SUSTAINMENT OF SOIL FERTILITY IN THE TRADITIONAL RICE FARMING, DRY ZONE, SRI-LANKA

Changes in soil fertility during the rice cropping season and the subs equent fallow period were investigated in the dry zone of Sri Lanka. T he effects of grazing by cattle and soil drying during the fallow on s oil fertility was also studied. A build-up of plant N took place durin g the fallow plant succession, through the immobilization of soil N in the plants. The release of N from dying plants diminished during the fallow, thus retaining more N in organic forms in the soil. The minera lizable N pool was depleted under grazing while the clay-fixed NH4+ po ol increased. Cattle grazing also controlled annual weeds in the succe eding rice crop. Soil drying during the fallow enhanced the release of mineral N following rewetting, which was reflected in improved crop g rowth during the following season. Drying and rewetting cycles also en hanced the germination of rice. It was hypothesized that the loss of N from harvest of the crop was replenished through associative and free -living N2 fixation during the fallow and cropping phases, thus sustai ning soil fertility in this traditional rice farming system.