The Right Microbe Biome Balance for Your Crop

Elizabeth Pearce @ SymSoil
4 min readApr 26, 2020


Soil Nerds who find soil life forms and their interactions fascinating, often forget to step back and discuss how this microbial community changes as the grower’s crops change. In the wild, as above-ground ecosystem complexity increases, the soil ecosystem diversity will also increase, with old forests having the greatest fungal dominance and total biodiversity.

Robust Compost, or BioComplete Compost, has broad biodiversity across those seven critical life forms. Most discussion focus on the ratio of the fungal biomass to the total bacterial biomass, the F:B ratio. Think of the soil microbe biome as an investment portfolio based upon the S&P 500. There are funds that invest in value stocks with high yields, and other funds focused on growth stocks. Some funds emphasize technology companies or healthcare companies.

Similarly, there is not one single best F:B ratio for every farm. The best F:B ratio for you depends upon the historic origin of your crop. In the wild, the soil of the grasslands is bacterial dominate, and the soil of old hardwood or redwood forests are fungal dominate.

Brassicas, for example, originated in the grasslands, and do best with an F:B ratio of 0.5. Brassicas include cabbage, mustard, cauliflower, kale, bok choy, broccoli, and brussels sprouts.

Other crops that originated at the edge of the forest, prefer a F:B ratio close to 1.0. Cannabis plants, for example, produce the maximum THC levels at 0.9 to 1.0. Hemp plants, maximize CBD production when the soil’s F:B ratio is closer to 1.2.

Most vines, including grapes, prefer a higher ratio. Grapes for wine, for example, maximize their flavor and productivity when the soil’s F:B ratio is 1.6 to 2.0.

Most orchards need a high ratio. Olive trees, for example, would find 4.0 to 10.0 to be the optimal ratio. Most nut trees would prefer even more fungi.

In other words, while it is difficult to attribute a specific ratio healthy soil, it is possible to move towards ratios that are optimal for a specific crop and to monitor biological composition in order to detect changes in the soil environment.

On a farm, properties such as water quality, ambient temperatures and soil moisture affect the composition of the soil microbe biome. Of course, fungal and bacterial activity follows seasonal fluctuations, with a peak during optimal conditions of temperature and soil moisture. Monoculture farms will have lower biodiversity than farms that use crop rotation. Adding organic materials will increase the amount of biology in the soil. Adding products with broad biodiversity, such as robust compost or compost extracts or teas will reseed the soil microbe biome.

A sure way to decrease your F:B ratio is to till your soil, which is why most biological farmers use minimal tilling. Tillage disturbs both the non-living and biological components of soils, with fungi being among the most sensitive to these changes. Tillage destroys fungal hyphae, reduces the stability of soil aggregates. To increase the F:B ratio, focus on robust compost products with a high fungal potential, that is, a high spore count in your robust compost.

Measuring Your Biology

One of the most important characteristics is the water quality. If you are working with well water, we strongly recommend having your water tested. With city water, you have chlorine and can get information on the minerals and hardness from your water utility. In general, low pH is associated with fungal dominance, whereas a high pH is more typically found in soil with bacterial dominance.

SymSoil has a soil lab, and soil tests (with and without consultation) can be ordered through our website. You can also use any Soil Food Web Lab to get a biological assessment.

The SymSoil lab offers basic biological assessment, for example, if you want to know if there is a specific pathogen. At the other extreme, the Soil Mavens can review your chemical soil tests, as well as your water tests and your full biological assessment and offer a comprehensive analysis and recommendation based upon your farms’ challenges and goals.

This is the fifth article in the series, which begins with 21st Century Farmer’s Guide to Sustainable Farming, An Overview which can be found here. We encourage you to follow us for all the updates.

More on the 21st Century Grower’s Guide to Sustainable Farming

Part 1 Overview

Part 2 Soil Microbe Biome

Pt 3 Benefits of a Healthy Soil Microbe Biome

Part 4 Healthy Soil has 7 Types of Life

Part 5 The Right Microbe Biome Balance for Your Crop

Part 6 Engineered Soils

Part 7 20th Century Farming vs Biological Farming

For more information, please visit our website

SymSoil Inc. is an evidence-based, soil health company with products and services for regenerative agriculture. Our flagship products are Robust Compost, Fungal Infused Biochar and Grow Cubes for the cannabis and hemp cultivators. Our science team has 35 years of experience developing solutions to growers’ problems based upon a deep understanding of the complete soil microbe biome. A core belief is regional soil microbes can be restored to regenerate the soil, which will significantly increase plant health, crop yields, flavor profile and nutrient density, as the plants access nutrients the way nature intended.

SymSoil was named as a 2019 AgTech Company to Watch. SymSoil is seeking funding from Accredited Investors and/or relationships with Agricultural Co-ops, learn more here and here.



Elizabeth Pearce @ SymSoil

We recreate the complete soil microbe biome to improve farmer profits. #RegenAg #ClimateAction #100KTrees