Building Soils for Better Crops, Third Edition

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.
  • 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 nutrient sources more efficiently.
  • Use localized placement of fertilizers whenever possible.
  • Split fertilizer application if leaching or denitrification losses are a potential problem (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.

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 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, as additional organic fertilizers are generally very expensive. On crop-livestock farms these soil organic N and P sources are typically sufficient to meet the crop’s demand, but not always.

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.

Since most plant-available P in soils is relatively strongly adsorbed by organic matter and clay minerals, estimating P availability is routinely done by 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.

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 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 temps increase N mineralization) and soil wetness (saturated soils cause large denitrification losses, especially when soils are warm).

Figure 19.3 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 rapidly and take up large quantities of N faster than new N is converted to mineral forms.

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 corn germinates in the spring, it takes a while until it begins 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 N may be needed to compensate for relatively high mineral N loss from soil (figure 19.3). However, in dry years— especially drought spells during the critical pollination period—corn yield 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. The actual amount of required N depends on the complex and dynamic interplay of crop growth patterns with weather events, which is difficult to predict. In fact, optimum N fertilizer rates for corn without organic amendments in the U.S. corn belt may vary from as little as 0 pounds per acre in one year to as much as 250 pounds per acre in another year. Those are the extremes, but, nevertheless, it is a great challenge to determine the optimum economic N rate.

Fixed and Adaptive Methods for Estimating Crop N Needs

e 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, recent 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 means (1) that larger and healthier root systems can take up more N, and (2) that frequently the weather pattern stimulates the presence of higher levels of nitrate in the soil.Several general approaches are used to estimate crop N needs, and they can be grouped into fixed and weather-adaptive approaches. Fixed approaches assume that the N fertilizer needs do not vary from one season to another based on weather conditions but may vary because of the previous crop. They are useful for planning purposes and work well in dryer climates, but they are very imprecise in a humid climate.

Several leading U.S. corn-producing states have adopted the maximum return to N (MRTN) approach, another fixed approach, which largely abandons the mass-balance method. It provides generalized recommendations based on extensive field trials, modelfitting, 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 for many seasons, it does not account for the soil and weather factors that affect N availability.

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 to 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 U.S.

The pre-plant nitrate test (PPNT) measures soil nitrate or soil nitrate plus ammonium in the soil (typically from 0 to 2 ft) early in the season to guide N fertilizer applications at planting. It is effectively used in dryer climates—like the U.S. Great Plains—where seasonal gains of inorganic forms of N are more predictable and losses are minimal. The PPNT 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 using reflectance spectroscopy allow adaptive approaches based on seasonal weather and local soil variation. Leaf chlorophyll meters or satellite, aerial, or tractor mounted sensors that measure light reflecting from leaves are used for assessing leaf or canopy N status, which can then guide sidedress N applications. These methods generally require a reference strip of corn that has received high levels of N fertilizer. This approach has been proven effective for spring N topdressing in cereal production, especially of winter wheat, but so far there has been limited success using this with corn due to more complicated crop and soil N dynamics.

Environmental information systems and simulation models are now also being employed for N management, with successful applications for wheat and corn. This is an adaptive approach that takes advantage of increasingly sophisticated environmental databases— like radar-based high-resolution precipitation estimates—that can be used to provide input information for computer models. N mineralization and losses are simulated together with crop growth to estimate soil N contributions and fertilizer N needs.

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. 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 two 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.

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