Gases and water can move through the pores in soil particles. The exchange of oxygen between the soil and the atmosphere is particularly important, as plant roots and many soil organisms require oxygen to breathe.
The binding of sand, silt, and clay particles forms aggregates. These aggregates are the matrix or skeleton for the chemical and biological processes that are essential for soil function.
Nitty-gritty
The formation of aggregates is largely dependent on soil microorganisms and organic matter.
Organic matter can be thought of as the mortar or glue that holds soil particles together. To be effective, organic matter must be decomposed. This is where soil bacteria come into play.
Every gram of soil contains one billion bacteria. This is about the same as the population of China and India combined or 50 times more than the Australian population.
This number does not include soil fungi. These bacteria and fungi coexist because their needs are different, or they don’t interact.
A bacterium that is on one side of an aggregate will be isolated from another bacterium located on the opposite side. Even if the two bacteria are similar, they do not compete.
It is also possible that they do not compete because they are not both active at the same time. Only a small portion of the bacteria population is active in a particular soil condition. Most bacteria are dormant (inactive) and only come alive when conditions are ideal.
The huge diversity is another reason why there are so many microbes per gram of soil. Around a million species are found in a gram of soil. There are both generalists and specialists. For example, there are those that can fix nitrogen from the air or degrade wood.
Organic matter is the main food source for soil microbes. This is mostly plant matter. As soon as plant matter is introduced into the soil, soil microbes attack it for nutrients and energy.
The soil animals, such as earthworms and mites, are a great help in this attack. They fragment the plant material in the soil, allowing it to be more easily contacted by soil organisms and plant matter.
Microbes produce carbon dioxide, which plants use to absorb nutrients. Microbes that have died and plant components that are difficult to decompose bind soil particles.
The long filaments from fungi hyphae also help in this binding. This method produces stable aggregates, which are important for supplying air and water to soil organisms and plant roots.
When aggregates are unstable, as they are in sodic soils, for example, the water and air supply is restricted, and plants and soil organisms suffer.
Feeding your soil
For soil health and function to be maintained, a certain amount of organic matter must remain in the soil. Organic matter is important for soil health and function. It not only helps to keep aggregates stable, but it also acts as a sponge that holds water. This is especially true in sandy soils.
Organic matter also serves as a source of nutrients for plants. Decomposition of plant material releases nutrients, and can also bind nutrients that can be made available later.
Organic matter decomposes and needs to be replaced on a regular basis. This can be done by adding organic matter to your soil either directly or after composting.
Compost is more stable than litter or straw in soil because the composting process has already decomposed it. Compost is good for binding soil particles together and binds nutrients. However, it does not provide much nutrition for plants.
Fresh, young plant material is a great source of nutrients. However, it decomposes quickly and can initially release more nutrients than the plants are able to absorb.
Mature cereal straw decomposes very slowly and is low in nutrients. For a good supply of nutrients and long-term soil production, you might have to use a mixture of different organic amendments.
You can also bypass the organic loop by using inorganic fertilizers. The plants will grow, but it is important to carefully match the supply of fertilisers with plant demand, as most are quickly dissolved.
