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United States Department of Agriculture

Agricultural Research Service


item Clapp, Charles
item Chen, Yona
item Magen, Hillel

Submitted to: International Humic Substances Society Conference
Publication Type: Abstract Only
Publication Acceptance Date: 3/24/1999
Publication Date: N/A
Citation: N/A

Interpretive Summary:

Technical Abstract: Stimulatory effects of humic substances (HS) on plant growth have been observed and widely documented. Studies have shown positive effects on seed germination, root initiation and total plant biomass. The consistency of these observations has been uncertain, mainly due to the lack of understanding of the plant growth promotion mechanism. Often these effects were attributed to hormones and the term "hormone-like activity" was used to describe the plant stimulation. Yet, investigators were unable to prove that growth regulators are present in HS preparations. An alternative hypothesis suggesting that enhancement of plants grown in nutrient solution containing HS is the result of an improved micronutrient availability, Fe in particular, has been tested. Nutrient solutions containing macro- and micronutrients at concentrations optimal for plant growth were tested for solubility of Fe and Zn. In addition to control solutions at pHs of 5.0 to 7.5, a leonardite humic acid (HA) at 50 mg/L were tested for Fe and Zn solubility. The HA greatly enhanced the maintenance in solution of Fe and Zn, especially at pH 7 and 7.5. Plant growth experiments were performed on both dicotyledonous plants (melons) and monocotyledonous plants (creeping bentgrass), because of the major difference in their Fe uptake mechanism. Plants grown in the absence of Fe exhibited severe deficiency that could only partially be corrected with the addition of mineral Fe salts. The addition of HA did not result in a growth enhancement or a remedy of the Fe deficiency, suggesting that no growth hormones were present. However, the addition of Fe, Zn and HA resulted in healthy, chlorophyll rich plants and enhanced growth, thereby proving that improved Fe is a major mechanism of plant growth stimulation by HS.

Last Modified: 05/27/2017
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