Transfer of phosphate ions between soil and solution: Perspectives in soiltesting

Phosphorus inputs must be estimated accurately to optimize the economic ret urn to farmers and minimize P loss from seas to surface waters, Currently, P recommendations are based on the diagnosis of field crop responses by che mically extracted soil P. However, the inability of chemical extraction to characterize plant-available P limits the reliability of these recommendati ons. Major sources of P mobilized by plant roots include P ions in solution and those from soil constituents, which replenish and buffer solution. A m echanistic evaluation of soil P supply should therefore be based on the des cription of P ion transfer between soil constituents and solution. Sorption , desorption, electro-ultrafiltration (EUF), and isotopic exchange studies show that an adequate modeling of this quantity [Q((CP,t))] of P ions must account for both the concentration of P ions in soil solution (C-P) and tim e (t). In one long-term field experiment, the Q((CP,t)) description was not affected by crop rotation and mineral fertilization histories; therefore, Q((CP,t)) changes are fully explained by C-P changes, In two field experime nts, Cp changes were linearly correlated with the cumulative P budget, inpu ts, and outputs over years. In three field experiments, the soil type effec t on the relative maize (Zea mays L.) response curve was taken into account using the ability of soil P to replenish solution P for 1 d. The residual variance of this diagnosis is halved compared to Olsen's extraction. Althou gh more information is necessary, accuracy is improved when soil testing is based on mobility of P ions.