Response of No-Till Soybean [Glycine max (L.) Merr.] to Timing of Preplant and Foliar Potassium Applications in a Claypan Soil

Received for publication September 10, 2004. Potassium availability in agronomic crops has recently decreased due to periodic drought conditions, soil compaction, reduced K applications, lower frequency of soil testing, and higher K fertilizer requirements because of increasing corn (Zea mays L.) yields and larger soybean [Glycine max (L.) Merr] acreage. Little research has evaluated the effects of foliar K applications on low to medium soil test K claypan soils, which are susceptible to drought and surface compaction. Field research was conducted in 2001 and 2002 to determine soybean response to foliar-applied K fertilizer compared with a preplant application and evaluate the cost-effectiveness of these treatments. Potassium fertilizer (K2SO4) was either broadcast-applied at 140, 280, and 560 kg K ha^–1 as a preplant application or foliar-applied at 9, 18, and 36 kg K ha^–1 at the V4, R1–R2, and R3–R4 stages of soybean development. Soybean grain yield increased 727 to 834 kg ha^–1 when K was foliar-applied at 36 kg ha^–1 at the V4 and R1–R2 stage of development in 2001 and 2002. Foliar-applied K at the R3–R4 stage of development increased grain yield but not as much as V4 or R1–R2 application timings. Treatment cost-effectiveness ranked preplant K at 280 kg ha^–1 = preplant K at 140 kg ha^–1 (less than) preplant K at 560 kg ha^–1 = V4 or R1–R2 foliar-applied at 36 kg ha^–1. Foliar K did not substitute for preplant K in this research; however, foliar K may be a supplemental option when climatic and soil conditions reduce nutrient uptake from the soil.