Physiological Effects of Liquid Applications of a Seaweed Extract and a Humic Acid on Creeping Bentgrass

A variety of organic materials such as humic substances, seaweed extracts (SWE), organic matter, and amino acids are being used as fertilizer supplements in commercial turfgrass management. Among them, SWE and humic acid (HA) are widely used in various biostimulant product formulations. These compounds have been reported to contain phytohormones and osmoprotectants such as cytokinins, auxins, polyamines, and betaines. Manufacturer claims are that these products may supplement standard fertility programs by reducing mineral nutrient requirements while improving stress tolerance. There is a lack of season-long, fi eld-based evidence to support these claims. This study was conducted to investigate the infl uence of monthly fi eld applications of SWE, HA, and high and low seasonal fertilization regimes on the physiological health of fairway-height creeping bentgrass (Agrostis stolonifera L.). Plots were treated monthly with SWE at 16 mg·m -2 and HA (70% a.i.) at 38 mg·m -2 alone, or in combination, and were grown under low (20 kg·ha -1 /month) or high nitrogen (50 kg·ha -1 /month) fertilization regimes during 1996 and 1997. Endogenous anti- oxidant superoxide dismutase (SOD) activity, photochemical activity (PA), and turf quality were measured in July of each year. Superoxide dismutase activity was increased by 46% to 181%, accompanied by a PA increase of 9% to 18%, and improved visual quality of bentgrass in both years. There was no signifi cant fertilization × supplement interaction. Although not part of our original objectives, it was noted that signifi cantly less dollar spot (Sclerotinia homoeocarpa F.T. Bennett) disease incidence occurred in supplement-treated bentgrass. Our results indicate that increased SOD activity in July due to SWE and/or HA applications improved overall physiological health, irrespective of fertiliza- tion regime. This suggests that these compounds may be benefi cial supplements for reducing standard fertilizer and fungicide inputs, while maintaining adequate creeping bentgrass health. mic acid (HA) are two novel materials that have shown promise for protecting turfgrasses against oxidative stress. Much early work by Schmidt and collaborators focused on refi ning appropri- ate dilution rates of seaweed extracts and HA based on turfgrass responses such as leaf growth rate and senescence (Goatley and Schmidt, 1990), nutrient uptake (Yan, 1993), root mass (Nabati et al., 1994), and photochemical activity (Zhang, 1997). These studies have shown that certain rates of generic and commercial formulations of SWE and/or HA can improve turfgrass quality and resistance to environmental stresses such as drought (Zhang, 1997) and salinity (Nabati, et al., 1994). Further research indicated that improved stress resistance was associated with increases in anti- oxidant contents and activities (Zhang and Schmidt, 1997; Zhang and Schmidt, 1999, 2000b). All of these studies were conducted under uniform fertilization. Because the chemical components of SWE and HA have yet to be completely characterized and tested for specifi c activity, we do not currently understand the physiological or biochemical bases of SWE and/or HAʼs effects on antioxidants and other stress responses. However, information from other researchers has provided some clues as to constituents and possible modes of action. Fike et al. (2001) reported that SWE derived from A. no- dosum contains various compounds including amino acids and micronutrients; they also reported hormonal activity equivalent to 50 mg·L-1 kinetin. Additionally, auxin and cytokinins have been identifi ed and quantifi ed in SWE using GC-MS techniques (Crouch and Van Staden, 1993; Sanderson and Jameson, 1986; Sanderson et al., 1987; Senn, 1987). Lastly, three betaine forms have been quantifi ed in A. nodosum extracts (Blunden et al., 1986). Quarternary ammonium compounds such as the betaines are thought to play a pivotal role in plant cytoplasmic adjustment