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. Most of the soil dwelling critters are things like bacteria, fungi, nematodes and protozoa which can’t be seen with the naked eye, but includes the things we can see like worms, springtails, and slaters. Bacteria and fungi break down and feed on inorganic mineral particles and organic matter in the soils, incorporating minerals in their bodies as they go. The nutrient is not available to plants in this form. As a plant grows, it absorbs energy through photosynthesis and carbon dioxide. Up to 40% of this energy gets sent to the plant roots and is exuded into the soil as proteins and sugars. The bacteria and fungi feed on these exudates and colonise around the plant roots. Nematodes and other organisms feed on the bacteria and fungi, releasing the absorbed nutrients directly at the plant roots, making it available to the plant. The soil food web depends on living plant roots exuding protein and sugars to feed on, and the plant depends on the soil food web delivering minerals to grow. Some microbes will live on inorganic matter alone, but there will be a bigger and more diverse community where there are living plant roots. A diverse community of microbes will make more nutrient available to the plant roots, effectively increasing the reach of the roots into different layers of the soil. 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 and make it available in the root zone. Being a good organic gardener means being a microbe farmer.
How do we create a healthy soil food web?
Mostly by doing things not to harm it. Excessive mechanical tilling and chemicals are the biggest killers of the soil food web. For it to thrive it needs to be disturbed as little as possible, and have oxygen, water, energy, (the protein and sugars exuded from plant roots) and nutrient, which is extracted from organic and inorganic matter (compost and mineral particles in the soil). Having a greater diversity of plants growing together will attract a greater diversity of soil microbes. Growing perennial plants leads to a healthier soil food web because the soil doesn’t get disturbed. Annual plants like vegetables prefer a bacterial soil, and perennial plants prefer a fungal soil. As the soil food web becomes healthier and more diverse, disease resistance improves as there is less room for pathogens to breed and more ‘good guys’ to fight them off. Developing the soil biology like this is often called biological farming, or bio-intensive growing. The soil structure and drought resilience also improves.
How does this compare to using chemical fertilisers?
Chemical fertilisers are water soluble, that is, they dissolve in water and come into direct contact with the roots to get taken up by the plant. Chemical fertilisers, herbicides and insecticides will kill off the microbes in the soil making the naturally occurring nutrients unavailable to the plants. Its then dependent on the chemical fertiliser and will only take up what is delivered in water to its roots. Nitrate nitrogen is only ever taken up with water so high nitrate levels means a high dilution rate and this rapid watery growth will attract insects and diseases. As the soil biology plays a roll in improving the structure of the soil and absorbing water, killing off the biology changes what the soil looks and feels like. It becomes dirt, not soil. Being water soluble, the nutrient is quickly able to leach from the soil and pollute waterways in heavy rain.
Nutrient density and brix testing
Having a healthy soil food web with a full range of microbes stretching deep into the soil and far beyond the root zone of the crop means that a full range of nutrient, including micro-nutrients are made available to the plant as it needs it and the high levels of organic matter means greater water retention in the soil. The plant is able to access all the nutrient it needs and as a result develops complex sugars, compared to the simple sugars that are present in the watery, nitrate heavy growth that comes with chemical fertilisers. Pests are less likely to attack because they want to eat simple sugars, many people believe that if you have a pest problem on your vegetables it’s because you are growing insect food! Diseases are less likely to take hold because the diversity of microbes already present can crowd out and attack pathogens. The complexity of the sugars can measured by placing a small drop of plant sap on a refractometer to read the brix level. The higher the brix level, the higher the 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. Another problem is that if a soil has become deficient in a nutrient, making compost of crop residues grown in that soil will just 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). There is no guarantee that being organic will lead to nutrient dense food being grown unless the soil is tested and all minerals added in the quantities needed, and the growing plants brix tested.
Why buy fertiliser if the food web provides it all?
I’m still trying to understand this. Most advocates of biological farming still use products to either add minerals or increase biological activity and there are a few companies specialising in creating biological fertilisers to enhance this process. Increasing the humus content of soil is also important because it provides food for microbes. Some growers recommend using compost tea and nothing else. Compost tea is a ‘home brew’ of microbes made in special buckets which are heated and aerated for 24 hours then sprayed on crops to increase the microbe population. Some say the microbes can extract all the nutrients ever needed and that nature contains everything a growing plant needs if we just get the correct microbes, and others say our soils are depleted and we need to bring in the minerals before we can hope to grow nutrient dense crops. I don’t know who is right! I’m using a few different products to lift the brix levels of my crops but I hope that eventually I can get the mineral levels and biological activity high enough that I can provide all my own fertility through compost and green manure.
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