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SOIL FERTILITY

Soybean Yield and Nutrient Composition as Affected by Early Season Foliar Fertilization

^a Dep. of Agronomy, Iowa State Univ., Ames, IA 50011 USA

apmallar{at}iastate.edu

The response of soybean [Glycine max (L.) Merr.] to foliar fertilization during early growth stages has received little attention. Recent Iowa research showed a 15% probability of positive yield response to a 3–15–8 (N–P–K) fertilizer applied at the V5 growth stage. This study evaluated the effects of fertilizers varying in N–P–K ratio on soybean grain yield and tissue nutrient composition. Twenty-seven field trials were conducted in soils that tested at or above optimum soil P and K levels for soybean. Six treatments included a control and nonfactorial combinations of rates and application frequency of 28 to 56 L ha^-1 of 3–8–15, 10–4–8, and 8–0–7 fertilizers sprayed at the V5 stage. Differences between treatments were inconsistent across sites. Some or all treatments increased or decreased yields significantly at six sites. The mean yield increase or decrease for responsive sites was 400 kg ha^-1. The 3–8–15 fertilizer caused no leaf damage and other fertilizers caused little or no damage, although the damage was not clearly related with yield decreases. Analyses by site showed that fertilization seldom increased tissue N–P–K composition, nutrient uptake, photosynthesis, or plant weight measured at the R2 growth stage. Multivariate analyses across sites showed that 27% of the variation in yield response was explained by a combination of N, P, and K availability, vegetative growth, and rainfall. Positive yield responses tended to occur when soil or weather conditions reduced plant growth and nutrient availability. Foliar fertilization across all conditions will not offset the application costs.

Abbreviations: CEC, cation exchange capacity • CER, leaf CO₂ exchange rate

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