Humic acid, a weak acid and macromolecular organic amphoteric substance, plays a crucial role in soil improvement. The hydrogen ions released by the acidic functional groups of humic acid can undergo a neutralization reaction with alkaline substances in the soil, producing water and reducing soil alkalinity. Functional groups such as aldehyde and carboxyl groups in humic acid can, on one hand, form humate salts, creating a buffer system for the mutual conversion between humic acid and humate salts. On the other hand, they can combine with various cations in the soil, increasing the cation exchange capacity. The base exchange capacity of humic acid is 2 – 3 mol/kg, 10 – 20 times that of soil clay minerals. This enables salt ions (Na+, Cl-, HCO3-, CO32-, Mg2+) in the soil solution to exchange with humic acid, reducing the base content in the soil.
The primary reason for the generally acidic nature of soil is the strong leaching of base ions, leading to the accumulation of a large amount of absorbable hydrogen and aluminum ions, especially the increase in absorbable aluminum ions, which intensifies the process of water ionization to produce hydrogen ions. Therefore, aluminum ions are the main source of hydrogen ions in the soil. In strongly acidic soils, the activity of microorganisms decreases, and the available phosphorus in the soil is easily fixed and inactivated by free iron and aluminum ions, making it difficult for crops to absorb and utilize. Humic acid can form relatively stable complexes with free aluminum and iron ions in the soil, inhibiting the generation of hydrogen ions, thus alleviating soil acidity. It also reduces the toxicity of aluminum and iron to crops and the fixation of phosphorus. A study shows that 120 days after transplanting cherry tomato plants, the pH of the potting soil treated with 0.20‰ – 0.30‰ alkaline humic acid nutrient solution is greater than 5.60, while that of the soil treated with compound fertilizer is less than 5.00. Evidently, alkaline humic acid nutrient solution can effectively improve acidic soil.
Saline-alkali soils are divided into two main categories: saline soils and alkaline soils. Saline soils mainly contain a large amount of soluble salts in the soil, mainly sodium chloride and sodium sulfate. Alkaline soils mainly contain a large amount of sodium bicarbonate, sodium carbonate, and exchangeable sodium, resulting in an excessively high soil alkalinity, with a pH value generally around 9.00. Saline-alkali soils have a high degree of soil particle dispersion, poor soil structure, poor air permeability, and poor tillage properties, making them severely degraded soils that require improvement. The basic formation law of saline-alkali soils is “salts come with water, water dissipates with gas, and salts remain when gas dissipates.” Applying humic acid-containing fertilizers to such soils can enhance the soil’s cation exchange capacity and promote the formation of soil aggregate structure, thereby preventing the upward movement of salts, slowing down the accumulation of surface soil salts, reducing the surface soil salt content, and playing a “salt-barrier” role. Acidic groups such as carboxyl groups on humic acid molecules can neutralize soil alkalinity to a certain extent and accelerate the removal of sodium ions. Humic acid has a strong water-holding capacity for soil moisture, improving the soil’s water retention ability. To some extent, it reduces the upward movement of soil moisture, thereby decreasing the process of salt accumulation due to surface evaporation. In addition, humic acid has strong physiological activity, promoting root development, carbon and nitrogen metabolism, water metabolism, and the activity of various enzymes in crops, thus enhancing the crops’ resistance to saline-alkali stress.