Long-term nitrogen, phosphorus and potassium fertilization of cassava influences soil chemical properties in North Vietnam

The long-term effects of Yearly fertilizer applications on soil chemical pr operties and nutrient availability are not well documented for cassava (Man ihot esculenta) production in Vietnam. In 1990, research plots were establi shed (randomized complete block design) with 12 treatments to test effects of different rates of N, P and K on soil properties in Acrisols (FAO-UNESCO Soil Classification) at Thai Nguyen University, North Vietnam. In June 199 8, composite soil samples (0- to 10-cm and 10- to 20-cm depth) were collect ed from each plot. Nine years of N application significantly reduced the la bile pools of soil inorganic P, total soil P and soil extractable K and Mg. Total soil N was only slightly increased in the 10- to 30-cm depth. Long-t erm applications of P significantly increased soil inorganic P fractions, b ut reduced concentrations and supply rates of nitrate and K. Moreover, 9 yr of K application significantly increased soil organic C (two depths), soil total N (10-30 cm) and soil extractable K in the 0- to 10-cm depth. Genera lly, the results show that long-term applications of mineral fertilizers in a ratio of N:P2O5:K2O of 2:1:2 (at the rates of 80 N:40 P2O5:80 K2O or 160 N: 80 P2O5:160 K2O) are effective in maintaining the total content and ava ilability of applied nutrients, but when applied alone are associated with decreases in other nutrients.