Irrigated soybean response to granular fertilizer potassium application timing

Amelioration of K deficiency during the growing season requires knowledge of critical tissue concentrations and crop yield response to fertilization time. Our objectives were to characterize the yield and uptake responses of K‐deficient irrigated soybean [ Glycine max (L.) Merr.] to in‐season fertilizer‐K application time and rate, evaluate fertilizer‐potassium‐recovery efficiency (FKRE), and evaluate how leaflet‐K concentration responds to K‐fertilization time. Six trials were established on silt loam soils. Muriate of potash was applied pre‐plant and compared to an equivalent K rate applied post‐emergence on six or seven dates. Grain yield, trifoliolate‐K concentration, and K uptake were measured. Relative soybean yields were regressed across days after planting (DAP) for two situations of K‐responsive soybean: season‐long K‐deficiency symptoms or few symptoms (hidden hunger). The maximum yield increases from K fertilization ranged from 524 to 1948 kg ha −1 among trials producing relative yields that were 59−90% of the maximum yield produced with the greatest pre‐plant‐applied K rate. A linear‐plateau model showed maximal yields of soybean with hidden hunger could be produced with in‐season fertilizer K applied as late as 83 DAP or 44 d after R1 stage (DAR1). For soybean experiencing season‐long K deficiency, K fertilization from pre‐plant until 60 DAP or about 20 DAR1 produced similar relative yields. The FKRE of pre‐plant‐applied fertilizer K ranged from 36 to 75% among trials. Regardless of the severity of K deficiency, fertilizer K applied post‐emergence into the R2 development stage was taken up efficiently and produced similar yields as equal pre‐plant‐applied K rates.