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United States Department of Agriculture

Agricultural Research Service

Title: Stimulation of Plant Growth by Humic Substances: Effects on Iron Availability

Authors
item Clapp, Charles
item Chen, Y - HEBREW UNIV OF JERUSALEM
item Magen, H - HEBREW UNIV OF JERUSALEM
item Cline, V - TORO COMPANY

Submitted to: International Humic Substances Society Conference
Publication Type: Proceedings
Publication Acceptance Date: September 1, 1999
Publication Date: N/A

Interpretive Summary: Studies on effects of humic substances, the major components of soil organic matter, on plant growth, are important to agriculture and horticulture. A review of earlier work has indicated that these naturally- occurring components of soil systems can provide a complexing and transport mechanism for trace metals such as iron and zinc to stimulate plant growth. .Experiments in Rehovot, Israel and St. Paul, Minnesota have shown that plants such as melons and turfgrasses were increased in leaf green color (chlorophyll) and enhanced in shoot and root weights. Other studies have proven that humic materials can play an important role in micronutrient nutrition of plants under field conditions. The results of these experiments have been shared with turfgrass scientists, horticultural specialists, and golf course managers. The impact will be to provide a simpler and less costly method for supplying required trace metals like iron to agricultural and horticultural crops, while decreasing risk of possible 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 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: 7/31/2014
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