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United States Department of Agriculture

Agricultural Research Service

Title: Plant Growth Stimulation by Humic Substances and Their Complexes with Iron

Authors
item Chen, Yona - HEBREW UNIVERSITY
item Magen, H - HEBREW UNIVERSITY
item Clapp, Charles

Submitted to: Fertilizer Research
Publication Type: Proceedings
Publication Acceptance Date: October 15, 2001
Publication Date: N/A

Interpretive Summary: Natural humic substances, 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. Creeping bentgrass, a turfgrass, was used as a test plant for the evaluation of responses to humic substances. Results of experiments at St. Paul, Minnesota have indicated that the humic substances extracted from Minnesota peats exhibit positive effects on turfgrass growth. Dry weights of both plant shoots and roots increased in a 'growth pouch' laboratory screening method. Similar results were also found through a 'microplate' experiment with commercially- produced humic substances and Minnesota peats. Decreasing plant growth was also observed by some treatments compared with a fertilizer nutrient control. Promoting root growth rather than shoot growth, in some cases, has been characterized as a good sign for stimulation of turfgrasses. This enhancement can help plants to resist unfavorable natural environments such as a drought or infertile soil. Our results can provide some new techniques to assist turfgrass growers and researchers in evaluating or interpreting use of humic substances on golf courses, sports turf and lawns.

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. Often these effects were attributed to plant growth hormones and the term "hormone-like activity" was used to describe the plant growth stimulation. Yet, investigators were unable to prove that plant growth regulators are presen in HS preparations. An alternative hypothesis suggesting that growth enhancement of plants grown in nutrient solution containing HS is the result of an improved micronutrient availability, Fe in particular, has been postulated and tested in the present study. In addition to control solutions of pH 6.0 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 and Zn solubility. The HA greatly enhanced the maintenance in solution of Fe and Zn, especially at pH 6.0 and 7.5. Plant growth experiments were performed on both dicotyledonous plants (melons) and monocotytedonous plants (ryegrass), due to the major difference in their Fe uptake mechanism. The addition of HA or fulvic acid (FA) did not result in a growth enhancement or a remedy of 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.

Last Modified: 12/27/2014