A Phosphate Sorption Index for Soils

Abstract

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.

Grouping soils of the Montreal Lowlands (Quebec) according to fertility and P sorption and desorption characteristics

“…The Psi has been proposed for the estimation of SPSC (Bache and Williams 1971). The Psi showed significant correlations with Al ox (r = 0.76***), Fe ox (r = 0.61***), CEC (r = 0.57***), OC (r = 0.53***) and M3Al (r = 0.48***) in these soils (Table 2).…”

“…Water-soluble P (Pw) was measured in a 1:27 (weight/volume) ratio of soil/water according to modification of the Sissingh (1971) procedure ). The P sorption index (Psi) was estimated by the Bache and Williams (1971) procedure. Mehlich-3 (M3) extractable P, K, Al, Fe.…”

Binding of inorganic sulphate and phosphate in humid-climate soils measured using column leaching, equilibration and extraction methods

“…The binding of phosphate was shown to be strongly related to solution P concentrations by regression equations (r 2 from 0.84 to 1.00), provided appropriate transformations of the treatment or the binding factor values were made. In 6 of the 11 soil samples, binding of the treatment phosphate decreased relatively consistently with increasing treatment concentration such that a log-log transformation, which is equivalent to Freundlich type of adsorp- tion (Bache and Williams 1971;Barrow 1978), gave the best regression fit. In the remaining five soil samples, the proportion of phosphate peaked at intermediate treatment concentration.…”

“…Further, the Langmuir equation calculation method of determining P saturation was not found to be reliable for many of the soil samples examined in this study. A single point equilibration measurement of a sample with a relatively low P treatment concentration (e.g., 15 to 20 mg P kg -1 ) has been proposed for determining maximum P binding of soils (Bache and Williams 1971;Beck et al 2004) . However, the Langmuir equation functioned well for all the soil samples they studied.…”

“…Depending on the assessment of residuals, either or both of the amount of anion bound and the equilibration treatment concentrations were logarithmically transformed. Relationships between the logarithm of the amount of bound anion and the logarithm of the concentration of the anion treatment were considered to follow the Freundlich equation (Bache and Williams 1971;Barrow 1978). Where only the concentration of the anion treatment was logarithmically transformed, the relationship was considered to follow the Tempkin equation.…”