Keywords: Precision farming, Fertilizer application, Spatial variability, GPS, Mapping.

The concept of what is currently referred to as precision farming has matured over a period of perhaps ten years from a series of relatively disconnected techniques into what can truly now be described as an all-embracing concept. However, rather than being a new system of farming, it is in reality a more focussed application of knowledge which is already in use. That is not to say that precision farming poses no questions, but in general they are not new questions.

This paper focuses on the specific issue of fertilizers in precision farming, but although fertilizers and plant nutrition are an essential constituent of good farming and farm management, they are only a part of overall precision management.

Precision farming must be an integrated management system if it is to provide real benefit. Its objectives can be seen as both economic and environmental, through more precise utilisation of resources within the current unit of management, the field.

The key to the management of variability is the measurement of the variables, which is perhaps another way of putting what Francis Bacon wrote in about 1600 that 'Knowledge itself is power'.

The principles and methods of the sampling and analysis of soil for the purposes of developing fertilizer recommendations are discussed in the context of variability within field. The combination of yield maps and crop nutrient offtakes is used as a guide to the generation of a sustainable phosphorus and potassium fertilizer policy.

Chris J Dawson, Consultant, Strensall, York, UK.

43 pages, 15 figures, 7 tables, 9 refs.