Table 1. Soil-test-based phosphorus recommendations for soybean production in Louisiana.

Potassium

Severe K deficient soybean produces irregular yellowing along the leaf margin and yields significantly lower than its potential. The soil-test-based K recommendations for soybean production in Louisiana is shown in Table 2. The critical soil-test K concentrations (i.e., medium soil K level) for soybean production depend on soil types. For example, the critical soil-test K concentration ranges from 107 to 141 ppm (214 to 282 lbs/acre) for soybean production in alluvial silt loam or loam soils but 177 to 264 ppm in alluvial silty clay loam or clay loam soils (Table 2). The critical soil-test K concentration also depends on the type of sediment deposition such as alluvial vs. upland soils. Therefore, care should be taken in interpreting soil-test K concentration and K fertilizer recommendations for soybean production in a particular field in Louisiana.

Soybean is somewhat more responsive to K than P fertilization. Soybean yield response to K fertilization is almost certain in soils with below critical K level. However, like P, soybean yield response to K fertilization is seldom expected in soils with critical K level and not expected in soils with above critical K level. Therefore, soybean must be fertilized with K for soils with less than critical/medium K level (i.e., very low and low level) and K fertilization is not recommended for soils with more than critical/medium K level (i.e., high and very high level). For K fertilization, use granular muriate of potash (MoP; 0-0-60) and apply either in the Fall or in the Spring at or before planting. The information about the effect of K application timing on soybean yield can be found at “Phosphorus and potassium fertilizers application in spring vs. fall”.

Soil-test-based potassium recommendations for soybean production in Louisiana.

Loamy Sand, Sandy Loam

*Mehlich-3 Soil-test K Concentration (ppm)

Very Fine Sandy Loam, Fine Sandy Loam

*Mehlich-3 Soil-test K Concentration (ppm)

Loam, Silt Loam

*Mehlich-3 Soil-test K Concentration (ppm)

Clay Loam, Silty Clay Loam

*Mehlich-3 Soil-test K Concentration (ppm)

Silty Clay, Clay

*Mehlich-3 Soil-test K Concentration (ppm)

Sulfur

Sulfur is often recommended when Mehlich-3 soil-test S concentration falls below 12 ppm (24 lbs/acre). In this case, about 20 lbs S, as sulfate, is recommended per acre. Sulfur can be applied at or after planting. There are many S products available in the market. Gypsum (16% S) is a very good source of S for soybean production. Ammonium sulfate (21-0-0-24S) is another readily available S source, but not recommended for soybean production since it contains 21 percent N. This small amount of N may negatively affect soybean nodulation and possibly reduce yield. There are some soil-applied liquid S sources available in the market such as potassium thiosulfate, magnesium thiosulfate, etc. and these products are as good as dry S fertilizers when applied as sidedress.

Zinc

Zinc is one of the important micronutrients that often limits yield if deficient in soils. Zinc is not usually recommended for soybean. However, for high yielding soybean fields, it is advised to apply 10 lbs of Zn/acre if the Mehlich-3 soil-test Zn concentration falls below 1 ppm, 5 lbs of Zn/acre for soil-test Zn concentration of 1 to 2.25 ppm, and no Zn for soil-test Zn concentration more than 2.25 ppm. Zinc sulfate or zinc chelate is the most common Zn source and can be applied at or after planting as broadcast (dry formulation) or sidedress (liquid formulation).

Molybdenum

Molybdenum is a vital component of the nitrogenase enzyme that helps Rhizobium bacterial to fix atmospheric N for soybean plants. Molybdenum is not typically recommended for soybean production in Louisiana since most of Louisiana soils have enough Mo for optimal soybean growth. However, Mo availability is drastically decreased if the soil pH falls below 6.0. Therefore, Mo should be applied as seed treatment at planting for soils with less than 6.2 pH if lime is not applied in the Fall. If Rhizobium inoculum is used as seed treatment, Mo should not be used unless seeds are planted immediately after treating. Otherwise, Mo salt will reduce the viability of inoculum, resulting in poor nodulation. Since nutrient availability is maximum between soil pH 6.5 and 7.0, it is better to raise soil pH to near neutral (7.0) rather than applying Mo.

Soybean is a very nutrient-intensive crop. One bushel soybean requires about 5 lbs nitrogen (N), 1 lb di-phosphorus penta-oxide (P2O5), and 3.8 lbs di-potassium oxide (K2O) per acre. Since soybean can meet its own N requirement by fixing atmospheric N, the crop is mainly fertilized with phosphorus (P) and potassium (K) and sometimes sulfur (S), zinc (Zn), and molybdenum (Mo). Soybean fertilization in Louisiana mainly depends on Mehlich-3 soil-test nutrient concentrations of soil samples collected from 0- to 6-inch depth except for Mo which depends on soil pH.

Phosphorus

Phosphorus deficient soybean does not usually display any striking visual symptom, but stunted growth is the most common characteristics of P deficiency. The soil-test-based P recommendations for soybean production in Louisiana is shown in Table 1. The medium soil-test P level (21 – 35 ppm or 42 – 70 lbs/acre; 1 ppm = 2 lbs/acre) is considered as the critical soil P concentration. Soybean yield response to P fertilization is often expected in soils with below critical P level (< 21 ppm), seldom expected in soils with critical P level (21 – 35 ppm), and not expected in soils with above critical P level (> 35 ppm). Therefore, soybean must be fertilized with P for soils with less than 21 ppm P and P fertilization is not recommended for soils with more than 35 ppm P. For P fertilization, use granular triple super phosphate (TSP; 0-46-0) and apply either in the Fall or in the Spring at or before planting. The information about the effect of P application timing on soybean yield can be found at “Phosphorus and potassium fertilizers application in spring vs. fall”.