Abstract

In a 3-year experiment on a typic Haplustert low in soil test sulfur (S), residual effects of S applied to either soybean or wheat in a soybean–wheat system were measured for seed yields, recoveries of added S, and changes in the soil test S. Sulfur as gypsum was applied at 5 rates (0–80 kg S/ha) to soybean and 4 rates (0–60 kg S/ha) to wheat during the first year. In subsequent years the residual effects were studied in relation to fresh application of 40 kg S/ha to each crop. Seed yields of soybean and wheat were increased significantly by the application of S to each crop. The rate 80 kg S/ha applied to soybean showed residual effects in 2 succeeding crops, while 60 kg S/ha applied to soybean or wheat showed residual effect in only 1 succeeding crop. The S applied to wheat was more effciently utilised than that applied to soybean in rotation. The rates 53 and 70 kg S/ha (calculated via regression equations) applied to soybean gave 90% of the seed yield of freshly applied S at 40 kg/ha in the succeeding wheat and soybean crops, respectively, and 57 kg S/ha applied to the wheat crop gave 90% of the seed yield in the succeeding soybean crop. The recoveries of added S were greater with smaller rates of added S and were greater in the first 2 residual crops. Soil test S was adequate only in the first year and fell below critical level in the subsequent cropping period. Cumulative S uptake determined the levels of available S in the soil.