Keywords: Phosphorus, soil extraction, fertilizer recommendations, P surplus

Concentrations of extractable soil phosphorus (P) form the basis of most fertilizer P recommendation systems, although until recently the principle application has been the identification of soils with a low P status. The extent to which interpretation and subsequent fertilizer recommendations are modified through the use of additional, site specific, information varies greatly between individual countries. A range of different extraction procedures are in current use and even where a common technique is employed the comparability between derived soil P indices are often poor. input:output balances for phosphorus (P) calculated at either individual farm or national scales indicates a net annual surplus. Comparisons made between different farm enterprises show a significant range in the magnitude of any P surplus which is especially large for areas having intensive livestock. The P surplus for arable situations appears to be smaller. The cumulative effect of any annual P surplus should be reflected by soil P analysis. The extent to which current fertilizer strategies have recognised temporal changes in soil P status is not always that clear.

There have been numerous changes to agricultural practices, other than just the rate of P application, during the last 30 years that could influence the P concentration obtained from soil analysis. Considerable scope exists to improve the usefulness of soil P test data, however a balance between additional cost and the expected value of any improvement in efficiency of P use must be determined. Improvements to P fertilizer recommendations would have a dual significance in terms of reducing variable costs of agricultural production whilst also helping to minimise possible environmental impacts arising from P loss.

A C Edwards, Macaulay Land Use Research Institute, Craigiebuckler, Aberdeen, UK.
P J A Withers, ADAS Bridgets Research Center, Winchester, UK.
T J Sims, Department of Plant Science, University of Delaware, Newark, USA.

23 pages, 11 tables. 37 refs.