Response of Soybean Grain Oil and Protein Concentrations to Foliar and Soil Fertilization

Numerous studies investigated fertilization effects on soybean Glycine max (L.) Merr. grain yield, but few focused on oil and protein concentrations. This study determined fertilization effects on soybean grain oil and protein concentrations in 112 field trials conducted in Iowa from 1994 to 2001. Forty‐two trials evaluated foliar fertilization (N–P–K mixtures with or without S, B, Fe, and Zn) at V5–V8 growth stages. Seventy trials evaluated preplant broadcast and banded P or K fertilization (35 P trials and 35 K trials). Replicated, complete block designs were used. Foliar and soil P or K fertilization increased ( P < 0.05) yield in 20 trials. Foliar fertilization increased oil concentration in one trial (1 g kg −1 ) and protein in one trial (5 g kg −1 ) but decreased protein in two trials (6 g kg −1 ). Phosphorus fertilization increased oil concentration in two trials (6 g kg −1 ) and protein in five trials (5 g kg −1 ) but decreased oil in five trials (4 g kg −1 ) and protein in two trials (6 g kg −1 ). Potassium fertilization increased oil in four trials (3 g kg −1 ) and protein in two trials (9 g kg −1 ) but decreased oil in two trials (4 g kg −1 ) and protein in two trials (11 g kg −1 ). Total oil and protein production responses to fertilization tended to follow yield responses. Fertilization increased oil production in 20 trials and protein production in 13 trials. Fertilization that increases soybean yield has infrequent, inconsistent, and small effects on oil and protein concentrations but often increases total oil and protein production.