ANION AND CATION-EXCHANGE RESIN MEMBRANES TO ASSESS THE PHOSPHORUS STATUS OF SOME PORTUGUESE SOILS

Ion exchange resin methods were applied to 78 different soils to asses s their phosphorus (P) status for predicting their response to P ferti lization. The techniques used were anion exchange resin membranes elut ed with hydrochloric acid (HCl) (AEM) and cation-anion exchange resin membranes eluted with HCl (CAEM-HCl), sodium chloride (NaCl) (CAEM-NaC l) or water with directly color development (CAEM-H2O). Greenhouse stu dies were conducted with the same soils in order to validate laborator y data. Ryegrass was grown with two levels of P: nil and 150 mg P kg(- 1) of soil. Results indicate that soil P levels are significantly corr elated (p<0.001) if extracted with AEM or CAEM, both eluted with HCl, although the CAEM technique had extracted larger amounts of P. Concern ing the type of elution, results did not show significant differences (p<0.05) between CAEM-HCl and CAEM-NaCl, but both were significantly c orrelated (p<0.001) with the results obtained with CAEM-H2O. All the t echniques used to measure extractable P correlated significantly with relative yield and P uptake by ryegrass, showing their ability to pred ict soil P availability. Nevertheless, CAEM extraction had higher valu es of r(2). Among the three techniques for elution, the levels of corr elation with the biological parameters were equivalent. From these res ults, it was concluded that: (i) exchange resins, specially CAEM, is a n accurate method to assess the P fertility status of soils, and (ii) the traditional step of elution can be avoided, allowing the process t o be less time consuming, thus more suitable for routine use.