Des bilans de phosphore majoritairement négatifs pour les systèmes de grandes cultures biologiques sans élevage en Midi-Pyrénées. Quels impacts sur le phosphore biodisponible des sols et l’état de nutrition des cultures ?

Abstract

Soil phosphorus (P) fertility management in organic cropping systems is a debated issue in the general context of decreasing resources in a near future. A survey showed that in Midi-Pyrenees a majority of stockless organic cropping systems present overdrawn P balances. We investigated the impacts of P balances on the soil plant available P and nutrition status on the long-term field experiment of La Hourre (CREAB, Auch, France), which is representative of rainfed systems set on clayey calcareous soils. The changes in soil plant available P were monitored between 2002 and 2012 using P Olsen soil test. The results specified the declining rate of P availability in time and according to the amplitude of the negative balance. The levels of soil available P did not still lead to degradation of the crop P nutrition status; in this context, N inputs are low, so consequently crop P needs remain limited. However the problems associated with long term management of soil P fertility remain. Beyond the desirable scientific perspectives, we propose a first analysis of the factors on which it would be relevant to act in order to allow the farmers to improve their practices