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

hidden hunger (i.e., invisible symptoms of growth disorder caused by nutrient deficiency). It may be possible Leaf K concentrations needed for optimum soybean [Glycine max to alleviate the hidden hunger and prevent further de(L.) Merr.] production under conservation tillage systems may be different from those in conventional tillage (moldboard plow) because velopment into severe nutrient deficiencies (Tisdale et soil properties (such as soil-test K distribution) and soybean root al., 1985). Many crop producers, therefore, rely on crop distribution within the soil profile under conservation tillage systems monitoring to prevent nutrient deficiencies from limdiffer from those in conventional tillage. Little information is available iting potential crop yield. about adequate leaf K concentrations for soybean on conservationCritical nutrient concentration is defined as the contilled soils with significant vertical soil-test K stratification. This study centration of a specific nutrient within a specific plant was conducted at three locations in Ontario, Canada from 1998 part at which growth or yield begins to decline (Ulrich through 2000 to estimate the critical leaf K concentrations for conserand Hills, 1967). According to this approach, a single vation-till soybean on K-stratified soils with low to very high soil-test concentration value is assigned to a point where the K levels and a 5to 7-yr history of no-till management. Three K plant nutrient shifts from deficient to adequate. Because fertilizer placement methods (band placement, surface broadcast, and zero K), two conservation tillage systems (no-till and fall tandem of variations in the soil, climate, and other production disk), and two soybean row widths (19 and 38 cm) were used to create environments, a range of concentrations is also used to a wide spectrum of production environments. For maximum seed represent critical nutrient concentration. In Ontario, the yield, the critical leaf K concentration at the initial flowering stage critical trifoliate leaf K concentration was 12.0 g kg 1 (R1) of development was 24.3 g kg 1. This concentration is greater for soybean at the initial flowering stage (Ontario Ministhan the traditional critical leaf K values for soybean that are being try of Agriculture, Food, and Rural Affairs, 1997). This used in Ontario and in many U.S. Corn Belt states. Critical leaf K value had not changed with the advent of conservation values for the maximum concentrations of K, oil, and isoflavone in tillage practices and narrow-row soybean production. seed were 23.3, 24.1, and 23.5 g kg 1, respectively. The extent of The K sufficiency range of 17.1 to 25.0 g kg 1 for the vertical soil-test K stratification seems to be one of the factors contribupper fully developed trifoliate leaves of soybean— uting to apparently higher critical leaf K concentrations for conservation-till soybean. sampled before pod set—proposed by Small and Ohlrogge (1973) in Ohio is still widely used in many Corn Belt states. Another adequate leaf K range for soybean at the initial flowering stage was estimated to be 17.5 A of nutrient concentrations in plant tissue at to 25.0 g kg 1 by Plank (1979) in Georgia. For the end certain critical growth stages has often been used of flowering stage, an average critical leaf K concentraas an effective tool to diagnose nutrient disorder probtion of 21.5 g kg 1 was reported by de Mooy and Pesek lems in field crop production. By analyzing plant tissue, (1970) in Iowa. The critical leaf K value for soybean at one can compare the nutrient concentrations with the the early pod stage was found to be approximately 20 g recommended critical values (a single value or range of kg 1 in Florida (Sartain et al., 1979). All of the latter critical concentrations) to confirm nutrient deficiency when visual symptoms are present and to determine critical leaf K values were established for soybean prowhether nutrient concentrations are adequate to production in conventional tillage and (predominantly) in duce maximum plant growth or seed yield (Plank, 1979). wide row widths. Another important value of plant analysis is the prevenThe use of conservation tillage for soybean production of severe nutrient deficiency in plants (Ulrich and tion in North America has increased remarkably since Hills, 1967), because plant analysis can be used to detect the late 1980s. Currently, about 50% of soybean acreage is under some kind of conservation-till management in X. Yin, Dep. of Agronomy, Iowa State Univ., Ames, IA 50011-1010. the USA. Production management shifts such as those T.J. Vyn, Dep. of Agronomy, Purdue Univ., West Lafayette, IN 47907from conventional tillage to conservation tillage and 2054. Research was supported by Purdue Research Foundation, Agthe use of narrow instead of wide rows, combined with ricultural Adaptation Council of Canada, Ontario Soybean Growers’ overall yield improvements, have raised new concern Marketing Board, and Ontario Ministry of Agriculture, Food, and about the applicability of critical leaf K concentrations Rural Affairs. Received 9 Jan. 2003. *Corresponding author (tvyn@ purdue.edu). originally designed for conventional-till soybean to conservation-till soybean production. This concern may be Published in Soil Sci. Soc. Am. J. 68:1626–1634 (2004). most acute on long-term no-till fields where significant  Soil Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA vertical soil-test K stratification has occurred. There-