Verdure: Creeping into microbial activity

University of Maryland researchers evaluated the utility of natural organic fertilizers and amendments for creeping bentgrass establishment and maintenance.


A newly constructed putting green is a bit of a nutritional wasteland. High in sand and without clay, these new soils may benefit from the addition of materials that stimulate microbial activity, which could enhance creeping bentgrass growth. One method for doing this is through the use of granular natural organic fertilizers or amendments, such as humates. Researchers at the University of Maryland (John Kaminski and Cale Bigelow, who are now professors but were students of Pete Dernoeden, Ph.D., at the time) decided to evaluate the utility of natural organic fertilizers and amendments for creeping bentgrass establishment and maintenance in a sand-based root zone.

A newly built sand-based putting green was used for the experiment. The main fertilizer amendment treatments were: (1) a conventional synthetic surface-applied program of a 19N-11P-4K material applied at 1 pound/1,000 square feet (49 kg/hectare); (2) treatment No. 1 plus granular humate (0.9N-0.1P-0.1K) incorporated to 2.4 inches (6 cm) at a rate of 1,785 pounds/acre (2,000 kg/hectare); (3) a natural organic fertilizer (10N-1P-5K), which also contained strains of Bacillus and Trichoderma, surface-applied at 437 pounds/acre (490 kg/hectare); and (4) treatment No. 3, but incorporated to a depth of 2.4 inches (6 cm) at a rate of 875 pounds/acre (980 kg/hectare). All of these materials were applied, the green smoothed, and three bentgrass cultivars seeded (Penn G-2, Providence and Crenshaw) in individual plots. All fertilizer treatments were continued monthly during the establishment year at a rate of 44 pounds/acre (49 kg N/hectare), with supplemental phosphorus and potassium as needed to balance all the plots to uniformity. After establishment, the synthetic fertilizer source was changed to a slow-release methylene urea for the maintenance portion of the test.

Collected data included percent cover as the bentgrass established, visual turfgrass quality, and turfgrass disease assessments. Other data were root length, root mass, thatch depth and soil microbial activity. Microbial activity was measured by removing samples 10 times over two years and assessing total microbial activity in the lab.

For establishment, creeping bentgrass was quickest to cover when the synthetic fertilizer was included in the treatment. Adding the incorporated humate to the synthetic fertilizer did not speed establishment. The slowest establishment was always in the plots in which the organic fertilizer was only surface-applied, with slightly faster establishment when that organic fertilizer was also incorporated into the greens mix. In general, the readily available soluble nitrogen from the synthetic fertilizer was needed for the most rapid establishment, and incorporation of the humate did not speed establishment.

Did the addition of the organic amendments spur increased microbial activity? Yes, it did. For the first 14 months of the study, soil treated with the natural organic fertilizer often had higher microbial activity. After that initial period, however, no differences were observed among treatments, and there were never any observed effects on disease suppression. In general, the positive effects of the natural organic fertilizers or amendments occurred early in the study. For example, the incorporation of the humate into the greens mix significantly increased root length at about 100 days after seeding, but differences were not observed after that. The only longer-term effect from the addition of the organic fertilizer was a slight reduction in thatch depth (1.5 to 3 mm less) when compared with thatch depth from plots fertilized with the synthetic fertilizer. Finally, effects of the fertilizer/amendment on any disease were mixed, and not particularly substantial.

Overall, visual turf quality was best when a plant-available synthetic fertilizer was part of the fertilization program. Nitrogen availability was the most important factor affecting establishment. Addition of the organic granular humate did increase root length and density. It was noted that this work focused on the short-term (two years or less) benefits of organic additions, and examinations of the long-term benefits of organic fertilizers and amendments are needed.

Source: Kaminski, J.E., P.H. Dernoeden and C.A. Bigelow. 2004. Soil amendments and fertilizer source effects on creeping bentgrass establishment, soil microbial activity, thatch, and disease. HortScience 39:620-626.

Beth Guertal, Ph.D., is a professor in the Department of Crop, Soil and Environmental Sciences at Auburn University in Auburn, Ala., editor-in chief for the American Society of Agronomy, and president-elect of the Crop Science Society of America. She is a 20-year member of GCSAA.