Potential Changes in Soil Fertility from Arable Farming, Including Organic Systems

Keywords: Soil fertility, Sustainability, Long-term experiments, Organic matter, Nitrogen, Phosphorus, Potassium, Legumes, Leys.

The well-being of future generations depends crucially on two factors, the maintenance of soil fertility so that agricultural production can be sustained at a level to feed all people adequately a wholesome water supply. Present concern about the level of nitrate and pesticides in some water supplies has focused attention on current production systems for arable crops. However, nitrogen inputs, whether as fertilisers or organic manures at recommended rates or by leguminous crops; and agrochemicals to control weeds, pests and diseases simply determine the level of production; they have little immediate effect on long-term fertility. Such fertility arises from complex interactions between biological, chemical and physical factors. This paper considers some aspects of the chemical attributes of soil fertility. Results are taken from a number of long-term experiments started during the latter half of the last century on silty clay loams at Rothamsted, on sandy loams at Woburn and on sandy clay loams at Saxmundham. All three sites are in south eastern England. Arable crops were grown in a traditional Norfolk 4-Course Rotation in which no weedkillers or pesticides were used. In most experiments inorganic NPK fertilisers were compared with farmyard manure and the benefits of leguminous crops could be assessed. The level of production and its sustainability are discussed together with changes in those soil fertility parameters which can be estimated using soil analysis. Yields were always similar with both fertilisers and manures when fertiliser nitrogen inputs were limited but the level of production would not be sufficient to meet current demand. The continuous use of inorganic fertilisers over periods of up to 140 years had no greater adverse effect on soil fertility than did the use of organic manures at rates typically available in commercial practice. Where organic manures were used large amounts of nitrogen were lost from the crop: soil system.

A E Johnston, Soils and Agronomy Department, AFRC Institute of Arable Crops Research, Rothamsted Experimental Station, Harpenden, UK.

38 pages, 9 figures, 18 tables, 23 refs.