Can compost suppress soilborne diseases?
Compost can help protect plants against soil pathogens. Effectiveness depends on its biology, the pathogen and the crop.
The Soil Food Web School links plant health with nutrient cycling and soil structure that supports aeration and roots. Plant protection forms part of overall soil function.
What is suppressive compost?
The term describes compost reducing disease occurrence or severity under tested conditions. Products can differ even with similar feedstocks.
Logo and colleagues tested 37 composts from seven commercial producers in 2025, using cress and cucumber with Globisporangium ultimum, formerly Pythium ultimum, and Rhizoctonia solani. Thirty-three suppressed disease in at least one plant–pathogen combination; only three were effective across all three combinations. This shows both potential and the importance of matching compost to use.
Changes in the community
LeBlanc and Harrigian added green-waste compost to soil containing Fusarium oxysporum in greenhouse and growth-chamber experiments reported in 2024. Molecular methods tracked fungi and bacteria. Compost reduced F. oxysporum biomass in all experiments and changed bacterial composition. Sixteen bacterial genera consistently increased, raising questions about their roles.
The study demonstrates reduction of that fungus under experimental conditions. Disease and crop-yield effects need corresponding field assessment.
Are many microbes enough?
In the 37-compost study, suppression was not consistently associated with total microbial activity or bacterial and fungal diversity. A biologically rich picture alone cannot predict protection from a specific disease. Community function within the plant–pathogen system must be examined.
Practical assessment
Small comparisons can use the same crop and care with and without compost. Record product origin, rate, growth, symptoms and yield. Identifying a disease’s pathogen helps interpret the outcome.
Our field work
We combine compost microscopy with comparative crop trials, connecting biology with measurable plant health and growth outcomes.