Polyphosphates differ significantly from orthophosphate with respect to their chemical properties. The adsorption by soils and clay minerals of a series of polyphosphates containing from 2 to 65 phosphate units was examined. The affinity coefficients of the Langmuir model and the intercepts of the Freundlich model indicated that soils have greater affinities for polyphosphates than for orthophosphate and generally greater affinities for pyrophosphate than other polyphosphates. The affinities of soils for ortho-, pyro-, and tri-phosphate were significantly correlated with the clay content, while the affinities of soils for longer polyphosphates were significantly correlated with dithionite-extractable Fe and Al contents. Increasing the temperature of equilibration from 15(DEGREES) to 45(DEGREES)C increased the adsorption of ortho-, pyro-, and tri-phosphate, but decreased the adsorption of long chain polyphosphate (P(,35)). Trimetaphosphate (TMP) was not adsorbed by soils;A method was developed for determination of TMP in the presence of other inorganic P compounds added to soils. Use of this method in studies of enzymic and nonenzymic hydrolysis of TMP in soils showed that the optimum buffer pH (0.1 M Tris) for trimetaphosphatase activity in soils occurred at pH 8, whereas the nonenzymic hydrolysis of TMP generally increased with pH. Maximum trimetaphosphatase activity occurred in soils at an incubation temperature of 40(DEGREES)C. The nonenzymic hydrolysis of TMP increased with temperature up to 80(DEGREES)C, and with addition of Ca and Mg. Studies of the kinetic parameters of trimetaphosphatase activity in 3 soils showed that the K(,m) values of this enzyme ranged from 6.3 to 7.2 mM TMP. The average energy of activation for the enzymic hydrolysis of trimetaphosphatase in soils was 21 kJ/mole as compared with 32 kJ/mole for the nonenzymic hydrolysis. The inhibition of this enzyme by phosphate and its analogs and by EDTA showed noncompetitive kinetics. The enzyme was stable up to 3 months when soil samples were stored at 5(DEGREES)C;Trimetaphosphatase activity of soils was significantly correlated with the organic C and clay contents of 28 Iowa surface soil samples. It decreased with depth in four soil profiles examined. The nonenzymic hydrolysis of TMP was significantly correlated with soil pH, organic C, water-extractable Ca and Mg contents of soils.