Phosphorus reserves and apparent phosphorus saturation in four weakly developed cultivated pedons

Phosphorus (P) reserves and related characteristics of soil horizons were studied in two clay (Inceptisols) and two coarse-textured (Inceptisol, Spodosol) cultivated soils in Finland. The soils were young and they all had been formed of multiple parent materials, the deposition of which were linked to the evolutionary stages of the Baltic Sea. In all C horizons, the content of total P and measured Chang and Jackson P fractions were very similar with 80–90% of the fractionable P in H2SO4-soluble form. The dominance of this fraction from the approximate depth of 70 cm down demonstrated the low degree of weathering of the soils. In the B horizons, soil formation had decreased total P and the proportion of H2SO4-soluble P while fertilizer P had accumulated in the Ap horizons, leading to an anthropic epipedon in one of the four soils. Accumulation of secondary P was studied by measuring the degree of phosphorus saturation (DPS) of the horizons as a molar ratio of P to the sum of Al and Fe in acid ammonium oxalate extracts. Oxalate extracted from 1.2 to 8.5 times more P than the sum of NH4Cl–, NH4F– and NaOH–soluble P fractions, suggesting that a substantial amount of the oxalate-extractable P was primary acid-soluble P. The method thus led to erroneously high DPS values in the poorly weathered soils. Phosphorus saturation calculated as a molar ratio of the sum of NH4Cl–, NH4F– and NaOH–soluble P fractions to oxalate-extractable Al and Fe did not indicate a high P saturation in any of the subsurface horizons studied. The result was in accordance with the very low amounts of water-extractable P in the respective horizons.