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Management of Nitrogen and Phosphorus
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
The management of nitrogen and phosphorus
is discussed together because both are needed in large amounts by
plants and both can cause environmental harm when present in excess.
We don't want to do a good job of managing one and, at the same
time, do a poor job with the other. The main environmental concern
with nitrogen is the leaching of soil nitrate to groundwater and
excess nitrogen in runoff. The drinking of high-nitrate groundwater
is a health hazard to infants and young animals. In addition, nitrate
stimulates the growth of algae and aquatic plants just like it does
for agricultural plants. The growth of plants in many brackish estuaries
and saltwater environments is believed to be limited by nitrogen.
So, when nitrate leaches through soil, or runs off the surface and
is discharged into streams, eventually reaching water bodies like
the Gulf of Mexico or the Chesapeake Bay, undesirable microorganisms
flourish. In addition, the algal blooms that result from excess
nitrogen and phosphorus cloud water, blocking sunlight to important
underwater grasses that are home to numerous species of young fish,
crabs, and other bottom-dwellers.
Phosphorus is the nutrient that appears to limit the
growth of freshwater aquatic weeds and algae. Phosphorus damages
the environment when excess amounts are added to a lake from human
activities (agriculture, rural home septic tanks, or urban sewage
or runoff). This increases algae growth, making fishing, swimming,
and boating unpleasant or difficult. When excess aquatic organisms
die, decomposition removes oxygen from water and leads to fish kills.
All farms should work to have the best nitrogen and
phosphorus management possible -- for economic as well as environmental
reasons. This is especially important near bodies of water that
are susceptible to accelerated weed or algae growth (eutrophication).
However, don't forget that nutrients from farms in the Midwest are
contributing to problems in the Gulf of Mexico over 1,000 miles
away.
There are major differences between the way nitrogen
and phosphorus behave in soils (see table 17.1
and figure 17.1 below). Besides fertilizer
sources, nitrogen is readily available to plants only from decomposing
organic matter, while plants get their phosphorus from both organic
matter and soil minerals. Nitrate, the primary form in which plants
use nitrogen, is very mobile in soils; phosphorus movement in soil
is very limited.
Most unintentional nitrogen loss from soils occurs
when nitrate leaches or is converted into gases during denitrification,
or when surface ammonium is volatilized. Large amounts of nitrate
may leach from sandy soils, while denitrification is generally more
important in heavy loams and clays. On the other hand, almost all
unintended phosphorus loss from soils is carried away in sediments
eroded from fields (see figure 17.1 for a comparison between relative
pathways for nitrogen and phosphorus losses). Except when coming
from highly manured fields, phosphorus losses from healthy grasslands
are usually quite low mainly as dissolved phosphorus in the runoff
waters because both runoff water and sediment loss are very low.
Biological nitrogen fixation carried on in the roots of legumes
and by some free-living bacteria actually adds new nitrogen to soil,
but there is no equivalent reaction for phosphorus or any other
nutrient.
Improving nitrogen and phosphorus management can help
reduce reliance on commercial fertilizers. A more balanced system
with good rotations and more active organic matter should provide
a large proportion of crop nitrogen and phosphorus needs. Better
soil structure and attention to use of appropriate cover crops can
lessen loss of nitrogen and phosphorus by reducing leaching, denitrification,
and/or runoff. Reducing the loss of these nutrients is an economic
benefit to the farm and, at the same time, an environmental benefit
to society. The greater nitrogen availability may be thought of
as a "fringe benefit" of a farm with an ecologically based
cropping system. In addition, the manufacture, transportation, and
application of nitrogen fertilizers is very energy intensive. Of
all the energy used to produce corn (including the manufacture and
operation of field equipment), the manufacture and application of
nitrogen fertilizer represents close to 40 percent. So relying more
on biological fixation of nitrogen reduces depletion of a non-renewable
resource. Although energy has been relatively inexpensive for many
years, it may very well become more expensive in the future. Although
phosphorus fertilizers are less energy consuming to produce, a reduction
in their use helps preserve this non-renewable resource.
Management of Nitrogen and Phosphorus
Nitrogen and phosphorus behave very differently in
soils, but many of the management strategies are actually the same
or are very similar. Management strategies include the following:
a) Take all nutrient sources into account.
- Use soil tests to assess available nutrients.
- Use manure tests to determine nutrient contributions.
- Consider nutrients in decomposing crop residues (for N only).
b) Reduce losses / enhance uptake.
- Use nutrient sources more efficiently.
- Use localized placement of fertilizers whenever possible.
- Split fertilizer application if leaching or denitrification
losses are a problem (for N only).
- Apply nutrients when leaching or runoff threats are minimal.
- Reduce tillage.
- Use cover crops.
- Include perennial forage crops in rotation.
c) Balance farm imports and exports once crop needs
are being met.
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Figure 17.1 Different pathways for nitrogen
and phosphorus losses from soils (relative amounts indicated
by width of arrows). Based on an unpublished diagram by D. Beegle.
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Taking All Nutrient Sources Into Account
Soil testing for nitrogen and phosphorus and interpreting
soil test results are discussed in chapter
19.
Credit nutrients in manures, decomposing sods,
and other organic residues. Before applying commercial fertilizers
or other off-farm nutrient sources, you should properly credit the
various on-farm sources of nutrients. In some cases, there is more
than enough fertility in the on-farm sources to satisfy crop needs.
If manure is applied before sampling soil, the contribution of much
of the manure's phosphorus and all its potassium should be reflected
in the soil test. One nitrogen soil test, the Pre-Sidedress Nitrate
Test (PSNT), reflects the nitrogen contribution of the manure (see
chapter 19 for a description of nitrogen
soil tests). The only way to really know the nutrient value of a
particular manure is to have it tested before applying it to the
soil. Many soil test labs will also analyze manures for their fertilizer
value. (Without testing the manure or the soil following application,
estimates can be made based on average manure values, such as those
given in table 9.1) Because significant nitrogen losses can occur
in as little as one or two days after manure application, the way
to derive the full nitrogen benefit from manure is to incorporate
it as soon as possible. Much of the manure-nitrogen made available
to the crop is in the ammonium form, and losses occur as ammonium
is volatilized when manures dry on the soil surface. A significant
amount of the manure's nitrogen also may be lost when application
is a long time before crop uptake occurs. About half of the nitrogen
value of a fall manure application even if incorporated may be lost
by the time of greatest crop need the following year.
Legumes, as either part of rotations or as cover crops, and well-managed
grass sod crops can add nitrogen to the soil for use by the following
crops (table 17.2). Nitrogen fertilizer decisions should take into
account the amount of nitrogen contributed by manures, decomposing
sods, and cover crops. If you correctly filled out the form that
accompanies your soil sample, the recommendation you receive may
take these sources into account. However, soil testing labs may
not take these into account when making their recommendations; most
do not even ask whether you've used a cover crop. If you can't find
help deciding how to credit nutrients in organic sources, take a
look at chapters 9 (animal manures), 10
(rotations), and 11 (cover crops). For
an example of crediting the nutrient value of manure and cover crop,
see the section "Making Adjustments to Fertilizer Application
Rates" in chapter 19.
Rely on legumes to supply nitrogen to following
crops. Nitrogen is the only nutrient for which you can "grow"
your own supply. High-yielding legume cover crops, such as hairy
vetch and crimson clover, can supply most, if not all the nitrogen
needed by the following crop. Growing a legume as a forage crop
in rotation (alfalfa, alfalfa/grass, clover, clover/grass) also
can provide much, if not all of the nitrogen for row crops. The
nitrogen-related aspects of both cover crops and rotations with
forages were discussed in previous chapters (chapters 10
and 11).
Animals on the farm or on nearby farms? If you have
ruminant animals on your farm or on nearby farms for which you can
grow forage crops, (and perhaps use the manure on your farm), there
are many possibilities for actually eliminating the need to use
nitrogen fertilizers. A forage legume, such as alfalfa, red clover,
or white clover or a grass/legume mix, can supply substantial nitrogen
for the following crop. Frequently, nutrients are imported onto
animal farms as various feeds (usually grains and soybean meal mixes).
This means that the manure from the animals will contain nutrients
imported from outside the farm and this reduces the need to purchase
fertilizers.
No animals? Although land constraints don't usually
allow it, some vegetable farmers grow a forage legume for one or
more years as part of a rotation, even when they are not planning
to sell the crop or feed it to animals. They do so to rest the soil
and to enhance the soil's physical properties and nutrient status.
Also, some cover crops, such as hairy vetch growing off-season in
the fall and early spring can provide sufficient nitrogen for some
of the high-demanding summer annuals. It's also possible to undersow
sweetclover and then plow it under the next July to prepare for
fall brassica crops.
Reduce Nitrogen and Phosphorus Losses
Use nitrogen and phosphorus fertilizers more efficiently.
If you've worked to build and maintain soil organic matter, you
should have plenty of active organic materials present. These readily
decomposable small fragments provide nitrogen and phosphorus as
they are decomposed, reducing the amount of fertilizer that's needed.
The timing and method of application of commercial
fertilizers and manures affect the efficiency of use by crops and
the amount of loss from soils especially in humid climates. In general,
it is best to apply fertilizers close to the time when they are
needed by plants. Losses of fertilizer and manure nutrients are
also frequently reduced by soil incorporation with tillage.
If you're growing a crop for which a reliable in-season
nitrogen test is available (see discussion in chapter
19) then you can hold off applying fertilizer until the nitrogen
test indicates a need. At that point, apply nitrogen as a sidedress.
Otherwise, you may need to broadcast some nitrogen before planting
to supply sufficient nutrition until the soil test indicates if
there is need for more nitrogen (applied as a sidedress). For row
crops in colder climates, about 15 to 20 lbs. of starter N per acre
(in a band at planting), is highly recommended.
Some of the nitrogen in surface-applied urea, the
cheapest and most commonly used solid nitrogen fertilizer, is lost
as a gas if it is not rapidly incorporated into the soil. If as
little as 1/4-inch rain falls within a few days of surface urea
application, nitrogen losses are usually less than 10 percent. However,
losses may be as large as 30 percent or more in some cases (50 percent
loss may occur following surface application to a calcareous soil
that is over pH 8). When urea is used for no-till systems, it can
be placed below the surface. When fertilizer is broadcast as a topdress
on grass or row crops, you might consider the economics of using
ammonium nitrate. Although it is more costly than urea per unit
of nitrogen, nitrogen in ammonium nitrate is generally not lost
as a gas when left on the surface. Anhydrous ammonia, the least
expensive source of nitrogen fertilizer, causes large changes in
soil pH in and around the injection band. The pH increases for a
period of weeks, many organisms are killed, and organic matter is
rendered more soluble. Eventually, the pH decreases and the band
is repopulated by soil organisms. However, significant nitrogen
losses can occur when anhydrous is applied in a soil that is too
dry or too wet. Even if stabilizers are used, anhydrous applied
long before a crop needs it significantly increases the amounts
of nitrogen that may be lost in humid regions.
If the soil is very deficient in phosphorus, phosphorus
fertilizers are commonly incorporated into the soil to raise the
general level of the nutrient. Incorporation is not possible with
no-till systems and, if the soil was initially very deficient, some
phosphorus fertilizer should have been incorporated before starting
no-till. Nutrients accumulate near the surface of reduced tillage
systems when fertilizers or manures are repeatedly surface-applied.
In soils with optimal phosphorus levels, some phosphorus
fertilizer is still recommended, along with nitrogen application,
for row crops in cool regions. (Potassium is also commonly recommended
under these conditions.) Frequently, the soils are cold enough in
the spring to slow down both root development and mineralization
of phosphorus from organic matter, reducing phosphorus availability
to seedlings. This is probably why it is a good idea to use some
starter phosphorus in these regions even if the soil is in the optimal
phosphorus soil test range.
Use perennial forages (sod-forming crops) in rotations.
As we've discussed a number of times, rotations that include a perennial
forage crop help reduce the amount of runoff and erosion; build
better soil tilth; break harmful weed, insect, and nematode cycles;
and build soil organic matter. Decreasing the emphasis on row crops
in a rotation and including perennial forages also helps decrease
leaching losses of nitrate. This happens for two main reasons:
1) There is less water leaching under a sod because
it uses more water over the entire growing season than does an annual
row crop (which has a bare soil in the spring and after harvest
in the fall).
2) Nitrate concentrations under sod rarely reach anywhere near those
under row crops.
So, whether the rotation includes a grass, a legume,
or a legume/grass mix, the amount of nitrate leaching to groundwater
is usually reduced. (A critical step, however, is the conversion
from sod to row crop. When a sod crop is plowed, a lot of nitrogen
is mineralized. If this occurs many months before the row crop can
use nitrogen, high nitrate leaching and denitrification losses occur.)
Using grass, legume, or grass/legume forages in the rotation also
helps with phosphorus management because of the reduction of runoff
and erosion and the effects on soil structure for the following
crop.
Use cover crops to prevent nutrient losses.
High levels of soil nitrate may be left at the end of the growing
season if drought causes a poor crop year or if excess nitrogen
fertilizer or manure has been applied. The potential for nitrate
leaching and runoff can be reduced greatly if you sow a fast-growing
cover crop like winter rye. One option available when using cover
crops to help manage nitrogen is to use a combination of a legume
and grass. The combination of hairy vetch and winter rye works well
in the mid-Atlantic region. When nitrate is scarce, the vetch does
much better than the rye and a large amount of nitrogen is fixed
for the next crop. On the other hand, the rye competes well with
the vetch when nitrate is plentiful, and less nitrogen is fixed
(of course, less is needed) and much of the nitrate is tied up in
the rye and stored for future use.
In general, having any cover crop on the soil during
the off season is helpful for phosphorus management. A cover crop
that establishes quickly and helps protect the soil against erosion
will help reduce phosphorus losses.
Reduce tillage. Because most phosphorus is
lost from fields by erosion of sediments, environmentally sound
phosphorus management should include reduced tillage systems. Leaving
residues on the surface and maintaining stable soil aggregation
and lots of large pores helps water to infiltrate into soils. When
runoff does occur, less sediment is carried along with it than if
conventional plow-harrow tillage is used. Reduced tillage, by decreasing
runoff and erosion, usually decreases both phosphorus and nitrogen
losses from fields.
Reducing tillage usually leads to marked
reductions in nitrogen and phosphorus loss in runoff and nitrate
leaching loss to groundwater. However, there are two complicating
factors that should be recognized:
- If intense storms occur soon after application
of surface-applied urea or ammonium nitrate, nitrogen is
more likely to be lost via leaching than if it had been
incorporated. Much of the water will flow over the surface
of no-till soils, picking up nitrate and urea, before entering
wormholes and other channels. It then easily moves deep
into the subsoil.
- Phosphorus accumulates on the surface of
no-till soils (because there is no incorporation of broadcast
fertilizers, manures, crop residues, or cover crops). Although
there is less runoff and fewer sediments and less total
phosphorus lost with no-till, the concentration of dissolved
phosphorus in the runoff may actually be higher than for
conventionally tilled soils.
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Working Towards Balancing Nutrient Imports and
Exports
Nitrogen and phosphorus are lost from soils, in many ways, including
runoff that takes both nitrogen and phosphorus, leaching of nitrate
(and sometimes significant amounts of phosphorus), denitrification,
and volatilization of ammonia from surface applied urea and manures.
Even if you take all precautions to reduce unnecessary losses, some
loss of nitrogen and phosphorus will occur anyway. While you can
easily overdo it with fertilizers, use of more nitrogen and phosphorus
than needed also occurs on many livestock farms that import a significant
proportion of their feeds. If a forage legume, such as alfalfa,
is an important part of the rotation, the combination of biological
nitrogen fixation plus imported nitrogen in feeds may exceed the
farm's needs. A reasonable goal for farms with a large net inflow
of nitrogen and phosphorus would be to try to reduce "imports"
of these nutrients on farms (including legume nitrogen), or increase
exports, to a point closer to balance.
Managing High Phosphorus
Soils
High-phosphorus soils occur because of a history
of either excessive applications of phosphorus fertilizers
or more commonly application of lots of manure. This is a
problem on livestock farms with limited land and where a medium-to-high
percentage of feed is imported. The nutrients imported in
feeds may greatly exceed nutrients exported in animal products.
In addition, where manures or composts are used at rates required
to provide sufficient nitrogen to crops, more phosphorus than
needed usually is added. It's probably a good idea to reduce
the potential for phosphorus loss from all high-phosphorus
soils. However, it is especially important to reduce the risk
of environmental harm from those high-phosphorus soils that
are also likely to produce significant runoff (because of
steep slope, fine texture, poor structure, or poor drainage).
There are a number of practices that should
be followed with high-phosphorus soils:
- First, deal with the "front end"
and reduce animal phosphorus intake to the lowest levels
needed. A recent survey found that the average dairy herd
in the U.S. is fed about 25 percent more phosphorus than
recommended by the standard authority (the National Research
Council, NRC). In addition, research indicates that the
NRC recommendations may be 10 to 15 percent higher than
actually needed. This is costing dairy farmers about $3,500
to feed a 100-cow herd supplemental phosphorus that the
animals don't need and that only ends up as a potential
pollutant!
- Second, reduce or eliminate applying extra
phosphorus. For a livestock farm, this may mean obtaining
the use of more land to grow crops and to spread manure
over a larger land area. For a crop farm, this may mean
using legume cover crops and forages in rotations to supply
nitrogen without adding phosphorus. The cover crops and
forage rotation crops are also helpful to build up and maintain
good organic matter levels in the absence of importing manures
or composts or other organic material from off the farm.
The lack of imported organic sources of nutrients (to try
to reduce phosphorus imports) means that a crop farmer will
need more creative use of crop residues, rotations, and
cover crops to maintain good organic matter levels.
- Third, reduce runoff and erosion to minimal
levels. Phosphorus usually is only a problem if it gets
into surface waters. Anything that helps water infiltration
or impedes water and sediments from leaving the field decreases
problems caused by high-phosphorus soils reduced tillage,
strip cropping along the contour, cover crops, grassed waterways,
riparian buffer strips, etc. [Note: Significant phosphorus
losses in tile drainage water have been observed, especially
from fields where large amounts of liquid manure are applied.]
- Fourth, continue to monitor soil phosphorus
levels. Soil test phosphorus will slowly decrease over the
years, once phosphorus imports as fertilizers, organic amendments,
or feeds are reduced or eliminated. Testing soils every
two or three years should be done for other reasons anyway.
So, just remember to keep track of soil test phosphorus
to confirm that levels are decreasing.
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On crop farms, as well as animal farms with low numbers of animals
per acre, it's fairly easy to bring inflows and outflows into balance
by properly crediting nitrogen from the previous crop and nitrogen
and phosphorus in manure. On the other hand, it is a more challenging
problem when there are a large number of animals for a given land
base and a large percentage of the feed must be imported. This happens
frequently on "factory"-type animal production facilities,
but can also happen on family-sized farms. At some point, thought
needs to be given to either expanding the farm's land base or exporting
some of the manure to other farms. Another option is to compost
the manure which makes it easier to transport or sell and causes
some nitrogen losses during the composting process stabilizing the
remaining nitrogen before application. On the other hand, the availability
of phosphorus in manure is not greatly affected by composting. That's
why using compost to supply a particular amount of "available"
nitrogen usually results in applications of larger total amounts
of phosphorus than plants need.
Phosphorus and Potassium Can Get too High When
Using Organic Sources
Manures and other organic amendments are frequently applied to soils
at rates estimated to satisfy nitrogen needs of crops. This commonly
adds more phosphorus and potassium than the crop needs. After many
years of continuous application of these sources to meet nitrogen
needs, soil test levels for phosphorus and potassium may be in the
very high (excessive) range. Although there are a number of ways
to deal with this issue, all solutions require reduced applications
of fertilizer phosphorus and phosphorus-containing organic amendments.
If it's a farm-wide problem, some manure may need to be exported
and nitrogen fertilizer or legumes relied on to provide nitrogen
to grain crops. Sometimes, it's just a question of better distribution
of manure around the various fields getting to those fields far
from the barn more regularly. Changing the rotation to include crops,
such as alfalfa, for which no manure nitrogen is needed can help.
However, if you're raising livestock on a limited land base, you
should make arrangements to remove some of the manure from the farm.
Sources
Brady, N.C., and R.R. Weil. 1999. The Nature and Properties of
Soils. 12th ed. Macmillan Publ. Co. New York, NY.
Jokela, B., F. Magdoff, R. Bartlett, S. Bosworth,
and D. Ross. 1998. Nutrient Recommendations for Field Crops in
Vermont. UVM Extension, University of Vermont. Burlington, VT.
Magdoff, F.R. 1991. Understanding the Magdoff pre-sidedress
nitrate soil test for corn. Journal of Production Agriculture
4:297305.
National Research Council. 1988. Nutrient requirements
of dairy cattle, 6th rev. ed., National Academy Press, Washington,
D.C.
Sharpley, A.N. 1996. Myths about phosphorus. NRAES-96.
Proceedings from the Animal Agriculture and the Environment North
American Conference, Dec. 1113, Rochester, NY. Northeast
Region Agricultural Engineering Service. Ithaca, NY.
Vigil, M.F., and D.E. Kissel. 1991. Equations for
estimating the amount of nitrogen mineralized from crop residues.
Soil Science Society of America Journal 55:757761.
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