Potassium status in different particle size fractions of some Finnish soils

Authors

• Armi Kaila University of Helsinki, Department of Agricultural Chemistry

DOI: https://doi.org/10.23986/afsci.71673

Abstract

Samples of fine sand, silt and clay soils from the surface and the deeper layers were separated without destruction of organic matter into the following fractions: clay <2 μ, silt 2—20 μ, and fine sand 20—200 μ. The separates were analysed for their content of various forms of potassium and the fixation of added potassium against an extraction with ammonium acetate. The mean content of total potassium was in the clay fraction 3.02 ± 0.11 %, in the silt fraction 2.75 ± 0.10 %, and in the fine sand fraction 2.16 ± 0.13 %. Clay was richest in readily exchangeable potassium, with an average content of 31 ± 7 mg/100 g in the 29 samples analysed, while the corresponding values were in the 28 samples of silt 6 ± 2 mg/100g, and in the 20 samples of fine sand 2 ± 1 mg/100 g. The content of nonexchangeable potassium released by HCI at 50°C was, on the average, in the clay samples 400 ± 70 mg/100 g, in the silt samples 230 ± 50 mg/100g, and in the fine sand samples 70 ± 20 mg/100 g. Without drying, the 29 clay samples fixed 32 ± 8 per cent (9 to 90 per cent) of the 97.5 mg K added per 100 g of the material. This »wet» fixation was by the 28 silt samples 27 ± 9% (5 to 96 %), and by the 20 fine sand samples 12 ± 4 % (2 to 44 %). Drying of the KCI-suspension at 80°C resulted in fixation of 69 ± 10 % (21 to 95 %) by clay material, 54 ± 10 % (16 to 100 %) by silt, and 25 ± 6 % (7 to 55 %) by fine sand. In all fractions a low correlation was found between the contents of readily exchangeable and nonexchangeable acid soluble potassium. The »wet» fixation did not depend on these forms of potassium, but in the clay fraction and the silt fraction the »dry» fixation was negatively, even if slightly, correlated with the exchangeable potassium, r = —0.67***, and r = —0.55**, respectively. The relationship between the »wet» and the »dry» fixation was closest in the silt fraction (r = 0.86***) and poorest in the clay fraction (r = 0.69***). The relatively close relationship between the clay and the silt separates in regard to their contents of exchangeable (r = 0.91***) and nonexchangeable potassium (r = 0.87***), and the fixation of potassium under »wet» conditions (r = 0.93***) and »dry» conditions (r = 0.92***) may be taken to support the supposition that these fractions differ from each other more in their quantitative than in their qualitative mineral composition. Between the silt and fine sand fractions fairly high correlation was found for their contents of exchangeable potassium (r = 0.97***), their contents of nonexchangeable potassium (r = 0.86***), and for their values of »wet» fixation (r = 0.90***), while a lower correlation existed between the results of »dry» fixation (r = 0.66**). In spite of the sources of errors, examined in the discussion, it was found justifiable to conclude that the silt fraction is likely to play an important role in the release and fixation of potassium in Finnish mineral soils, and that the fine sand fraction, although significantly less inactive, may in some soils be noteworthy in these respects.