Influence of Humic Acid on Moisture Retention in Simulated Usga Putting Greens

Humic substances are routinely applied to creeping bentgrass putting greens during times of stress to improve turf health. Confounding effects of added nutrients and other ingredients make it difficult to separate the effects caused by the organic material from additional effects of the supplemental ingredients. Claims made in advertising and marketing of the effectiveness of humic substance products are often unsubstantiated, and the claim of increasing soil moisture retention has not been studied. Humic substance effects are also difficult to evaluate in the field because of confounding factors and variability, so pure humic acid without added nutrients or commercial additives was evaluated in a research greenhouse using simulated USGA putting greens. Organic acids, including a humic acid, tannic acid and citric acid, were applied at normalized carbon rates of 250 mg C L as watering solutions through an automated irrigation system to creeping bentgrass. None of the organic acids improved moisture retention in sand. Humic acid actually decreased soil water holding capacity of the soil, drying down more quickly and requiring more frequent irrigations than the control. No increase in tissue phosphorus concentration was observed after treatment with humic acid, and no differences in top-growth, biomass production or dry shoot mass, were observed. Root growth and distribution was affected by the humic acid treatment, as turf irrigated with the humic acid produced significantly deeper roots than the control. All organic acids 22 induced hydrophobic tendencies in the sand, and root depth increases may have been influenced by this hydrophobicity. Surface soil hydrophobicity appeared to facilitate the movement of water into the subsurface, and consequently root growth may have followed water distribution. Golf course superintendents looking to reduce water and phosphorus fertilizer applications may not see a benefit using humic substance products.