Bacteria and fungi: their ratio and seven stages of ecological succession
The fungal-to-bacterial ratio describes one part of the microbial community. We relate it to the crop, available food and the functioning of the whole soil food web.
What F:B means
F:B stands for fungi:bacteria. In Elaine Ingham’s Soil Biology Primer comparison, it means fungal relative to bacterial biomass. A ratio of 1:1 represents equal estimated biomass; 2:1 means twice as much fungal biomass. Biomass accounts for both organism numbers and size.
Succession: from pioneer plants to old forests
Ecological succession is gradual change in plant communities and ecosystems. In Ingham’s educational model, the transition from early herbaceous vegetation to shrubs, trees and old forests is associated with an increasing fungal-to-bacterial biomass ratio.
| Plant community | Fungi:bacteria |
|---|---|
| Pioneer annuals / early weeds | 0.1:1 |
| Early-successional grasses | 0.3:1 |
| Mid-successional grasses and vegetables | 0.75:1 |
| Late-successional grasses and row crops | 1:1 |
| Shrubs and vines | 2:1 to 5:1 |
| Deciduous trees | 5:1 to 100:1 |
| Conifers and old forests | 100:1 to 1000:1 |
These are indicative biomass ratios from an educational framework. Conifers and old forests occupy its upper end. For a crop, we consider its successional position and soil conditions. An olive grove combines trees and herbaceous cover, so both must inform assessment.
Two connected nutrient pathways
Hunt and colleagues, including Elaine Ingham, modelled nitrogen transfers in a shortgrass prairie food web in 1987, combining population, feeding preference and nitrogen-content data. The model described bacterial and fungal pathways linked at higher feeding levels. Both depended on consumers of microbial biomass.
The ratio needs context
A 2026 Soil Food Web School article by Wes Sander connects ratio assessment with research on decomposition, nutrition and succession. We examine biomass alongside microbial consumers. The same ratio can arise from very different total quantities, so both absolute estimates and the broader community matter.
Food and management influence the picture
The Primer relates organisms to resources: complex residues support decomposer fungi, while readily available root-zone compounds feed bacteria. Management can change their relative abundance.
Consistent measurement
We use consistent sampling and preparation, recording dilution, field of view and group estimates to compare successive measurements. The School’s microscopy training includes preparation, assessment and interpretation.
Our field work
We monitor soil and compost. With the target plant’s successional position in mind, we select suitable microbiology and inoculate through compost and its liquid derivatives, checking their composition microscopically.