Stover and potassium management in an upland rice-soybean rotation on an Indonesian ultisol

Fertilizer and stover management greatly determine the extent of K deficiency on weathered, low-K soils in the humid tropics. This study quantified the effects of K fertilization and stover management on soil properties and crop yields on a Typic Kanhapludult in Indonesia. Cowpea [Vigna unguiculata (L.) Walp. subsp. unguiculata]cowpea-rice (Oryza sativa L.)-soybean [Glycine max (L.) Merr.]rice-soybean were grown during a 2-yr period. Fertilizer KCl was applied as 70 and 250 kg K ha(-1) to the first crop only, and as a total of 250 and 600 kg K ha(-1) applied to several crops. The effect of stover removal or return was examined for each K rate. Critical soil K levels were 0.14 cmol(c) kg(-1) for the final rice crop and 0.14 and 0.16 cmol, kg(-1) for the two soybean crops. By returning stover, a single application of 70 kg K ha(-1) to the first crop was adequate to maintain soil K above the critical level for all six crops. When stover was removed, a total of 250 kg K ha(-1) applied during several crops maintained soil K above critical levels. A one-time 250 kg K ha(-1) application to the first crop, however, resulted in yield declines by the fifth crop. A maintenance rate of about 45 kg K ha(-1) per crop was required when stover was removed. Stover removal also hastened soil Mg depletion, and thus a maintenance rate of about 6 kg Mg ha(-1) crop(-1) is recommended.