Soybean is an important crop in Minnesota and provides a significant return in many farm enterprises. Yields will be reduced when essential nutrients are deficient. Therefore, profitable fertilizer programs must be developed to maximize yields.
The soybean is a legume and if properly inoculated, can use the nitrogen in the atmosphere (N2) for plant growth. Therefore, nitrogen fertilizer is not needed for soybean production in most situations.
Nitrogen fertilizer use for soybean production in the Red River Valley deserves special consideration. Research in the region has shown that the use of fertilizer N may increase yields when producers have experienced problems in getting good nodulation and the amount of NO3--N to a depth of 24 inches is less than 75 lb./ acre. For these situations, use of some N (50 to 75 lb./acre) in a fertilizer program may be beneficial. The amount of NO3--N to a depth of 24 inches should be measured before nitrogen fertilizer is applied.
Phosphate and Potash
Phosphate fertilizer guidelines for soybean production are listed in Table 52. The suggestions for potash use are provided in Table 53. The suggested rates of phosphate and/or potash are not adjusted for placement. A summary of research from Minnesota and neighboring states leads to the conclusion that neither banded nor broadcast placement is consistently superior if adequate rates of phosphate and/or potash are used. Soybean seeds are very sensitive to fertilizer placed on or near the seed row so fertilizer should not be banded with the seed as a "popup" fertilizer application at the time of planting.
Timing of the phosphate application is an important consideration when fertilizing soybeans, especially when the soil pH is 7.4 or higher. The phosphate should be applied in the spring before planting to reduce the time interval for contact between soil and fertilizer.
Table 1. Phosphate guidelines for soybean production.
|Phosphorus (P) Soil Test, ppm*|
|bu./acre||— P2O5 to apply (lb./acre) —|
|Less than 30||50||30||0||0||0|
* Use one of the following equations if a phosphate guideline for a specific soil test and a specific expected yield is desired.
P2O5 rec = [1.752 — (0.0836) (Bray P, ppm)] (Expected Yield)
P2O5 rec = [1.752 — (0.1114) (Olsen P, ppm)] (Expected Yield)
Table 2. Potash guidelines for soybean production.
|Expected Yield||Potassium (K) Soil Test, ppm*|
|Less than 30||55||30||15||0||0|
* Use the following equation if a potash guideline for a specific soil test and a specific expected yield is desired.
K2O rec = [2.2 — (0.0183) (Soil Test K, ppm)] (Expected Yield)
Research in Minnesota has shown that soybeans do not respond to the application of other nutrients in a fertilizer program. Manganese deficiency has been reported in glyphosate resistant soybeans in areas of the Eastern Corn Belt in areas traditionally responses to the nutrient, but research conducted in Minnesota has not shown a benefit to soil or foliar applied manganese on soybean.
Iron deficiency chlorosis (IDC) is a serious concern in western Minnesota. In these situations, there is an ample supply of iron in the soil. However, for reasons that are not completely understood, the soybean plant is not able to take up the iron needed for optimum growth. Research has shown a significant greening effect on soybeans and sometimes a yield increase when a 6% ortho-ortho EDDHA iron product is placed with the seed at rates of 1-3 lb. of product per acre. For best results apply EDDHA chelate must be placed in contact with the seed at planting. Planting an oats companion crop has also been shown to decrease the severity of IDC seeding at a rate of 1.5 bu./acre prior to planting soybeans. However, the oats must be left to grow to and sprayed with a herbicide by the time they are 10 inches tall to prevent over competition with soybean.