Abstract
We investigated which of two fertilization methods would be more suitable for organic rice cultivation when using cotton-mulch sheets in low-fertility paddy fields of southwestern Japan. The two methods were (i) application of fertilizer before puddling (fertilizer before puddling) and (ii) application of fertilizer before laying the cotton-mulch sheets (fertilizer before laying). With fertilizer before puddling, the measured soil inorganic nitrogen concentration was the same as in non-fertilized plots both before and after laying the cotton-mulch sheet, and the number of tillers that developed was also the same as in non-fertilized plots. With the fertilizer before laying method, soil inorganic nitrogen increased, the number of tillers and straw weight increased, and the yield of rice was greater. With topdressing at the panicle formation stage, the number of grains per unit area, thousand-kernel-weight, unhulled rice weight, and brown rice yield all increased. The rice growth and yield did not improve with intermediate topdressing (i.e., between planting and panicle formation). The recovery rate of nitrogen from fertilizer applied before laying was 35% and from the fertilizer applied in topdressing at the panicle formation stage was 58%. There was no apparent nitrogen recovery from the intermediate topdressing. These results indicate that intermediate topdressing is unnecessary for rice cultivation with cotton-mulch sheets. In terms of yield performance, lodging resistance, and taste when using fertilizer before laying in low-fertility paddy fields, we found that the optimum nitrogen application rate is 8g-Nm^<-2> followed by topdressing with 4g-Nm^<-2> or less at the panicle formation stage.
