron E. Janes, Ph.D., Asso. Hort.
R. A. Dennison, Ph.D., Asso. Hort.
R. Showalter, M.S., Asso. Hort.
R. J. Wilmot, M.S.A., Asst. Hort.
R. D. Dickey, M.S.A., Asst. Hort.
Victor F. Nettles, M.S.A., Asst. Hort.
F. S. Lagasse, Ph.D., Asso. Hort.2

W. B. Tisdale, Ph.D., Plant Pathologist'
Phares Decker, Ph.D., Asso. Plant Path.
Erdman West, M.S., Mycologist and Botanist
Lillian E. Arnold, M.S., Asst. Botanist


F. B. Smith, Ph.D., Microbiologist s
Gaylord M. Volk, Ph.D., Chemist
J. R. Henderson, M.S.A., Soil Technologist
J. R. Neller, Ph.D., Soils Chemist
Nathan Gammon, Jr., Ph.D., Soils Chemist
C. E. Bell, Ph.D., Associate Chemist
L. H. Rogers, Ph.D., Biochemist
R. A. Carrigan, B.S., Asso. Biochemist
H. W. Winsor, B.S.A., Assistant Chemist
Geo. D. Thornton, M.S., Asso. Microbiologist
R. E. Caldwell, M.S.A., Soil Surveyor
Wade McCall, B.S.A., Asst. Chemist
J. B. Cromartie, B.S.A., Soil Surveyor

SHead of Department.
2 In cooperation with U. S. D. A.
s Cooperative, other divisions, U. of F.
In Military Service.
SOn leave.



J. D. Warner, M.S., Vice-Director in Charge
R. R. Kincaid, Ph.D., Plant Pathologist
W. H. Chapman, M.S., Asso. Agron.
R. C. Bond, M.S.A., Asso. Agronomist
L. G. Thompson, Ph.D., Soils Chemist
Frank S. Baker, Jr., B.S., Asst. An. Husb.

Mobile Unit, Monticello
R. W. Wallace, B.S., Associate Agronomist

Mobile Unit, Marianna
R. W. Lipscomb, M.S., Associate Agronomist

Mobile Unit, Wewahitchka
J. B. White, B.S.A., Associate Agronomist

Mobile Unit, DeFuniak Springs
R. L. Smith, M.S., Associate Agronomist

A. F. Camp, Ph.D., Vice-Director in Charge
W. L. Thompson, B.S., Entomologist
J. T. Griffiths, Ph.D., Asso. Entomologist
R. F. Suit, Ph.D., Plant Pathologist
E. P. Ducharme, M.S., Plant Pathologist6
J. E. Benedict, B.S., Asst. Horticulturist
B. R. Fudge, Ph.D., Associate Chemist
C. R. Stearns, Jr., B.S.A., Asso. Chemist
James K. Colehour, M.S., Asst. Chemist
T. W. Young, Ph.D., Asso. Horticulturist
J. W. Sites, M.S.A., Horticulturist
H. O. Sterling, B.S., Asst. Horticulturist
J. A. Grange B.S.A., Asst. Horticulturist
H. J. Reitz, M.S., Asso. Horticulturist
Francine Fisher, M.S., Asst. PI. Path.
I. W. Wander, Ph.D., Soil Chemist
A. E. Willson, B.S.A., Asso. Soil Phys.
R. W. Jones, Asst. Plant Path.
J. W. Kesterson, M.S., Asso. Chemist
C. W. Houston, Ph.D., Asso. Chemist

R. V. Allison, Ph.D., Vice-Director in Charge
F. D. Stevens, B.S., Sugarcane Agron.
Thomas Bregger, Ph.D., Sugarcane
B. S. Clayton, B.S.C.E., Drainage Eng.2
W. T. Forsee, Jr., Ph.D., Chemist
R. W. Kidder, M.S., Asso. An. Husb.
T. C. Erwin, Assistant Chemist
Roy A. Bair, Ph.D., Agronomist
C. C. Seale, Asso. Agronomist
L. O. Payne, B.S.A., Asst. Agronomist
Russel Desrosiers, M.S., Asst. Plant Path.
N. C. Hayslip, B.S.A., Asso. Entomologist
J. C. Hoffman, M.S., Asso. Hort.
C. B. Savage, M.S.A., Asst. Hort.
Geo. Van den Berghe, B.S., Asst. Fiber Tech.

Geo. D. Ruehle, Ph.D., Vice-Director in
D. O. Wolfenbarger, Ph.D., Entomologist
R. W. Harkness, Ph.D., Asst. Chemist

C. D. Gordon, Ph.D., Geneticist in Charge2

W. G. Kirk, Ph.D., Vice-Director in Charge
E. M. Hodges, Ph.D., Associate Agronomist
D. W. Jones, B.S., Asst. Soil Tech.
E. R. Felton, B.S.A., Asst. An. Husb.

R. W. Ruprecht, Ph.D., Vice-Director in
A. Alfred Foster, Ph.D., Asso. PI. Path.
J. W. Wilson, Sc.Dl., Entomologist
Ben F. Whitner, Jr., B.S.A., Asst. Hort.

H. W. Lundy, B.S.A., Asso. Agronomist


G. K. Parris, Ph.D., Plant Path. in Charge

Plant City
A. N. Brooks, Ph.D., Plant Pathologist

A. H. Eddins, Ph.D., Plant Path. in Charge
E. N. McCubbin, Ph.D., Horticulturist

S. O. Hill, B.S., Asst. Entomologists '
A. M. Phillips, B.S., Asso. Entomologist2

J. R. Beckenbach, Ph.D., Horticulturist in
E. G. Kelsheimer, Ph.D., Entomologist
David G. Kelbert, Asso. Horticulturist
E. L. Spencer, Ph.D., Soils Chemist
Robert 0. Magie, Ph.D., P1. Path., Glad. Inv.
J. M. Walter, Ph.D., Plant Path.
Donald S. Burgis, M.S.A., Asst. Hort.

Warren O. Johnson, B.S., Meteorologist2

SHead of Department.
SIn cooperation with U. S.
SCooperative, other divisions, U. of F.
SIn Military Service.
5 On leave.



IMPORTANCE OF ORGANIC MATTER IN SOILS....................-- --.----------------------. 5

Relation of Organic Matter to Erosion and Leaching Losses........---..... 6

Influence of Organic Matter on Chemical Properties of Soils....---...... 6

Influence of Organic Matter on Biological Activities......------..................... 8

Influence of Organic Matter on Crop Yields---------................... ---------- 8

RATE OF Loss OF ORGANIC MATTER IN SOILS---................---- -----..............--- 9

MAINTENANCE OF ORGANIC MATTER IN FLORIDA SOILS -------................................ 12

Green Manures and Cover Crops ..................------------------- 13

Crop Residues ...................-------------------- ------------------------------.. 13

SUMMARY .............-- ------------------ --------------- 14

LITERATURE CITED --.....--....-------:-------- -...------------ 15

Organic Matter in Florida Soils

Man's interest in the soil is mainly economic. The soil is one
of the nation's great resources and, unlike some of our other
natural resources, it can be used and still maintained perman-
ently in a high state of productivity. Analyses of unproductive
soils where civilization flourished in ancient times show that
most fertility constituents are still present in abundance, strong-
ly indicating that by proper management these soils might be
made productive again.
The maintenance of soil fertility demands attention to certain
details. Soils are usually more productive when they are first
brought under cultivation than after they have been cultivated
for a time. One important difference between virgin and culti-
vated soil is the content of organic matter. Cultivation of soils
stimulates decomposition processes and hastens the decay of J
humus. Also, the supply of organic residues added to the soil
may be reduced greatly under certain systems of cropping. Good
soil management calls for a system of cropping which permits a
renewal of the supply of organic residues. In most cultivated soils
the content of organic matter decreases rapidly -at first, due to
the marked difference between virgin and cultivated soil condi-
tions, and is a perfectly natural consequence of use. But good
soil management practices reduce the rate of disappearance and
maintain a normal or optimum amount of organic matter in
cultivated soils. Without sufficient organic matter yields always
The crop-producing power of the soil depends upon a number
of factors. The organic matter content is only 1 factor but a
significantly important one. The amount of organic matter in
the different soil types in the state is being determined as the
statewide Soil Survey progresses, and these results will be re-
ported in a later publication. It is the purpose of this bulletin
to point out the importance of organic matter in soils and to sug-
gest how it may be maintained under Florida soil conditions.

Importance of Organic Matter in Soils
Organic matter is an important constituent of the soil body.
It plays a prominent role in soil formation and is 1 of the major
factors determining the characteristics of the soil profile. The
beneficial effects which it has on the physical properties alone

6 Florida Agricultural Experiment Station

would justify the effort necessary to maintain an adequate
supply in the soil. Decomposed organic matter or humus has a
tendency to bind loose sandy soils together and to make stiff clay
N more open and porous. Humus increases the water-holding
capacity of sands and promotes the free movement of water
and air in the clays. Humus makes the soil a favorable environ-
ment for the growth and activities of microorganisms.
The effect of humus in conferring these beneficial character-
istics on the soil is manifest to a degree that is quite dispropor-
tionate to the small amount present in most soils. Depletion of
the humus seems to bring about a deterioration in the desirable
physical characteristics, especially of the heavier soils.

Relation of Organic Matter to Erosion and Leaching Losses.-
Organic matter on the surface of the soil and that mixed with
the soil aid in preventing soil erosion by water and wind. A
soil that is well supplied with humus is much more resistant to
Erosion than a soil low in organic matter. Humus tends to pre-
vent leaching losses because it combines with certain mineral
plant food nutrients such as soluble mineral fertilizers and holds
them in a readily available form for plant growth.

Influence of Organic Matter on Chemical Properties of Soils.-
Decomposing organic matter renders some of the inorganic ele-
ments of the soil more readily available to the plant, thus in-
creasing soil fertility. It improves the buffering capacity-the
ability of the soil to avoid rapid changes in reaction. It helps
to correct toxic soil conditions caused by the use of excessive
amounts of artificial fertilizers or by the presence of spray resi-
dues and similar materials. It supplies certain catalytic agents
and certain growth substances beneficial to plant growth.
From a soil fertility standpoint the most important chemical
elements in the soil organic matter are carbon and nitrogen.
Phosphorus, iron, sulfur and other elements occur in lower con-
centrations. These elements are not available to plants until
the organic matter is decomposed. Organic matter from some
plants decomposes more rapidly than that from others. When
used as a green manure on Norfolk fine sand (2)1, it was found
that Crotalaria spectabilis Roth. decomposed more rapidly than
either C. striata D.C. or C. intermedia Kotschy. In 1 year the

1Italic figures in parentheses refer to "Literature Citea" in the back of
this bulletin.

Organic Matter in Florida Soils

following percentages of constituents of the crotalaria were

Percent Liberated
SI Phosphoric I Calcium
CarbonI Nitrogen ] Acid Potash lime
_(C) (N) I (P205) (K20) (CaCOa)
Crotalaria striata 88 89 89 99 80
Crotalaria spectabilis 95 94 93 99 93
Crotalaria interinedia 89 87 81 98 89

Normally, only a few pounds of nitrate nitrogen can be found
in the soil at any 1 time and, unless an adequate supply is fur-
nished in fertilizer, plant growth depends upon the continuous
liberation of nitrate from the organic matter throughout the
growing season. Generally, high crop yields are more closely
correlated with the nitrogen supply than with any other element.
The base exchange capacity of a soil may be defined as the
capacity of the soil to absorb and retain a group of elements such
as calcium, magnesium, potassium, manganese, copper and zinc,
which are known as exchangeable bases. The total amount of
exchangeable bases any soil can hold is a more or less fixed
quantity for a given condition, and is determined by the quan-
tity of organic matter and clay present in the soil. Since most
Florida soils are very low in clay content, the decomposed or-
ganic matter or humus constitutes the main part of the exchange
capacity in these soils. On an equal weight basis, humus far ex-
ceeds clay in its capacity to combine with and hold exchangeable
bases. The availability of the exchangeable bases held by the.
organic matter does not differ appreciably from that retained
by the inorganic part of the soil.
Many of the soils of Florida are composed almost entirely of
sand, which is inert and chemically inactive. However, there is a
very small amount of clay in most soils, even in sands. Clay
and decomposing organic matter or humus form an intimate
mixture known as the colloidal complex, chemically the most
active portion of the soil.
Since the decomposed organic matter or humus constitutes the
main source of colloidal matter in many Florida soils, a slight
increase in the content of organic matter produces a marked

Florida Agricultural Experiment Station

increase in the total exchange capacity. Since these organic
colloids are extremely active, an increase of 1 percent in decom-
posed organic matter may double the exchange capacity. The
converse is true also. A slight decrease in the content of humus
in the soil brings about a marked decrease in its exchange

Influence of Organic Matter on Biological Activities.-Soil or-
ganic matter not only is a storehouse for plant food, but also it
is the food supply for soil microorganisms. When organic matter
is plowed under or when a sod is broken the food supply is in-
creased materially. The number of microorganisms in the soil
is controlled by the available food supply. Thus, a soil low in
digestible food has a small population, whereas a soil rich in food
has a very large population. The soil microorganisms feed on
the proteins of the plant residues, liberating ammonia which is
then oxidized by .other soil bacteria to nitrites and finally to
Much of the protein is used by the soil microorganisms to build
their own cells. Each individual is very small, but there are
such vast numbers present that many pounds of nitrogen in the
form of protein are present in the living microorganisms and
many more pounds in the residues of the dead organisms. As
the food supply decreases, the number of microorganisms that
can be supported diminishes and thus the excess nitrogen is
liberated as ammonia and then changed to nitrates which can
be used by higher plants.
Since microbial activities can proceed the year around in Flor-
ida soils decomposition is more rapid and the content of organic
matter is usually lower than in northern soils where low temper-
atures keep microbial activity at a minimum during winter

Influence of Organic Matter on Crop Yields.-Experiments
have shown that it is possible to build up the fertility of poor
sandy soils by growing leguminous green manure crops.
A study (6) of the effects of summer cover crops on yields and
on the soil (Norfolk medium fine sand, deep phase) was made
from 1925 to 1928, inclusive. Average annual yields of sweet
potatoes and corn following the various cover crops were as

Organic Matter in Florida Soils

Cover Sweet Potatoes Corn
Crop (bu. per acre) (bu. per acre)

Crotalaria 38.9 16.6
Cowpeas | 27.9 14.2
Velvet beans 26.9 16.8
Beggarweed 23.4 12.0
"Florida pusley" | 20.9 8.7

Rate of Loss of Organic Matter in Soils
Organic matter decomposes rapidly in Florida soils or as a
mulch on top of them. A humid atmosphere, high temperatures
through most of the year and well drained sandy soils which are
thoroughly aerated combine to give ideal conditions for rapid de-
composition. Consequently, for best results it is necessary to
add organic matter to the soil every year, since there is no perm-
anent accumulation of organic matter in well drained Florida
sandy soils.
In a lysimeter study (3) it was found that after 596 days de-
composition had removed 72 percent of the organic matter of
crotalaria used as a mulch and 87 percent of that incorporated in
the soil. With Natal grass (Tricholaena repens (Willd.) Hitchc.)
losses by decomposition were 63 percent when used as a mulch
and 95.9 percent when incorporated in the soil. After 20 months
78 percent of the crotalaria was'decomposed and had liberated
84 percent of the nitrogen.
The content of well decomposed organic matter was increased
slightly in the soil from 0 to 9 and from 9 to 18 inches deep by
large additions of Crotalaria straita and velvet beans. The
crotalaria was added at the rate of approximately 23 tons of dry
matter per acre, the velvet beans at the rate of 21 tons per acre.
Even these large applications showed only a slight increase in
the amount of soil organic matter after 20 months, as stated
At Lake Alfred (1), analyses of soils from most of the sum-
mer legumes and Natal grass plots showed about 0.80 percent
organic matter, while soils under crotalaria which was allowed
to reseed contained 1.123 percent, besides a large accumulation
of residue on top of the soil.
It was observed (2) in a cover crop experiment on virgin Nor-

Florida Agricultural Experiment Station

folk sand that annual additions of legume or non-legume summer
cover crops maintained approximately the original organic mat-
ter, nitrogen and carbon content of the surface 0-8 inches of soil.
Clean culture definitely and progressively decreased the content
of organic matter, nitrogen and carbon of the soil during a 10-
year period.
In a study of the rate of decomposition of various plants incor-
porated in the soil (4) it was found that after 35 days 56.8 per-
cent of the velvet bean material was decomposed but only 28.8
percent of the crabgrass was decayed. However, after 374 days
there was still 20.2 percent of the velvet bean material remaining
in the soil as humus but all the crabgrass had been destroyed.
Numerous other experiments have shown that materials of a
wide carbon-nitrogen ratio decompose more slowly than materials
with a narrow carbon-nitrogen ratio; but more humus is pro-
duced from materials, especially leguminous materials, which
have a narrow carbon-nitrogen ratio. This is of particular im-
portance in Florida, especially on the loose, open sandy soils
where organic matter disappears rapidly.
In 1933 (7) a land resting experiment was established by the
Agronomy Department of the Florida Station. Soil samples were
taken at that time and again (4) in 1938. Results (Table 1) in-
dicated less organic matter in the soil which had been in continu-
ous corn and peanuts than in soil which was cropped to corn and
peanuts in alternate years. The soil which was cropped to corn
and peanuts 1 year and crotalaria the next showed a significant
increase in undecomposed organic matter. So did the soil planted
Table 1. Percentages of undecomposed (coarse) organic matter in soils
under different cropping system after 5 years.

Cropping system Plot No. I Percent organic matter

Continuous corn and peanuts 1 0.23
Same, with crotalaria, last cultiva- |
tion. Oats hbtween crotalaria and I
peanuts in fall 2 0.20
Corn and peanuts alternate with 1 |
year of crotalaria 3 0.55
Corn and peanuts alternate with I
1 year rest I 4 0.40
1 I
Corn and peanuts 1 year and rested I
2 years 5 0.55

Organic Matter in Florida Soils

to corn and peanuts 1 year and rested 2 years. In view of
the important role the organic matter of the soil plays in the
growth of plants and microorganisms, this increase is regarded
as significant.
The decomposition of humus may be relatively rapid under
certain conditions of moisture, temperature and aeration and
with certain microorganisms found abundantly in most of the
soils of Florida. In experiments conducted at Gainesville (5) it
was found that actinomyces make up a high proportion of the
total microbial population. In an average loam the actinomyces
rarely make up more than 15 percent of the total population,
but in many Florida soils the percent of actinomyces is often
higher than this. Since the actinomyces are able to withstand
drying conditions and are highly efficient and extremely active,
they can decompose the most resistant organic complexes. The
coarse texture of these soils, the long periods of low rainfall and
the relatively high temperature favor a vigorous flora of actino-
myces which decompose the organic matter rapidly and com-
pletely. Thus, conditions are favorable at all times of the year
for relatively rapid decomposition of the organic matter. Con-
sequently, under normal conditions these soils are never able to
accumulate a high content of organic matter. Therefore, to
maintain an adequate supply of organic matter in these soils
annual green manure crops, especially legumes, should be used
in short rotations.
Environmental factors possibly are the most important ones
that limit rate of decomposition of organic matter, at least from
the standpoint of soil management. More important among these
are moisture, temperature, aeration, reaction and nutrient ele-
Since microorganisms must have water to grow and work, it
is obvious that they do not work if there is not sufficient mois-
ture. About 50 percent of the saturation capacity of the soil
appears to be about the optimum moisture content for the growth
of most beneficial microorganisms. If the moisture content of
the soil is increased much beyond this amount aeration is greatly
decreased and a different group of microorganisms becomes ac-
tive. Some microorganisms can grow without free oxygen from
the air but they do not decompose organic matter as rapidly nor
as completely as do aerobic bacteria. Also, some of the inter-
mediate decomposition products which may accumulate under
these conditions may become highly toxic.

Florida Agricultural Experiment Station

Soil temperature has a marked influence on the rate and
amount of decomposition. Minimum temperature for the growth
of most soil microorganisms is about 100C. (500F.), although
some may grow at OOC. (320F.). Temperatures of 50 to 600C.
(122 to 1400F.) will kill most soil microorganisms. The best
temperature for rapid decomposition is about 300C. (860F.) and
where the winters are mild the soil temperatures are favorable
for rapid microbiological activities during most of the year.
The reaction acidity or alkalinity of the soil has an im-
portant influence on the growth of soil microorganisms. Bac-
teria and actinomyces do not grow well in acid soils but molds
are tolerant of acid conditions and bring about most of the de-
composition of organic matter in extremely acid soils. Microbial
action tends to adjust the recation of the medium, by either low-
ering or raising it, to a point favorable for the growth of the
microorganisms. Since there is usually a lack of bases in ex-
tremely acid soils, the injurious effect in many cases is due to
the deficiency of available calcium and magnesium rather than
to a harmful effect of the acid conditions.
Nearly all of the microorganisms of the soil are lower forms
of plants and they have need for most essential elements required
by higher plants. The elements required in largest amounts are
possibly nitrogen, phosphorus, potassium, calcium, sulfur and
magnesium. Elements required in smaller amounts are iron,
manganese, copper, zinc, boron and possibly others. A deficiency
of any of these elements will affect the growth of microorganisms
and rate of decomposition of organic matter.

Maintenance of Organic Matter in Florida Soils
Since organic matter decomposes rapidly and completely under
many Florida soil conditions it is necessary to make regular
additions at frequent intervals. This may be done by a variety
of soil management practices, such as the use of green manures
and cover crops, crop residues and composts. Where available,
barnyard and poultry manures are excellent sources of organic
matter. The growing of crops to be plowed under as a green
manure and alternating cultivated crops with those not inter-
tilled are good practices for the general farm. Some of these
practices may be followed profitably on the truck farm and
under grove conditions, but it may be advisable to bring in or-
ganic matter from outside areas frequently to supplement that
which can be grown under these conditions.

Organic Matter in Florida Soils

Green Manures and Cover Crops.-The organic matter content
of the soil may be more nearly maintained by the use of green
manures and cover crops than by any other single practice. The
plowing under of a crop of green manure may add 1 to 2 tons of
dry matter per acre. In addition to the nitrogen taken from the
soil, a well inoculated legume crop may contain 75 to 100 pounds
of nitrogen which, when turned under, would increase the soil
reserves of nitrogen by that amount. Non-legume green ma-
nures return to the soil the nitrogen which was taken up by the
crop and add organic matter. When incorporated in the soil the
green manures decompose rapidly, leaving comparatively little
resistant residue but liberating the nitrogen and other nutrients
for plant growth.
Where it is possible to grow legumes successfully they should
always constitute the green manure crop. A neutral or nearly
neutral soil is required for the best growth of most legumes.
Some legumes will grow in more acid soils than others, especial-
ly if the soils contain sufficient amounts of available calcium,
but most legumes will grow much better on soils well supplied
with lime. To secure best results legumes should be inoculated.
The bacteria which form nodules on the roots of legumes may
not be present in the soil or may be weakened and inefficient.
Generally it is a good practice to inoculate legumes even on soil
which recently has grown a crop successfully.
Cover crops are of special importance as they protect the soil
from surface erosion; take up soluble plant food, especially nitro-
gen which would be leached out of the soil if no crop occupied
the land; and add organic matter to the soil when they are plowed
Crop Residues.-Crop residues such as corn stalks, straw,
weeds and grass are high in carbon and low in nitrogen. Their
decomposition temporarily uses up some or all of the available
nitrogen in the soil. So they must be incorporated some time
before planting of the next crop, especially if the crop to be
planted is a rapidly growing one. It is advisable under many
conditions to supplement these materials with a mineral fertilizer
high in nitrogen. Since they usually consist largely of hemi-
cellulose, cellulose and lignins, they leave appreciable amounts of
resistant residue in the soil. Experiments have shown that if
nitrogen is added when these materials are incorporated in the
soil, larger amounts of humus are left than if the material de-
composed slowly without added nitrogen. One ton of crop resi-

Florida Agricultural Experiment Station

dues probably would be as effective in increasing the humus con-
tent of the soil as an equal amount of fresh manure.
Liberation of the nitrogen contained in crop residues is rela-
tively slow, so that increases in yield in the immediately succeed-
ing crops would not be expected. Decreased yields may be caused
by plowing under crop residues low in nitrogen. Materials low
in nitrogen and high in carbon stimulate the growth of the molds
and certain other groups of microorganisms which compete suc-
cessfully with the higher plants for available nitrogen. There-
fore, nitrogen should be added to the soil when such materials
are incorporated, especially if a crop is to be planted on the land
soon after the treatment is made.
Burning crop residues destroys the organic matter and liber-
ates the nitrogen they contain. The practice is indefensible under
normal conditions, though at times it may be necessary for the
control of diseases and insect pests.
The maintenance of soil organic matter by the use of artificial
manure or compost cannot be recommended for general farm
use, but where materials are available at little cost and conditions
are favorable artificial manure may be produced which will be
equal in value to barnyard manure. The truck farmer, the
market gardener and the greenhouse manager may find artifi-
cial manure of great value. Composting offers a good means of
supplementing the inadequate supply of soil organic matter.

Recognition of the economic importance of soil implies an
understanding that good soil management is necessary in any
system of permanent agriculture. Good soil management calls
for the maintenance of soil organic matter.
Organic matter has beneficial effects on the physical, chemical
and biological properties of soils; decreases erosion and leaching
losses; and increases crop yields.
Organic matter disappears rapidly under Florida soil condi-
tions and it is necessary to make frequent and regular additions
to maintain an adequate supply. Green manures, cover crops
and crop residues are important for maintaining the supply of
soil organic matter.

Organic Matter in Florida Soils

Literature Cited

1. BARNETTE, R. M. Organic matter in citrus soils. FIa. State Hort. Soc.,
Proc. 42: 21-26. 1929.
2. BARNETTE, R. M. Determination of the effect of green manures on the
composition of the soil. Fla. Agr. Exp. Sta., Ann. Rpt., p. 64. 1937.
3. BARNETTE, R. M., H. W. JONES AND J. B. HESTER. Lysimeter studies
with the decomposition of summer cover crops. Fla. Agr. Exp. Sta.
Bul. 327. 1938.
4. SMITH, F. B. Determination of the effect of green manure on the com-
position of the soil. Fla. Agr. Exp. Sta., Ann. Rpt., p. 82. 1939.
5. SMITH, F. B., AND O. E. GALL. Types and distribution of microorgan-
isms in some Florida soils. Fla. Agr. Exp. Sta. Bul. 396. 1944.
6. STOKES, W. E., R. M. BARNETTE AND J. B. HESTER. Effects of summer
cover crops on crop yields and the soil. Fla. Agr. Exp. Sta. Bul. 301.

7. STOKES, W. E., J. P. CAMP AND G. E. RITCHEY. Crop rotation studies
with corn, cotton, crotalaria and Austrian peas. II. Corn and run-
ner peanuts rotating with crotalaria and with native cover crops.
Fla. Agr. Exp. Sta., Ann. Rpt., pp. 40-42. 1939.

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