Which soil P test reflects best the phosphorus availability for leaching?

Soil phosphorus (P) tests are important for estimating the P status in soil. A variety of soil P tests is used worldwide, ranging from mild to relatively harsh soil extraction methods. The historical development of most tests is based on the need for fertilisation advice, i.e. the result of the test should reflect the P availability in the soil for the crops. Nowadays, P fertilisation is restricted in order to reduce P losses to ground and surface water to diminish eutrophication. In some NW European countries or regions the maximum allowed P fertilisation dose depends upon the soil P availability measured with a certain soil P test. It is however unclear if the soil P availability, measured with a method developed for crops, is also a good measure for the availability for P losses.
In Flanders (Belgium) the suitability of the standard soil P test (ammonium lactate extraction) for measuring the P availability for leaching, is compared to other soil P tests. Twenty-one soils, with textures ranging from sand over sandy loam to loam, were tested for their P leaching risk in a soil column experiment. Disturbed soil columns were irrigated with artificial rainwater with a flow rate representative for Flemish net rainfall conditions. A vacuum pomp created an underpressure of 10 kPa to mimic field capacity. During one month, filtrate was sampled twice a week. In the filtrate, Fe, Al, Ca and P concentrations were measured. The P concentration in the leachate stabilized mostly two weeks after the start of the experiment.
Soil P availability was measured in the 21 soils by different methods: (1) extraction with ammonium lactate and acetate at pH 3.75 (Egnér et al., 1960); (2) extraction with 0.5 M NaHCO3 (Olsen et al., 1954); (3) extraction with 0.01 M CaCl2 (Houba et al., 2000); (4) extraction with ammonium oxalate (Schwertmann, 1964), with possibility to calculate the phosphate saturation degree; (5) diffusive gradient in thin film technique (DGT) (Degryse et al., 2009). For each method, the stabilized P concentrations in the leachate was plotted versus the measured soil P availability. A curve was fitted on the data. The method for which the uncertainty on the “critical soil P availability”, i.e. the soil P availability related to unacceptable P concentrations for surface waters, is the smallest among the tested availability methods, is selected as the best method for measuring the soil P availability for leaching. Results will be presented at the conference.