Mtf. Wong et Dl. Rowell, LEACHING OF NUTRIENTS FROM UNDISTURBED LYSIMETERS OF A CLEARED ULTISOL, AN OXISOL COLLECTED UNDER RUBBER PLANTATION AND AN INCEPTISOL, Interciencia, 19(6), 1994, pp. 352-355
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.