When applied to clay soils, humic acid can help break up compacted soils, allowing for enhanced water penetration and better root zone growth and development. When applied to sandy soils, humic acid adds essential organic material necessary for water retention thus improving root growth and enhancing the sandy soil's ability to retain and not leach out vital plant nutrients.
As mentioned above, one way plant growth is improved is through the structural improvement of both clay and sandy soil allowing for better root growth development.
Plant growth is also improved by the ability of the plant to uptake and receive more nutrients. Humic acid is especially beneficial in freeing up nutrients in the soil so that they are made available to the plant as needed. For instance if an aluminum molecule is binded with a phosphorus one, humic acid detaches them making the phosphorus available for the plant. Humic acid is also especially important because of its ability to chelate micronutrients increasing their bio-availability.