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

The conventional Bray 1, Egner-Riehm, Olsen, and the innovative cation/ anion 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 variability 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 loamy sand, sandy loam, loam, silty loam, clay loam, silty clay loam, were recognized. 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 exchange capacity 1.5-57.6 cmol(c) kg(-1). Soil-P (mg kg(-1)) ranges were: 0.2-185.9 for Bray 1, 0.6-307.7 for Egner-Riehm, 1.0-100.5 for Olsen, 1.1-69.4 for Pi, and 0.4-48.8 for CAERM. The P extraction efficiency decreased along with the order Bray 1 > Egner-Riehm > Olsen > Pi > CAERM, with mean values of 39.7, 34.6, 20.5, 19.0, and 14.8 mg P kg(-1) soil, respectively. All methods, with the relevant exception of Egner-Riehm, were almost uniformly affected by the considered pedological, chemical and physical features of soils, and well correlated each other. The following conversion equations, by simple 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+2.722*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 significant 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 resin 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 predicting plant P uptake and yield response.