,,5 Fertilizer Management-
Key to a Sound Water
by Del Bottcher and Dean Rhue*
PRODUCING GOOD FOOD WHILE PROTECTING FLORIDA'S WATER
Itm1VRSITY OF FLORIDA LIBRARIES
Key to a Sound Water
by Del Bottcher and Dean Rhue*
Fertilization, the process by which essential nutrients are ar-
tificially supplied to plants, is possibly the single most important
practice used by growers to assure high yields. Each year Florida
growers apply about 1.8 million tons of fertilizer at a cost of over
$250 million to grow over three billion dollars worth of crops. The
net financial return from fertilization has been extremely high,
however recent evidence indicates we may be doing too much of
a good thing, that is, over-fertilizing.
Over-fertilization leads to two serious concerns. The first is
simply that of cost. Every dollar spent on fertilizer that does not
return at least the same amount in increased yield is wasted.
Besides wasting money, over-fertilization leads to a second, off-
farm concern: pollution of groundwater supplies and down-
stream ecosystems due to transport of fertilizer nutrients. Dis-
charges of nutrients into surface and subsurface waters can be
Spreading fertilizer to enrich sandy soil.
*Associate Professor, Agricultural Engineering Department and Associate Pro-
fessor, Soil Science Department, respectively, Institute of Food and Agricultural
Sciences, University of Florida, Gainesville, FL 32611.
detrimental to the beneficial uses, aesthetic appeal, flora and
fauna of receiving water bodies. When this happens, growers suf-
fer only indirectly; society as a whole bears the brunt of the cost.
Good fertilizer management conserves
fertilizer and preserves water quality.
It would be impossible to tell Florida growers and water
regulators what ought to be done on all farms to prevent fertilizer
loss and subsequent water pollution because every farming
situation is different. Instead, this publication is designed to im-
part a general understanding of the fertilization process so that
predictions can be made for each particular situation. It will
discuss the soil-plant environment and the process of plant up-
take as these concepts relate to transport of nutrients in water. A
basic understanding of these principles is the key to a fertilizer
management program that conserves fertilizer and preserves
Plant nutrients contained in fertilizer are guaranteed. Crop yields
depend on the right amounts of essential nutrients.
NUTRIENTS AND THE SOIL-PLANT SYSTEM
Soil particles, water within the soil, soil microorganisms and
the soil atmosphere form a complex system in which numerous
chemical, physical and biological reactions occur. The fate of
nutrients and crop responses to them are influenced by each of
the above components in a very complex fashion. Nitrogen,
phosphorus and potassium are macronutrients extremely impor-
tant to farming. Micronutrients such as zinc, copper, molyb-
denum, manganese, iron and boron also play important roles in
the soil-plant system. Among these nutrients, nitrogen and
phosphorus represent the greatest cost to farmers and also
cause the bulk of water quality problems. Therefore, this circular
will focus on only nitrogen and phosphorus and the most impor-
tant processes involved in their availability and transport.
Conceptual phosphorus and nitrogen cycles are presented in
Figures 1 and 2, respectively. Apparent in these figures are the
numerous forms nitrogen and phosphorus can assume and the
many transformations and pathways between these forms. The
extent to which one form or another occurs and the process in-
volved is determined by the soil-plant system and climatic condi-
tions. The more important parameters are soil type, temperature,
tillage, moisture, vegetation type and fertilizer amount and form.
Growers have complete or partial control over the last four
parameters and to the extent that they can control them, they
can control fertilizer losses.
S . .. Erosion
Fertilizer --- ---
Minerals PO 4 Organic
Desorption 4 Mineralization Matte
Figure 1. Phosphorus Cycle for Soil-Plant System
Nitrogen and phosphorus cycles differ by the type of transfor-
mations involved, but have similar inputs and outputs. The exter-
nal sources of nitrogen and phosphorus to the soil-plant system
are rainfall, fixation from atmosphere (nitrogen only) and fertiliza-
tion. Important internal sources are decomposition of organic
material, soil mineral weathering, and chemical desorp-
tion the release of nutrients bonded to soil particles. Sandy
soils common to Florida receive much of their nitrogen and
phosphorus from external sources, while highly organic soils like
Everglades muck and some midwest prairie soils already contain
large amounts of these nutrients.
Figure 2. Nitrogen Cycle for Soil-Plant System
PLANT UPTAKE AND GROWTH RESPONSE TO
NUTRIENTS: THE CONCEPT OF LIMITING
Plant uptake of nutrients has been studied extensively. With
few exceptions these studies have shown that plant growth in-
creases with nitrogen and phosphorus fertilization up to a cer-
tain level of application. Very large amounts of fertilizer,
however, can be detrimental to growth. Modern testing tech-
niques and nutrient management technology have made it possi-
ble to predict, to some extent, the break-even point at which fur-
ther fertilization is counter-productive.
Plant growth in response to nitrogen
and phosphorus is reflected in the
concept of limiting factors.
Plant growth in response to nitrogen and phosphorus is
reflected in the concept of limiting factors. That is, plants will
grow as rapidly within their genetic capability as limiting
nutrients or other conditions will allow. For crops to grow well,
limiting factors need to be removed. For example, irrigation will
reduce moisture stress, pesticides will reduce weed, insect and
disease stress, and, of course, fertilization will reduce nutrient
deficiencies. As one constraint is removed, growth will quickly
increase, but only until another limiting condition is reached.
Determining and overcoming these limiting factors, and broaden-
ing genetic capability have been the means by which researchers
and progressive farmers have steadily increased agricultural pro-
Figure 3 shows a relationship between plant growth and con-
centrations of nitrogen and phosphorus in the soil and
soil-water. The dashed line represents growth assuming no
limiting factors exist except inherent genetic characteristics
while the solid line depicts growth under more typical field condi-
Availability of P or N in soil water (Increasing) --
Figure 3. Hypothetical Growth Response for N or P: Ideal curve
is for genetically limited growth whereas field curve is for
typical in-field conditions. Optimal economic ranges are
The nutrient range for native soil in Figure 3 is representative
of typical Florida mineral soils. However, some soils naturally
high in phosphorus exist in Florida for which that nutrient would
not be limiting. The shape and slope of individual nitrogen and
phosphorus growth-response curves usually differ only in
magnitude and scale, with nitrogen showing a stronger potential
growth response than phosphorus, particularly in mineral soils.
In general, yield response to phosphorus is not as consistent or
dramatic as response to nitrogen. Any fertilization which raises
nutrient concentration above the optimal zone is counter-
productive. Decrease in growth at higher nitrogen or phosphorus
levels can be the result of salt build-up, acidity, or direct in-
terference with chemical processes occurring within the plant.
In summary, plants will take up nutrients and respond accord-
ing to their physiological state and the presence of other factors
limiting growth. Many such factors may be present in the field, so
maximum potential growth is rarely achieved. Since these fac-
tors limit a plant's potential growth in response to fertilizer ap-
plication, fertilizer rates should accordingly be adjusted
PROCESSES AFFECTING TRANSPORT:
CYCLES OF NITROGEN AND PHOSPHORUS
The previous section pointed out the relationships between
the soil, plants, and nutrients related to growth; but how are
these nutrients, often considered to be pollutants, leaving the
field? At first glance the answer is simple: WATER. Where water
flows so will the substances contained within it. However, con-
trolling these losses becomes complicated when trying to deter-
mine how nutrients get into water and how long they will remain
At first glance the answer is simple:
WATER. Where water flows so will
the substances contained within it.
The solution lies partially in an understanding of the transport
capacities of water and in nitrogen and phosphorus cycles of
transformation. In order for water to transport a substance the
substance must be physically within the water, either suspended
as particulate matter or dissolved in solution. Water movement
within the soil by percolation or leaching usually will filter out the
suspended materials so that only dissolved materials are
transported to groundwater. Sometimes, in fractured soils with
cracks or root channels, or in well-structured soils, fine colloidal
particles can also be leached. Surface runoff is not filtered, so
suspended particles such as soil colloids are transported with it.
Since only dissolved materials usually move in percolating
water, the soluble forms of nitrogen and phosphorus will be most
likely to move downwards. For nitrogen, important soluble forms
are nitrate, ammonium, and soluble organic nitrogen. For phos-
phorus, the important soluble forms are ortho-phosphate and
soluble organic phosphorus. The relative abundance of these
forms is dependent on soil properties, vegetation, climate, and
Nitrate is by far the most abundant soluble form of nutrient
found in soil-water. Since most agricultural soils are well
aerated, other forms of nitrogen, such as ammonium, are quickly
oxidized with the help of bacteria and the nitrification process
-to nitrate. Nitrate, which is readily taken up by plants and
microorganisms, also has a very low affinity, or ability to bond, to
either soil particles or organic matter. Therefore, once dissolved
in water, nitrate will remain dissolved until biologically removed,
either by plants or by anaerobic microbial conversion to nitrogen
gas, through denitrification. When nitrate is moved below the
root zone into the less biologically-active lower soil profile, it is
not likely to be altered, and may eventually reach groundwater
supplies or move laterally to surface waters.
Nitrate dissolved in water will remain
dissolved until biologically removed.
Soluble organic nitrogen is typically the second most plentiful
form of nitrogen transported to groundwater. It has a low affinity
for soil particles and appears to be less stable in the ground-
water than nitrate, and therefore is less persistent.
Ammonium ions have an affinity for soil particles so that ad-
sorption in addition to nitrification results in very little am-
monium reaching groundwater. The adsorption process in this
case occurs when positively charged ammonium ions are at-
tracted to negatively charged surfaces in the soil. Organic matter
and clay particles have strong negatively charged surfaces,
therefore increases in either organic matter or clay content will
increase ammonium retention in soil.
Phosphate, the most common form of phosphorus, has a much
lower solubility than nitrate, which means a smaller portion of
phosphate present in soil can dissolve. Phosphorus, in either
ortho-phosphate or organic form, has a strong affinity to bond to
mineral soil particles by way of adsorption. This process can
remove phosphorus from percolating water even below the root
zone. Adsorption of phosphorus forms is usually associated with
aluminum and iron complexes in the soil. There are some mineral
soils, particularly in southern Florida, with little iron or aluminum
which have an extremely poor ability to adsorb, allowing signifi-
cant movement of phosphorus to the groundwater. However,
these soils, almost pure sands, will develop a significant adsorp-
tion capacity when limed. Clay particles will also increase rates
of phosphorus adsorption, while organic matter in soil has little
influence. In general, for most mineral soils, only a very limited
amount of phosphorus ever reaches the groundwater.
The processes involved in the soluble phase of subsurface
movement discussed above also apply to surface runoff, but in a
different physical situation. Subsurface processes involve soil
particles fixed within the soil. In surface runoff, particles may be
detached and mixed within the fluid. Thus, the ratio of water to
soil is much greater in surface runoff, so more dissolution can
occur. Also, soil particles in runoff are typically from the organic
and nutrient-rich upper soil horizon.
Surface runoff is influenced by quantity and intensity of rain-
fall. Heavy rains will increase both erosion and runoff volume.
Rainfall also contains nitrogen and phosphorus which can repre-
sent a significant portion of these nutrients found in runoff.
Significant amounts of inorganic phosphorus can also be
washed off foliage. In turn these factors will influence release
and adsorption of chemicals as rainwater mixes with soil.
Eroded soil particles carry adsorbed chemicals with them,
especially phosphorus which is adsorbed strongly by most
mineral soils. Organic matter, crop debris, applied fertilizer and
animal waste can also be physically transported by runoff water.
The dissolution of nutrients from all such transported suspended
material will continue or perhaps increase in receiving water
bodies, sometimes causing serious water quality problems.
MANAGEMENT PRACTICES TO CONTROL
NITROGEN AND PHOSPHORUS LOSSES
The way to control nutrient losses is straightforward: limit the
amount of soluble nutrients in the soil-water and reduce the
transport capacity of the water either by reducing flow rate or
total flow. Important practices which reduce nutrient loss are
soil conservation and efficient irrigation and fertilization tech-
The way to control nutrient losses is
to limit the amount of soluble nutri-
ents in the soil-water and reduce the
transport capacity of the water.
Soil conservation practices such as strip cropping, terracing,
minimum tillage and cover-cropping affect nutrient transport by
reducing surface flow velocities thereby reducing detachment
and transport of soil particles and providing a greater oppor-
tunity for infiltration. While lessened surface runoff will reduce
nutrient loadings to streams, it may increase percolation losses.
However, the disadvantage of increased percolation losses of
nutrients is usually outweighed by the benefits of reduced sur-
A second practice that reduces nutrient losses is efficient ir-
rigation. Irrigation should always be limited to wetting only the
root zone, because excessive irrigation can transport nutrients
below the root zone through leaching. Proper scheduling and
uniform water distribution are necessary to assure control.
Overhead and trickle irrigation systems require careful schedul-
ing because it is difficult to detect over-irrigation by these
methods, whereas seepage and furrow over-irrigation is visually
obvious in overflow and ponding.
Fertilizer management is by far the most critical practice for
controlling nutrient losses and crop production. In essence, the
more fertilizer that is applied to a field, the greater is the poten-
tial for losses. Therefore, a good fertilizer program is one that ap-
plies the minimum amount of fertilizer needed to obtain the most
Research indicates that a few years after a management
change, plant-available nutrient levels in agricultural soils usu-
ally reach equilibrium, fluctuating annually only within a given
range. For example, if a grower increases cultivation or steps up
fertilization a new equilibrium will develop. Cultivation will cause
an initial decrease in nutrient levels due to oxidation of ac-
cumulated organic matter, while fertilization will cause an initial
increase. Yet, studies indicate that over time, nitrogen levels re-
tained in the soil stabilize until management again changes.
Therefore, at least as much nitrogen will eventually leave the
soil-plant system as was applied in the form of fertilizer.
Available phosphorus will also stabilize, however, accumulation
of unavailable phosphorus will occur because applied
phosphorus strongly binds to soil particles.
It would be ideal if all nitrogen and phosphorus applied ended
up in the crop, but in fact uptake rates range from 40-80 percent
of nitrogen applied and only 5-20 percent of phosphorus applied.
As more fertilizer is applied, uptake becomes less efficient. That
is, the percentage of applied fertilizer taken up by plants goes
down as application amounts go up, thereby increasing potential
nutrient loss through runoff, leaching and volatization in the form
of gaseous nitrogen. Losses from leaching and runoff combined
are between /2 and 5 percent of the phosphorus applied and be-
tween 5 and 30 percent of the nitrogen applied.
How then, should growers fertilize in order to ensure efficient
plant uptake and keep nutrient losses by way of other pathways
to a minimum? The method by which fertilizer is applied, its
chemical form and its rate of application are factors which they
The method by which fertilizer is
applied, its chemical form and its rate
of application are factors which
growers must consider.
The method of application can significantly influence ac-
cessibility of nutrients to plants. For example, banding, or apply-
ing fertilizer in strips along crop rows, increases nutrient ac-
cessibility to roots. Broadcasting, or applying fertilizer uniformly
over an entire soil surface, can place some fertilizer completely
outside the root zone of certain plants and thus require a propor-
tionately larger application to assure growth.
The form in which fertilizer is applied is another important fac-
tor influencing plant uptake and nutrient loss. For instance,
since soluble nitrogen usually oxidizes to nitrate within a few
days of application, its chemical form upon application makes
little difference to its chances of groundwater transport.
However, volatile forms of nitrogen, such as anhydrous am-
monia, may be lost to the atmosphere before conversion to
nitrate if improperly injected. Urea nitrogen may also be volatil-
ized if improperly applied. In addition, fertilizer in slow-release
form affects the rate of fertilization and in this sense influences
plant uptake and nutrient loss.
Rate of application is probably the single most complex and
controversial aspect of efficient fertilizer management, as il-
lustrated by Figure 4. This figure is a general representation of
several studies conducted at the UF and elsewhere. It compares
single applications of soluble and slow-release fertilizer to
multiple applications of soluble nitrogen. Effects of fertilizer
placement are not shown in Figure 4, but the general response to
nitrogen it shows would apply to either broadcasting or banding.
Figure 4 shows the estimated amounts of fertilizer needed for
each application method. The dashed line shows the theoretical
amount of applied nitrogen needed to meet optimal crop re-
quirements, 90 kilograms per hectare, and is included for com-
parison purposes. This curve drops after harvest due to the
release of nitrogen from plant decomposition. From Figure 4 it is
evident that a single application of soluble nitrogen requires the
largest amount of fertilizer to assure growth (200 kg/ha) while a
multiple application of soluble nitrogen required the least (120
Figure 4 also shows that the effectiveness of multiple applica-
tions of fertilizer depends on both knowledge of optimal plant up-
take of nutrients and scheduling of application. Multiple applica-
tions will usually give better control of nutrient releases but will
also increase costs, forcing a compromise. Fertigation-
applying fertilizer through an irrigation system enables fre-
quent applications with minimal management and costs, as long
as irrigation is also needed. Trickle irrigation and under-tree, low-
volume sprinkler systems are well-suited to fertigation of tree
crops because these systems do not wet foliage and fallow areas
between rows. In some cases, overhead irrigation systems are
just as satisfactory.
Two release curves are shown in Figure 4 for slow-release fer-
tilizer, one matched to plant uptake and the other mismatched.
According to Figure 4, both slow-release curves were somewhat
in-between single and multiple application in their effec-
tiveness. Rates depended upon how fast fertilizer granules
dissolved and released nitrogen. When slow-release fertilizer is
matched with a crop's uptake requirement, fertilizer require-
ments are sharply reduced. Often, however, release rates of
slow-release fertilizer are even more mismatched than shown in
Figure 4. The matching of release rates with crop need is ex-
tremely difficult because of changing rates of uptake due to
Leaching Rainfall Events
-- \F -- m-
SINGLE APPLICATION OF SOLUBLE N
plants / (200 kg/ha-N)
7 APPLICATIONS OF SOLUBLE N
to (120 kg/ha-N)
plants (180 20g- a-N N)
mismatched release curve SINGLE APPLICATION OF SLOW
N ............. well matched release curve RELEASE N, TWO RATES
fe 00z t (125 kg/ha-N) (in^ p t ) s..... a e .......h
0o *I-I- I I
Jan. & Fert.) Apr. (Harvest) J july Oct. Jan.
*For mismatched case. TIME
Figure 4. Available nitrogen for plant uptake (solid and dotted
lines) for three fertilization methods as compared to hypo-
thetical plant requirements, 90 kg/ha-N (dashed line). The
fertilization rate (in parentheses) is set at a level which ex-
actly meets plant requirements. Relative distribution of
leached N is given below each figure.
plant growth and weather conditions. Slow-release fertilizers
also reestablish soluble nitrogen levels in the soil-water more
slowly after a leaching rainfall than soluble fertilizers. Controlled
slow-release fertilizers have potential, but unless their release
rates are improved and their costs reduced, they are not likely to
play a major role in field-scale agriculture in the near future.
Some of the nutrients in this fertilizer are intended to become
soluble as needed by crop.
Figure 4 also shows the relative amount of nitrogen lost from
the various fertilization rates due to leaching. It shows that the
amount of nitrogen leached is proportional to the amount of solu-
ble nitrogen in the soil at the time of rain. Note also that annual
losses which were about 30, 17, and 27 kilograms per hectare for
single, multiple and mismatched slow-release methods respec-
tively, are proportional to the total amount of nitrogen applied.
The application method determines how much nitrogen must be
applied to meet crop needs, and therefore how much of it will be
lost. Thus, fertilizer management that maintains yield while ap-
plying less fertilizer is the most effective way to reduce nutrient
losses to the environment. Yet, such practices may involve more
intensive management and greater cost, so environmental
benefits must be weighed against increased production costs
before a new fertilization practice is adopted.
Sometimes, residual fertilizer is unavoidable after harvest,
however, residual nutrients can be partially utilized by multi-
cropping, where a new planting immediately follows harvest.
Multi-cropping combined with multiple applications or slow-
release nutrients offer the best chance to use and not lose valu-
SOIL TESTING AND PHILOSOPHIES
Growers must know their crop nutrient needs and soil nutrient
availability in order to evaluate the effectiveness of various fer-
tilization practices. To guess at crop needs can be wasteful
regardless of how good the fertilizer management practice is.
Therefore, growers must obtain the most accurate information
possible for their specific growing condition. How is this done?
The best way is through a good program of soil testing including
proper soil sampling, good sample analysis and adherence to
recommended application rates for specific crops. Both the
Cooperative Extension Service and private laboratories provide
To guess at crop needs can be wasteful
regardless of how good the fertilizer
management practice is.
There are two major philosophical approaches to fertilizer
recommendations. The first, which is used by most university
soil testing laboratories including IFAS, is to recommend a suffi-
cient amount of each nutrient which will provide maximum yield
for the grower for a particular crop. Some laboratories use
another philosophy: that the soil itself should be maintained at
some general optimal nutrient balance usually determined
with little regard to specific crop requirements. This philosophy
of optimal nutrient balance is an insurance program because it
tries to assure that maximum growth will always occur for a
variety of crops by supplying sufficient fertilizer for the most
nutrient-demanding crops. The theory is to keep the soil-
nutrient level high so changes in farming practices will not be
limited from year to year. In reality, many Florida sands do not
have the nutrient-holding capacity that would enable them to be
enriched over time. So while this approach generally works very
well for maintaining optimal plant growth, it can be costly, both
economically and environmentally. The philosophy of soil testing
laboratories should be carefully considered before selecting one.
Good fertilizer management for every agricultural commodity
is essential for profitable production and for environmental pro-
tection. The key to success is knowing crop needs and matching
fertilization programs to those needs. Several methods dis-
cussed here could deliver required nutrients while minimizing
losses to the environment. Rules to follow are simply to apply as
little fertilizer as necessary to meet production requirements, to
seek professional advice in determining correct application and
to practice other conservation measures recommended to
reduce nutrient losses.
For additional information contact your local Cooperative Ex-
tension Service Office.
PRODUCING GOOD FOOD WHILE PROTECTING FLORIDA'S WATER
COOPERATIVE EXTENSION SERVICE, UNIVERSITY OF FLORIDA, INSTI-
TUTE OF FOOD AND AGRICULTURAL SCIENCES, John T. Woeste, Director, in
cooperation with the United States Department of Agriculture, publishes this
information to further the purpose of the May 8 and June 30, 1914 Acts of Congress;
and is authorized to provide research, educational information and other services
only to individuals and institutions that function without regard to race, color, sex,
age, handicap or national origin. Single copies of extension publications (excluding L]
4-H and youth publications) are available free to Florida residents from county
extension offices. Information on bulk rates or copies for out-of-state purchasers is available from C.M.
Hinton, Publications Distribution Center, IFAS Building 664, University of Florida, Gainesville, Florida
32611. Before publicizing this publication, editors should contact this address to determine availability.