Maxwell Coffin1, Nathan A. Slaton2, Trenton L. Roberts2 and Russell E. DeLong1, (1)University of Arkansas, Fayetteville, AR (2)Crop, Soil, and Environmental Sciences, University of Arkansas, Fayetteville, AR
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.