It’s hard to believe just how many benefits compost provides for soil. Even when spread on the surface as a mulch, and long before it actually enters the soil, compost helps to suppress weeds, retains soil moisture in dry conditions, and protects soil from the damaging effects of wind and heavy rain.
But it’s when organic matter gets into the soil that it really starts to work its magic. Soil structure depends almost entirely on organic matter. In healthy soil, the organic humus and inert mineral particles are stuck together in tiny crumbs a fraction of an inch across. These crumbs are held together by fine fungal strands, or hyphae, and by organic glues produced by trillions of bacteria.
The crumb structure of a healthy soil is an important factor in maintaining the soil’s water supply, because tiny pores between the crumbs form a kind of “sponge” that is normally full of water. The soil is also full of larger channels, which fill with water after rain but are normally full of air.
This air supply is vital, since plant roots and soil animals need to breathe. Most of these channels are created by earthworms, which eat you guessed it organic matter. The more compost you make and use on the soil, the better will be its crumb and pore structure, and the less need there will be to water your plants in dry weather.
Compost is also a valuable source of mineral nutrients, such as nitrogen (N) and phosphorus (P), that plants need for growth. As organic matter breaks down, the minerals are slowly released and made available to plants. This breakdown happens faster in warmer weather, which of course is when the plants need them most. Some kinds of organic matter break down and release their nutrients relatively quickly, while others go on to form humus, which provides a much longer-lived bank of nutrients in the soil.
Mycorrhizas, specialist symbiotic fungi that live on and in plant roots, also break down organic matter and transfer the nutrients directly to plants, helping to stop them from being washed out of the soil and lost.