Sustainable Agriculture Research and Education - Publications

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.

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:

Use soil tests to assess available nutrients.
Use manure tests to determine nutrient contributions.
Consider nutrients in decomposing crop residues (for N only).

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.

fig 17.1 different pathways for nitrogen and phosphorus losses from soils

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.

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.

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.

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.

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.