LEACHING OF NUTRIENTS FROM UNDISTURBED LYSIMETERS OF A CLEARED ULTISOL, AN OXISOL COLLECTED UNDER RUBBER PLANTATION AND AN INCEPTISOL

The transition from shifting cultivation to more settled agriculture r equires a better understanding of the mechanism of soil fertility loss during cultivation. In the humid tropics, the leaching of plant nutri ents accumulated during the fallow period is important. Leaching of ni trate, sulfate, chloride, calcium, magnesium, potassium and sodium was measured with undisturbed lysimeters of a freshly cleared Ultisol in the humid forest region of Nigeria. The field experiment was complemen ted with detailed laboratory experiments using smaller undisturbed lys imeters of an Oxisol collected under a Malaysian rubber plantation and a Colombian Inceptisol to determine the effect of positive charge in delaying the leaching of nitrate and its accompanying cations. Main an ions leached from the three soils were nitrate followed by small amoun ts of chloride and sulfate. The leaching of nitrate through the three soils was delayed with respect to water flow. Between 1.8 to 4.2 pore volumes were required for its displacement (1 pore volume is equal to the water content of the lysimeter at field capacity). The delay was d irectly proportional to the positive charge. In the Nigerian soil, the benefit of the delay was offset by excessive rain (2420mm) resulting in a considerable loss of plant nutrients. Calcium was present in the greatest amount in the drainage water followed by magnesium and small amounts of potassium. In the three soils, the charge concentration of calcium and magnesium matched closely to that of nitrate in the draina ge water. Their concentration peaked at 1.8 to 4.2 pore volumes as did that of nitrate. Equivalent of 92 and 390kg ha-1 of native nitrate-ni trogen were leached from the Malaysian and Colombian soil columns resp ectively. The Nigerian soil lost the equivalent of 144kg N ha-1 in the first year of cultivation. In that year, 27 percent of the sum of the exchangeable Ca in the soil profile and the added calcium, and 29 per cent of the corresponding sum for magnesium were leached with the anio ns. The rapid loss of basic cations will increase the acidification of the soil by rapidly decreasing its base saturation. Split fertilizer applications and the use of deep rooted trees such as in agroforestry should alleviate this type of soil degradation.