Fertility Planning

Table 1
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Many growers are experiencing record soybean yields in South Dakota this fall. Early indications from soil samples in those fields are that potassium (K) use by soybeans has been high and soils will need K fertilizer to replenish the reservoir for a corn crop. Nutrient balance in the soil and the fertility plan must be maintained to support high yield potential. Realistic yield goals should be set to provide direction for fertility planning and reduce nutrient losses.

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Farmers can estimate the amount of nutrient removal as a guide to planning for fertility applications (Table 1). Fall fertilizer applications can have advantages such as reduced soil compaction, spreading out the workload, and potentially lower fertilizer costs. The crop nutrient removal calculator can be used to estimate crop nutrient removal of nitrogen (N), phosphorus (expressed as P2O5), potassium (expressed as K2O), and sulfur (S) for many field crops (http://ipni.info/calculator).

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Potassium is associated with movement of water, nutrients, and carbohydrates in plant tissue. If K is deficient or not supplied in adequate amounts, growth is stunted and yields are reduced. Various research efforts have shown that potassium stimulates early growth, increases protein production, improves water use efficiency, is vital for stand persistence, longevity, and winter hardiness of alfalfa, and improves resistance to diseases, nematodes, and insects.1 A corn crop may require nearly 240 pounds of K2O to produce the roots, stems, leaves, and grain. The nutrient reservoir of many soils may be inadequate to supply the total K needs of high yielding corn, especially during the peak demand period just prior to silking.

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Higher crop yields coupled with reduced tillage can result in nutrient deficiencies, nutrient stratification, or soil mining, particularly with less mobile nutrients like phosphorous (P) and K. Compaction, loose, or dry soil conditions can result in K deficiency. Decreased soil mixing in reduced-till systems combined with surface applications of P and K can result in higher nutrient concentrations in the top few inches of soil. Research has demonstrated that P and K values are 3.5 times higher in the upper 2 inches of soil than at the 2 to 6 inch depth.2 Horizontal stratification of P and K can occur with banded fertilizer applications.

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Table 2
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Soil pH is an important but sometimes overlooked element of soil fertility management. Some producers focus on replenishing depleted nutrients each season but forego actions to manage pH. Soil pH is a measure of acidity or alkalinity and is influenced by parent material, time, topography, climate, and soil organisms. Crop performance responds differently to pH (Table 2). The solubility and availability of many of the nutrients required by plants changes with changes in pH. P and most of the macro-nutrients are most available at pH 6.5 to 7.5, and drops off on either side of this range. Research at South Dakota State University has shown that an average of 18 to 20 lb/acre P above the amount needed to replace P used by the previous crop is required to raise soil test P by one part/million (ppm).3 K is less sensitive to pH and is equally available across a broad range of pH levels. In contrast, iron (Fe), manganese (Mn), boron (B), copper (Cu), and zinc (Zn) are most available at acidic pH levels, but are still sufficiently available for crop use in the 6.5 to 7.5 range.