Ways in which soil microbes and plants interact

A very important part of the success of this project is an understanding of the soil food web, in other words, how organisms that live in the soil interact with plants, writes Bill Kerr.

Ways in which soil microbes and plants interact
When the soil has developed a structure like this, it will produce wonderful crops.
Photo: Bill Kerr
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The soil food web is a relatively new concept and the science behind it is still not fully understood. What has been proved is that having this web functioning and having increased the humus content to at least 3%, plants will be much healthier and more productive.

Without this web functioning and with a low humus content, chemical fertilisers become the only basis for plant nutrition. There are complex mineral interactions between chemicals, and their balance becomes vital for successful production. A deep knowledge of their functions and interactions becomes vital for successful production.

When the organic content is restored with the associated soil life, the soil begins to take care of itself, and knowledge of fertiliser elements and their interactions become virtually unnecessary.

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All we really need to know is how to get the soil humus content to the sustainable level and keep the food web functioning through not tilling the soil and feeding these organisms with crop residue and applications of manure, compost or cover crops.

The soil food web consists of shredders such as wood lice, earthworms, fungi, protozoa, beneficial nematodes, actinomycetes and bacteria. In a no-till environment these will develop an equilibrium with one another.

There are an incredibly large number of species of bacteria each having a different function in the soil, different species at different soil depths, for example.

When we till the soil, we disturb this equilibrium and also cause carbon to leave the soil as CO2.

Plants know what is good for them

Plants have a better idea of what benefits them than we do. They use up to 30% of the energy derived from sunlight to form complex sugars, which are exuded through the roots to feed soil organisms around their roots.

Different species formulate different mixtures to increase the volume of species of organisms that will benefit them. This area is referred to as the rhizosphere.

We became aware that plants use energy to produce nectar to attract pollinating insects, but it was only centuries later that we found out about plants feeding beneficial organisms around their roots.

There are species of fungus called mycorrhizae that attach themselves to the plant roots and then make a mass of hyphae far exceeding the root system of the plant.

In exchange for sugars and lipids from the plant, the mycorrhizae supply the plant with water and, importantly, phosphorus. It is able to extract phosphorus much more effectively than the plant is able to do.

This fungus is not able to survive in cultivated soil, but is very useful in untilled soil. It will also not have a symbiosis with Brassica species or Chenopodium. It is of particular value where soils are low in phosphorus. The hyphae contain glomalin, which makes a very stable form of humus.

Bill Kerr is a vegetable specialist and breeder.