EFFECT OF SOIL pH AND ORGANIC MATTER ON LABILE ALUMJNIUM IN SOILS UNDER PERMANENT GRASS

The rates of extraetíon of Na, K, Mg, Ca, and Al with 1 M NH. NO. from the mineraIand organic-rieh layers of sorne Park Grass (Rothamsted) soils were measured at the pH of the soil. Below pH 3.7 exehangeable Al, deríved from the kineties curve, inereases with decreasing soil pH and is less in the organic-rich Iayer. The sum of the basic exchangeable eations, I;(Na + K + Mg + Ca), increases with increasing soil pH and is more in the organíc-rieh layer. The extraction of exchangeable Al obeys first order kinetics, the rate constant being similar for all the soils (mean value 36 ±7 x 10 -61s -1 ), which implies that exchangeable Al is relea sed from surfaees with similar properties for the adsorption of Al, and that the rate ís not affected by soil pH and organic matter. The rate of extraction of non-exchangeable Al ís the same in the mineraland organic-rieh layers of eaeh soil, and is maxírnal at about pH 3.7, deereasing sharply at more and less acid pH values.