Soil Organic Matter Content: Is There A Need to Update The Interpretation for Agricultural soils?

Incorporating the SOM:clay ratio in the Principles of Agricultural Crop Fertilisation in Switzerland (PRIF) allows us to give more specific agronomic advice at each site based on a broader consideration of soil quality.

Soil organic matter (SOM) is the main indicator of soil quality. The SOM content of agricultural soils is strongly influenced by agricultural practices, and a certain percentage of these soils are in a degraded state. It is therefore crucial, particularly for farmers, to be able to interpret the SOM levels revealed by the soil analysis in order to guide crop management choices.

The Principles of Agricultural Crop Fertilisation in Switzerland (PRIF) provide a grid for interpreting the SOM content of soils which generally serves as a reference for the interpretation of the analyses. However, this grid has scarcely evolved since the 1980s and nowadays focuses on the function of supplying nitrogen (N). This suggests a significant potential for optimisation.

The SOM:clay ratio is a simple soil quality indicator whose thresholds are based on the soil structure quality potential. The SOM:clay ratio interpretation scheme is calibrated to the ’VESS’ (Visual Evaluation of Soil Structure quality) method.

The study conducted here aimed to evaluate the relevance of the SOM:clay ratio for interpreting the SOM content of the agricultural plots with a view to laying the foundations for a possible evolution of the PRIF interpretation grid. To this end, the thresholds defined in the PRIFs were compared with those of the SOM:clay ratio and the differences in assessment depending on method were highlighted for a selection of 184 FRIBO (Fribourg Soil Monitoring Network) plots.

The SOM:clay ratio provides greater nuance

The results of the study show that the PRIFs divide the plots into two groups: on the one hand, the majority of the arable land comprising the soils with the least SOM, classified as having an adequate SOM content, and on the other hand, a group comprising the arable land with the most SOM and the permanent grasslands, classified as ‘rich’. The assessment based on the SOM:clay ratio provides greater nuance on the SOM content of the arable land, indicating in particular that this content is considered inadequate in one-third of all cases.

Towards more specific agronomic advice

The study thus highlights the fact that in its current form, the PRIF interpretation grid does not constitute an adequate tool for effectively guiding farmers in managing the SOM of their fields. Incorporating the SOM:clay ratio in the PRIFs would enable a better differentiation between fields with very different SOM content but which are all currently classified as being at least adequate. The knowledge acquired will serve to update the PRIF chapter ‘Soil Properties and Analyses’.