The use of nitrogen fertiliser in agriculture. Where do we go practically and ecologically?

Nitrogen fertilisers were produced in 72 countries in 1982, total world capacity being 99 mt of N, having been 50 mt in 1970. Consumption was 31.8 mt in 1970, rising to 60.3 mt in 1980 (Av. annual growth rate 7%). Forecasts suggest N use of 90mt in 1990 rising to between lll-134mt by the year 2000. The large amount of N added to only some 11 % of the earth's land surface as fertilisers, coupled with concurrent increases in biological N fixation, mainly by grain legumes, is bound to result in increases in the total N content of soils, waters, crop residues and municipal wastes. The need to use N to produce sufficient food and fibre for the 7 billion humans must be set against the need to maintain a 'good' and safe environment. Nitrate levels are increasing in both surface and ground water supplies. The amount of ammonia and oxides of nitrogen in the atmosphere produced by volatilisation and denitrification from soils and animal excreta is also rising. Such increases may have detrimental environmental effects to human health and to the ecology of downstream or 'polluted' non-agricultural ecosystems though the severity and extent of these effects requires verification. As yet, there is little hard evidence of direct damage to human health due to high levels of nitrate in diet or of NH₃ and NO X in the atmosphere, but effects on natural and forest ecosystems in some areas are proven. With this background, strategies are examined which should help to increase the efficiency with which N is utilised by crops and animals and so decrease losses of nitrogen from farmland. These include the selection of optimum N fertiliser practices based on knowledge of plant requirements, soil N supply, and the use of carefully chosen times, methods and forms of N fertiliser application. Other technological approaches such as use of slow release fertilisers, chemicals that inhibit certain biological transfers of N in soils and amendments added to N fertilisers, to soils or to animal excreta to alter their chemical properties could be developed. Greater use of legumes and enhanced levels of N₂-fixation may also help to lessen the need for N fertiliser. To achieve further improvement in the ways of using N in agriculture, more precise knowledge is needed of the dynamics of nitrogen turnover in soils, of translocation and assimilation in plants, and of interactive flows between soil, plants and animals, and the atmosphere. Only with full understanding of the many biological processe