. . . an economical use of fertilizers requires that they merely supplement the natural supply in the soil, and that the latter should furnish the larger part of the soil material used by the crop.
—t.l. lyon and e.o. FiPPin, 1909
Both nitrogen and phosphorus are needed by plants
of N. So, when nitrate leaches through soil, or runs off
in large amounts, and both can cause environmen-
the surface and is discharged into streams, eventually
tal harm when present in excess. They are discussed
reaching water bodies like the Gulf of Mexico or the
together in this chapter because we don’t want to do a
Chesapeake Bay, undesirable microorganisms flourish.
good job of managing one and, at the same time, do a
In addition, the algal blooms that result from excess
poor job with the other. Nitrogen losses are a serious
N and P cloud water, blocking sunlight to important
economic concern for farmers; if not managed prop-
underwater grasses that are home to numerous species
erly, a large fraction (as much as half in some cases) of
of young fish, crabs, and other bottom dwellers. The
applied N fertilizer can be lost instead of used by crops.
greatest concern, however, is the dieback of the algae
Environmental concerns with N include the leaching
and other aquatic plants. These plants settle on the bot-
of soil nitrate to groundwater; excess N in runoff; and
tom of the affected estuaries, and their decomposition
losses of nitrous oxide, a potent greenhouse gas. For P,
consumes dissolved oxygen in the water. The result is
the main concerns are losses to freshwater bodies.
an extended area of very low oxygen concentrations in
High-nitrate groundwater is a health hazard to
which fish and other aquatic animals cannot live. This is
infants and young animals because it decreases the
a serious concern in many estuaries around the world.
blood’s ability to transport oxygen. In addition, nitrate
Denitrification is a microbial process that occurs
stimulates the growth of algae and aquatic plants just
primarily in surface layers when soils are saturated with
as it stimulates the growth of agricultural plants. The
water. Soil bacteria convert nitrate to both nitrous oxide
growth of plants in many brackish estuaries and salt-
(N2O) and N2. While N2 (two atoms of nitrogen bonded
water environments is believed to be limited by a lack
together) is the most abundant gas in the atmosphere
Photo by Dennis Nolan
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damages the environment when excess amounts are
volatilization and
added to a lake from human activities (agriculture, rural
denitrification
crop uptake
home septic tanks, or urban sewage and street runoff).
This increases algae growth (eutrophication), making
fishing, swimming, and boating unpleasant or difficult.
N
When excess aquatic organisms die, decomposition
organic
NO -, NH +
removes oxygen from water and leads to fish kills.
3
4
runoff and
All farms should work to have the best N and P
erosion
management possible—for economic as well as environ-
mental reasons. This is especially important near bodies
of water that are susceptible to accelerated weed or algae
leaching
growth. However, don’t forget that nutrients from farms
NITROGEN
in the Midwest are contributing to problems in the Gulf
vs.
of Mexico—over 1,000 miles away.
PHOSPHORUS
There are major differences between the way N and
P behave in soils (figure 19.1, table 19.1). Both N and P
crop
can, of course, be supplied in applied fertilizers. But aside
uptake
from legumes that can produce their own N because of
the bacteria living in root nodules, crop plants get their
N from decomposing organic matter. On the other hand,
P
organic
plants get their P from both organic matter and soil min-
& mineral
erals. Nitrate, the primary form in which plants absorb
runoff and
erosion
nitrogen from the soil, is very mobile in soils, while P
movement in soils is very limited.
leaching
Most unintentional N loss from soils occurs when
nitrate leaches or is converted into gases by the process
Figure 19.1. Different pathways for nitrogen and phosphorus losses
of denitrification, or when surface ammonium is volatil-
from soils (relative amounts indicated by width of arrows). Based on an
ized. Large amounts of nitrate may leach from sandy
unpublished diagram by D. Beegle, Penn State University.
soils, while denitrification is generally more significant
and not of environmental concern, each molecule of
in heavy loams and clays. On the other hand, most unin-
N2O gas—largely generated by denitrification, with some
tended P loss from soils is carried away in runoff or sedi-
contribution from nitrification—has approximately 300
ments eroded from fields, construction sites, and other
times more global warming impact than a molecule of
exposed soil (see figure 19.1 for a comparison between
carbon dioxide.
relative pathways for N and P losses). Phosphorus leach-
Phosphorus losses from farms are generally small in
ing is a concern in fields that are artificially drained.
relation to the amounts present in soils. However, small
With many years of excessive manure or compost appli-
quantities of P loss have great impacts on water quality
cation, soils saturated with P (often sands with low P
because P is the nutrient that appears to limit the growth
sorption capacity) can start leaking P with the percolat-
of freshwater aquatic weeds and algae. Phosphorus
ing water and discharge it through drain lines or ditches.
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ProbLeMs usInG excess n fertILIzer
There are quite a few reasons you should not apply more N than needed by crops. N fertilizers are now quite expensive, and many farmers are being more judicious than when N was relatively cheap. However, there are other problems associated with using more N than needed: (1) ground and surface water become polluted with nitrates; (2) more N2O (a potent greenhouse gas and source of ozone depletion) is produced during denitrification in soil; (3) a lot of energy is consumed in producing N, so wasting N is the same as wasting energy; (4) using higher N than needed is associated with acceleration of decomposition and loss of soil organic matter; and (5) very high rates of N are frequently associated with high levels of insect damage.
Also, liquid manure can move through preferential flow
to 30%. Although energy was relatively inexpensive
paths (wormholes, root holes, cracks, etc., especially in
for many years, its cost has fluctuated greatly in recent
clay soils) directly to subsurface drain lines and con-
years, as has the cost of fertilizers, and is expected to
taminate water in ditches, which is then discharged into
be relatively high for the foreseeable future. So relying
streams and lakes (see also chapter 17).
more on biological fixation of N and efficient cycling in
Except when coming from highly manured fields, P
soils reduces depletion of a nonrenewable resource and
losses—mainly as dissolved P in the runoff waters—from
may save you money as well. Although P fertilizers are
healthy grasslands are usually quite low, because both
less energy consuming to produce, a reduction in their
runoff water and sediment loss are very low. Biological N
use helps preserve this nonrenewable resource—the
fixation carried on in the roots of legumes and by some
world’s P mines are expected to run out in the next fifty
free-living bacteria actually adds new N to soil, but there
to one hundred years.
is no equivalent reaction for P or any other nutrient.
Improving N and P management can help reduce
Table 19.1
Comparing Soil N and P
reliance on commercial fertilizers. A more ecologically
Nitrogen
Phosphorus
based system—with good rotations, reduced tillage, and
more active organic matter—should provide a large pro-
Nitrogen becomes available from Phosphorus becomes available
decomposing soil organic matter. from decomposing soil organic
portion of crop N and P needs. Better soil structure and
matter and minerals.
attention to use of appropriate cover crops can lessen
N is mostly available to plants
P is available mainly as dissolved
loss of N and P by reducing leaching, denitrification,
as nitrate (NO –3)—a form that is
phosphate in soil water—but
very mobile in soils
little is present in solution even in
and/or runoff. Reducing the loss of these nutrients is an
fertile soils, and it is not mobile.
economic benefit to the farm and, at the same time, an
Nitrate can be easily lost in
P is mainly lost from soils by
environmental benefit to society. The greater N avail-
large quantities by leaching to
runoff and erosion. However,
ability may be thought of as a fringe benefit of a farm
groundwater or by conversion to liquid manure application on
gases (N
well-structured soils and those
with an ecologically based cropping system.
2, N2O).
with tile drainage has resulted in
In addition, the manufacture, transportation, and
P loss to drainage water.
application of N fertilizers are very energy intensive. Of
Nitrogen can be added to soils by No equivalent reaction can add
all the energy used to produce corn (including the man-
biological N fixation (legumes).
new P to soil, although many
bacteria and some fungi help
ufacture and operation of field equipment), the manu-
make P more available to plants.
facture and application of N fertilizer represents close
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MANAgEMENT OF N AND P
expensive. On crop-livestock farms these soil organic N
Nitrogen and phosphorus behave very differently in
and P sources are typically sufficient to meet the crop’s
soils, but many of the management strategies are actu-
demand, but not always.
ally the same or very similar. They include the following:
Since most plant-available P in soils is relatively
1. Take all nutrient sources into account.
strongly adsorbed by organic matter and clay minerals,
• Estimate nutrient availability from all sources.
estimating P availability is routinely done by soil tests.
• Use soil tests to assess available nutrients.
The amount of P extracted by chemical soil solutions can
• Use manure and compost tests to determine
be compared with results from crop response experi-
nutrient contributions.
ments and can provide good estimates of the likelihood
• Consider nutrients in decomposing crop residues
of a response to P fertilizer additions, which we discuss
(for N only).
in chapter 21.
2. Reduce losses and enhance uptake.
Estimating N fertilizer needs is more complex, and
• Use nutrient sources more efficiently.
soil tests generally cannot provide all the answers. The
• Use localized placement of fertilizers whenever
primary reason is that the amounts of plant-available
possible.
N—mostly nitrate—can fluctuate rapidly as organic
• Split fertilizer application if leaching or denitrifi-
matter is mineralized and N is lost through leaching or
cation losses are a potential problem (for N only).
denitrification. These processes are greatly dependent
• Apply nutrients when leaching or runoff threats
on soil organic matter contents, additional N contribu-
are minimal.
tions from organic amendments, and weather-related
• Reduce tillage.
factors like soil temperature (higher temps increase N
• Use cover crops.
mineralization) and soil wetness (saturated soils cause
• Include perennial forage crops in rotation.
large denitrification losses, especially when soils are
3. Balance farm imports and exports once crop needs
warm). Mineral forms of N begin to accumulate in soil
are being met.
Estimating Nutrient Availability
Good N and P management practices
take into account the large amount of
plant-available nutrients that come from
period of significant
total amount
leaching and denitrification
of mineral
soil mineral N,
the soil, especially soil organic matter
N available
normal year
and any additional organic sources like
during the
season
manure, compost, or a rotation or cover
crop. Fertilizer should be used only to
soil mineral N,
supplement the soil’s supply in order to
wet spring
provide full plant nutrition (figure 19.2).
spring
summer
fall
Organic farmers try to meet all demands
through these soil sources, as additional Figure 19.2. Available N in soil depends on recent weather. After increasing for a period, mineral N
organic fertilizers are generally very
decreases during a wet spring because leaching and denitrification losses are greater than N being converted to mineral forms. More mineral N is available for plants when the spring is drier.
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per acre in another year. Those are the
plant N
extremes, but, nevertheless, it is a great
challenge to determine the optimum
normal year
economic N rate.
sidedress
N
soil or
period of significant
fertilizer
Fixed and Adaptive Methods for
plant N
leaching and denitrification
needed
soil mineral N,
Estimating Crop N Needs
normal year
Several general approaches are used
to estimate crop N needs, and they can
soil mineral N,
be grouped into fixed and weather-
wet spring
adaptive approaches. Fixed approaches
spring
summer
fall
assume that the N fertilizer needs do
not vary from one season to another
based on weather conditions but may
Figure 19.3. Need for supplemental N fertilizer depends on early-season weather.
Note: The amount of mineral N in soil will actual y decrease (not shown) as plants begin to vary because of the previous crop. They
grow rapidly and take up large quantities of N faster than new N is converted to mineral forms.
are useful for planning purposes and
Soil N shown in figure 19.2 and here is the total amount made available by the soil during the work well in dryer climates, but they are
growing season.
very imprecise in a humid climate.
in the spring but may be lost by leaching and denitrifica-
The mass-balance approach, a fixed approach, is
tion during a very wet period (figure 19.2). When corn
the most commonly used method for estimating N fertil-
germinates in the spring, it takes a while until it begins
izer recommendations. It is generally based on a yield
to grow rapidly and take up a lot of N (figure 19.3).
goal and associated N uptake, minus credits given for
Weather affects the required amount of supplemental
non-fertilizer N sources such as mineralized N from soil
N in two primary ways. In years with unusually wet
organic matter, preceding crops, and organic amendments.
weather in the spring, an extra amount of sidedress N
However, recent studies have shown that the relation-
may be needed to compensate for relatively high mineral
ship between yield and optimum N rate is very weak for
N loss from soil (figure 19.3). However, in dry years—
humid regions. While higher yields do require more N, the
especially drought spells during the critical pollination
weather pattern that produces higher yields means (1) that
period—corn yield will be reduced, and the N uptake and
larger and healthier root systems can take up more N, and
needed N fertilizer are therefore lower (not shown in
(2) that frequently the weather pattern stimulates the pres-
figure 19.3). However, you really don’t know at normal
ence of higher levels of nitrate in the soil.
sidedress time whether there will be a drought during
Several leading U.S. corn-producing states have
pollination, so there is no way to adjust for that. The
adopted the maximum return to N (MRTN)
actual amount of required N depends on the complex
approach, another fixed approach, which largely aban-
and dynamic interplay of crop growth patterns with
dons the mass-balance method. It provides generalized
weather events, which is difficult to predict. In fact, opti-
recommendations based on extensive field trials, model-
mum N fertilizer rates for corn without organic amend-
fitting, and economic analyses. It is only available for
ments in the U.S. corn belt may vary from as little as 0
corn at this time. The rate with the largest average net
pounds per acre in one year to as much as 250 pounds
return to the farmer over multiple years is the MRTN,
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and the recommendations vary with grain and fertilizer
which can then guide sidedress N applications. These
prices. Adjustments based on realistic yield expectation
methods generally require a reference strip of corn that
are sometimes encouraged. The MRTN recommenda-
has received high levels of N fertilizer. This approach
tions are based on comprehensive field information,
has been proven effective for spring N topdressing in
but owing to generalizing over large areas and for many
cereal production, especially of winter wheat, but so far
seasons, it does not account for the soil and weather fac-
there has been limited success using this with corn due
tors that affect N availability.
to more complicated crop and soil N dynamics.
The adaptive approaches, described in the follow-
Environmental information systems and sim-
ing paragraphs, attempt to take into account seasonal
ulation models are now also being employed for N
weather, soil type, and management effects and require
management, with successful applications for wheat and
some type of measurement or model estimate during the
corn. This is an adaptive approach that takes advantage
growing season.
of increasingly sophisticated environmental databases—
The pre-sidedress nitrate test (PSNT) measures
like radar-based high-resolution precipitation esti-
soil nitrate content in the surface layer of 0 to 12 inches
mates—that can be used to provide input information
and allows for adaptive sidedress or topdress N applica-
for computer models. N mineralization and losses are
tions. It implicitly incorporates information on early-
simulated together with crop growth to estimate soil N
season weather conditions (figure 19.2) and is especially
contributions and fertilizer N needs.
successful in identifying N-sufficient sites—those that
do not need additional N fertilizer. It requires a special
Evaluation at the End of the Season
sampling effort during a short time window in late spring,
To evaluate the success of a fertility recommendation,
and it is sensitive to timing and mineralization rates dur-
farmers sometimes plant field strips with different
ing the early spring. The PSNT is usually called the late
N rates and compare yields at the end of the season.
spring nitrate test (LSNT) in the midwestern U.S.
The lower stalk nitrate test is also sometimes used
The pre-plant nitrate test (PPNT) measures
to assess, after the growing season, whether corn N
soil nitrate or soil nitrate plus ammonium in the soil
rates were approximately right or too low or too high.
(typically from 0 to 2 ft) early in the season to guide N
These two methods are neither fixed nor adaptive
fertilizer applications at planting. It is effectively used in
approaches for the current year since evaluation is
dryer climates—like the U.S. Great Plains—where sea-
made at the end of the season, but they may help farm-
sonal gains of inorganic forms of N are more predictable
ers make changes to their fertilizer application rates in
and losses are minimal. The PPNT cannot incorporate
following years. Adaptive management may therefore
the seasonal weather effects, as the samples are analyzed
also include farmer-based experimentation and adjust-
prior to the growing season, which inherently limits its
ment to local conditions.
precision compared to the PSNT.
Recent advances in crop sensing using reflectance
PlANNINg FOR N AND P MANAgEMENT
spectroscopy allow adaptive approaches based on
Although N and P behave very differently in soils, the
seasonal weather and local soil variation. Leaf chlo-
general approaches to their management are similar
rophyll meters or satellite, aerial, or tractor-
(table 19.2). The following considerations are important
mounted sensors that measure light reflecting from
for planning management strategies for N and P:
leaves are used for assessing leaf or canopy N status,
Credit nutrients in manures, decomposing
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Table 19.2
Comparison of N and P Management Practices
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