Nutrient Management

By Dr. Dick Bohling, CCA

The primary goal of a sound nutrient management program is to take advantage of the existing nutrients supplied by your soil and to supplement those nutrients with fertilizer applications that can optimize crop yield and maximize profits. Start with soil testing. It is the single most important guide you have for cost-effective application of fertilizer and lime. In fact, if you aren’t sampling and testing your soil and following the recommendations from your test, you are losing money. Applying more fertilizer than the crop needs is expensive and wasteful. But not applying enough for the crop to reach its yield potential will cost you in lost profits. Test your soil and get started on the best nutrient management program you can.

Nitrogen

Harvested crops remove nitrogen more than any other nutrient. As profit margins narrow and environmental concerns increase, it’s imperative that you use nitrogen fertilizer as efficiently as possible.

Nitrogen recommendations in humid regions have been based primarily on expected yield, with adjustments for previous crop, manure applications and other management practices. Although this method has worked well, it ignores differences between soils in their ability to supply nitrogen. Soils differ in their capacity to mineralize nitrogen (release nitrogen from organic matter) and in their capacity to prevent nitrogen losses (denitrification and leaching). These differences have encouraged researchers to develop reliable and practical soil nitrogen tests that can be used to modify application rates to enhance crop profits without harming the environment.

Although soil tests for nitrogen have been developed, their use is currently limited. In many areas, none of the nitrogen soil procedures have given adequate crop nitrogen recommendations. In these areas, yield goal is still the preferred method for determining nitrogen needs. Following is a brief discussion of these two methods.

The Yield Goal Method -- A realistic yield goal is one of the major considerations in determining the optimum rate of nitrogen fertilizer to apply to each crop. A yield goal should be established for each field based on soil type and planned management level. A reasonable yield goal could be the average of the three highest yields from the past five production years for that crop in a given field. If yield records are not available for a particular field, check with your Agricultural Extension Service for the productivity-index value for the soil types involved. For fields under exceptionally high management, a 15 to 20 percent increase in the recommended yield goal would be reasonable.

The basis of the yield goal method for determining nitrogen needs is the numerous research trials that have been conducted under various soil and climatic conditions. Yield response curves, commodity prices and nitrogen costs are then used to determine the optimum nitrogen rate for each crop.

For some crops, trial results are used to calculate an efficiency index (unit of applied nitrogen required per unit of production), which is used for determining nitrogen application rates. In many corn production areas, for example, 1.2 to 1.3 pounds of nitrogen are recommended for each bushel of production. The lower rate of application (1.2 pounds) would be recommended when the value of a bushel of corn is low, relative to the price of a unit of nitrogen. Relatively large increases in yield would be necessary to pay for additional nitrogen. This relationship is referred to as the corn:nitrogen price ratio. When the corn:nitrogen price ratio is high, higher rates of nitrogen are recommended (1.3 pounds of nitrogen per bushel of production). In this case, relatively small increases in yield would be necessary to pay for additional nitrogen.

Many agronomists consider a corn:nitrogen price ratio of 20:1 as the break point for low and high nitrogen recommendations. At 20:1 or above, multiply your yield goal by 1.3 pounds of nitrogen. Below 20:1, multiply your yield goal by 1.2 pounds of nitrogen. For example, the recommended nitrogen rate for 140 bushel per acre corn would be 168 (140 x 1.2) pounds per acre at the low corn:nitrogen price ratio. It would be 182 (140 x 1.3) pounds per acre at the high corn:nitrogen price ratio. Remember, when the corn:nitrogen price ratio is low, a bushel of corn buys less nitrogen than at a high price ratio. The yield response curve stays the same, but the economic optimum rate varies with the corn:nitrogen price ratio -- and it decreases as the ratio decreases.

Once you have determined the nitrogen rate, adjustments should be made for the previous crop, manure applications and planting dates. Most grass species (corn, sorghum, small grains, etc.) benefit from residual nitrogen left in the soil by a legume crop (soybeans, alfalfa, clover, etc.) grown in rotation. Credits from residual nitrogen can be significant and should be deducted from the requirements for your new crop. For example, in a corn-soybean rotation, soybeans contribute approximately 40 pounds of nitrogen to the following corn crop. Another approach that some producers prefer is to give a credit of 1 pound of nitrogen per bushel of soybean yield. Regardless of the method you use, the contribution is significant. Nitrogen contributions by alfalfa and clover are even greater. Credits to a corn crop can exceed 100 pounds per acre. Credits to small grains are generally less because the release of nitrogen from legumes is not as rapid in early spring, when the nitrogen needs of small grains are greatest.

Manure is a good source of all major and micro-nutrients, including nitrogen, the nutrient that generally has the greatest value. The nitrogen content of manure varies with type and method of application, so analyzing a sample of the manure you use is essential for good nutrient management. On farms with a deficiency of nutrients, manure can greatly reduce fertilizer needs. On farms with an excess of manure, however, these nutrients can represent an environmental threat if they are not used properly. For greatest efficiency, manure should be incorporated during or immediately after application. Even when incorporated shortly after application, only 50 to 60 percent of the total nitrogen will be available to the crop grown during the year of application.

The final adjustment to nitrogen rates is based on planting date. In some areas, where the planting date is delayed, less nitrogen is needed for optimum yields. But this adjustment is only possible if nitrogen is sidedressed. A good rule of thumb is to reduce the nitrogen rate about 20 pounds per acre for each week that planting is delayed past the optimum planting date. However, don’t go below a minimum of 70 to 90 pounds per acre. Also, don’t delay planting to apply nitrogen fertilizer. It’s usually best to plant, then sidedress.

Nitrogen Soil Test Systems -- Nitrogen recommendations based on yield goals make no adjustments for the effects of soil type and weather on nitrogen availability. In the past, this was acceptable because nothing better was available. But research during the past decade has produced soil and tissue tests that now can be used to address soil and weather variability.

The early spring nitrate nitrogen test for corn involves collecting soil samples in 1-foot increments to a depth of 2 to 3 feet in early spring. Nitrogen recommendations are then reduced by the amount found in the soil profile. This procedure has been used for several years in arid parts of the country with reasonable success. Results are mixed as you move east into areas with higher precipitation. In these areas, much of the nitrogen detected through early testing is lost through denitrification or leaching before the plant has an opportunity to use it.

The late spring nitrate nitrogen test involves collecting soil samples to a 1-foot depth when corn plants are 6 to 12 inches tall. Sidedress nitrogen fertilizer recommendations are then based on the amount of nitrate nitrogen found. In general, no additional nitrogen is recommended if soil test levels exceed 25 parts per million (50 pounds per acre). Below 10 parts per million, the full rate of nitrogen should be applied. Between 10 and 25 parts per million, proportional adjustments in nitrogen rates should be applied. A portion (50 to 70 percent) of the normal nitrogen application should be applied preplant to minimize the potential for a nitrogen shortage prior to sidedressing.

A third test available for nitrogen management is the end-of-season stalk nitrate test. In this test, an 8-inch segment of the lower stalk is analyzed for nitrate levels. If the stalk nitrate content exceeds an optimum range, there’s a g