Phosphate sorption isotherms covering a wide concentration range (10-" to 5 x I O -~M phosphate) were determined for 42 soil samples at 20 "C by a standardized technique. The slope of a plot of the sorption, x, against the logarithm of the equilibrium solution phosphate concentration, log c, measured at c = IO-'M, proved a suitable reference index to characterize the phosphate sorbing properties of the soils. Several single-point methods were tested by statistical correlation against this reference index. Of these, the sorption, x, from one addition of 150 mg P/IOO g soil gave Y = 0.951, but Y = 0.974 when the equilibrium concentration was also taken into account in the quotient x/log c. This quotient is therefore suggested as a simple yet adequate way of indicating a fundamental soil property, its phosphate sorption isotherm.

The implications of the different methods available for measuring the cation exchange capacity (CEC) of soils are examined in the light of cation exchange mechanisms, and the issues involved in selecting a suitable method are discussed.

The toxicity of dissolved aluminium to many plant and animal species is one of the major deleterious effects of the acidification of the environment. AI mobilization from insoluble forms in minerals and rocks is governed mainly by the pH of the solution surrounding weathering minerals. To a first approximation, the solubility product of gibbsite gives an indication of AI concentrations expected in solution. Secondary silicate minerals such as kaolinite, imogolite and allophane, and sulphate minerals such as alunite, maintain lower AI concentrations in solution than does gibbsite, particularly at pH < 5 . The cation exchange system of acid soils provides a large reserve of ionic AI, which can be brought into solution when soluble salts percolate through soils. Ligands such as fluoride and organic anions, which form soluble AI complexes, combine with much of the AI in solution when they are present in suitable concentrations, and therefore maintain higher concentrations of AI than might be expected from mineral solubility equilibria, particularly at pH 5-7.Slow reactions involving solid phases, and uncertain data for possible AI species present, limit the rigorous application of equilibrium thermodynamics to these systems, but the factors mentioned provide a reasonable understanding of the major features of aluminium mobilization.

After 1 (H)h reaction with CI-resin and 300h reaction with HCO3-resin (approaching equilibrium). the concentration of anions complementary to phosphate was the critical variable affecting the transfer of P from soil to resin. Solution ccmcentrations of H30+, ca?+ and phosphate indicated that desorption of P by OH-, and dissolution of Ca phosphates, controlled P release from soils. P extracted by HC03-resin was much greater than by CI-resin from an acid soil. due to lower total anion and higher desorbing anion concentrations. but there was little difference between the two resins with a calcareous soil. HCO-resin extracted a constant proportion of isotopically-exchangeable P from different soils whereas CI-resin did not. Anion exchange resins provide a convenient means for producing P desorption curves for soils. introduc lion A N I O N -E X C H A N G E resins are frequently used to extract plant-available phosphate from soils (Amer et al., 1955; Cooke and Hislop, 1963). The resin method gives better correlation with P uptake by plants than other methods using single extractants (Gunary and Sutton, 1967; Bache and Rogers, 1970; Metwally et al., 1975; Kadeba and Boyle, 1978) presumably because it simulates the desorbing effect of plant roots better than the usual chemical extractants. Desorption from soil occurs as a result of the low P concentration that the resin maintains in solution, and Barrow and Shaw (1977) concluded that this concentration governs the P desorbed in a given time. Vaidyanathan and Talibudeen (1970) suggested that the rate of P transfer to the resin was controlled by diffusion within resin particles rather than by chemical reactions between soil and solution, whereas Sibbesen (1978) concluded that P desorption from soil to water was the rate-determining step.We report here P adsorbed by the resin as a function of time, resin concentration, salt concentration and the desorbing anion, and also the pH and concentrations of Ca and P in the soil-resin-solution system. The results provide further information on mechanisms of P release from soil by resins.

Experiments on model and real soil blocks designed to assess the feasibility of using magnetic resonance imaging for three-dimensional mapping of the time-varying spatial distribution of water in structured soils are reported. The results show that, notwithstanding inherent problems in imaging natural soils with a significant iron content, experimental parameters can be identified which allow satisfactory images to be obtained. Magnetic resonance imaging may therefore provide important information on soil structure and water movement in dual porosity soils, with attendant benefits for the calibration of models of non-Darcian flow in such soils.