EXTRACTABLE PHOSPHORUS IN SURFACE AND SUBSURFACE LAYERS OF STE. ROSALIE AND ST. BLAISE SOIL SERIES IN QUEBEC

Abstract

The relationship between extractable P and soil characteristics including pH, clay content (CLAY), cation exchange capacity (CEC), organic matter content (OM), and phosphorus retention (PRET) was determined using stepwise multiple regression analysis. Similarly, the influence of pH, CLAY, CEC, OM and PRET on the relationship between extractable P and relative yield (Yo/Yp) of young corn plants in the greenhouse was determined. Surface (0- to 20-cm) and subsurface (20- to 40-cm) layers from two soil series (St. Blaise and Ste. Rosalie) were collected from farm fields in the region south of Montreal. Twenty-eight sites of the St. Blaise series and 25 sites of the Ste. Rosalie series were sampled from fields of which the majority were in cultivation for 100–200 yr. Three P extractants were used: Bray 1 (P1) (0.03 N NH4F, 0.025 N HCl), Bray 2 (P2) (0.03 N NH4F, 0.1 N HCl) and 0.5 M NaHCO3 (P3). Extractable P by all three extractants was negatively correlated with pH of the St. Blaise surface soils. However, extractable P by the P2 extractant was positively correlated with pH of the Ste. Rosalie surface soils. Although significant, the R2 values for these relationships were low, ranging from 0.15 to 0.17 for the surface soils. Extractable P was related, to variable extents, to one or more of pH, PRET, OM and CEC of the subsurface soils depending on the soil series and P extractant in an unexplainable manner. However, the R2 values were generally much higher for the subsurface than surface soils suggesting that extractable P in subsurface soils was more closely related to the measured soil characteristics. There was no relationship between extractable P with any extractant and CLAY for either series. The highest correlation between relative yield (Yo/Yp) and extractable P occurred with P3 for the surface soils of both series. None of the soil characteristics was included with P3 in the regressions with the surface soils. PRET was included along with P2 in the regression with Yo/Yp for the St. Blaise surface soils, whereas a P × CLAY interaction term occurred with the P1 and P2 extractants for the Ste. Rosalie surface soils. A linear relationship was obtained between Yo/Yp and P1 or P3 for the Ste. Rosalie surface soils and P2 for the St. Blaise surface soils. A curvilinear relationship was obtained with P1 or P3 for the St. Blaise surface soils and with P2 for the Ste. Rosalie surface soils. Extractable P was not included as an independent variable in any of the regressions with Yo/Yp as the dependent variable for the subsurface soils. This suggests that extractable P with any of the extractants used in this study does not reflect the availability of P in subsurface soils.