Building Soils for Better Crops, Third Edition

Planning for N and P Management

  Table 19.2: Comparison of N and P Management Practices
Use fixed-rate approaches for planning purposes and adaptive approaches to achieve precision.Test soil regularly (and follow recommendations).
Test manures and credit their N contribution.Test manures and credit their P contribution.
Use legume forage crops in rotation and/or legume cover crops to fix N for following crops and properly credit legume N contribution to following crops.No equivalent practice available.
Time N applications as close to crop uptake as possible.Time P application to reduce runoff potential.
Reduce tillage in order to leave residues on the surface and decrease runoff and erosion.Reduce tillage in order to leave residues on the surface and decrease runoff and erosion.
Use sod-type forage crops in rotation to reduce nitrate leaching and runoff.Use sod-type forage crops in rotation to reduce the amount of runoff and erosion losses of P.
Use grass cover crops, such as winter rye, to capture soil nitrates left over following the economic crop.Use grass cover crops, such as winter rye, to protect soil against erosion.
Make sure that excessive N is not coming onto the farm (biological N fixation + fertilizers + feeds).After soil tests are in optimal range, balance farm P flow (don’t import much more onto the farm than is being exported).

Although N and P behave very differently in soils, the general approaches to their management are similar (table 19.2). The following considerations are important for planning management strategies for N and P:

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 onfarm 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 P and all its potassium should be reflected in the soil test. The presidedress nitrate test can estimate the N contribution of the manure (see chapter 21 for a description of N 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 12.1.) Because significant ammonia N losses can occur in as little as one or two days after manure application, the way to derive the full N benefit from manure is to incorporate it as soon as possible.

Much of the manure N made available to the crop is in the ammonium form, and losses occur as some is volatilized as ammonia gas when manures dry on the soil surface. A significant amount of the manure’s N may also be lost when application is a long time before crop uptake occurs. About half of the N value of a fall manure application—even if incorporated—may be lost by the time of greatest crop need the following year.

Legumes, either as part of rotations or as cover crops, and well-managed grass sod crops can add N to the soil for use by the following crops (table 19.3). Nitrogen fertilizer decisions should take into account the amount of N contributed by manures, decomposing sods, and cover crops. If you correctly fill out the form that accompanies your soil sample, the recommendation you receive may take these sources into account. However, not all soil testing labs do take them into account; 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 10 (cover crops), 11 (rotations), and 12 (animal manures). For an example of crediting the nutrient value of manure and cover crop, see the section “Making Adjustments to Fertilizer Application Rates,” in chapter 21.

Table 19.3: Examples of Nitrogen Credits for Previous Crops*
Previous CropN Credits (lbs/acre)
Corn and most other crops0
Grass (low level of management)40
Grass (intensively managed)70
2-year stand red or white clover70
3-year alfalfa stand (20-60% legume)70
3-year alfalfa stand (>60% legume)120
Hairy vetch cover crop (excellent growth)110
*Less credit should be given for sandy soils with high amounts of leaching potential.

Relying on legumes to supply N to following crops. Nitrogen is the only nutrient of 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, of the N needed by the following crop. Growing a legume as a forage crop (alfalfa, alfalfa/ grass, clover, clover/grass) in rotation also can provide much, if not all, of the N for row crops. The N-related aspects of both cover crops and rotations with forages were discussed in 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 N fertilizers. A forage legume, such as alfalfa, red clover, or white clover, or a grass-legume mix can supply substantial N for the following crop. Frequently, nutrients are imported onto livestock-based 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—grown off-season in the fall and early spring—can provide sufficient N for some of the high-demanding summer annuals. It’s also possible to undersow sweet clover and then plow it under the next July to prepare for fall brassica crops.

Reducing N and P Losses

Use N and P 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 N and P 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 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 adaptive method is available, like the PSNT, spectroscopy, or a computer model, you can hold off applying most of the fertilizer until the test or model indicates a need. At that point, apply N as a sidedress or topdress. However, if you know that your soil is probably very N deficient (for example, a sandy soil low in organic matter), you may need to band-apply higher than normal levels of starter N at planting or broadcast some N before planting to supply sufficient

N nutrition until the soil test indicates whether there is a need for more N (applied as a sidedress or topdress). For row crops in colder climates, about 15 to 20 pounds of starter N per acre (in a band at planting) is highly recommended. When organic farmers use fishmeal or seed meals to supply N to crops, they should plan on it becoming available over the season, with little available in the first weeks of decomposition.


Corn is a tropical plant that is more efficient at utilizing N than most other crops—it produces more additional yield for each extra pound of N absorbed by the plant. But corn production systems as a whole have low efficiency of fertilizer N, typically less than 50%. Environmental N losses (leaching, denitrification, and runoff) are much higher for corn than for crops such as soybean and wheat, and especially when compared to alfalfa and grasses. This can be attributed to different crop growth cycles, fertilizer rates, fertilizer application schedules, timing of crop water and N uptake, and rooting depth. Intensive corn production areas have therefore become the focus of policy debates that address environmental concerns like groundwater contamination and low dissolved oxygen levels in estuaries.

Nitrogen management for corn is still mostly done without recognition of the effects of seasonal weather—particularly precipitation—that can cause high N losses through leaching and denitrification. The PSNT was the first approach that addressed these dynamic processes and therefore provided inherently more precise N fertilizer recommendations and eliminated a lot of unnecessary N applications. Still, many farmers like to apply additional “insurance fertilizer” because they want to be certain of an adequate N supply in wet years. But they may actually need it in only one out of four seasons. For those other years excess N application creates high environmental losses.

In addition to the PSNT, new technologies are emerging that allow us to more precisely manage N. Computer models and climate databases can be employed to adapt N recommendations by accounting for weather events and in-field soil variability. Also, crop reflectance of light, which is affected by the degree of N nutrition in the plant, can be measured using aerial and satellite images or tractor-mounted sensors and used to adjust sidedress N fertilizer rates on the go.

Some of the N in surface-applied urea, the cheapest and most commonly used solid N fertilizer, is lost as a gas if it is not rapidly incorporated into the soil. If as little as a quarter inch of rain falls within a few days of surface urea application, N losses are usually less than 10%. However, losses may be 30% or more in some cases (a 50% 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 ammonium nitrate is more costly than urea per unit of N and not always readily available, its N is generally not lost as a gas when left on the surface. Anhydrous ammonia, the least expensive source of N 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 N 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 crop uptake significantly increases the amount of N that may be lost in humid regions.

If the soil is very deficient in P, P fertilizers are commonly incorporated 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 P fertilizer should be incorporated before starting no-till. Nutrients accumulate near the surface of reduced tillage systems when fertilizers or manures are repeatedly surface-applied. This is when band application may be preferred.

In soils with optimal P levels, some P fertilizer is still recommended, along with N 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 root development, P diffusion toward the root, and mineralization of P from organic matter, reducing P availability to seedlings. This is probably why it is a good idea to use some starter P in these regions—even if the soil is in the optimal P soil test range.

Reducing tillage usually leads to marked reductions of N and P 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, N 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. It is best not to broadcast fertilizer and leave it on the surface with a no-till system.
  • P 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, fewer sediments, and less total P lost with no-till, the concentration of dissolved P in the runoff may actually be higher than for conventionally tilled soils.

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 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 help 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 as high as 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 N is mineralized. If this occurs many months before the row crop takes it up, high nitrate leaching and denitrification losses occur.) Using grass, legume, or grass-legume forages in the rotation also helps with P 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 N 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 immediately after the main crop has been harvested. One option available to help manage N is to use a combination of a legume and grass. The combination of hairy vetch and winter rye works well in cooler temperate regions. When nitrate is scarce, the vetch does much better than the rye and a large amount of N is fixed for the next crop. On the other hand, the rye competes well with the vetch when nitrate is plentiful; less N 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 P management. A cover crop that establishes quickly and helps protect the soil against erosion will help reduce P losses.

Reduce tillage. Because most P is lost from fields by erosion of sediments, environmentally sound P management should include reduced tillage systems. Leaving residues on the surface and maintaining stable soil aggregation and lots of large pores help 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 P and N losses from fields. Recent studies also showed that reduced tillage results in more effective N cycling. Although N fertilizer needs are generally slightly higher in early transition years, long-term no-tillage increases organic matter contents over conventional tillage and also, after some years, results in 30 to 50 pounds per acre more N mineralization, a significant economic benefit to the farm.

Working Toward Balancing Nutrient Imports and Exports

Nitrogen and phosphorus are lost from soils in many ways, including runoff that takes both N and P, leaching of nitrate (and in some situations P, as well), denitrification, and volatilization of ammonia from surface-applied urea and manures. Even if you take all precautions to reduce unnecessary losses, some loss of N and P will occur. While you can easily overdo it with fertilizers, use of more N and P 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 N fixation plus imported N in feeds may exceed the farm’s needs. A reasonable goal for farms with a large net inflow of N and P would be to try to reduce imports of these nutrients on farms (including legume N), or increase exports, to a point closer to balance.

On crop farms, as well as livestock-based farms with low numbers of animals per acre, it’s fairly easy to bring inflows and outflows into balance by properly crediting N from the previous crop and N and P in manure. On the other hand, it is a more challenging problem when there are a large number of animals for a fixed land base and a large percentage of the feed must be imported. This happens frequently on factory-type animal production facilities, but it can also happen on smaller, 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. In the Netherlands, nutrient accumulation on livestock farms became a national problem and generated legislation that limits animal units on farms. One option is to compost the manure—which makes it easier to transport or sell and causes some N losses during the composting process— stabilizing the remaining N before application. On the other hand, the availability of P in manure is not greatly affected by composting. That’s why using compost to supply a particular amount of “available” N usually results in applications of larger total amounts of P than plants need.

Using Organic Sources of Phosphorus and Potassium

Manures and other organic amendments are frequently applied to soils at rates estimated to satisfy N needs of crops. This commonly adds more P and potassium than the crop needs. After many years of continuous application of these sources to meet N needs, soil test levels for P 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 P and P-containing organic amendments. If it’s a farm-wide problem, some manure may need to be exported and N fertilizer or legumes relied on to provide N 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 N is needed, can help. However, if you’re raising livestock on a limited land base, you should make arrangements to have the manure used on a neighboring farm or sell the manure to a composting facility.

Managing High-P Soils

High-P soils occur because of a history of either excessive applications of P 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 the nutrients exported in animal products. In addition, where manures or composts are used at rates required to provide sufficient N to crops, more P than needed usually is added. It’s probably a good idea to reduce the potential for P loss from all high-P soils. However, it is especially important to reduce the risk of environmental harm from those high-P 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-P soils:

  • First, deal with the “front end” and reduce animal P intake to the lowest levels needed. Not that long ago a survey found that the average dairy herd in the U.S. was fed about 25% more P than recommended by the standard authority (the National Research Council, or NRC). Using so much extra can cost dairy farmers thousands of dollars to feed a 100-cow herd supplemental P that the animals don’t need and that ends up as a potential pollutant.
  • Second, reduce or eliminate applications of extra P. 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 N without adding P. 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 P imports) means that a crop farmer will need more creative use of crop residues, rotations, and cover crops to maintain good organic matter levels. Also, don’t use a high-P source to meet N demands. Compost has many benefits, but if used to provide N fertility, it will build up P over the long term.
  • Third, reduce runoff and erosion to minimal levels. P is usually a problem only if it gets into surface waters. Anything that helps water infiltration or impedes water and sediments from leaving the field—reduced tillage, strip cropping along the contour, cover crops, grassed waterways, riparian buffer strips, etc.—decreases problems caused by high-P soils. (Note: Significant P losses in tile drainage water have been observed, especially from fields where large amounts of liquid manure are applied.)
  • Fourth, continue to monitor soil P levels. Soil test P will slowly decrease over the years, once P imports, as fertilizers, organic amendments, or feeds, are reduced or eliminated. Soils should be tested every two or three years for other reasons, anyway. So just remember to keep track of soil test P to confirm that levels are decreasing.

Phosphorus accumulates especially rapidly in the surface of no-till soils that have received large applications of manure or fertilizer over the years. One management option in these cases is a one-time tillage of the soil to incorporate the high-P layer. If this is done, follow practices that don’t result in building up surface soil P once again.

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