Comparing different extraction methods for estimating phosphorus solubility in various soil types

In areas with intensive animal livestock farming, agricultural soils are enriched with phosphorus (P). These soils exhibit an increased risk for P transfer to the sub-soil and surface water via leaching. Besides the presence of hydrological pathways between a field and surface water, P in soil solution should be studied for evaluating the environmental risk. For this purpose, soil P extraction methods can be used. In this study, we tested the relation between various extraction methods and P in soil solution, simulated by a water extraction at a soil-to-solution ratio of 1:2 (w/v) using field-moist topsoils sampled from the major Dutch soil types (noncalcareous and calcareous sand and clay, reclaimed peat, and peat). The following methods were used: Pw (1:60 [v/v] water-extractable P), 0.01 M CaCl2 (1:10 [v/v]), FeO-strip, and acid ammonium oxalate-extractable P, Al, and Fe. Phosphorus in the 1:2 water extracts was mainly present as molybdate-reactive P (MRP). Extraction methods with the highest ability to predict MRP in 1:2 water extracts across different soil types were CaCl2, Pw, and FeO-strip, the latter two normalized for [Al + Fe]ox. However, for the peat and noncalcareous clay soils, also estimation of molybdate-unreactive P (MUP) is important because MUP dominates in the 1:2 water extracts of these soils. Thus, an extraction method that only determines MRP will not suffice, and further research is needed on the environmental risk of MUP in soil solution from these soil types. The calcareous sandy soils deviated significantly from the above mentioned relationships. For this soil type, it should be tested whether a single water extraction (e.g., Pw) suffices for determining the environmental risk.