Evaluating Effects of Humic and Fulvic Acid Products

Evaluating Effects of Humic and Fulvic Acid Products for Improving Bentgrass Putting Green Quality and Root Growth during Summer Stress

By Xunzhong Zhang, Ph.D. and Mike Goatley, Ph.D., School of Plant and Environmental Sciences, Virginia Tech, Blacksburg, VA

Introduction and Objective

Humic acid (HA ) is the fraction of humic substances that is not soluble in water under acidic conditions (pH less than 2), but is soluble at higher pH values. Fulvic acid (FA) is the fraction of humic substances that is soluble in water under all pH conditions. Humic acids have higher molecular weight than fulvic acis. HA and FA may enhance root growth due to auxin-like activity and improved mineral nutrient availability (Schmidt and Zhang, 1999). In our recent growth chamber study, we found that both fulvic acid (FA) and humic acid (HA ) improved creeping bentgrass quality under heat and mild drought stress. O’Dennell (1973) noted that HA at 0.05% to 0.1% could mimic auxin to promote root growth under non-stress conditions. Zhang and Schmidt (1999) found that HA at 11.6 g/1000 ft 2 (=0.33%) improved drought tolerance in Kentucky bluegrass. Similarly Zhang and Ervin (2004) found that HA at 11.1 g/1000 ft 2 (= 0.32%) enhanced drought tolerance of creeping bentgrass. In a putting green field trial in 2017, we observed that HA at 1.78 oz/1000 ft 2 (~0.2%) provided a superior drought response to the lower rate (0.78 oz/1000 ft 2 , ~0.1%). We also found that FA at 1.2 oz/1000 ft 2 did not show consistent beneficial effects as it did in the growth chamber trial. We speculate that the optimum FA rate may be higher in field situations relative to that under the controlled environment. The objective of this research was to evaluate two rates of HA and FA under field conditions and examine effects of FA and HA at both rates on turf quality and root growth of a creeping bentgrass putting green during summer stress.

Research procedures

The study was carried out on the bentgrass putting green at the Virginia Tech Turfgrass Research Center, Blacksburg, VA. There were 6 treatments with 4 replications arranged in a randomized complete block design with 6 x 6 ft plots. Regular mowing and irrigation practices for a putting green were performed. The HA and FA products were provided by Harrell’s.

The humic acid product (Diamond Grow-Humi [K] liquid, Humic Growth Solution, FL) contains 12% humic acid based on analysis by A&L Western Ag Lab. The fulvic acid product [Diamond Grow – Ful-Grow Gold liquid concentrate, Humic Growth Solution, FL] contains 9% fulvic acid. Based on our mung bean bioassay research and previous reports from the literature, a 1% solution of each product was applied as the low rate at 3.5 L per 1000 ft 2 . In this way, the application solution of HA product contained 0.12% HA (0.89 fl oz/1000 ft 2 ). We used 2% solution of HA product as the high rate, which contained 0.24% HA . We used two rates of FA (1.2 fl oz/1000 ft 2 and 2.4 oz/1000 ft 2 ). The two products contained 0.1% N, and total N application rate for each treatment was equalized by calculating N input from each product.

The 5 treatments are as follows:

1. Fertilized control (0.15 lb (68.1g) N per 1000 sq ft every 14 days from 28-8-18 complete fertilizer).

2. Fulvic acid at 1.2 oz per 1000 sq ft every 14 days, with 68.0644 g of N from 28-8-18 to equalize N inputs.

3. Fulvic acid at 2.4 oz per 1000 sq ft every 14 days, with 68.0288 g of N from 28-8-18 to equalize N inputs.

4. Humic acid at 0.89 oz per 1000 sq ft every 14 days, with 68.074 g of N from 28-8-18 to equalize N inputs.

5. Humic acid at 1.78 oz per 1000 sq ft every 14 days, with 68.048 g of N from 28-8-18 to equalize N input.

The trial lasted for 10 weeks from June 19 through August 27 with a total of five applications. The following measurements took place at day 0, 14, 28, 42, 56 and 70 after initial treatments:

Leaf color was rated on a visual scale of 1 – 9 with nine indicating the dark green color, and one indicating brown color. Photochemical efficiency (PE ) was measured based on the Fvm, which is the ratio of variable chlorophyll fluorescence (Fv) to maximum chlorophyll fluorescence (Fm) (Zhang et al., 2017). Leaf samples were collected and chlorophyll content was determined using spectrophotometer method (Zhang et al., 2005).

At the end of the trial, three root samples (1-inch diameter and 6-inch deep cores) were collected from each plot, cores were washed, and root dry weight was determined after the samples were dried at 70° C for 72 h.

In late August, irrigation was stopped to induce mild drought stress and then resumed after drought symptoms appeared. The data were collected on August 27. The root length, root diameter, root surface area, and length density were analyzed using WinRhizo technology. Briefly, after fine cleaning of each root sample, the samples from each plot were divided into multiple subsamples. Each subsample was scanned using WinRhizo and all root morphological parameters were generated and analyzed. In addition, root viability was analyzed using the TTC method (Zhang et al., 2017).

Results and Discussion

FA and HA at the two rates improved leaf color from rating dates from July 16 through August 27, but there were no differences in response between the two rates of HA or FA (Table 1). High rate of FA and HA improved PE as measured on July 30 through August 27 except for FA at August 13 (Table 2). FA at high rate improved chlorophyll content on July 30 through August 27 (Table 2). FA at both rates and HA at high rate increased root biomass (Table 3). FA at low rate and HA at high rate increased root length and surface area. HA at high rate increased root volume. FA at high rate and HA at both rates increased root viability as measured at end of trial (August 27). In summary, we found FA and HA improved turf quality, photochemical efficiency, and root characteristics and viability of creeping bentgrass when applied biweekly. FA at low rate (1.2 oz per 1000 ft 2 ) and HA at high rate (1.78 oz/1000 ft 2 ) had better effects on root growth. These data continue to support previous Virginia Tech research that documents the benefits of supplemental organic acid applications prior to and during environmental stress periods on creeping bentgrass.

Acknowledgements

We thank Mr. Jonathan Dickerson for the help and Harrell’s for the financial support of this study.

References O’Donnell, R.W. 1973. The auxin-like effects of humic preparations from leonardite. Soil Science 116:106-112.

Schmidt, R.E. and X. Zhang. 1998. How humic substances help turfgrass grow. Golf Course Management. 66(7):65-67.

Zhang, X. and E. H. Ervin. 2004. Cytokinin-containing seaweed and humic acid extracts associated with creeping bentgrass leaf cytokinins and drought resistance. Crop Sci. 44:1737-1745.

Zhang, X., E.H. Ervin, W. Wu, N. Sharma, and A. Hamill. 2017. Auxin and trinexapac-ethyl impact on root viability and hormone metabolism of creeping bentgrass under water deficit. Crop Sci. 57:s130-137.

Zhang, X. and R.E. Schmidt. 1999. Antioxidant response to hormonecontaining seaweed extract in Kentucky bluegrass subjected to drought. Crop Sci. 39:545-551.