Effects of humic substances on plant growth

Studies of the effects of humic substances on plant growth, under conditions of adequate mineral nutrition, consistently show positive effects on plant biomass. Stimulation of root growth is generally more apparent than stimulation of shoot growth. Both increases in root length and stimulation of the development of secondary roots have been observed for humic substances in nutrient solutions. The typical response curve shows increasing growth with increasing humic substance concentration in nutrient solutions, followed by a decrease in growth at very high concentrations. Shoots generally show similar trends in growth response to humic substances but the magnitude of the growth response is less. Foliar sprays can also enhance both root and shoot growth. The stimulatory effects of humic substances has been correlated with enhanced uptake of macronutrients. Humic substances can complex transition metal cations, which can sometimes result in enhanced uptake and sometimes result in competition with the roots resulting in decreased uptake. A small fraction of lower molecular weight components in humic substances can be taken up by plants. These components seem to increase cell membrane permeability and may have hormone-like activity. In soils, addition of composts can stimulate growth beyond that provided by mineral nutrients presumably because of the effects of humic substances. Addition of Fe-enriched organic materials can alleviate high-lime chlorosis. Soil additions of prepared humic substances is not economical, but the response to foliar sprays has the potential to be economical because of the relatively small quantities needed.