Every living thing eventually becomes humus. Humus is the result of decomposition. Organic matter in soil becomes "humus" when the decomposition process has reached a point where further change slows to a crawl and the matter is relatively stable. Humus contributes to topsoil's ability to support plant growth.
Soil Structure Improvement
Humus improves soil structure by binding small soil particles together into larger aggregates. This makes clay soils looser and improves drainage. In sandy soils the aggregation tightens everything up and improves water retention. The aggregation also provides for better air infiltration and better root growth.
Cation Exchange Improvement
The negative charge of humus attracts the positive charge of certain plant nutrients (potassium, calcium, magnesium and ammonium) and prevents them from leaching. By holding onto the elements, the humus keeps them available to plants, rather than washed away into the watershed.
Slow Release of Nutrients
The slow breakdown of humus releases nitrogen, phosphorus and sulfur to plants over a very long period of time, unlike synthetic sources that cause plants to grow rapidly and need to be replenished frequently.
Humus stabilizes the acid/alkaline balance of soil by buffering the processes that change soil pH. A soil rich in humus will remain relatively neutral, that is, neither too acidic nor to alkaline.
Topsoil erodes when exposed to water runoff and to wind. Soils rich in humus are less vulnerable to erosion than humus-poor soils. The aggregation properties that improve water and air infiltration also make the soil particles heavier than sand, silt or clay particles alone. Those heavier particles resist the erosive effects of wind and water.