Mycorrhizal inoculation success depends on soil health and crop productivity.

As the human population grows, so does the demand for higher agricultural yields. As a result, agricultural intensification practices are increasing while soil health is often declining. Integrating the benefits of microorganisms into agricultural management systems can reduce the need for external resource inputs. One particular group of plant symbionts that can help plants to acquire additional nutrients and promote plant growth are arbuscular mycorrhizal fungi (AMF). The application of AMF in agricultural practice has been hampered by the variability in the success of mycorrhizal inoculation and the lack of consistency in different fields. Here, we tested whether it is possible to predict mycorrhizal inoculation success based on soil health and productivity. We hypothesised higher inoculation success on fields with poor soil health because in such fields, mycorrhiza can improve nutrient uptake and biotic resistance to pathogens. We calculated a soil health index by aggregating six biotic and abiotic variables from 54 maize fields and tested its correlation with the mycorrhizal growth response (MGR). The MGR was linked to soil health and significantly higher in less healthy soils and less productive fields. This implies that soil inoculation with AMF has most potential in fields with poor soil health and low productivity. Based on these findings, we propose a soil health framework that highlights the potential benefits of AMF field inoculation.