Many producers are reducing or skipping preplant fertilization for soybean due to high fertilizer prices, and in 2008 many fields were planted late or replanted due to excess rainfall with colder than normal temperatures. Therefore, producers and crop consultants wonder if foliar-applied fluid fertilizer could improve soybean growth and grain yield. Prior to the 1990s, research in Iowa and the Midwest had focused mainly on foliar fertilization at late soybean reproductive stages (R4 to R7). Hundreds of trials from the 1970s to the middle 1980s included nitrogen (N), phosphorus (P), potassium (K), sulfur (S), and micronutrients treatments. The soybean plant has a sharp decline in root activity during late seed development stages with large nutrient translocation from leaves and pods into the developing seed. Researchers theorized that nutrients applied to the foliage at this time could increase yield by delaying leaf senescence and seed starvation. A few early experiments in Iowa suggested that spraying nutrients in the ratio 10-2.3-3.6-0.5 (N-P2O5-K2O-S) between the R5 and R6 growth stages could increase yield by 7 to 8 bu/acre even after preplant fertilization. However, more than 200 subsequent trials in the region showed inconsistent results, with mostly no yield increases and frequent yield decreases. Work mainly during the late 1990s and early 2000s in rain-fed conditions of the Midwest has shown mostly yield decreases when N sources were sprayed at late growth stages. The more positive results were observed under very high yield conditions and irrigation in some starts of the Great Plains region. These results have discouraged further research and adoption of foliar fertilization of soybean at late reproductive stages. Because of concerns about Asian Soybean Rust spreading north and evidence of soybean grain yield response to midseason application of fungicides, however, some producers are considering mixing fluid fertilizers and fungicides for midseason foliar application to soybean.
Soil Science Extension
University of Wisconsin Madison