19.3: Planning for N and P Management

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.

Table 19.2 Comparison of N and P Management Practices

Nitrogen	Phosphorus
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 is available (although cover crop and cash crop mycorrhizae help mobilize soil P already there, making it more available to plants).
Time N applications as close to crop uptake as possible, and place to reduce runoff or gaseous losses.	Time and place P application to reduce runoff potential.
Reduce tillage in order to leave residues on the surface and to decrease runoff and erosion.	Reduce tillage in order to leave residues on the surface, to decrease runoff and erosion, and to keep mycorrhizal network intact.
Use sod-type forage crops in rotation to reduce nitrate leaching and runoff, making N more available to following crops.	Use sod-type forage crops in rotation to reduce the amount of runoff and erosion losses of P, making P more available to the following crop.
Use grass cover crops, such as cereal rye, to capture soil nitrates leftover following the economic crop.	Use grass cover crops, such as cereal rye, to protect soil against erosion.
Make sure that excessive N is not coming onto the farm (biological N fixation plus fertilizers plus feeds).	After soil tests are in the optimal range, balance the farm’s P flow (don’t import much more onto the farm than is being exported).

Figure 19.4. A soybean crop generally provides 20–40 pounds N per acre to a following corn crop, which needs to be accounted for in making N recommendations.

Credit nutrients in manures, rotation crops, decomposing sods, cover crops 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 P and all its potassium will be reflected in the soil test. The pre-sidedress 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 for its fertilizer value before applying it to the soil; many soil test labs also analyze manures. (Although a manure analysis test is recommended and will provide the most accurate result, 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 application, the way to derive the full N benefit from surface-applied manure (or urea for that matter) 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 happens a long time before crop uptake occurs. Even if incorporated, about half of the N value of a fall manure application may be lost by the time it is needed by the crop in the following year.

Manure Application, Tillage And N Loss

When using some tillage, it makes sense to incorporate manure as soon after application as weather and competing work priorities allow. With no-till there are low-disturbance manure injectors that place liquid manure in the soil with minimal N loss.

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 next crop (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 that; 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, discussed as part of integrated livestock-cropping systems). Also, some of the adaptive simulation models described above can incorporate such credits into recommendations, while also accounting for variable weather conditions. For an example of crediting the nutrient value of manure and cover crops, see the section “Making Adjustments to Fertilizer Application Rates” in Chapter 21.

Table 19.3 Examples of Nitrogen Credits for Previous Crops

Previous crop	N credits (pounds per acre)
Corn and most other crops	0
Soybeans^2	0–40
Grass (low level of management)	40
Grass (intensively managed, using N fertilizer for maximum economic yield)	70
2-year stand of red or white clover	70
3-year alfalfa stand (20–60% legume)	70
3-year alfalfa stand (>60% legume)	120
Crimson clover	110
Winter peas	110
Hairy vetch cover crop (excellent growth)	110
^1Less credit should be given for sandy soils with high amounts of leaching potential. ^2Some labs give 30 or 40 pounds of N credit for soybeans, while others give no N credit. Credits represent the amount of N that will be available to the crop (not the total amount contained in residue). Although the actual amount of N that will become available can be higher in dry years and lower in wet years (Figure 19.2), we still can’t accurately predict the growing season weather. When following cover crops, the stage of growth and the amount of growth will strongly influence the amount of N available to the following crop.

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.

Cover Crops Enhance P For Following Crop

Cover crops mobilize and take up a significant amount of P through mycorrhizae and other organisms of the root microbiome. Later, as they decompose, this P becomes available for the following crops to use. While this is a very different mechanism than N fixation by legumes, it is another example of a crop together with microorganisms helping the following crop obtain particular nutrients.

Animals on the farm or on nearby farms? There are many possibilities for actually eliminating the need for N fertilizer 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). 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 off the farm, and this reduces the need to purchase fertilizers. When planting vegetable crops following a manure application, keep in mind the regulation that requires 120 days from application to harvest (see discussion in Chapter 12 for manure use and food safety issues).

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 and biological 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, planning for fall brassica crops the following year. (If tillage is used, it can be plowed under the next July to prepare for the fall crop.) Sunn hemp and cowpeas growing as cover crops in the Southeast during the summer months have been found to replace one-third to one-half of the N needed for fall broccoli.

Reducing N and P Losses

Manage N and P fertilizers more efficiently. You should have plenty of organic nutrients present if you’ve worked to build and maintain soil organic matter. These readily decomposable fragments provide N and P as they decompose, thereby reducing the amount of fertilizer that’s needed.

When applying commercial fertilizers and manures, the timing and method of application 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, which is especially important when it involves N. Losses of surface-applied fertilizer and manure nutrients are also frequently reduced by soil incorporation with tillage (even a light incorporation can help a lot). Liquid N fertilizer, especially when dribble applied, penetrates the surface, better protecting it from possible gaseous loss. And no-tilled soils that have continual living roots by using cover crops tend to have vastly greater water infiltration and less runoff and gaseous losses.

If you’re growing a crop for which a reliable in-season adaptive method is available, like the PSNT, a sensor or a computer model, you can hold off applying most of the fertilizer until the crop 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). About 15–20 pounds of starter N per acre (in a band at planting) is highly recommended for crops in colder climates. Even more starter N is needed when some cover crops like cereal rye or triticale are allowed to grow near maturity. The large amount of biomass, with its high C:N ratio, will tie up mineral sources of soil N for some weeks following cover crop termination. 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 released in the first weeks of decomposition. On the other hand, N contained in feather meal may become available more rapidly.

In-season topdressing N on wheat and on some other annual cereal or oilseed crops is sometimes needed, especially when wet conditions cause significant losses of available soil N. It’s helpful if farmers put high-N strips within fields, in which they apply N at rates of 40–50 pounds per acre higher than other areas. The length and width of the strips aren’t that important. The purpose of the strips is to see if you can tell the difference between the wheat in the high-N strip and the rest of the field. Top dressing N is recommended if the difference is very noticeable.

If the soil is very deficient in phosphorus, P fertilizers have traditionally been incorporated by tillage to raise the general level of the nutrient. Incorporation is not possible with no-till systems, and if a soil is 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. If P levels are good to start with, in later years small amounts of surface-applied P will work its way deeper into the soil surface. And P can be band applied as starter fertilizer at planting, or it can be injected, keeping it below the surface.

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, thereby reducing P availability to seedlings. No-tilled soils with plentiful surface residue will stay cool for a longer period in the spring, thereby decreasing both N and P availability. However, if cover crops are used together with no-till—a combination that provides many benefits—soils will dry and warm more rapidly, lessening the concern with early P deficiency in row crops. But for no-till without cover crops in cool climates it is a good idea to use a small amount of starter P for the young crop—even if the soil is in the optimal P soil test range.

Use the right fertilizer products. 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 or surface applied in the form of chemically stabilized urea, greatly reducing N loss. Stabilized urea is the most economical source when N fertilizer is broadcast as a topdress on grass, on cereals such as wheat, or on row crops. Solutions of urea and ammonium nitrate (UAN) are also used as a topdress or are dribbled on as a band. (Although once widely used, solid ammonium nitrate fertilizer is expensive and not always readily available due to concerns about explosivity. But like calcium ammonium nitrate [CAN], its N is generally not lost as a gas when left on the surface and therefore is a good product for topdressing.)

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. In humid regions, even if stabilizers are used, anhydrous applied long before crop uptake significantly increases the amount of N that may be lost. For this reason, fall-applied anhydrous ammonia is a practical N source only in the more arid western portion of the Corn Belt, and only after the soil has cooled below 50 degrees F. But fall application of anhydrous ammonia remains relatively common even in the more humid parts of the region due to price and logistical benefits, but this raises environmental concerns.

In some cases, nutrients are applied individually through separate fertilizer products, while multi-nutrient compounds (like monoammonium phosphate) or blended materials are used in other cases. When applying multiple nutrients at once, aim to use combinations that proportionally fit the nutritional needs of your crop, thereby reducing unnecessary applications and buildup of nutrients that are overapplied. Or otherwise use multi-nutrient fertilizer in combination with single-nutrient products to achieve the right proportions.

Use nitrogen efficiency enhancement products. Field nitrogen losses can be high depending on the soil, the practices used and the conditions of the growing season, especially weather. With urea-based nitrogen fertilizers and manure, ammonia losses into the atmosphere can be considerable if the material is left on the surface, especially when conditions following application are dry and soil pH is high. Several products on the market reduce ammonia losses by suppressing the activity of the urease enzyme. These urease inhibitors reduce the production of ammonia by naturally occurring soil enzymes, lessening N losses as well as concerns about air pollution and unwanted nitrogen deposition in nearby areas. Nitrification inhibitors are another type of products for use with N fertilizers. These suppress conversion of ammonium to nitrate by naturally occurring soil microorganisms. Ammonium is strongly held by negative charges on soil particles (the cation exchange complex) and does not leach from soils, while the negatively charged nitrate ion can wash through the soil when a lot of rain occurs. This is especially a concern with sandy soils. Also, in finer-textured soils, nitrate can be lost during wet periods through denitrification and volatilization of N₂ and N₂O into the air. Of course, the leaching and gaseous losses are detrimental to farm profitability as well as to the environment. The role of the nitrification inhibitor is to maintain nitrogen in the ammonium form for longer periods, slowly making nitrate available as the growing crop develops, thereby increasing use efficiency. A third type of product, similar to nitrification inhibitors, focuses on controlled release by using a coating on fertilizer material that causes it to slowly dissolve and release the nitrogen fertilizer.

Soil Reactions with N Fertilizers

Urea is converted to ammonia (lost to the atmosphere or dissolved in water to form ammonium as a gas, or converted to nitrate).

Ammonia and ammonium are nitrified to nitrate (easily lost by leaching and/or denitrification).

The choice of enhanced efficiency products depends on the fertilization strategy. Urease inhibitors are appropriate when using urea-based fertilizers without incorporation. When applying ammonia/ammonium-based fertilizers well before crop uptake, consider adding a nitrification inhibitor or using coated materials. In some cases, a combination of products is appropriate. In general, the use of these products reduces N losses, but it depends on the production environment in a particular growing season. It may prevent yield losses in some years or allow reductions in overall N fertilizer rates by reducing the need for using higher levels of fertilizer as “insurance.”

Common Nitrogen Fertilizer Efficiency Enhancement Products

Mode of action	Formulation and use	Common enhanced efficiency products^1
Urease inhibition	Additive for urea-based; manure	NBPT, MIC
Nitrification inhibition	Additive for anhydrous ammonia, urea- and ammonium-based	Nitrapyrin, DCD, MIC
Urease and nitrification inhibition	Stand-alone fertilizer product	Ammonium and calcium thiosulfates
Controlled release	Stand-alone fertilizer product	Polymer-coated prilled nitrogen or other nutrients
^1This list is not comprehensive but includes the most widely used products. Inclusion or omission of a product in this list does not imply an endorsement by the authors or publisher. Source: Cantarella, H., R. Otto, J.R. Soares and A.G. de Brito Silva. 2018. Agronomic efficiency of NBPT as a urease inhibitor: A review. Journal Advanced Research 13: 19–27.

New Technology For Corn Nitrogen Fertilization

Corn is a tropical plant that is more efficient at utilizing N than are 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 soybeans 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 depths. Intensive corn production areas have therefore become the focus of policy debates that address environmental concerns like groundwater contamination and hypoxia zones in estuaries.

Nitrogen management for corn is still mostly done without recognizing how seasonal weather, particularly precipitation, 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, say, one out of four seasons. For those other years, excess N application creates high environmental losses.

New technologies are emerging in addition to the PSNT 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 can then be used to adjust sidedress N fertilizer rates, even for small zones in a field (precision management).

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; improve beneficial aggregation; 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 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 reduced runoff and erosion, and the effects on soil structure for the following crop.

Use cover (catch) 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 significantly reduced if you sow a fast-growing cover crop like cereal rye immediately after the main crop has been harvested. Such cover crops are commonly referred to as “catch crops” because their fast-growing roots can capture the remaining nutrients in the soil and store them in their biomass. One option available to help manage N is to use a combination of a legume and grass. The combination of hairy vetch and cereal rye or triticale works well in cooler temperate regions. When nitrate is scarce, the vetch or crimson clover does much better than the rye, and a large amount of N is fixed for the next crop. Conversely, 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. Crimson clover with either cereal rye or oats works similarly in the South, with the clover growing better and fixing more N when soil nitrate is scarce, and with cereal rye growing faster when nitrate is plentiful.

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 sediment erosion, 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 when conventional plow-harrow tillage is used. Reduced tillage, by decreasing runoff and erosion, usually decreases both P and N losses from fields. Recent studies have also shown that reduced tillage results in more effective N cycling. Although N fertilizer needs are generally slightly higher in early transition years, long-term no-till increases organic matter contents over conventional tillage and also, after some years, results in 30 pounds (or more) per acre greater N mineralization, which is a significant economic benefit to the farm.

Tillage, Nutrient Loss, And Fertilizer Application Methods

Reducing tillage usually leads to marked reductions of nitrate leaching loss to groundwater as well as to runoff and, therefore, N and P loss in runoff. But, questions have come up about potential problems with broadcasting N and P fertilizers in reduced tillage systems, especially in no-till. The main attractiveness of broadcast fertilizer is that you can travel faster and cover more land than with injection methods of application—around 500–800 acres in eight hours for broadcast versus about 200 acres for injection. However, there are two complicating factors.

• If intense storms occur soon after application of surface-applied urea, 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 N fertilizer and to leave it on the surface with a no-till system. This is particularly true for urea, since surface residues contain higher levels of the urease enzyme, facilitating fast conversion to ammonia, which is rapidly lost as a gas. Fertilizer N may be applied at different stages: before planting, with the seed at planting, or as a sidedress. Using liquid N as a sidedress results in better soil contact than a solid fertilizer would achieve.

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 usually less runoff, fewer sediments and less total P lost with no-till, the concentration of dissolved P in the runoff is often higher than for conventionally tilled soils. Phosphorus should be applied below the surface to reduce such losses.

Working Toward Balancing Nutrient Imports and Exports

In addition to being contained in the products sold off the farm, nitrogen and phosphorus are lost from soils in many unintended ways, including runoff that takes both N and P, nitrate leaching (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 N and P loss will occur. While you can easily overdo it with fertilizers, using more N and P than is 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 through feed would be to try to reduce imports of these nutrients onto the farm (including legume N), or to increase exports, to a point closer to balance.

On crop farms, as well as on 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. But 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. It causes some N losses during the composting process, but stabilizes 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 a crop’s N need. 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 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.