Arbuscular mycorrhizal fungi help plants absorb phosphorus. Inoculating crop soils with these beneficial fungi should improve yields. Often it does — growth increases up to 40%. Sometimes it doesn't — growth decreases by 12%. Across 54 Swiss farms, the response to the same inoculation varies wildly.
Lutz et al. (Nature Microbiology, 2023) asked what predicts whether inoculation will work. The answer was not what agronomists expected. Soil nutrient levels — the obvious candidates — were weakly predictive. The abundance of pathogenic fungi already living in the soil was the strongest single predictor, explaining 33% of the variation in crop growth response. The broader soil fungal community explained 53%. Soil microbiome indicators together captured 86% of the variation.
The finding inverts the standard model of agricultural intervention. We typically ask: what are we adding, and how good is it? The mycorrhizal inoculum is the same across all 54 fields — same species, same preparation. The intervention is constant. What varies is the resistance it encounters. Fields with high pathogenic fungal loads respond dramatically to inoculation because the mycorrhizal fungi provide protection the plants need. Fields with low pathogenic loads show minimal response because the plants don't need help. The intervention's value is set by the enemy, not the ally.
More precisely: the obstacle predicts the outcome of intervention more accurately than the intervention predicts its own success. You learn more about whether a treatment will work by studying what it has to overcome than by studying the treatment itself. The resistant field, not the remedy, carries the information.