PYROPHOSPHATE HYDROLYSIS IN MINERAL FRACTIONS OF SOILS, GOETHITE, KAOLINITE, AND MONTMORILLONITE

Condensed inorganic phosphates, including pyrophosphate (PP), are of interest in soil fertility because of their water solubility and high P content. However, phosphate (P) is largely taken up by plants as orthophosphate (OP). Plant uptake, therefore, requires PP hydrolysis. Hydrolysis of pyrophosphate (PP) was studied in autoclaved and nonautoclaved soils from Quebec. The samples studied were two surface soils from cultivated fields of St. Bernard (Typic Hapludoll) and Dalhousie (Mollic Haplaquept) and three minerals: Goethite, kaolinite, and montmorillonite. Soil and mineral samples were fractionated into two separates. Each soil and mineral fraction was treated with PP and incubated under anaerobic conditions at 10, 20, and 30°C for periods ranging from 1 to 28 d. The percentage of PP hydrolyzed was significantly lower in goethite samples than in soil, kaolinite, and montmorillonite samples. There was less PP hydrolysis in fine fractions (2 to 0.2 μm) than coarser fractions (20 to 2.0 μm) in both autoclaved and nonautoclaved fractions regardless of soil or mineral type. Rates of PP hydrolysis were 2 to 5 times greater in nonautoclaved than autoclaved soil fractions, and rates of PP hydrolysis increased with increased temperature from 10 to 30°C. The effect of temperature on hydrolysis rates, however, was more obvious in the autoclaved than nonautoclaved samples. Added orthophosphate (OP) prior to PP addition reduced the amount of PP remaining nonhydrolyzed, probably due to reduced PP sorption. Increasing the level of PP applied to soil reduced the percentage of PP hydrolysis regardless of the level of OP in the OP-amended soils. © 1990 Williams & Wilkins.