Trials with a Bacteriological Method For Measuring Available Phosphorus in Soils

The basis of most laboratory tests for determining the availability of plant food constituents in soils is the assumption that the materials dissolving in some particular solvent are available for plant use. Crop growth, however, has not always been correlated with the solubility of the constituents and various solvents have been employed in the attempt to obtain a definite correlation. One of the first methods proposed for determining the availability of phosphorus in soils consisted in the determination of the phosphorus extracted from the soil by a 1 per cent solution of citric acid. It was claimed that the root sap of most plants and a 1 per cent solution of citric acid were about the same in acidity, hence, this solution should extract about the same amount of phosphorus from the soil as the plant would be able to obtain. A 0.002 N sulfuric acid solution has also been proposed as the extracting solution. Both methods correlate fairly well with crop response to phosphate fertilizers on some soils but neither method is satisfactory for all soils.