Numerous trials conducted in Iowa since the 1970s have shown that foliar fertilization results in small, infrequent, and difficult to predict soybean yield responses. Work in the 1970s suggested that spraying N-P-K-S mixtures at late growth stages increased yield, but subsequently hundreds of trials in the Midwest and other regions showed few yield increases. More recent research evaluated N-P-K mixtures with or without sulfur and micronutrients at early stages. It found inconsistent differences between mixtures, a 15% probability of response, and that a single spray with 3 gallon/acre of 3-18-18 gave the most consistent response. Recently, the threat of Soybean Asian Rust has prompted questions about use of fungicide for soybean, alone or in mixture with foliar fertilizers.