Estimation of the Requirement of Liming by Determination of Exchangeable Soil Aluminium

Abstract

Soil pH (in water) is not entirely satisfactory for predicting where liming is required to obtain optimal growth of most crops. Elimination of the toxic effects of Al on plants was considered to be more important than neutralization of soil acidity to a certain pH value. A test for active soil Al was developed. Extraction with 0.1 M (NH4)2SO4 according to Stahlberg as well as with M KCl was performed, and the method of Dougan and Wilson was applied for determination of Al in the extracts. Samples (130) of cultivated soils were investigated. Both ammonium sulfate (AS) and KCl can be used to extract active soil Al. There is a rather close correlation between the 2 extraction methods (r2 = 0.925). As the AS method can be used also for estimation of plant available P, K and Mg it was chosen as the most suitable method for practical purposes. It was assumed that the diluted AS solution will extract the most active and toxic fraction of Al from the soil. An Al-AS value of 1.0 mg/100 ml soil, corresponding to an Al-KCl value of 1.5 mg/100 ml, is quite acceptable for most Swedish crops such as various cereals, grasses, clover and oil seeds. Virtually no economic gain can be expected from liming of soils with Al-AS values < 1.1 mg/100 ml. Liming may be recommended for soils with Al-AS values > 1.0 mg/100 ml. Liming can be expected to be economic under most circumstances with Al-KCl values > 3.1 mg/100 ml or Al-AS values > 2.0 mg/100 ml. All soils with pH lower than 5.1 need liming in order to reduce their contents of active Al. Very few soils with pH higher than 6.0 require liming for that purpose. About 85% of the soils with pH 5.1-5.5 and 47% of those with pH 5.6-6.0 require liming to obtain acceptable Al-AS and Al-KCl values.