Turf on the left received subsurface cooling and aeration that resulted in increased turf quality, shoot density, rooting depth and root dry matter compared to untreated turf on the right.

Subsurface air injection increased turf quality, density, rooting depth and root dry matter at the end of summer, three years after establishment of the green. A-1 and Crenshaw improved in quality as much as older varieties Pennlinks and Penncross when they all received subsurface air injection. Severe root-zone temperatures were moderated somewhat by the air injection.

Maintaining creeping bentgrass (Agrostis palustris) turf quality during the hot, humid summers of the southern United States is quite challenging (5). Equipment manufacturers and golf course superintendents assert that air movement through the soil profile reduces summer bentgrass decline by cooling the root zone and increasing the oxygen available to roots.

To test this idea, we conducted research on a specially constructed 12,000-square-foot chipping green at Wild Wing Plantation in Conway, S.C. (6). The green was partitioned into quadrants with vertical sheets of plastic so air treatments could be applied independently to each quadrant. A 7.5-hp SubAir blower supplied air to two of the four quadrants from March through September 1998 and from May through September 1999.

Within each quadrant were four 25-by-60-foot plots, each containing a different cultivar of creeping bentgrass: A-1, Crenshaw, Pennlinks or Penncross. We wanted to determine whether the older cultivars, Pennlinks and Penncross, benefited more from subsurface cooling and aeration than the newer heat-tolerant cultivars, A-1 and Crenshaw.

Turf quality, shoot density, rooting depth and root dry matter were measured periodically from September 1997 through November 1999. During the second growing season after establishment (1998), subsurface air injection increased shoot density, but showed no effects on turf quality or rooting (3).

We theorized that the benefits of subsurface aeration might increase as the green matured. Indeed, substantially better turf quality occurred with air injection in 1999 at the close of the third summer. Turf quality of the four cultivars averaged nearly one point higher with subsurface air injection than without, and turf quality of all four cultivars remained near acceptable levels (6.0 or above) with subsurface air injection.

Turf quality was unacceptable without air-injection treatment. Fungicide application during this period was minimal, further stressing the bentgrass. Continued evaluation revealed that differences in turf quality, first evident in late August 1999, remained similar in magnitude through Nov. 15 as the turf struggled to recuperate from summer decline.

The most obvious result of air injection on turf quality was a reduction in dead turf. Shoot density also increased. Similar increases in shoot density occurred in September the year before, but aeration in that year did not result in an overall increase in turf quality. Surprisingly, enhancements in turf quality and shoot density were no greater in the old cultivars than in the new cultivars.

Peak temperatures at the end of August without subsurface cooling and aeration were as high as 93 F at 2 inches below the turf surface, well above what is typically considered optimal for bentgrass rooting and turf quality (about 60 to 70 F) (1,2). Subsurface cooling and aeration treatment decreased peak temperatures 3 to 4 F.

Air injection also improves the gas composition of the root zone. Carbon dioxide, which is toxic to plant roots, accumulates in the root zone when soil temperatures are warm (4). Earlier research on our green demonstrated that subsurface air treatments reduced carbon dioxide to ambient levels minutes after injection began, and levels remained low for hours after injection ended (3).

Rooting depths of A-1, Crenshaw and Penncross were 0.5 to 1 inch deeper with air injection than without. Air injection also dramatically increased root dry matter of A-1, Pennlinks and Penncross 50 to 75 percent, and that of Crenshaw 20 percent. Better rooting probably increased turf quality by providing greater access to water and nutrients.

Summary
Substantial increases in bentgrass quality and rooting occurred with subsurface air injection at the end of summer in the third growing season. Severe root-zone temperatures were moderated somewhat by air injection. Deeper and more extensive rooting contributed to better turf quality and increased shoot density. Improvement with subsurface cooling and aeration was just as great for the heat-tolerant cultivars, A-1 and Crenshaw, as for the old standbys, Pennlinks and Penncross.

Literature cited

1. Beard, J.B., and W.H. Daniel. 1965. Effect of temperature and cutting on the growth of creeping bentgrass (Agrostis palustris Huds.) roots. Agronomy Journal 57:249-250.
2. Beard, J.B., and W.H. Daniel. 1967.Variations in the total, nonprotein, and amide nitrogen fractions of Agrostis palustris Huds. leaves in relation to certain environmental factors. Crop Science 7:111-115.
3. Dodd, R., B. Martin and J. Camberato. 1999. Subsurface cooling and aeration. Golf Course Management 67(9):71-74.
4. Ervin, E.H., and B.S. Corwin. 1999. Carbon dioxide: culprit in bentgrass summer decline? Golf Course Management 67(4):66-70.
5. Nus, J. 1994. Microenvironment manipulation. Golf Course Management 62(1):204-209.
6. Trusty, S., and S. Trusty. 1998. Hot town, cool bentgrass. Golf Course Management 66(4):186-191.

James J. Camberato, Ph.D.; Roy B. Dodd, Ph.D.; and Bruce Martin, Ph.D., are associate professors at Clemson University in South Carolina. Sheila Godwin is a research technician at Clemson.