19.2: Management of N and P

Nitrogen and phosphorus behave very differently in soils, but many of the management strategies are actually the same or very similar. They include the following:

1. Take all nutrient sources into account.
   • Estimate nutrient availability from all sources.
   • Use soil tests to assess available nutrients. (Nitrogen soil tests are not available for all states. Some make N fertilizer recommendations based on fertilizer trials and estimates of cover crop contributions. Other methods for making N recommendations are discussed later in this chapter.)
   • Use manure and compost tests to determine nutrient contributions.
   • Consider nutrients in decomposing crop residues (for N only).
2. Reduce losses and enhance uptake (use 4R-Plus principles, fertilizer application using the right rate, at the right time, in the right place, in the right amount, plus conservation practices; see Chapter 18).
   • Use nutrient sources more efficiently.
   • Use localized placement of fertilizers below the soil surface whenever possible.
   • Split fertilizer application if leaching or denitrification losses are a potential problem (almost always for N only).
   • Apply nutrients when leaching or runoff threats are minimal.
   • Reduce tillage.
   • Use cover crops.
   • Include perennial forage crops in rotation.
3. Balance farm imports and exports once crop needs are being met.

Cover crops combined with minimal or no tillage is a set of practices that work well together. They improve soil structure; reduce the loss of nutrients through leaching, runoff and erosion; reduce denitrification loss of nitrates; and tie up N and P that otherwise might be lost between cash crops by storing these nutrients in organic forms.

Estimating Nutrient Availability

Figure 19.2. Available N in soil depends on recent weather. After increasing for a period, mineral N decreases during a wet spring because leaching and denitrification losses are greater than N being converted to mineral forms. More mineral N is available for plants when the spring is drier. (Gains and losses are greater when large amounts of organic applications, for example manure, are made.)

Good N and P management practices take into account the large amount of plant-available nutrients that come from the soil, especially soil organic matter and any additional organic sources like manure, compost, or a rotation crop or cover crop. Fertilizer should be used only to supplement the soil’s supply in order to provide full plant nutrition (Figure 19.2). Organic farmers try to meet all demands through these soil sources because additional organic fertilizers are generally very expensive. This is typically done by incorporating a legume as a crop or cover crop into the rotation or by adding high-N organic nutrient sources. When using organic fertilizers, the higher the percent N in the compost or in the other material, the more N will become available to plants. Little to no N will be available to plants if the amendment is around 2% N or less (corresponding to a high C:N ratio). But if it’s around 5% N, about 40% of the N in the amendment will be available. And if it’s 10% or 15% N (corresponding to a very low C:N ratio), 70 percent or more of the N in the amendment will be available to crops. On integrated crop-livestock farms soil organic N and P sources are typically sufficient to meet the crop’s demand, but not always.

Since most plant-available P in soils is relatively strongly adsorbed by organic matter and clay minerals, estimating P availability is routinely done through soil tests. The amount of P extracted by chemical soil solutions can be compared with results from crop response experiments and can provide good estimates of the likelihood of a response to P fertilizer additions, which we discuss in Chapter 21.

Figure 19.3. The need for supplemental N fertilizer depends on early season weather. Note: The amount of mineral N in soil will actually decrease (not shown) as plants begin to grow. They grow rapidly and take up large quantities of N faster than new N is converted to mineral forms.

Estimating N fertilizer needs is more complex, and soil tests generally cannot provide all the answers. The primary reason is that the amounts of plant-available N, mostly nitrate, can fluctuate rapidly as organic matter is mineralized and as N is lost through leaching or denitrification. These processes are greatly dependent on soil organic matter contents, additional N contributions from organic amendments, and weather-related factors like soil temperature (higher temperatures increase N mineralization) and soil wetness (saturated soils cause large leaching and denitrification losses, especially when soils are warm). Mineral forms of N begin to accumulate in soil in the spring but may be lost by leaching and denitrification during a very wet period (Figure 19.2). When plants germinate in the spring, it takes a while until they begin to grow rapidly and take up a lot of N (Figure 19.3). Weather affects the required amount of supplemental N in two primary ways. In years with unusually wet weather in the spring, an extra amount of sidedress (or topdress) N may be needed to compensate for relatively high mineral N loss from soil (Figure 19.3). The increasing rainfall intensity in some regions makes the use of sidedress N even more important. Research on corn in Minnesota from 2015 to 2019—where 75 percent of the sites evaluated had one month during the growing season with 150 percent of normal rainfall—indicated using sidedress N with some N applied before planting didn't decrease yields and actually increased yields by an average of 11 bushels an acre in a quarter of situations.

On the other hand, in dry years, especially drought spells during the critical pollination period, yields will be reduced, and the N uptake and needed N fertilizer are therefore lower (not shown in Figure 19.3). However, you really don’t know at normal sidedress time whether there will be a drought during pollination, so there is no way to adjust for that. For a field with a given soil type and set of management practices, the actual amount of required N also depends on the complex and dynamic interplay of crop growth patterns with weather events, which are difficult to predict. In fact, optimum N fertilizer rates for corn without organic amendments in the U.S. corn belt have been found to vary from as little as 0 pounds per acre to as much as 250 pounds per acre. Those are the extremes, but, nevertheless, it is a great challenge to determine the optimum economic N rate. There may be different issues arising in other regions. In the Northwest’s maritime region, large amounts of winter rainfall normally result in very low levels of available N in spring. Without much year-to-year carryover of mineral N and with low organic matter decomposition during the cool season, it is especially important to be sure that some readily available N is near the developing seedling of spring planted crops.

Fixed and Adaptive Methods for Estimating Crop N Needs

Several approaches are used to estimate crop N needs, and they can be grouped into fixed and adaptive approaches. Fixed (static) approaches assume that the N fertilizer needs do not vary from one season to another based on weather conditions, which may work well in drier climates but are very imprecise in a humid climate. Adaptive methods recognize that precise N fertilization requires additional data from field samples, sensors or computer models to modify the N rate for a particular production environment.

The mass-balance approach, a fixed approach, is the most commonly used method for estimating N fertilizer recommendations. It is generally based on a yield goal and associated N uptake, minus credits given for non-fertilizer N sources such as mineralized N from soil organic matter, preceding crops and organic amendments. However, studies have shown that the relationship between yield and optimum N rate is very weak for humid regions. While higher yields do require more N, the weather pattern that produces higher yields also implies 1) that larger and healthier root systems can take up more soil N, and 2) that frequently the weather pattern stimulates the presence of higher levels of nitrate in the soil. Conversely, very wet conditions cause reduced yields due to insufficient soil aeration and low soil N availability.

Several leading U.S. corn-producing states have adopted the maximum return to N (MRTN) approach, another fixed method that largely abandons the mass-balance approach. It provides generalized recommendations based on extensive field trials, model-fitting and economic analyses. It is only available for corn at this time. The rate with the largest average net return to the farmer over multiple years is the MRTN, and the recommendations vary with grain and fertilizer prices. Adjustments based on realistic yield expectation are sometimes encouraged. The MRTN recommendations are based on comprehensive field information, but owing to generalizing over large areas and over many seasons, it does not account for the soil and weather factors that affect N availability and is therefore inherently imprecise for an individual field.

The adaptive approaches, described in the following paragraphs, attempt to take into account seasonal weather, soil type and management effects, and require some type of measurement or model estimate during the growing season.

The pre-sidedress nitrate test (PSNT) measures soil nitrate content in the surface layer of 0–12 inches and allows for adaptive sidedress or topdress N applications. It implicitly incorporates information on early season weather conditions (Figure 19.2) and is especially successful in identifying N-sufficient sites: those that do not need additional N fertilizer. It requires a special sampling effort during a short time window in late spring, and it is sensitive to timing and mineralization rates during the early spring. The PSNT is usually called the late spring nitrate test (LSNT) in the midwestern United States.

Pre-plant nitrate and labile N tests measure soil nitrate, soil nitrate plus ammonium, or readily available organic nitrogen in the soil early in the season to guide N fertilizer applications at planting. These approaches are more effective in drier climates, like in the U.S. Great Plains where seasonal gains of inorganic forms of N are more predictable and losses from leaching or denitrification are generally minimal. Fall soil sampling can provide valuable information for N management for winter wheat while early spring season sampling is preferable for evaluating N needs for corn. These approaches cannot incorporate the seasonal weather effects, as the samples are analyzed prior to the growing season, which inherently limits its precision compared to the PSNT. Recent advances in crop sensing and modeling allow adaptive approaches based on seasonal weather and local soil variation. Leaf chlorophyll meters that measure light transmission in leaves and satellite, aerial, drone or tractor-mounted sensors that determine light reflection from leaves are used for assessing leaf or canopy N status and biomass, which can then guide sidedress N applications. Environmental information systems and dynamic simulation models are now also being employed for N management, with successful applications for wheat and corn. This approach takes advantage of increasingly sophisticated environmental databases, such as radar-based, high-resolution precipitation estimates and detailed soil databases, and can be used to provide input information for computer models. We discuss these further in Chapter 21.

Evaluation at the End of the Season

To evaluate the success of a fertility recommendation, farmers sometimes plant field strips with different N rates and compare yields at the end of the season. This can be done for vegetable crops as well as for crops like grain corn. Another option is to sample for soil nitrate after harvest, sometimes called a “report card” assessment, to evaluate residual levels of available N. The lower stalk nitrate test is also sometimes used to assess, after the growing season, whether corn N rates were approximately right or too low or too high. These methods are neither fixed nor adaptive approaches for the current year, since evaluation is made at the end of the season, but they may help farmers make changes to their fertilizer application rates in following years. Adaptive management may therefore also include farmer-based experimentation and adjustment to local conditions.