Timing and Rate of K Application for Wheat‐Soybean Double Cropping on a Sandy Soil 1

The wheat [ Triticum aestivum (L.) em. Thell.] and soybean ( Glycine max (L.) Merr.] double cropping sequence is an important cropping system in the Southern Coastal Plain of the USA. A study was conducted for two crop cycles to determine the optimum timing of supplying adequate K on a Bonifay sand (loamy, siliceous, thermic grossarenic, Plinthic Paleudult). Treatment variables consisted of five K application timings at two Mg rates (0 and 40 kg ha −1 ) in the first crop cycle, and four K application timings at four rates of K (90,135,180, and 225 kg ha −1 ) in the second crop cycle. Significant leaching of K from the 0 to 15 and 15 to 30 cm depth was measured when all of the K fertilizer was applied at wheat planting. Potassium uptake and grain yield of wheat were significantly greater for treatments in which one‐half of the total K fertilizer was applied at wheat planting and the remainder in February or as one‐fourth at wheat planting, one‐half in February and the remainder at 6 to 8 weeks after soybean planting compared with the application of all of K fertilizer at wheat planting, application of K at planting of both crops or split application of K in four equal increments on both crops. However, soybean yields were not significantly affected by the treatments in either cropping cycle. Application of Mg in the first cycle increased both soil and plant Mg concentrations but had no significant effect on yields. Increasing rates of K linearly increased wheat yield and soil K at soybean planting but had no significant effects on soybean yield or soil K at the end of the season. We concluded that delaying the application of at least one‐half of the K fertilizer until spring is essential for maintaining adequate K in the rhizosphere of wheat plants on deep sands.