Fertilizer Potassium Recovery Efficiency By Irrigated Soybeans.

Irrigated soybeans (Glycine max L.) are responsive to potassium (K) fertilization when grown on soils with low K availability. Limited information is available regarding K fertilizer recovery efficiency (FKRE) by irrigated soybeans. Our research objective was to determine FKRE as affected by K fertilization time. Granular fertilizer K (112 kg K ha^-1) was applied at planting, V6, R2-R3, and R5 growth stages on two K-deficient soils in 2016. Whole plant samples for each application timing and soybean receiving no potassium fertilizer were collected at the R7 stage (1.25 m²), and analyzed for biomass and K concentration. The soils were mapped as a Calhoun (46 mg Mehlich-3 K kg^-1) and a Calloway (64 mg Mehlich-3 K kg^-1) silt loams. The FKRE was calculated using the difference method. Plant biomass was not affected by K fertilization time on the Calloway soil. Severe lodging of soybean grown on the Calloway soil occurred before plant sampling, making plant sampling difficult and increased the variability to a questionable level. On the Calhoun soil, biomass declined linearly as fertilization was delayed from preplant (12,123 kg ha^-1) to maturity (8,204 kg ha^-1). Soybean receiving no fertilizer K contained significantly lower aboveground K content (56 kg K ha^-1) compared to soybean receiving fertilizer K (125-146 kg K ha^-1). The FKRE values ranged from 61 to 80% and were not different among K fertilization times, but the lowest FKRE occurred for the last K application time. It is interesting to note that in both trials whole-plant K concentrations, aboveground K contents, and FKRE were numerically greatest when K was applied at the R2-3 stage. Information indicates that irrigated soybean can rapidly take up surface-applied granular fertilizer K during reproductive growth.