Animal excreta: Fertilizer or pollutant?

The difficulty of reconciling human food production and safeguarding the environment is well illustrated by nitrate and phosphate pollution resulting from animal excreta. Animal excreta contain, at the point of application to the land, some 4 g N, 2 g P, and 2.5 g K per kg. The average output from a pig, for example, is some 2000 kg excreta per annum, and the capacity of crops to utilize N and P is about 150 kg and 100 kg per hectare per annum respectively. Thus crops can utilize per hectare per annum the nutrients from about 20 pigs. Over-application has resulted in the need for legislation in order to limit the amount of excreta applied to agricultural land. But the material itself is the essential mainstay of organic and sustainable farming initiatives, allowing food production with avoidance of artificial aids and minimizing the use of non-renewable resources. It is not that there is too much excreta; rather the problem is one of concentration caused by the location of many intensive livestock units into certain geographical areas. The difficulty can be resolved by a change in farming structure, such as encouraging diversity and dispersal of intensive livestock production units. The creation of mixed farming systems will facilitate the required improvement in balance between crop area and housed livestock density, and will also provide an ecologically more varied habitat in the countryside.