A complex black substance that is formed when organic matter decomposes is called humus which contains humic acid.
What are Humic and Fulvic acids?
Components of humus, these organic acids (Humic and Fulvic Acids) are naturally present in soil organic matter. They are extremely important in increasing and maintaining soil fertility.
What do these acids do?
Possessing an overall negative charge, they can attract positively charged nutrients such as K+ (Potassium). Humus can be thought of as a bank which holds nutrients and can release them in response to plant or microorganism needs. Their molecules can attract and bind with trace minerals in the soil that would otherwise be unavailable to the plant.
As plant roots grow through the soil in search of nutrients, they feed on the humus. Each plant root is surrounded by a halo of hydrogen ions that is a by-product of respiration by roots. These hydrogen ions also carry a positive electric charge. The roots actually 'bargain' with the humus, exchanging some of its positively charged hydrogen ions for positively charged nutrient ions stuck onto the surface of the humus.
Humus attracts water molecules as well to then improve moisture holding capacity of garden soils. Humus is a Carbon source, being organic matter, so this becomes a source of energy for microbes in the soil. The end result is a more active, healthy soil.