Phosphorus (P) is essential for plants and animals because of its role in vital life processes, such as in photosynthesis in plants and energy transformations in all forms of life. It also has a significant role in sustaining and building up soil fertility, particularly under intensive systems of agriculture.

Chemistry of Phosphorus Transformations in Soil

15 s. The 4 other pools, B, C, D and F, branch directly off the central pool. In this system studied in steady-state, the pool B contains ions which can exchange with the phosphate ions of the soil solution in between 1 min and 1 d. The pool C contains the ions able to enter the solution in steady-state between 1 d and 3 months. The pool D contains ions which can exchange with phosphate ions in the solution in 3 months to 1 yr. The pool F contains phosphate ions exchangeable after > 1 yr. This model concerns only inorganic P, organic P fol- lowing other rules of transformation, such as those of microbial mineralization. A schematic functional model has been developed to summarize this information. The model applied to various soil samples taken from 2 long-term field ex- periments showed that repeated fertilization or exhaustive cropping can modify all the pools. This scheme shows 2 kinds of phosphate buffering capacity (PBC) for soils. The first kind, which depends on the P pool, is called 'instanta- neous PBC'; the second kind, which depends on the 4 other pools, is called 'delayed PBC': its effect on the mainte- nance of phosphate ion concentration in the soil solution is time-dependent.

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Le phosphore assimilable des sols : sa représentation par un modèle fonctionnel à plusieurs compartiments

Four adsorption equations were fitted to phosphate isotherm data for 38 soils from northern New South Wales. The two-surface Langmuir equation provided the best fit to the data. The Freundlich equation, however, was almost as effective, provided that an estimate of native adsorbed phosphate was included. It required fewer adsorption points because the transformed data produced a straight line. A regression constant, a, from the Freundlich equation although underestimating adsorption capacity, calculated from the Langmuir II equation, was highly correlated with this parameter (r = 0.984) and a buffering index (r = 0.986), calculated from the Langmuir I equation. The other Freundlich regression constant, n, was significantly correlated (P <1%) with the intensive parameters of adsorption from the one- and two-surface Langmuir equations. Since only two adsorption points are required for fitting the Freundlich equation and the parameters are easier to derive, it is suggested that this equation may be more suitable for commercial soil-testing laboratories than the Langmuir one- and two-surface equations for routine determination of phosphate adsorption characteristics of soils.

A comparison of the Langmuir, Freundlich and Temkin equations to describe phosphate adsorption properties of soils

The chemistry of P in the spodic (Bh) horizon of Florida Spodosols has generated considerable interest in recent years, particularly where water quality problems have been reported. Early studies suggested that the Bh horizon can function as a sorption sink for P moving through nonretentive sands. However, the conditions under which P can be retained in or released from the Bh horizon have not been adequately investigated. This study evaluated the P sorption and desorption properties of the Bh horizon from 16 Florida Spodosols. The samples exhibited P sorption capacities that were reflective of their citrate-dithionite-bicarbonate (CDB)-extractable Al contents. Mean Langmuir sorption maximum (S max ) values of 7.55, 11.08, and 17.49 mmol P kg -1 were obtained for low-, medium-, and high-Al soils, respectively. The CuCl 2 -extractable Al (organic matter bound) was the single most important chemical property contributing to P retention, accounting for >60% of the variability in P sorption. Though anaerobic incubation altered P sorption and release, the effect was generally small and inconsistent. Such treatment reduced P sorption by low-Al soils and slightly increased that of high-Al soils. The lack of response to anaerobic incubation was attributed to generally low levels of reducible Fe in the soils. The samples also exhibited varying degrees of P sorption-desorption hysteresis. Irreversibility was most pronounced for high-Al soils, where >70% of the added P remained sorbed after nine successive extractions. The relationships between Al contents and P sorption capacities should be useful in formulating site criteria for control and management of wastewater from dairy and other agricultural operations located on Florida Spodosols.

Phosphorus Sorption and Desorption Properties of the Spodic Horizon from Selected Florida Spodosols

Relationships between phosphate sorption and iron oxides in Alfisols from a river terrace sequence of Mediterranean Spain

Seven phosphate buffering indices were evaluated by determining the additional variance accounted for when each index was added to a regression of plant phosphorus uptake on labile soil phosphate. The study was done on two groups of soils: one relatively homogeneous group of 24 soils all formed on the same parent material, and a heterogeneous group of 30 soils formed on a variety of parent materials. A separate pot experiment was done on each group, ryegrass being grown on the homogeneous soils and white clover on the heterogeneous soils. Only two indices did not account for a large and significant increase in variance in phosphate uptake. The extra variance accounted for was much greater in the heterogeneous group than in the homogeneous group, although the total variance accounted for by both variables was greater in the homogeneous group. The much smaller volume of soil used in the ryegrass experiment may explain the smaller buffering effect in the homogeneous soils. The three most effective and consistent indices were a simple index of the maximum buffer capacity, determined from the Langmuir isotherm over a standard range of equilibrium solution concentrations, the slope of the isotherm at the natural solution concentration, and the amount of adsorption at a standard equilibrium concentration of 0.3 pg phosphorus/ml.

I. C. R. Holford

Papers

Evaluation of soil phosphate buffering indices