By Rich Zollinger, Extension Weed Specialist
Question: Humic acid is getting some 'play' in the countryside. I understand it is sourced locally, derived from North Dakota Leonardite shale. We heard from farmers that humic acid has been suggested as a herbicide adjuvant to increase the activity of, and to improve the rainfastness of translocated herbicides.
I understand the trade is suggesting humic acid at 1 pint per to the spray solution. Sometimes it is recommended alone, other times it is recommended with non-ionic surfactant. An on-line query suggests many more uses:
• SOIL APPLIED: Apply 4-8 quarts per acre before or after a major fertilization. Apply 2-4 quarts per acre as needed through the crop cycle.
• AS AN ADDITIVE TO LIQUID FERTILIZER: Add 1-2 percent by volume to the nutrient solution to activate nutrients, provide organic matter and remediate salt problems. This product may be used in fertilizers for injection into drip or low volume irrigation systems. Do not mix with solutions that contain ammonium sulfate or phosphoric acid.
• FOLIAR APPLICATIONS: As a nutrient activator. apply 1-4 pints per acre in sufficient water to assure thorough coverage.
• Do you have ideas how humic acid may 'work' as an additive to fertilizer and herbicides?
Answer: As used as an adjuvant: Humic acid and humates may exhibit certain adjuvant properties (droplet retention, active ingredient deposition, and absorption).
There isn't a large database of information to support adjuvant claims and they may be oversold as to their actual capacities. There appears to be significant variability in the products. There is also agreement that the rate of products sold is often way too low to be of any help.
As used as a soil amendment, here are some comments from Dave Franzen, a soil scientist for NDSU Extension.
"Humic acid has only been known to improve yields in pure sand, and better establishment in very sandy, low OM golf-course greens has been documented. Iowa State University web site has a link for non-conventional additives and amendments. The web search engine allows to search papers on humic acid. Most papers do not show beneficial uses in agriculture. It has a high profit margin and is unregulated material."