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DIVISION S-4—SOIL FERTILITY & PLANT NUTRITION

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

^a Dep. of Agronomy, Iowa State Univ., Ames, IA 50011-1010
^b Dep. of Agronomy, Purdue Univ., West Lafayette, IN 47907-2054

^* Corresponding author (tvyn{at}purdue.edu)

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.

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