Potassium Fertilizer Rate and Timing on Recovering Yield Loss in Potassium Deficient Soybean.

Abstract

Potassium deficiency of is the most common nutrient disorder of irrigated soybean (Glycine max) grown on loamy soils in Arkansas. Unless the K deficeincy is severe, visual symptoms are often not apparent until the R5 growth stage. Information regarding soybean yield response to fertilizer-K appliction time is lacking. The research objective was to evaluate soybean yield response to fertilizer-K application time on a K-deficient silt loam. Fertilizer-K (56 kg K ha^-1 as muraite of potash) was applied several weeks preplant and seven times after emergence of a maturity group 4.7 variety. Non-linear regression was used to model soybean yield response to fertilizer-K application time by regressing seed yield against the time fertilizer-K was applied, expressed as days after planting (DAP). At the R2 growth stage, the recently mature trifoliolate leaves (no petiole) of soybean receiving no fertilizer-K averaged 9.85 g K kg^-1 compared to 12.6 to 13.6 g K kg^-1 for soybeans fertilized with 56 kg K ha^-1. Soybean response to the timing of 56 kg K ha^-1 was a quadratic function of DAP. The relationship (r² = 0.74) predicted maximum yield was produced by K applied 3 DAP and 5 and 10% yield losses were predicted for K applied 54 (R2 stage) and 75 (R5 stage) DAP, respectively. Yields were maximized (4289 kg ha^-1) only by preplant application of 112 kg K ha^-1 which yielded 512 kg ha^-1 more than soybean fertilized preplant with 56 kg K ha^-1 and 1297 kg ha^-1 more than soybean receiving no fertilizer-K.  Preliminary results from one trial conducted in 2015 indicate that K-deficient soybeans respond to fertilizer-K until the initiation of seed fill. Results from two additional trials conducted in 2016 using higher in-season K rates will be reported.