Use of Acid-Fluoride Solutions for the Extraction of Available Phosphorus in Calcareous Soils and in Soils to Which Rock Phosphate Has Been Added

In the chemical measurement of “available” phosphorus in calcareous soils from western Kansas, it was found that a method involving use of a solution of 0.025N HCl and 0.03N NH₄F worked very well. It was necessary to use a large volume of solution to effect accurate measurement of available phosphorus. Neither dilute acid alone nor 0.1N HCl plus 0.03N NH₄F proved satisfactory. Use of 0.5M NaHCO₃ buffered at pH 8.5 was not as satisfactory as the use of dilute acid and fluoride as mentioned above. It was better than the other techniques, however. Addition of fluoride to dilute acid (0.025N HCl) repressed the solubility of certain forms of phosphorus found in calcareous soils which were extracted by acid alone. Apparently the portion of the phosphorus not repressed by fluoride, insofar as its solubility is concerned, corresponded rather closely to that which was usable by wheat plants. Therefore, good agreement was obtained between this method of chemically extracting available phosphorus and plant growth response to added phosphorus. Addition of 1.0N CaCl₂ to the extracting solution, as a substitute for fluoride, did not aid materially in improving the usefulness of dilute acid solution as a means of extracting “available” phosphorus. It did repress somewhat the solubility of phosphorus, however. The acid-fluoride combination (0.025N HCl + 0.03N NH₄F), most acceptable for use with the calcareous soils, also was adapted for use with soils from central and eastern Kansas, most of which were acid in reaction. This method of extracting phosphorus was useful in evaluating availability of phosphorus in either untreated soils or in soils after the addition of rock phosphate. Likewise it was useful in measuring the release of phosphorus from rock phosphate to forms in the soil which may be regarded as available for plant utilization. Extraction of phosphorus from pure chemical compounds by means of the dilute acid-fluoride technique provided additional information.