Phosphate fertilising management: new indicators for bioavailability applied to carrot crops

New indicators for bioavailability, in link with the knowledge of the phenomena involved in plants' phosphate nutrition, were assessed in carrot crops, and experimental references were established. The trials conducted in the Aquitaine region, a major French carrot production area for the fresh market, consisted in analysing the response of carrot, in terms of yield, to a wide range of soil phosphorus supply levels. The synthesis of the data acquired during a three-year study shows the importance of the P concentration in the soil solution, i.e. the soluble P, noted as Cp, for diagnosing the soil's nutritional status. Measuring Cp may be sufficient in the case of slightly buffered soils, characteristic for carrot cultivation in Aquitaine. However, the quality of the relation between yield and soil supply improves when it is represented by the sum of soluble P and diffusible P, linked to the plant's functioning. This is indicator E. Threshold values were determined, corresponding to 1 mg P L-1 and 15.7 mg P kg-1 soil respectively for Cp and E. Those thresholds require further validation in a wider range of agronomic conditions. Furthermore, the newly acquired knowledge and technical references already make it possible to rectify fertilising practices in Aquitaine. In optimum fertilising conditions, the crop takes up 26 kg P ha-1, i.e. 60 kg P2O5 ha-1, and soil supply represents two thirds of the phosphorus uptake. This study offers perspective for new diagnosing methods, which take into account the functioning of the plant species. It shows that a more sparing use of phosphorus in carrot crops is possible without compromising yield. Changing fertilising practices accordingly will guarantee a better preservation of natural resources and water quality. Finally, the outcome of this study encourages us to apply those new concepts in soil phosphorus supply to a wider range of vegetables, in order to establish a reliable basis for integrated P management practices.