Behaviour in the soil of orthophosphate from pig slurry compared with that of KH2PO4

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Abstract
An assessment was made of the relative merits in the short term of KH2PO4 and pig slurry as sources of orthophosphate. This involved a comparison of these 2 sources of P with regard to adsorption of P by the soil from solution, maintenance of the solid-phase P in labile form and rate of release of the solid-phase P into solution. The adsorption isotherms made with 2 soils revealed that the amount of orthophosphate adsorbed from pig slurry was much higher than that from KH2PO4 at all levels of application. Conversely, in order to raise the orthophosphate concentration in the soil solution to a given level, much larger applications of orthophosphate would be required as pig slurry than that in the form of KH2PO4. Therefore, the risk of direct orthophosphate-P pollution of ground water from pig slurry is likely to be even less than that from soluble P fertilizers. Determinations of exchangeable-P in treated soils after different times from treatment showed that a larger fraction of the solid-phase P derived from slurry orthophosphate remains in an exchangeable form than that from KH2PO4. The rate of desorption of solid-phase P into solution measured using an anion exchange resin was higher in the KH2PO4-treated soil than that in the slurry-treated soil at different times from addition of equal amounts of orthophosphate-P.