In order to integrate organic fertilisers into mainstream agriculture, it is necessary to be able to predict with reasonable accuracy the supply of nutrients from a given product. This will allow organic fertilisers to be applied to the soil at correct rates (on their own and in combination with inorganic fertilisers) to meet the crop nutrient requirements and avoid any excess supply that might harm the environment. The focus of the research within this thesis was on the supply of plant available nitrogen (PAN) from recycled organic (RO) wastes and its prediction. The research documented in this thesis addressed the following four hypotheses; • Hypothesis 1 – The NLBAR (i.e. Nitrogen Limited Biosolids Application Rate) formula (New South Wales biosolid guidelines) and its percentage organic N mineralisation assumptions provide an accurate prediction of the mineral N supply from biosolids in the first year of a crop. • Hypothesis 2 – RO waste composition affects the proportion of its organic N mineralised in the soil. • Hypothesis 3 – Certain composition characteristics of RO wastes can provide a reliable prediction of the mineral N supply from RO wastes under controlled conditions. • Hypothesis 4 – Near-infrared spectrometry can be used to accurately predict the mineral N supply and recalcitrant C pools from RO wastes applied to the soil, under controlled conditions.