Have extractable phosphorus and potassium contents evolved in French agricultural soils since 2004?

In the last 25 years, amounts of phosphorus and potassium brought to soils through mineralfertilisers have decreased by 70% in France, without organic compensation. This situation con-duced to a reduction of nutrients balances. The aim of this study was to estimate if the decreasedfertilisation could be linked with a significant decrease of extractable P and K contents in soil. TheFrench National Soil Test (FNST) database was used to study this hypothesis. It gathers resultsof physico-chemical analysis performed on agricultural topsoils samples, at farmers’ request. Thespatial and temporal sampling strategy were not controlled. The sampling year is known andthe localisation is given by the municipality of origin of the sample. Two periods were compared:1990-2004 and 2005-2014. A total of 2,516,743 and 2,161,811 soil test results for K2O and P2O5respectively were available in the FNST database, equitably allocated between the two periods.Analytical results were grouped within administrative spatial entities (cantons - groups of munic-ipalities; small agricultural regions - 713 units in France), and statistical estimator of nutrientscontents were calculated after a resampling procedure (Monte Carlo approach) aiming at limitingsampling biases. Two indicators were studied: the evolution of extracible nutrients contents andthe evolution of the agronomic diagnosis performed with the software RegiFert. The agronomicdiagnosis evaluates the necessity of fertilisation to reach the potential yield considering the crop,the soil nutrient status and other soil characteristics. Results are provided according to 3 diagnosisclasses: low, intermediate and high fertility. Evolution of contents was assessed using the non-parametric Mann–Whitney test, and evolution of diagnosis was assessed using the adequation 2test with a limit p-value of 0.1, combined with expert rules on the evolution of the count of resultsinto each fertility class between the two periods. Extractable P contents exhibit a clear decrease: Pcontent decreased by more than 10% in 42.2% of the total cultivated area (TCA), with contrastedregional situations according to intensity of the animal production. The evolution of extractableK content was very low: it decreased for only 4.5% of the TCA. Evolution of agronomic diagnosisis very narrow, meaning that evolution in nutrients contents was too limited to significantly im-pact fertilisation recommendations. However, the overall decline in nutrients availability in soilsencourage vigilance and soil fertility monitoring through regular soil testing.