What is the soil food web?
The soil food web refers to the living biology of the soil and how it impacts plant growth and soil structure. It is the actions of the soil microbes that make nutrient available to plant roots which means being a good organic grower is being a good microbe farmer. Soil biology includes microbes like bacteria, fungi, nematodes and protozoa, and things we are familiar with like worms, springtails, and slaters. Bacteria and fungi break down and feed on inorganic mineral particles and organic matter in the soils and incorporate them in their bodies. Plants can’t directly access this mineral source but engage microorganisms to make it available for them. Plants send up to 40% of the energy they absorb through photosynthesis to their roots where it is exuded into the soil as proteins and sugars. The exudates attract microorganisms and a feeding frenzy develops in the rhizosphere (the area around the roots). Nematodes and protozoa feed on the bacteria, releasing the absorbed nutrients directly at the plant roots, making it available to the plant. Plant roots on their own have no ability to extract nutrient from organic matter, they are dependent on the soil biology to extract the nutrient they need to grow. A healthy and diverse soil food web works to increase the access plants have to nutrients, extending the reach of plant roots deep into the soil. Disease resistance improves as the soil is so full of life, there is less space for pathogens to occupy. Microorganisms play a role in converting organic matter to humus which improves the soil structure and makes it drought resistant. Developing the soil biology like this is often called biological farming, or bio-intensive growing.
How do we create a healthy soil food web?
Excessive mechanical tilling and chemicals are the biggest killers of the soil food web. The most important changes are to reduce tilling, eliminate chemicals and begin building soil organic matter with compost, green manures and cover crops. Having a greater diversity of plants growing together will support a greater diversity of soil microbes. Growing perennial plants leads to a healthier soil food web because the soil doesn’t get disturbed and the bigger root systems encourage a wider network of biological life. Annual plants thrive in bacterial soils and gardeners can encourage this by feeding the soil with fulvic acid and humates to promote bacterial life. Perennial plants prefer a fungal soil and are slower to develop, but almost all tree species have symbiotic relationships with fungi. Tillage and bare soil are particularly disastrous for fungi.
How does this compare to using chemical fertilisers?
Chemical fertilisers are water-soluble: the nutrients are dissolved in water and are watered directly over the roots to be absorbed on contact. Sitting there in the soil, the excess gets washed into waterways whenever it rains. Chemical fertilisers, herbicides and insecticides kill off the soil biology which makes it impossible for the plants to take up the nutrient naturally in the soil. Without the soil biology to provide soil structure and water retention, it is prone to erosion, flooding and compaction: its no longer soil, its dirt.
Nutrient density and brix testing
Having a healthy soil food web with a full range of microbes stretching deep into the soil makes a bigger range of nutrients available to the plants. Well-nourished plants develop complex sugars that are nutrient dense and of immense value to our health. Plants that are fed with nitrate-heavy chemicals can grow large but it will be watery, soft growth containing simple sugars. Insects are attracted to simple sugars and are more likely to be found on plants that don’t contain the complex sugars of nutrient dense plants. If insects want to eat it, its food fit for insects. The complexity of the sugars can be measured by placing a small drop of plant sap on a refractometer to read the brix level. A higher the brix level indicates a higher nutrient density and the greater nutritional value the plant has to us. Measuring the brix levels can tell us if the plants are getting the nutrient they need and can let us know if we are doing things right.
How does this compare to organics?
Many organic growers will have a healthy soil food web without having tried to create one, but many will not, especially if they have regularly tilled the soil. A problem is that if a soil has become deficient in a nutrient, making compost of crop residues grown in that soil will accentuate the deficiency. Also, many people will, with the best of intentions, add organic products like dolomite to the soil without understanding what minerals the soil actually needs. (Kay Baxter from Koanga gardens believes she depleted her own soil by doing just that). The balance of minerals needed to provide all the necessary nutrients and support a healthy soil web is unique to each property. Many soils have suffered years of abuse and can’t be remineralised without a program of testing and correcting which can take many years of work. Environmental Fertilisers provide this service in New Zealand. There is no guarantee that food grown organically will be nutrient dense, but there are many resources that can help understand the soil food web better and start on the way to restoring soils and improving the nutritional value of the food we grow.