Phosphate and potash fertiliser recommendations for cereals: current issues and future needs

Inputs of inorganic phosphorus (P) and potassium (K) fertilizers to cereal crops cost the UK agricultural industry about £185 million per year. With cereal prices falling well below £100/tonne and increasing EU, commercial and public pressure to farm in a manner which will not cause a risk to human health or cause a deterioration in environmental quality, there are strong economic and environmental arguments to reduce or omit P and/or K fertilizer inputs where they cannot be justified. As P is the more expensive fertilizer nutrient, and with increasing concern over the link between P and eutrophication of watercourses, and the build-up of cadmium in the soil from application of P fertilizers, there is a particular need to manage P inputs more efficiently. In contrast, K is the cheapest of the major nutrients and has no known deleterious effect on the environment.

Reductions in P and K fertilizers can be achieved by taking full account of the PK fertilizer value of organic manures and on soils which are oversupplied with nutrients, especially since cereal crops are not generally responsive to fresh PK inputs once residual soil PK levels have reached a critical level. However, the consequences of reducing or withholding PK fertilizer from modern high-yielding cereal varieties have not been adequately quantified. Better targeting of crop PK needs requires a level of precision which the current recommendation system was not developed for. Recent evidence from long-term field experiments and farmer experience indicates that standard soil PK tests and 'maintenance PK policies' are not performing adequately on all soil types. Soil types and their physical and chemical characteristics have a large influence on critical PK concentrations above which yield response is unlikely, on the amounts of fertilizer PK required to achieve and maintain a critical level, on the potential availability and uptake by crops of the pool of P and K in the soil which is not measured by standard soil tests, and on the risk of P loss in land run-off, yet these are not presently taken into account in current advice.

Protocols for plant sampling and analysis have recently been defined to help farmers assess adequacy or oversupply of plant nutrient supply but plant analysis has less predictive value than soil analysis in assessing fertilizer need. Information on the release and retention of P and K by readily and slowly exchangeable soil PK fractions on different soil types (topsoil and subsoil), the relative importance of crop rooting patterns and weather factors influencing crop uptake and the development of novel techniques for improving the efficiency of PK fertilizer use are required to help develop accurate decision support systems for farmers. Such systems should encompass both short-term and long-term impacts of fertilizer strategies on soil PK levels and crop yields, nutrient budgeting and include an economic appraisal. Intelligent zoning of farm and/or field areas is recommended as a means of using soil type information for better targeting of P and K inputs and improving nutrient and land management practices to maintain economic production levels and avoid accelerated P loss in land run-off. With the recent introduction of national guidelines to restrict P use in UK agriculture, there is increasing justification for more precise information on PK fertilizer strategies for economic and environmental sustainability.