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United States Department of Agriculture

Agricultural Research Service


item Clapp, Charles
item Cline, V
item Hayes, Mhb
item Palazzo, A
item Chen, Y

Submitted to: Bouyoucos Conference Proceedings
Publication Type: Abstract Only
Publication Acceptance Date: 6/23/2002
Publication Date: 6/23/2002
Citation: Clapp, C.E., Cline, V.W., Hayes, M., Palazzo, A.J., Chen, Y. 2002. Plant growth promoting activity of humic substances. Bouyoucos Conference Proceedings. p. 37.

Interpretive Summary:

Technical Abstract: Studies on the effects of humic substances (HS) on plants, under conditions of adequate mineral nutrition, consistently show stimulation of plant growth. Enhancement of root growth was usually more apparent than stimulation of shoot 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 a decrease in growth at high concentrations. Shoots generally showed similar trends in growth response to HS. Some researchers attributed the stimulative effects of HS to higher uptake of nutrients. Others, however, suggested that hormone activity of HS promotes plant growth. A small fraction of lower molecular weight components of HS can be taken up by plants. These components are considered to increase cell membrane permeability and to exhibit hormone-like activity. In soils, addition of composts was found to stimulate growth beyond that provided by mineral nutrients, presumably because of the effects of HS. The hypothesis that HS originating from various sources contain plant growth hormones was tested. Experiments were carried out using a 'pouch' method and a 'microsystem' method comparing commercially-produced and laboratory-prepared HS products with fertilizer controls for plant growth parameters. Plants involved included turfgrass, corn and soybean (some genetically-altered), tomato, and melon. The results will be discussed, in light of supporting literature data, showing that plant growth enhancement results from increased nutrient availability, iron and zinc in particular, due to their chelation by HS, rather than due to plant hormones which could not be found in these materials.

Last Modified: 06/22/2017
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