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DIVISION S-8—NUTRIENT MANAGEMENT & SOIL & PLANT ANALYSIS

Anion Exchange and Mehlich-III Phosphorus in Humaquepts Varying in Clay Content

^a Soils and Crops Research and Development Centre, Agriculture and Agri-Food Canada, 2560 Hochelaga Blvd., Sainte-Foy, QC, Canada G1V 2J3
^b Dep. of Soil and Agri-Food Engineering, Laval Univ., Sainte-Foy, QC, Canada G1K 7P4
^c Crops and Livestock Research Centre, Agriculture and Agri-Food Canada, 440 University Ave., Charlottetown, PE, Canada C1A 4N6

^* Corresponding author (leon-etienne.parent{at}sga.ulaval.ca)

Soil-test P is not only used for fertilizer recommendations but also as a component to predict risk of P transfer from soils to surface waters. The changes in soil-test P following P application are often related to soil texture. This study assessed the changes in anion-exchange membrane P (AEM-P) and Mehlich-III extractable P (M3P) in Humaquepts varying in clay content. Five soils from loamy sand to "heavy" clay received 0 (P0), 10 (P10), 20 (P20), or 40 (P40) mg P kg^-1 per each simulated growth season were successively cropped with barley (Hordeum vulgare L.) and soybean (Glycine max L. Merr.) in a growth chamber experiment. There were clay content mineral fertilizer P (MFP) interactive effects on soil AEM-P, M3P contents and plant P uptake. The AEM-P paralleled clay content and MFP rate. The average increase in M3P at P10 and P20 over P0 was smaller on clayey than on coarse-textured soils. The converse trend was true at P40 over P10 and P20. The increase in M3P per unit MFP addition in excess of plant uptake decreased linearly with the increase in clay at P10 and P20 but not at P40. There was a quadratic plateau relationship between AEM-P and M3P contents with a turning point at 4.2 µg AEM-P cm^-2 or 93.0 mg M3P kg^-1. Plant P uptake was more closely related to AEM-P than M3P in all soils, suggesting AEM-P is a more reliable indicator of labile P than M3P in Humaquepts for the tested crops, particularly in fine-textured soils. Clay content has a large influence on changes in AEM-P and M3P following MFP additions to Humaquepts.

Abbreviations: AEM, anion-exchange membrane • AEM-P, anion-exchange membrane P • DSPS, degree of soil P saturation • M3P, Melich-III extractable P • MFP, mineral fertilizer P • subscript ox, ammonium-oxalate extractable • P0, 0 mg P kg^-1 treatment • P10, 10 mg P kg^-1 treatment • P20, 20 mg P kg^-1 treatment • P40, 40 mg P kg^-1 treatment