In recent years, renewed attention has been given to the role of soil organic matter in sustainable food systems. As a result, a large number of meta-analyses has recently been performed (almost simultaneously) to assess the relationship between increasing soil organic matter and crop yields. These meta-analyses combined data from multiple field experiments into one response variable. Common response variables are, for example, the effect on agronomic nutrient use efficiency, changes in attainable crop yields, or a ratio between yields using mineral fertilisers versus using organic inputs.
This review highlights the impact of the choice of method and response variable when assessing the yield effect of increasing soil organic matter. We explain the differences between methods and classify them into nine categories. In addition, we synthesize the outcomes of 14 meta-analyses on the relationship between increasing soil organic matter (especially by using organic inputs) and crop yields. These meta-analyses used different response variables, included different crops and/or focused on different geographical regions. Some studies had global coverage, whilst others focused on a specific continent or country. Not surprisingly, the outcomes of these studies differ. Some general trends however were observed.
This review shows that most studies which reach a generally positive conclusion regarding the effect of increasing soil organic matter on crop yields did not control for the effects of the macro-nutrients nitrogen, phosphorus and/or potassium. Selecting only those studies which assessed the additional yield effect of soil organic matter (excluding macro-nutrient effects), indicates that cereals cultivated in temperate climates particularly seem to benefit relatively little from increasing soil organic matter, while most crops cultivated in tropical climates benefit more, but not always. Finally, adding farmyard manure gives more benefit than crop residues, even when the effects of macro-nutrients are excluded.
Understanding the additional yield effect of increasing soil organic matter is necessary to contextualize current policy proposals on soil carbon sequestration, and also for recommendations to farmers on soil management. Presenting simple correlations between soil organic matter and crop yields, or assuming a general percentage of increase in crop yield when using organic inputs, does not do justice to the complexity and diversity of crop yield responses. Only by distinguishing between the effects of nutrients and other effects of increasing soil organic matter, can a sound scientific basis be developed for targeted policies. More insights into underlying mechanisms (when additional yield effects are absent, negative or positive) is needed. A re-think is required on how the complexity and variety of yield responses to increasing soil organic matter can be captured, when researching and developing sustainable food systems.