Water dynamics and nutrient leaching through a cropped ferralsol in the Loyalty Islands (New Caledonia)

On the South Pacific islands, the change from traditional to more intensive agricultural practices is leading to more Fertilizer use, both extensively and intensively. Nutrient leaching should be minimized to avoid plant defi ciencies and ground water pollution. The fate of nitrate and potassium unde r corn (Zea mays L.) and perennial grass (Rhodes grass, Chloris gayana Kunt h.) was monitored during three wet seasons on a Ferralsol soil from Mare in the Loyalty Islands. In 1995, 130 and 41% of the applied NO3- and K+, resp ectively, leached beyond the root zone. Split application of the fertilizer s in 1996 decreased the amounts leached to 48 and 11%, respectively. This r eduction occurred even though the rainfall nearly doubled from 1995 to 1996 . Nitrate and potassium transport, however, can be decreased due to their r etention by this Ferralsol. Nitrate can be adsorbed on the positively charg ed surface of aluminum and ferric oxides. Due to the rapid drainage under t ropical rainfall, however, this phenomenon is not very effective. Potassium is usually tightly fixed on 2:1 clay minerals, but these are nonexistent i n our soil. Potassium also is bound to humic substances and the organic mat ter reaches 15% in the subsoil of this Ferralsol. Leaching of K+ was retard ed compared with NO3-, but was still quite high compared with others studie s on tropical soils.