Adding organisms randomly and hoping that they will increase the soil’s organic content is like leaving livestock on bare ground and expecting them to thrive.
I once witnessed an entire district of vegetable farmers conned into spending millions of rand on compost tea before they realised this fact. One farmer had his huge potato trial independently evaluated and discovered that he had suffered greater losses from rotting in the section where tea had been applied.
In any case, applying a few kilograms of bacteria will have a negligible effect as there are already tons of organisms in the soil. You might even end up causing harm by introducing pathogens.
This is not to say that you should never add microbes to the soil. In some situations, you can protect your crop from a specific threat by introducing a specially cultured species.
An example is the bacterium Bacillus chitenosporus, which feeds on the chitin that encases eelworm eggs. Farmers near factories that shell prawns often work the shells into the soil to build up the population of this organism.
In the same way, you could add Trichoderma fungi to the soil to protect the roots of plants against pathogenic organisms.
Put simply, it makes sense to add a specific organism to the soil for a specific function, but simply introducing a variety of random organisms is pointless. With the right food source and environment, soil organisms will proliferate anyway.
Plants bind sunlight energy to produce nutrients, which are then exuded from the roots to feed specific beneficial soil microbes.
About 30% of the energy that a plant generates through photosynthesis is used to produce ‘microbe food’ containing amino acids, organic acids, sugars, protein and other nutrients.
Each plant species has a ‘recipe’ for the organisms it requires to grow. For this reason, farmers often reason that a cover crop and a green manure crop should be made up of a mixture of species to create a greater diversity of microbe species in order to benefit the following crop.
It appears that in meadows a greater diversity is of benefit, but in other trials just one species or perhaps two are more beneficial. After all, the plant will feed those species that are beneficial for it, so what value is an unnecessary diversity of species?
What is important for green manure and cover crops is the correct carbon:nitrogen (C:N) ratio of the material. If a mixture is used, it should include enough legume so that the average C:N ratio is 30:1 or narrower. If it is not, less humus will be formed.
Legumes decompose more rapidly, an advantage for most vegetable farmers who require a quick turnabout.
For no-till growers planting into a mulch, a fair percentage of the mulch should be fibrous; this breaks down slowly, providing long-lasting surface soil protection.
Bill Kerr is a vegetable specialist and a breeder of a range of vegetables.