Soil properties affecting phosphorus extraction from portuguese soils by conventional and innovative methods

The conventional Bray 1, Egner-Riehm, Olsen, and the innovative cation/ ani on exchange resin membranes (CAERM) and iron oxide-impregnated filter paper strips (Pi) procedures were compared as phosphorus (P) extractants from 73 representative Portuguese soils. The soils were characterized by wide vari ability of their pedological, physical, and chemical properties. Eleven FAO -UNESCO soil units, with prevailing Anthrosol, Calcisol, Cambisol, Fluvisol , Leptosol, Luvisol, and 10 different textural classes, with prevailing loa my sand, sandy loam, loam, silty loam, clay loam, silty clay loam, were rec ognized. The soils showed the following properties: CaCO3 0.0-50.9 g 100 g( -1), organic matter 0.4-8.4 g 100 g(-1), pH-CaCl2 3.9 to 7.9, cation exchan ge capacity 1.5-57.6 cmol(c) kg(-1). Soil-P (mg kg(-1)) ranges were: 0.2-18 5.9 for Bray 1, 0.6-307.7 for Egner-Riehm, 1.0-100.5 for Olsen, 1.1-69.4 fo r Pi, and 0.4-48.8 for CAERM. The P extraction efficiency decreased along w ith the order Bray 1 > Egner-Riehm > Olsen > Pi > CAERM, with mean values o f 39.7, 34.6, 20.5, 19.0, and 14.8 mg P kg(-1) soil, respectively. All meth ods, with the relevant exception of Egner-Riehm, were almost uniformly affe cted by the considered pedological, chemical and physical features of soils , and well correlated each other. The following conversion equations, by si mple linear regressions, were calculated: Olsen-P=4.118+1-0.412*Bray 1-P, R -2=0.737; Olsen-P=-1.300+1.144*Pi-P, R-2=0.677; Olsen-P-0.760+1.330* CAERM- P, R-2=0.559; Bray 1-P=-3.064+2.249*Pi-P, R-2=0.602; Bray 1-P=-0.630 0+2.72 2*CAERM-P, R-2=0.538; Pi-P=3.417+1.053*CAERM-P, R2=0.677. After performing multiple regressions including the CEC as accessory variable, we obtained s ignificant equations only for the conversions between Egner-Riehm-P and Pi- P or CAERM-P values, as it follows: Egner-Riehm-P=-24.600+1.630*Pi-P+1.928* CEC, adj. R-2=0.587; Egner-Riehm-P=-23.303+2.183 *CAERM-P+1.747*CEC, adj. R -2=0.606. Thus, the innovative Pi-strips and the cation/anion exchange resi n membranes procedures could represent suitable tools for the assessment of soil-P status and for P fertilizer recommendations. As the future research step, we will verify the reliability of the contrasted soil-P tests in pre dicting plant P uptake and yield response.