Organic manures are a valuable resource. The maximum potential fertiliser value of the nitrogen, phosphorus and potassium (NPK) excreted annually by farm livestock in the UK has been estimated at over £575 million. Losses of these plant nutrients may occur during collection, storage or spreading and nitrogen losses can continue after application. Not all the nutrients applied are available for plant uptake. The actual potential value or fertiliser equivalent of these livestock manures is therefore considerably reduced and a value of less than £160 million, is more realistic for those wastes requiring handling and return to the land. By comparison, other organic manures are of minor importance.
Attempts at optimising the financial returns from organic manures are often made by using materials mainly as sources of P and K, and by supplementing with inorganic fertilisers, particularly nitrogen to balance crop nutrient requirements.
Agronomic experiments have demonstrated variable growth responses particularly of grass to nitrogen applied in cow slurry. Results suggest that frequent small doses of dilute slurry are more effective than single dressings of a high dry matter material. Such factors, together with storage and spreading costs, usually mean that it is not economically feasible to attempt to maximise the use of plant nutrients in manures. However, environmental constraints require that farmers do not cause pollution or a nuisance. The utilisation of livestock wastes as organic manures is an important facet of good waste management practice and one from which a significant financial return may be achieved.
Slurry injection, one means of improving the fertiliser efficiency, has a number of drawbacks which suggest that the technique may have limited application under UK conditions.
The potential fertiliser value of farm manures in the UK is not being realised at the present time. Environmental pressures will continue to encourage improvements but at the present time the question of optimal utilisation lies in the balance between economic considerations; the need to avoid pollution and to make best use of a resource which may otherwise be wasted.