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 deficiencies and ground water pollution. The fate of nitrate and potassium under corn (Lea mays L.) and perennial grass (Rhodes grass, Chloris gayana Kunth.) was monitored during three wet seasons on a Ferralsol soil from Mare in the Loyalty Islands. In 1995,130 and 41% of the applied NO−3 and K+, respectively, leached beyond the root zone. Split application of the fertilizers in 1996 decreased the amounts leached to 48 and 11%, respectively. This reduction occurred even though the rainfall nearly doubled from 1995 to 1996. Nitrate and potassium transport, however, can be decreased due to their retention by this Ferralsol. Nitrate can be adsorbed on the positively charged surface of aluminum and ferric oxides. Due to the rapid drainage under tropical rainfall, however, this phenomenon is not very effective. Potassium is usually tightly fixed on 2:1 clay minerals, but these are nonexistent in our soil. Potassium also is bound to humic substances and the organic matter reaches 15% in the subsoil of this Ferralsol. Leaching of K+ was retarded compared with NO−3, but was still quite high compared with others studies on tropical soils.