The most common reaction between pesticides and humates is adsorption, followed by a release to the soil solution at a rate dependent on the chemical structure of the pesticide. Degradation of the pesticide will be determined in part by the rate of release. Humic substances may be used in this case to control the concentration of pesticide in the soil solution, and to avoid toxicity hazards. A third case involves the mobility of pesticides by humic material. Some groups of compounds can form complexes with humates, which can then be absorbed by plant roots.
Humic acids can have a direct positive effect on plant growth in a number of ways.
They have been shown to stimulate seed germination of several varieties of crops. Both plant root and top growth have been stimulated by humates, but the effect is usually more prominent in the roots. A proliferation in root growth, resulting in an increased efficiency of the root system, is a likely cause of higher plant yields seen in response to humic acid treatment. Humic matter has been shown to increase the uptake of nitrogen by plants, and to increase soil nitrogen utilization efficiency. It can also enhance the uptake of potassium, calcium, magnesium and phosphorus.