An analysis of farmers' use of phosphorus fertiliser in industrial agriculture: a case study in the Bordeaux region (south-western France)

Global phosphorus (P) reserves are rapidly being depleted and agriculture is a major contributor to P consumption worldwide. Moreover, agricultural P use may lead to local excess of P in soils, resulting in water eutrophication. To remedy this situation, it is necessary to understand the key factors underlying P use by farmers within such a context. The objective of this article is to understand how farmers use P fertilisers under agricultural conditions. Particular attention was paid to the use of conventional decision-making tools such as soil analysis by farmers. Industrial agriculture, characterised by high levels of specialisation, input use and production yields, was considered as a case study. Forty farmers involved in industrial agriculture in south-western France were interviewed in 2008 about their P use. Soil analyses were collected when available. Input/output P balances were computed at the plot scale. Moreover, records of farmers' fertilisation practices were analysed from a database developed by a technical advisory group. Results showed that 90% of farmers had a crop-specific and yearly P fertilisation strategy. This yielded very positive P balances for vegetable crops. Few farmers used recommendations based on soil analyses: in most cases, the amounts of P used by the farmers were lower than those recommended. When asked for their opinion about the role of P in agriculture, most farmers explained that P fertilisation helped boost the growth of spring crops in the event of adverse weather conditions. This revealed the emphasis placed on "plant-based" strategies for P management, as opposed to "soil-based" strategies, thus reinforcing annual fertilisation strategy. This study provides a foundation for understanding key factors underlying P consumption in agriculture.