INFLUENCE OF LONG -TERM APPLICATION OF N, P AND K LEVELS ON SOIL PROPERTIES AND RICE YIELD IN THE CUU LONG DELTA, VIETNAM

Double-cropped, high-yield rice with NPK fertilizer has been practised in the Mekong River Delta nearly 20 years, but long-term effects on soil properties have not been examined. A long-term experiment that commenced in 1986 was used to examine the effect of nitrogen (N), phosphorus (P) and potassium (K) applications after 34 consecutive crops in intensive rice monoculture at CLRRI, Thoi Lai, Can Tho City, Vietnam. It showed that soils is good in development, soil texture is loam clay, bulk density varied from 0.93- 1.03 g/cm 3 , organic matter in height. Soil porosity is relative high and no consistency and structural stability is low. EC, pH (H2O) and pH (KCl) are decreased dramatically over initial. Removal all of rice straw on the field led to soil organic carbon decreasing. N, P, K elements in soil were analyzed to know the influence of long term application by N,P,K to soil properties and rice yield. Our results showed that after 34 rice seasons it decreased stronger. Total N content in soil increased in applying of P alone, and NK, PK, and NP combined treatments and slightly increased in treatment of NPK. Total N in soil was not changed in any treatments applying with N and decreased in control (without applying fertilizer) and K alone treatments. Bray extractable P content increased in treatments with P alone or PK and NP but not in the NPK treatment. Total P was not affected by long term N, P, K fertilizer application. Total K was still high in plots with and without K fertilizer. Exchangeable K was high in the plots supplied K. Overall, the levels of N, P and K in soils after 34 consecutive crops indicated minimal decline in total N, a 25% decline in total P and no change in total K for plots which had no fertilizer added. By contrast, with the recommended fertilizer rates of N, P and K, the total levels of these elements in the soil remained unchanged. However, recommended NPK fertilizer appears to increase nitrate N, and Bray-2 P but not exchangeable K. Cation exchange capacity (CEC) was not much changed. Cation exchange capacity (saturation) of soil was low, cation exchange capacity (unbuffer) was high, cation exchange capacity (buffer pH 8.1) was rather high, exchangeable Ca ++ and Mg ++ remained high. The main change in soil quality after 34 crops of rice and continuous fertilizer application caused the decline in soil organic matter, which was removed after every rice crop. There is yield declining phenomenon in rice with time in wet seasons and increase in dry seasons. In order to overcome this phenomenon, it is necessary applied P and returned rice straw to the field.