Aluminium and acidity in Finnish soils

Abstract

In the present study an attempt was made to study by statistical methods the proportion of Al of the exchange acidity of 298 soil samples of various kind, and to what extent the titratable nonexchangeable acidity in these soils is connected with Al, when Al soluble in Tamm’s acid oxalate was used as its indicator. Unbuffered N KCI replaced Al only from soil samples with a pH less than 5.3 in 0.01 M CaCl2 . In this part of the material, Al corresponded, on the average, to one third of the exchange acidity of mineral soil samples, and to 16 per cent of that of organic soils. The amount of Al was usually the higher the lower the soil pH, but the correlation was close only in the group of clay soils. Titratable nonexchangeable acidity was estimated as the difference of the amount of acidity neutralized at pH 8.2 and the corresponding amount of exchange acidity replaced by unbuffered KCI. In 100 clay soil samples it was, on the average, 12.0 ± 1.3 me/100 g, in 42 samples of silt and loam soils 8.8 ± 1.8 me/100 g, in 99 sandy soils 8.9 ± 1.1 me/100 g and in 57 organic soils 49.1 ± 6.8 me/100 g. There was no correlation between titratable nonexchangeable acidity and the clay content within various soil groups. In the clay soils exalate soluble Al alone explained 78.3 %, in the silt and loam soils 59.8 %, in the sandy soils 6.5 %, and in the organic soils 0.6 % of the variation in titratable nonexchangeable acidity. Taking into account the content of organic C increased the rate of explanation only to 82.1 % in clay soils, to 84.1 % in silt and loam soils, to 83,1 % in sandy soils, and to 63.7 % in the organic soils. Further, adding the soil pH increased the rate of explanation 5.8 to 9.6 per cent units in various soil groups, but considering of oxalate soluble Fe did no more distinctly increase the part of variation explained, except in the organic soils. Regression equations were calculated for the relationship of these variables. According to the partial correlation coefficients and to the β-coefficients, the relative importance of oxalate soluble Al in explaining the variation in titratable nonexchangeable acidity was in the clay soils higher than even that of organic C content, but in the other mineral soil groups it was less important than both C content and pH; in the organic soils even oxalate soluble Fe appeared to be slightly more important.