Separation of phosphorus bound to organic matrices from inorganic phosphorus in alkaline soil extracts by ultrafiltration

A method is described to separate inorganic phosphorus from phosphorus bound to humic substances in alkaline soil extracts. The method consists of the addition of polyethylenimine as a flocculant to the soil extract and subsequent ultrafiltration through 2September 1991 D cellulose acetate membranes. Inorganic P (Pi) is determined in the ultrafiltrate, organic P (Po) as the difference of total P, measured prior to ultrafiltration, and Pi. The procedure removed more than 90% of the organic carbon extracted at pH 11.2 from humic sandy soils. Substantial amounts of Pi determined in this way were released as Pi from the organic matrix when the alkaline extracts were acidified to pH < 4.0. The usual photometric methods for determining Pi require acidification of the solution to pH < 1.0. Therefore, if alkaline soil extracts are analysed without ultrafiltration, Po may be seriously underestimated. Acidification of the alkaline soil extracts to pH < 4.0 resulted also in a release of high percentages of Al and Fe bound to organic matter, the relation to pH being similar to that found for Pi. It is therefore concluded that the main part of Po is bound to the organic matrix by Al or Fe bridges.