Extraction of phosphorus, potassium, calcium, and magnesium from soils by an ion‐exchange resin procedure

Abstract A procedure for the simultaneous extraction of phosphorus, potassium, calcium and magnesium from soils, by an ion‐exchange resin procedure applicable to large‐scale advisory soil testing, is described. The important steps are the disaggregation of soil by shaking in water during 15 minutes with a glass marble, the transference of the elements from the soil to a sodium bicarbonate treated mixture of anion and cation exchange resins during a 16‐hour shaking period, the separation of the resin from the soil by sieving and extraction of the elements from the resin. The results of resin extractable calcium, magnesium and potassium were comparable to the results of these elements extracted with 1M NH4OAc, to calcium and magnesium extracted with 1M KCl, and to potassium extracted with 0.025M H2SO4. For phosphorus the resin extractable values were not comparable to the results obtained by the former routine method, based on the extraction with 0.025M H2SO4. The results of resin extractable P presented closer correlation with cotton response to phosphorus application in 28 field experiments (r = 0.85**) as compared with 0.025M H2SO4 extractable P (r = 0.68**), and also with P uptake by flooded rice in a pot experiment with eight lowland soil samples (r = 0.98**), as compared with extraction with 0.0125M H2SO4 in 0.050M HCl, for which the correlation was not significant. The reasons for the superiority of the extraction of P with the described procedure are discussed.