Critical Leaf Potassium Concentrations for Yield and Seed Quality of Conservation‐Till Soybean

Abstract

Leaf K concentrations needed for optimum soybean [Glycine max (L.) Merr.] production under conservation tillage systems may be different from those in conventional tillage (moldboard plow) because soil properties (such as soil‐test K distribution) and soybean root distribution within the soil profile under conservation tillage systems differ from those in conventional tillage. Little information is available about adequate leaf K concentrations for soybean on conservation‐tilled soils with significant vertical soil‐test K stratification. This study was conducted at three locations in Ontario, Canada from 1998 through 2000 to estimate the critical leaf K concentrations for conservation‐till soybean on K‐stratified soils with low to very high soil‐test K levels and a 5‐ to 7‐yr history of no‐till management. Three K fertilizer placement methods (band placement, surface broadcast, and zero K), two conservation tillage systems (no‐till and fall tandem disk), and two soybean row widths (19 and 38 cm) were used to create a wide spectrum of production environments. For maximum seed yield, the critical leaf K concentration at the initial flowering stage (R1) of development was 24.3 g kg−1 This concentration is greater than the traditional critical leaf K values for soybean that are being used in Ontario and in many U.S. Corn Belt states. Critical leaf K values for the maximum concentrations of K, oil, and isoflavone in seed were 23.3, 24.1, and 23.5 g kg−1, respectively. The extent of vertical soil‐test K stratification seems to be one of the factors contributing to apparently higher critical leaf K concentrations for conservation‐till soybean.