Protect soybean yield potential after slow and rainy planting season

• By David Moseley and Md Rasel Parvej •

The progress of 2021 soybean planting season has been behind the five-year average from the beginning due to weekly heavy rains throughout the state. The optimum soybean planting window is suggested to be from mid-April to mid-May.

potassium deficiency symptoms
Figure 1. Symptoms of potassium deficiency begin as irregular yellowing on the edges of leaves. The symptoms will be located mainly on older leaves in the lower canopy during early vegetative stages — photo by Dr. Rasel Parvej.

According to the U.S. Department of Agriculture-National Agricultural Statistics Service report, 45% of the crop was planted by May 16, compared to the five-year average of 77%. During the three weeks from May 16 to June 6, soybean producers were able to plant an additional 41% of the projected acres, slightly past the optimum window.

Soybean planting date research has indicated yield potential will decrease when planting late. However, the final yield from previous years with comparable slow planting progress indicate good soybean yield is still achievable this year.

The planting progress in 2013 and 2019 was comparable to this year. The yields were 48.5 and 48 bushels per acre in 2013 and 2019, respectively: approximately 95% of the average yield from 2016-2020 (51 bushels per acre).

Besides planting date, the final yield will also be determined by other factors such as environmental conditions and crop management (e.g. pest control, irrigation, and fertilization practices).

Managing in-season potassium (K) deficiency is one crop management practice that can have a positive effect on final yield. Normally, producers apply K in the fall or at planting based on soil test results. However, K deficiency can still occur across soil types for different reasons.

Coarse-textured soils, such as loamy sand to silt loam, have a low cation exchange capacity (CEC <10) where K can be prone to leaching below the root zone. Thus, the excessive rainfall experienced this spring can promote K deficiency in coarse-textured soils. In addition, coarse-textured soils also have poor water holding capacity.

If the weather pattern becomes dry, K deficiency can occur by inefficient K root uptake. If the prolonged saturated conditions this spring led to stunted roots, this decrease in root uptake may become more likely if the weather pattern shifts to drought conditions.

potassium deficiency in older beans
Figure 2. During the reproductive stages, potassium deficiency symptoms often appear on younger leaves in the upper canopy — photo by Dr. Rasel Parvej.

Potassium deficiency can limit yield potential by more than 50% in soybean if not corrected and can limit the plant’s ability to uptake phosphorus. A total of 2.9 pounds of K2O is estimated to be required for every bushel of soybean harvested.

For a 55-bushel crop, approximately 160 pounds of K2O per acre is required. If soybean plants are deficient in K, the symptoms will begin as irregular yellowing on edges of leaves (Figures 1 and 2). The leaves may eventually turn brown and die. However, not all K deficient soybean plants will show foliar symptoms even though the yield may be affected.

In-season tissue sampling can diagnose a hidden deficiency and research has suggested a timely K application can reduce yield loss. More information on K deficiency and corrective measures can be found in the Louisiana Crops Newsletter Volume 10, Issue 5 – June 2020, and the Louisiana Soybean Nutrient Handbook.

Dr. David Moseley is LSU AgCenter soybean specialist. He may be reached at Dr. Md Rasel Parvej is an assistant professor of soil fertility for the Northeast Region.

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