POTASSIUM FERTILIZATION OF DOUBLECROPPED WHEAT AND SOYBEANS UNDER 2 TILLAGE SYSTEMS

Because the yield response to K in a wheat (Triticum aestivum L.)-soybean [Glycine max (L.) Merr.] rotation on Atlantic Coastal Plain soils may be influenced by tillage type and K application timing, a study was conducted between 1986 and 1988 on a Bojac loamy sand (coarse-loamy, mixed, thermic Typic Hapludult) to investigate the effects of tillage, K rate, and K application timing on soil and plant K and crop yields. The experimental design was a split plot, with tillage as the main plot factor and K rate x K application timing as the subplot factor. Tillage treatments were conventional tillage (CT) and no-till (NT). Potassium fertilizer (KCl) rates of 0, 36, and 72 lb K/acre were applied (i) 100% prior to wheat planting; (ii) 50% prior to wheat planting, 50% at Zadoks 30 growth stage; and (iii) 50% prior to wheat planting, 50% prior to soybean planting. Wheat yields under continuous NT management declined from 84 bu/acre in 1987 to 53 bu/acre in 1988, while CT wheat yields remained constant at 76 bu/acre in 1987 and 75 bu/acre in 1988. No-till soybeans significantly outyielded CT soybeans by 48 to 33 bu/acre in 1987 and 61 to 46 bu/acre in 1988. Neither wheat nor soybeans responded to K rate or timing of application despite reductions in soil test K to levels diagnosed as insufficient. Leaching losses contributed significantly to declines in soil K, as crop removal accounted for only a portion of the ""missing''K. Testing for K on Coastal Plain soils should involve subsoil sampling, since both wheat and soybeans appear capable of procuring considerable amounts of K from deeper than 12 to 18 in. within the profile.