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United States Department of Agriculture

Agricultural Research Service

Title: Plant Growth Stimulation by Humic Substances

Authors
item Clapp, Charles
item Chen, Y - HEBREW UNIV. OF JERUSALEM
item Cline, V - TORO COMPANY
item Palazzo, A - USACOE
item Dowdy, Robert

Submitted to: International Humic Substances Society Conference
Publication Type: Proceedings
Publication Acceptance Date: October 23, 2000
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. Results of experiments at St. Paul, Minnesota have indicated that the humic substances extracted from natural sources exhibit positive effects on turfgrass and crop growth. Dry weights of both plant shoots and roots increased in a 'growth pouch' laboratory screening method. The results of these experiments have been shared with turfgrass scientists, horticultural specialists, and agricultural extension agents. 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: Studies of the effects of humic substances (HS) on plant growth, under conditions of adequate mineral nutrition, consistently show stimulation of plant growth. Both increase in root length and development of secondary roots have been observed for HS in nutrient solutions. Typical response curves showed enhanced growth with increasing HS concentration, followed by ya decrease in growth at high concentrations. Some researchers attributed the stimulative effects of HS to higher uptake of nutrients. Others, however, suggested that hormone activity of HS promotes plant growth. In soils, addition of HS was found to stimulate growth beyond that provided by mineral nutrients, presumably because of the effects of humic (HA) and fulvic (FA) acids. Results are discussed, in light of supporting literature data, showing that plant growth enhancement results from increased nutrient availability. Iron and possibly zinc, due to chelation by HS are probably responsible rather than due to plant hormones. Plant growth experiments were carried out on turfgrasses (creeping bentgrass and ryegrass) and agricultural crops (corn, soybean, and melon). The addition of HAs or FAs alone did not result in growth enhancement or a remedy of the Fe deficiency, suggesting that no plant growth hormones were present. However, addition of Fe, and either HAs or FAs resulted in healthy, chlorophyll-rich plants and enhanced growth, thereby proving that improved Fe availability is a major mechanism of plant growth stimulation by HS. Novel screening systems for HS effects on plants grown in nutrient solutions were used. The feasibility and impact of HS utilization in agriculture and horticulture are presented.

Last Modified: 9/20/2014
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