This article appeared in the June 2002 web issue of Horticulture Update,
edited by Dr. Douglas F. Welsh, and produced by Extension Horticulture,
Texas Cooperative Extension, The Texas A&M University System, College Station, Texas.

Humate and Humic Acid

By Dr. Wayne R. Kussow, Department of Soil Science,
University of Wisconsin - Madison

umerous products being sold for turf use as growth enhancers or growth stimulants contain humate or humic acid. Given the number of inquiries I’ve had about these products, the time seems right to asess their value in turfgrass culture. To begin, we need to understand something about humate and humic acid.

Humic acid can be extracted from any material containing well-decomposed organic matter - soil, coal, composts, etc. Extraction is by way of treatment of these materials with a solution of sodium hydroxide. This dissolves much of the organic matter present. If we then take this solution and add enough acid to drop its pH to about 2, organic material will begin to flocculate and can be separated from the liquid portion. The flocculated material is humic acid. What remains in solution is fulvic acid.

If we take the flocculated humic acid and dry it down to form a black mass that can be crushed and sized by dry sieving, we have humate. In other words, humate is humic acid in its solid state. Therefore, the chemical properties of humate and humic acid are basically the same.

Humic acid defies precise description except in very general terms. Black or very dark brown high molecular weight organic polymer is as good a description as any. The color of the materials is effectively used as a sales or advertising attribute. Black organic matter conjures up the image of dark fertile soils covered with lush plant growth.

Chemically, humic acid contains more carbon and less hydrogen than does the animal and plant residues from which it has formed through extensive biological decomposition. It also contains about 4 percent nitrogen. But don’t expect this N to be of any consequence as far as turfgrass growth is concerned. Because humic acid is one of the end products of the biological decay of organic matter, it has great resistance to further decomposition. Estimates of its microbial decay rate are often in the range of 0.3 percent per year under ideal laboratory conditions.

Two properties of humic acid that may have some benefit in turfgrass culture are its cation exchange capacity to form chelates with the metallic micronutrients, iron, copper, zinc and manganese. The cation exchange capacity (CEC) of commercially produced humic acid is in the range of 500 to 600 milliequivalents (me) per 100 grams. This is about five times greater than the CEC of good quality peat moss and twice as high as the CEC of soil humus.

To gain some perspective on the possibility of effectively making use of the high CEC of humic acid, we can examine the recommendations of one manufacturer that call for the addition of 2 lbs per cubic yard of 80:20 sand-peat rootzone mix or substitution of 3 lbs humate for the peat moss. By my calculations, assuming the pH of the rootzone mix and sand are near 7.0, 2 lbs of humate would contribute about 0.37 me CEC/100 g of the 80:20 mix. This would be in addition to the approximately 2.9 me of CEC provided by the peat moss. That turns out to be a rather expensive 13 percent increase in the CEC of the rootzone mix. When you substituted for the peat moss, you wind up with a rootzone mix with a CEC of about 0.55 me/100 g. Consider the fact the potassium leaches readily from sand-peat mixes with five times more CEC than in the sand-humate combination, this doesn’t seem like a wise substitution.

The chelating action of humic acid is sometimes used to produce chelated iron products. Without the addition of a nutrient such as iron, the claim is often made that humic acid has the ability to solubilize micronutrients already in the soil. This is a valid claim,but one has to realize that turfgras roots themselves excrete organic compounds that solubilize micronutrients.

Now let’s go to the research reports on the effects of humic acid additions on turfgrass. I have but one in my files. A search of the 17,000-plus entries in the Turfgrass Information Center revealed no reports where “humate” was a key word, four reports with “humic acid” asa key word, and three reports with “growth stimulant” as a key word. Only two of the seven literature citations were of relevance to this article. Both were studies that demonstrate how strongly humic acid can absorb fungicides and herbicides. Indications are that surface applications of humic acid or humate can significantly reduce the effectiveness of systemic pesticides by reducing their absorption by plant roots and soil-borne pathogens and insects.

The single research report in my files is fora study in which 14 “non-nutritional growth enhancers” were applied to a creeping bentgrass putting green. Several humic acid and humate products were among those tested. The focus of the study was the effects of the products on rooting and root development. Data averaged over all rooting depths for the entire growing season revealed that non of the products significantly affected bentgrass root length or root numbers.

Because so little research seems to have been done with humic acid products on turfgrass, there exists the possibility that there are situations where significant positive responses can occur. My assessment is that we should not expect positive effects over a wide range of conditions. Other than the possible reductions in the effectiveness of pesticide applications when the humate or humic acid resides on the soil surface, the products are rather harmless when applied at rates recommended by manufacturers. There is, however, not justification at this time for using them on more than a small scale, trial basis. Humic acid will not compensate for poor turfgrass cultural practices.