Effect of long-term intensive rice cultivation on the available silica content of sawah soils: Java']Java Island, Indonesia

The dramatic increases in rice productivity and cultivation intensity through the implementation of green revolution (GR) technology using high yielding varieties (HYVs) of rice and chemical fertilizers were not long lasting in Indonesia. The stagnancy of rice productivity in recent years without any scientific reasons presents a challenge for agronomists and soil scientists in Indonesia. This study describes the effects of long-term intensive rice cultivation on the change in available silica (Si) in sawah soil. The term sawah refers to a leveled and bounded rice field with an inlet and an outlet for irrigation and drainage. Soil samples collected by Kawaguchi and Kyuma in 1970 and new samples taken in 2003 from the same sites or sites close to the 1970 sites were analyzed and compared. From 1970 to 2003, the average content of available Si decreased from 1,512 +/- 634 kg SiO2 ha(-1) to 1,230 +/- 556 kg SiO2 ha(-1) and from 6,676 +/- 3,569 kg SiO2 ha(-1) to 5,894 +/- 3,372 kg SiO2 ha(-1) in the 0-20 cm and 0-100 cm soil layers, respectively. Cultivation intensity differences between seedfarms planted with rice three times a year and non-seedfarms rotating rice and upland crops appeared to affect the changing rates of available Si within the study period. In the 0-20 cm soil layer, the average content of available Si decreased from 1,646 +/- 581 kg SiO2 ha(-1) to 1,283 +/- 533 kg SiO2 ha(-1) (-22%) and from 1,440 +/- 645 kg SiO2 ha(-1) to 1,202 +/- 563 kg SiO2 ha(-1) (-17%) in seedfarms and nonseedfarms, respectively. Differences in topographical position also influenced the decreasing rate of available Si in this study. Using similar management practices and cultivation intensity, upland sampling sites lost more Si compared with lowland sites. Planted rice under a rain fed system with no Si addition from rain water in an upland position may be a reason for the higher loss of Si, particularly in non-seedfarms. The Si supply from irrigation water might have contributed to the slowdown in the decreasing rate of available Si in Java sawah soils.