Fall Sampling for Nitrogen

The majority of soil sampling for nutrients is conducted in the fall after harvest in South Dakota. Fall is also an excellent time to include soil sampling to estimate soybean cyst nematode infestations.

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Figure 1

Figure 1. Sampling areas based on variation. across the field as indicated by differences in soil type and an old fence line.

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Most fields have variable nutrient levels due to climate, topography, vegetation, natural soil variability, and soil management practices (Figure 1).1 Uniform fertilizer applications are likely to lead to excessive fertilization in some areas and inadequate fertilization in others. More intensive soil sampling schemes that subdivide a field into small units can help identify more variability and provide more information about soil-test levels compared with a traditional sampling procedure that uses one composite sample for an entire field or large sampling area. Less than one teaspoonful of soil from each sample is used for the laboratory analysis.1 In a traditional sampling scheme, one teaspoon or less of soil represents up to 40 acres. In a site-specific intensive sampling scheme the teaspoon represents up to a 2.5-acre area of the field, based on sampling design.

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Depth of sampling, timing of sampling, equipment, sample handling, and sampling procedures all have an effect on a good representative soil sample. Nitrogen (N) and sulfur (S) availability are affected by soil microbial activity. Therefore, fall soil sampling for N and S should be delayed until the soil temperature is below 50° F when microbial activity is minimal and there is negligible nutrient release.1

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Phosphorus (P), potassium (K), pH, soluble salt content, and micro- and most secondary nutrient soil tests are not affected by sampling time and can be taken any time during the year.

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A source of potential variability is the depth of soil sampling. The tillage system, fertilizer bands, compaction layers, and other factors can cause nutrient stratification. Higher crop yields coupled with reduced tillage can result in nutrient deficiency, stratification, or soil mining, particularly with less mobile nutrients like P and K. Depth of soil sampling should be consistent within and between fields and over years to get comparable nutrient tests. A 24-inch sample is needed for mobile nutrients such as N and S to get a good idea of the nitrate and sulfate concentrations potentially available to the crop. Typically, a 6 to 8-inch sampling depth is necessary for P, K, pH, organic matter, soluble salts, zinc (Zn), iron (Fe), manganese (Mn), copper (Cu), and boron (B). Samples may need to be subdivided into 2-inch increments if nutrient or pH stratification is suspected.

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The fall application goal is to use an N source that will stay in the ammonium form as long as possible because nitrate-N is susceptible to loss. Anhydrous ammonia (NH3) is the best option.2 This source eliminates the nitrifying bacteria at the point of application, and the activity of nitrifying bacteria within the ammonia retention zone is also inhibited for a couple of weeks because of the temporary high pH that develops as ammonia reacts with soil water to form ammonium. Nitrification inhibitors can help protect fall N against loss, but the greatest potential for loss occurs in the spring; by that time the inhibitor is less effective due to its breakdown over time.

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Fall applied urea (46-0-0) and UAN (28-0-0) should be incorporated within 3 days to a minimum depth of 3 inches.

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Consult the South Dakota State University publication Best Management Practices for Corn Production in South Dakota for nutrient recommendations. The corn nitrogen rate calculator can help determine the economically optimal N rate at various corn and N prices.3 The data used for this calculator came from trials where N loss was minimal, but it does not account for carryover N that might not have been used by the previous crop. Also, if you applied manure or the soil has high potential for N mineralization (as in the case of a field coming off of alfalfa), you will need to adjust the values derived from the calculator to reflect what will be available next year.1

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Because spring conditions have a substantial influence on N loss, N management plans should include planned N applications at-planting and in-crop, in addition to fall applications, to have N available in adequate amounts when corn demand is high.