FERTILISING VALUE OF PHOSPHORUS FROM URBAN AND AGRICULTURAL ORGANIC WASTE

Organic wastes contain quantities of phosphorus (P) potentially available for the plants, which justify their use in agriculture. However, in practice, organic waste spreading does not take sufficiently into account the corresponding phosphorus supply. The aim of this contribution is to present a synthesis of seve-ral studies of the relative phosphorus availability (RPA), i.e. the plant availability of waste P expressed as that of water soluble mineral P fertiliser, of sewage sludges, municipal composts, solid and liquid manu-res, following the same method. This method consists to cultivate ryegrass (Lolium perenne L.) in green-house experiments during three to four months. Before mixing with soil organic wastes or the soluble mineral phosphate, P ions of the soil are labelled with radioactive 32P. The determination of the total P and isotopic composition of P taken up by crop allows to differentiate P derived from the soil and P derived from the applied P source. Among several indicators of P availability, the contribution of the applied P ( = Pdff) is the most relia-ble. Approximately 60 (soil × wastes) systems were tested. The RPA values were generally higher than 80%, except for wastes that were stabilised by digestion and composting