Humic acids are big molecules, ranging in molecular size from 10,000 to 100,000.

The exact composition of humic molecules varies by source. Humic acids are not soluble in water in acid conditions, but are soluble in alkaline solutions. Humic acids work best in the soil, where they readily unlock nutrients and bind to them, making them available for uptake into plants. Humic acids are chelate toxic metals, preventing harmful material from entering the plant. Humic acids stimulate microbial activity in soil, increase water retention, and stimulate root and shoot growth. Humic acids also act as a dilator, increasing the cell wall permeability of plants so that nutrients can be absorbed. Humic acids can be thought of as the roads, bridges, and tunnels that allow for essential exchanges between the plant and soil.

Source: earthgreen.com