Clay particles, the smallest, average less than 0.002 mm in diameter, and have very good water-holding capacity , but poor aeration . Silt particles range in average diameter from 0.002 to 0.02 mm . The humus or organic matter content of a soil will also increase its water-holding capacity and aeration . Humus is lightweight and spongy in nature, and will swell as it absorbs water, then shrink as it dries. Humus also improves the ability of a soil to provide nutrients to plants.
Organic soils develop from swamps and marshes.
The organic matter of humus is broken down over time by soil microbes, and nutrients such as nitrogen and phosphorus are released. Because humus is constantly being broken down in mineral soils, it must be replaced through such practices as adding compost to a garden soil, or plowing down crop residues . In natural ecosystems, leaf fall and animal droppings replenish the organic matter content of soils. The grass sods of the great plains have high natural fertility because of the extensive grass root systems that developed each year. A thin film of water adheres or adsorbs to the surface of mineral particles , so the water-holding capacity of a soil is proportional to the combined surface area of the particles of which it is composed.