Responses to phosphorus and organic matter application in Rice-Based Cropping Systems of Rainfed Lowlands in North Sumatra, Indonesia

Soil fertility in rainfed lowlands is low because of continued cropping with little or no nutrient replenishment, inherently low soil fertility and high nutrient fixing ability. This study evaluated the effects of phosphorus and cow manure on soil properties and crop performance in a rice-rice-mungbean cropping sequence. Three experiments were conducted in Ultisol soils between June 2004 and June 2005, in North Sumatra, Indonesia. Treatments involved 0, 30, 60, and 90 kg/ha of P2O5 and 0, 3, and 6 t/ha of cow manure applied to rice. In the first transplanted and second direct seeded rice experiments, grain yield increased by 57 percent and 67 percent, respectively, when 60 kg/ha of P2O5 and 3 t/ha of cow manure were combined, and with no further enhancement with higher levels of both treatments. Higher grain yield was associated with increased level of available P, organic carbon, higher CEC and lower soil bulk density. Shoot biomass and P uptake, number of productive tillers, percent filled grains, and number of grains per panicle, 1000-grain weight, and H1 were also enhanced. In the third experiment, mungbean was grown after rice with no additional P or organic matter, to evaluate their residual effects. Similar responses were observed where mungbean grain yield increased by 120 percent. This increase in grain yield was associated with enhanced available P, organic carbon and CEC, and higher shoot biomass, crop growth rate, shoot P content and P uptake. Number of pods/sq m, seeds per pod, 100-grain weight, and Hl were also substantially improved. For both crops, application of P and organic carbon shortened the duration to flowering but extended the grain filling period. Apparently, 60 kg/ha of P2O5 and 3 t/ha of cow manure are appropriate for improving productivity of rice-based cropping systems in this study. However, more investigations are needed to evaluate their long-term effects on different soil types and crops.