The slogan "healthy soil, healthy food, healthy people" is repeated widely. What is less widely understood is the specific biological mechanism behind it — and how directly farming practice can enhance or destroy it.
What is the soil food web?
A teaspoon of healthy agricultural soil contains more individual organisms than there are humans on earth. Bacteria, fungi, protozoa, nematodes, earthworms, arthropods — all interacting in a complex web of relationships that is still being mapped by science. This diversity is not decorative. It is the engine of soil fertility and, ultimately, of nutritional quality in food.
The key process is mineralisation: the conversion of nutrients locked in organic matter into forms that plant roots can absorb. Nitrogen, phosphorus, potassium, calcium, magnesium, zinc, iron, copper — all must pass through soil biology before they reach the plant. It is not the mineral that matters so much as the biology that makes it available.
When soil biology is abundant and diverse, mineralisation is efficient and plant nutrition is comprehensive. When soil biology is suppressed — by tillage, by synthetic inputs, by monoculture — mineralisation slows, nutrient availability decreases, and the plant draws from a shallower nutritional pool. The consequences flow directly into the food.
How do mycorrhizal fungi affect the nutritional quality of crops?
Of all the relationships in the soil food web, the mycorrhizal symbiosis between fungi and plant roots is among the most important for nutritional outcomes — and among the most widely damaged by modern farming.
Mycorrhizal fungi form an extensive hyphal network in the soil — threads far finer than root hairs, reaching into pore spaces that roots cannot access. In exchange for carbohydrates from the plant, the fungi supply water and minerals, particularly phosphorus. Studies show that mycorrhizally colonised plants can access phosphorus from distances 10 to 1,000 times greater than the root system alone could reach.
Phosphorus is critical not only as a macronutrient in its own right but because it is a building block for the biochemical pathways that produce polyphenols and other secondary metabolites. A plant with adequate mycorrhizal phosphorus supply invests differently in its biochemistry — including in the production of the compounds most associated with human health benefits.
What damages mycorrhizal networks? Tillage physically severs the fungal threads. High phosphorus fertiliser application suppresses mycorrhizal colonisation because the plant no longer needs the fungal supply. Systemic fungicides kill or inhibit the fungi directly. Three of the most common practices in intensive arable farming are, together, among the most effective ways to dismantle the nutrient network.
Why is soil organic matter critical to nutritional quality?
Soil organic matter (SOM) is the storehouse from which much of this biological activity draws. It contains the carbon compounds that feed the microbial community, holds water and nutrients against leaching, and buffers soil chemistry to maintain conditions in which both plant and microbial life can thrive.
SOM levels in UK agricultural soils have declined significantly over the past century — a consequence of repeated tillage, monoculture, and the removal of organic matter from the system. Research published in the British Journal of Nutrition and elsewhere shows that crops grown in soils with higher organic matter consistently test higher for mineral content, vitamin concentrations, and antioxidant levels. The relationship is not incidental.
Building SOM is one of the highest-leverage interventions available to farmers seeking to improve nutritional quality. Cover cropping, composting, reduced tillage, and the integration of livestock into arable rotations all contribute to SOM accumulation. They take time — SOM builds slowly — but the nutritional payoff is measurable and durable.
How does the bacterial-fungal ratio affect nutrient cycling?
Soil biologists often characterise soil health in terms of the ratio between bacterial-dominated and fungal-dominated communities. Bacterial-dominated soils tend to cycle nutrients quickly but lose them easily — through leaching, volatilisation, and runoff. Fungal-dominated soils cycle nutrients more slowly but hold them more securely within the hyphal network, releasing them in response to plant demand.
Intensive tillage and high nitrogen fertilisation tend to push soils towards bacterial dominance. Reduced tillage, diverse rotations, and organic matter inputs tend to shift them towards fungal dominance — the condition associated with better mineral retention, stronger mycorrhizal networks, and more stable nutrient supply to crops.
This does not mean bacterial-dominated soils are uniformly bad — bacterial communities are essential for nitrogen cycling — but the balance matters. A soil dominated by fast-cycling bacterial communities with disrupted fungal networks is less able to deliver the full spectrum of minerals and trace elements that crops need for nutritional completeness.
What do these findings mean for farming practice?
The science of soil biology has several clear implications for farming practice — and for how we measure and reward food quality.
First, the most important variable in crop nutritional quality is not the crop variety or the fertiliser blend — it is the biological condition of the soil in which it grows. This is both challenging and liberating: challenging because soil biology is more complex than a fertiliser programme; liberating because it means improvement is possible on almost any farm, regardless of soil type or location.
Second, the practices most associated with improved soil biology — diverse rotations, cover crops, reduced synthetic inputs, minimal tillage, composting — are the same practices associated with improved nutritional outcomes in the evidence base. There is no tension between environmental sustainability and nutritional quality. They point in the same direction.
Third, measuring soil biology as part of a nutritional quality framework is both necessary and increasingly practical. Advances in metagenomic sequencing have made it possible to characterise soil microbial communities in detail at decreasing cost. Integrating biological indicators — microbial biomass, fungal-bacterial ratios, mycorrhizal colonisation rates — alongside chemical soil tests is becoming standard practice among high-performance farming operations.
This is the scientific foundation of the GroundUp Framework: the recognition that nutritional quality in food begins in the soil, that soil biological health can be measured, and that farming practices can be assessed for their likely nutritional impact before a crop is even planted.
