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 linearplateau 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.

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