Submitted to: Soil Science and Plant Nutrition (SSPN)
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/15/2004
Publication Date: 11/15/2004
Citation: Chen, Y., Clapp, C.E., Magen, H. 2004. Mechanisms of plant growth stimulation by humic substances: Role of organo-iron complexes. Soil Science and Plant Nutrition (SSPN). 50:1089-1095. Interpretive Summary: Natural humic substances (humic and fulvic acids), as components of soil organic matter, have played a vital role in soil fertility and environmental quality. The multiple parts played by these materials can greatly benefit plant growth. Examples are their contributions in plant growth enhancement, increasing fertilizer efficiency, or reducing soil compaction. Two types of plants - a dicot (melon) and a monocot (ryegrass) - were tested for growth stimulation with regard for iron availability. Dry weights of both plant roots and shoots increased over control treatments using a `growth pouch' laboratory screening method. Addition of iron with humic and fulvic acids resulted in healthy, chlorophyll rich plants and enhanced growth, proving that improved iron nutrition is a major mechanism of growth stimulation by humic substances. The impact will be to provide a simpler and less costly method for supplying trace metals like iron to agricultural and horticultural crops, while decreasing the risk of pollution by over-application of nutrients.
Technical Abstract: Stimulatory effects of humic substances (HS) on plant growth have been observed and widely documented. Studies have often shown positive effects on seed germination, root initiation and total plant biomass. The consistency of these observations has been uncertain, predominantly due to the lack of understanding of the plant growth promotion mechanism. Often these effects have been attributed to plant growth hormones and the term ¿hormone-like activity¿ has been used to describe the plant growth stimulation. Yet, investigators have been unable to prove that plant growth regulators are present in HS preparations. An alternative hypothesis suggesting that growth enhancement of plants grown in nutrient solution containing HS is the result of improved micronutrient availability, Fe in particular, has been postulated and tested in the present study. Nutrient solutions containing N, P, K, Ca, Mg, S, B, Mo, Cu, Mn, Zn, and Fe at concentrations considered to be optimal for plant growth were tested for solubility of the Fe, Zn and Mn 7 days after preparation. In addition to control solutions of pH 5, 6, 7 and 7.5, 0 to 200 mg L-1 of leonardite humic acid (HA) were added to the solutions and they were tested for Fe, Zn and Mn solubility. The HA greatly enhanced the maintenance in solution of Fe and Zn, especially at pH 7 and 7.5 and did not affect Mn solubility. Plant growth experiments were performed on both dicotyledonous plants (melons) and monocotytedonous plants (ryegrass), due to the major difference in their Fe uptake mechanism. Plants grown in the absence of Fe exhibited severe Fe deficiency that could only partially be corrected with the addition of mineral Fe salts. The addition of HA or fulvic acid (FA) did not result in growth enhancement nor did it provide a remedy for the Fe deficiency, suggesting that no plant growth hormones were present in these preparations. However, the addition of Fe, Zn and either EDTA, HA or FA resulted in healthy, chlorophyll rich plants and enhanced growth, thereby proving that improved Fe, and possibly Zn nutrition is a major mechanism of plant growth stimulation by HS.