Keywords: Agronomic efficiency, fertilizer, integrated nutrient management, manure, physiological efficiency, recovery efficiency.

Summary:
Efficient use of nutrients is a central component of sustainable cropping systems both in intensive production systems where losses can lead to environmental problems and in low input production systems where soil degradation through nutrient mining occurs. At its simplest, the concept of nutrient use efficiency provides a measure of the output per unit of input but, in practice, there are several components that contribute to the overall efficiency: agronomic, recovery and physiological efficiencies are all legitimate ways of considering nutrient efficiency depending on the context. A major means of achieving efficient nutrient use is to ensure that the availability of individual nutrients is matched to crop demand in time and space. Practical means of achieving this include optimal placement of fertilizers, and modifying quantities and timing to suit the particular crop. Site-specific and precision forms of agriculture are increasingly being developed and adopted to meet these aims.

Long-term experiments have shown that suitable inputs of organic materials are necessary to ensure the sustainability of production systems. Systems of integrated nutrient management, in which inorganic fertilizers and organic manures are added to crops, have the potential to supply a continuous supply of nutrients to crops at a rate matched to crop growth and to minimise wasteful losses in the process. Achieving this in practice is, though, not easy not least because the biological processes determining the release of nutrients from organic materials are imperfectly understood and can only be poorly managed at present, and because the labour costs inherent in making and applying organic manures are often unaffordable.

Genetic improvement of crop plants specifically to enhance nutrient use efficiency is at an early stage of development but offers exciting prospects. Plants and microbes have evolved several means of enhancing phosphate availability in soils low in P (e.g. mycorrhizal associations, rhizosphere pH changes, release of phosphatases) and the modification and exploitation of these processes in combined plant/soil management approaches within cropping systems remains a challenging prospect.

Peter J Gregory, Scottish Crop Research Institute, Invergowrie, Dundee, DD2 5DA, UK.
Tim S George, CSIRO Plant Industry, PO Box 1600, Canberra, ACT2601, Australia.

32 pages, 3 figures, 6 tables, 94 references.