Loss of water-soluble nutrients from A cultivated slope by occurrences of water erosion during the rainy season in northern Vietnam

Loss of water-soluble N, P, K, Ca and Mg from a cultivated slope by water erosion was monitored during crop cultivation in the 2000 rainy season in northern Vietnam. No fertilizer was applied to the crops. The magnitude of the loss of water-soluble nutrients for all twelve major occurrences of runoff followed the order of K > N > P > Ca > Mg. Concerning the temporal change, the loss of water-soluble N, P, Ca and Mg was larger for the first half than for the second half occurrences of runoff, showing that these nutrients cannot be replenished quickly once lost in slope soil. By contrast, a large amount of loss of water-soluble K occurred consecutively even during later occurrences of runoff. This suggests that a certain amount of K can be supplied anytime. Neither the effect of soil texture nor of land cover was identified as the cause for the loss of individual nutrient. The nutrients were considered to have been supplied from slope soil as follows: water-soluble N and P were supplied by mineralization from decomposed organic matter, water soluble K by leaching from raw organic materials such as crop residues, water-soluble Ca by release of exchangeable Ca, and water-soluble Mg by both mineralization of organic matter and release of exchangeable Mg.