Plant Growth Stimulation by Humic Substances and their Complexes with Iron

Keywords: Humic substances, Humic acid, Fulvic acid, Growth stimulation, Micronutrients, EDTA.

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 and Zn 7 days after preparation. In addition to control solutions at pH 6.0 and 7.5, 0 to 200 mg/l of leonardite humic acid (HA) were added to the solutions and they were tested for Fe and Zn solubility. The HA greatly enhanced the maintenance in solution of Fe at pH 6.0 and 7.5, and Zn at pH 7.5. Plant growth experiments were performed on both a dicotyledon (melon) and a monocotyledon (ryegrass), because there are major differences 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) alone 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 providing evidence that improved Fe, and possibly Zn nutrition is a major mechanism of plant growth stimulation by HS.

Y. Chen and H. Magen, Department of Soil and Water Sciences, The Hebrew University of Jerusalem, Rehovot 76100, Israel

C.E. Clapp, USDA-ARS and Department of Soil, Water, and Climate, University of Minnesota, St. Paul, MN 55108, USA

14 pages, 3 tables, 15 refs.