This research tests the hypothesis that conventional synthetic and organic fertilisers could be used together to maintain or increase yield of cereals, whilst reducing demands on conventional, mined, synthetic P supplies. This was done using a glasshouse experiment growing barley (Hordeum vulgare), and a field trial using baby corn (Zea mays L.). Both experiments were comprised of treatments of singular additions or combinations of compost and synthetic fertiliser. Leaf dry matter yield (DMY) increased significantly at a rate of 0.05 g DMY (±0.012 g) with each additional kilogram P per hectare applied as compost (with MAP making up the difference so that a total of 20 kg P/ha was added in each treatment). A significant increase in leaf P of 15.2 mg P/kg leaf (±6.7) with each additional kilogram of P applied per hectare as compost, with the remainder applied as MAP, was also found. In the field trial, however, yield was smaller in the treatments which had a partial substitution of synthetic fertiliser with compost, based on current local rates. These rates of compost addition were found to be insufficient to meet the nutritional requirements of the plant. A residual P effect was seen in the glasshouse experiment and suggests that benefits for future crops could exist following the application of compost to a paddock. Therefore, the use of composts, together with synthetic fertilisers, make it possible to reduce the quantities of synthetic fertilisers required and prolong the global availability of minable resources, such as P. This can be done without compromising yield, provided compost is applied at rates sufficient to meet plant demands.