Researchers in Switzerland treated 54 fields with Arbuscular mycorrhizal fungi (AMF), which can enhance plant nutrient uptake and reduce plant stress, to quantify the effects on maize (corn) growth, and found the results highly variable, ranging from negative 12 percent to positive 40 percent:
With few soil parameters and mainly soil microbiome indicators, we could successfully predict 86 percent of the variation in plant growth response to inoculation. The abundance of pathogenic fungi, rather than nutrient availability, best predicted (33 percent) AMF inoculation success.
More:
“On a quarter of the plots, the mycorrhizal fungi enabled up to 40 percent better yields. That’s huge,” says the study’s co-lead, Marcel van der Heijden, a soil ecologist at the University of Zurich and at Asgroscope. But there’s a catch: on a third of the plots, the yield did not increase and sometimes even decreased. The research team was initially unable to explain why this happened.
[…]
“We discovered that the inoculation functioned best when there were lots of fungal pathogens already in the soil,” says co-first author Stefanie Lutz from Agroscope, the federal center of competence for Agricultural Research.
“The mycorrhizal fungi act as a kind of protective shield against pathogens in the soil that would weaken the plants.” As a result, the normal yield can be maintained in fields where, without mycorrhizal fungi, there would have been losses. In contrast, mycorrhizal fungi had only a minor effect on fields that were not contaminated with pathogens.
“The plants there are strong anyway and grow excellently. The use of mycorrhizal fungi in such cases brings no additional benefits,” says the other first author, Natacha Bodenhausen from the Research Institute of Organic Agriculture.
TL;DR: Only take medicine when you’re sick.
What do they have to say about the Amazon’s weird soil?