Abstract
The concentration of phosphate ions in solution is critical in defining the ability of a soil to adequately meet crop demand. That concentration is regulated by adsorption/desorption and precipitation/dissolution processes, particularly its kinetics of reaction. This work was developed in order to find the kinetic model describing the sorption/desorption of phosphate in soils of the Argentinean Pampas region, to describe the processes associated with these reactions and to define the effect of temperature on them. A bi-linear model that adequately describes the data obtained in the adsorption and desorption experiments is proposed. Thus, it was possible to clearly differentiate two different kinetic mechanisms, each characterized by different reaction rates. Two very labile pools of phosphorus were determined in these soils. The rate at which these processes occur indicates the occurrence of chemical reactions of precipitation in highly soluble compounds and surface adsorption reactions with very low binding energy, as P is quickly released into the soil solution in the desorption process. However, since the amounts of P released in the desorption process are very much less than the adsorbed, is possible to infer that both processes occur at non-equilibrium conditions and there is a lack of reversibility of the reaction P-Soil – P-Solution in the soils studied. The intensity of these processes is differentially affected by temperature and, in general, there is a positive effect of phosphorus fertilization history, although the effect of this is not clearly seen in the amounts of extractable phosphorus with the classic Bray & Kurtz N°1 method.
Highlights
From the point of view of plant nutrition, the concentration of phosphate ions in solution is critical in defining the ability of a soil to adequately supply crop demand
It was possible to clearly differentiate two different kinetic mechanisms, each characterized by different reaction rates
The rate at which these processes occur indicates the occurrence of chemical reactions of precipitation in highly soluble compounds and surface adsorption reactions with very low binding energy, as P is quickly released into the soil solution in the desorption process
Summary
From the point of view of plant nutrition, the concentration of phosphate ions in solution is critical in defining the ability of a soil to adequately supply crop demand. In fast processes a slower change in the solid/liquid interface delays the equilibrium This is a consequence of the time required for phosphorus to dissolve and come into contact with adsorption sites within, which depends on diffusive flux of phosphate ions in the soil solution (Limousin et al 2007). Adsorption processes could be fast (when adsorption occurs on the surface of the particles) or slow (when adsorption occurs inside the particles) These processes are often difficult to differentiate and can be considered as a continuous one (Addiscott and Thomas 2000; Antelo et al 2007; Limousin et al 2007). This variation in adsorption rate is KEY WORDS Phosphorus fertilization, phosphorus sorption, kinetic models, Argentinean Mollisols
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