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Stimulation of Plant Growth by Humic Substances¹

ABSTRACT

Humic substances prepared by different techniques of extraction and from different sources of organic materials were tested for their effects on growth of corn seedlings and algae. Stimulating effects were confirmed with optimum concentrations about 5 ppm C as Na-humate for corn and 60 ppm for algae. With corn, the increase was 30 to 50% in nutrient solution or low organic matter soil; with algae, about 100%. Variation of effects among humic acids derived from different organic materials was not great.

The concentrations of elements in corn seedlings did not show any correlation with yield or humic acid level except for P and Fe. Phosphorus concentration was increased with increasing levels of humic acid regardless of the yield response. Higher Fe concentration in the plant tops and lower in roots was observed in the treatments with humic acid.

The application of humic acid to a soil low in organic matter or to nutrient solution gave the greatest growth response. Application to a high organic matter soil gave little growth response, or even a slightly negative response, indicating that the natural soil, without extraction, supplied optimum amount of humic substances to the plants. It is suggested that a test be developed to predict whether a given soil can furnish an optimum level of humic substances.

Different humic substances showed different degrees of stimulation of algal growth, with the most stimulation from a mixture of humic and fulvic acids. Humic substances in paddy soils may affect the growth of beneficial algae and of the rice (Oryza sativa L.) itself. As pollutants, humic substances may stimulate undesirable algal growth in bodies of water.

¹ Vermont Agr. Exp. Stn. J. Article no. 353. Presented before Div. S-3, Soil Science Society of America, Aug. 24–30 1975, Knoxville, Tenn.

² Graduate fellow and Professor, respectively, Dep. of Plant and Soil Science, Univ. of Vermont, Burlington, VT 05401.

Received for publication April 22, 1976. Accepted for publication August 9, 1976.