Bray-2 phosphorus as influenced by soil fineness and filtration time

Fine soil with a particle size less than 2 mm is generally used when determining Bray-2 phosphorus (P). The average aggregate size varies widely in the free soil of structural soils and may influence the reproducibility at Bray-2 extractions. Our objective was to study how this variation in fineness is related to filtering time and jointly influence Bray-2 P values in three different soil types and then to propose corrective measures to improve the reproducibility at Bray-2 extraction. In an Ultisol, there was no difference in Bray-2 P values between soil sieved through a 1-mm mesh compared to soil sieved through a 0.2-mm mesh. Bray-2 P was drastically decreased in the time (less than 0.2 mm) allophanic Andisols compared to soil passed through a 1- mm mesh. The difference disappeared gradually, when extending the extraction time from the method specific for 40 or 60 seconds and was not observed at 3 min extraction time or longer. The finer structure delayed the filtering time through filter papers which increased the resorption to allophanic soil constituents of the initially released P. Controlled filtering time by using micropore filters revealed an initial dissolution of P. The sorption rate became larger than the release rate after about 15 seconds, resulting in a net resorption in the allophanic Andisols. To the contrary, the Bray-2 P values increased for several minutes after addition of the extractant to a nonallophanic Andisol. The P values increased faster with the freer soil fraction compared to the coarser one. The variation in Bray-2 P values caused by soil fineness can be reduced by controlling the filtering time or prolonging the extraction time to 15 min. A prolonged extraction time will facilitate the laboratory work by permitting analyzing in batches.