Phosphorus is a resource in finite supply. Use of organic amendments in agriculture can be a sustainable alternative to inorganic P, provided it can meet crop requirements. However a lack of consistent knowledge of plant P availability following application of organic amendments, limits its potential. Studies suggest chemical extraction procedures, may not reflect plant available P. The Diffusive Gradients in Thin-films (DGT) technique is based on natural diffusion of P via a hydrogel and sorption to a ferrihydrite binding layer; which should accurately represent soil P (CDGT) in a plant available form. The aim of this research was to evaluate changes in soil P availability, following the addition of organic amendments, cattle farmyard manure (FYM), green waste compost (GW), cattle slurry (SLRY) and superphosphate (SP) using Olsen P and DGT. The research included incubation, and glasshouse studies, using ryegrass (Lolium perenne L.). Soils with a history of application of the aforementioned organic amendments were used (Gleadthorpe), as well as a soil deficient in P (Kincraigie). The hypotheses were as follows H1 A build-up of P available by diffusive supply, from historic treatment additions and subsequent availability from fresh treatment additions will be demonstrated by DGT. H2 Historical treatment additions are more important at determining yield and P uptake than fresh additions. H3 DGT can detect changes in P available by diffusive supply following addition of different treatments and subsequently following lysis of microbial cells on a soil deficient in P. H4 DGT will provide a more accurate indication of plant P availability than organic amendments in a soil deficient in P. H5 P measurements using DGT will be lower from organic amendments than superphosphate.H6 DIFS simulations of soil kinetic parameters will provide additional information about how treatments influence P resupply from solid phase to solution following DGT deployment. Cont/d.