Meeting in the middle

Considerable research has been conducted over the years in developing the technology for constructing better golf greens. U.S. Golf Association specifications for sand-based greens have come to be regarded as the standard for putting greens and sports fields.

However, USGA-type rootzones are not trouble-free. One particular concern has been the tendency of their performance to degrade over time as a result of particle migration through and out of the sand-based rootzone. This is a normal phenomenon in sand-based rootzones. But the result eventually is a rootzone that may become coarse and very porous. This has several negative effects, including a serious reduction in the rootzone's ability to retain water and soluble nutrients.

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The University of Tennessee recently experienced this problem with the installation of its new football field based on 1993 USGA specifications. After the turf began requiring several times the expected amount of fertilizer to maintain good quality, it was found that the sand-based rootzone was becoming more porous and coarse as a result of particle migration. In addition, the pea gravel sub-drain layer was being extensively infiltrated with rootzone sand.

Many turf-industry professionals and researchers believe that the installation of an intermediate layer between the sand-based rootzone and the sub-drain coarse aggregate can help resolve many of the problems associated with particle migration. In fact, the 1993 USGA revisions to its green specifications include options for an intermediate sand layer. However, the issue is controversial. Any strategy must solve problems without creating others, such as the loss of the essential saturated wetzone critical to turf growing on a high-sand content USGA-type rootzone.

Unfortunately, long-term studies that could help determine the usefulness of an intermediate layer are lacking. This was part of the impetus for initiating research at the University of Tennessee (UT) to investigate intermediate layers.

Both sand/gravel layering and geotextiles have been considered candidates for intermediate layers. The primary objective of the UT research was to determine the effectiveness, reliability, durability and performance of ten commercially available geotextiles as intermediate-layer drainage separators, in comparison with USGA and USGA-type profiles with and without a coarse sand intermediate layer.

How the study was conducted

The study was conducted with 48 rhizosphere chambers (essentially large rootzone-profile holding containers that allow close monitoring of all material entering and exiting the system) constructed according to USGA 1973 specifications. The rhizospheres were established with Penncross creeping bentgrass and maintained under greens conditions. The treatments consisted of ten different geotextile intermediate layers, one sand intermediate layer and one with no intermediate layer (see Table 1, page 16).

The study began in 1987 and continued through 1994. The researchers measured water-infiltration rates, water-holding capacity, particle migration and other characteristics over the course of the study. The data were used to assess which type of profile was most effective, reliable, durable and showed optimum performance.

What the study found

None of the intermediate layers used in the UT research were able to completely prevent particle migration, especially of the small and very small soil fractions of clay, silt and very fine sand. However, none of the treatments experienced any detrimental effects such as complete plugging of pores in intermediate layers.

  • Infiltration and percolation

    Although particle migration was extensive, progressive rootzone settling over the 7 years of this study helped offset the effects this should have had on infiltration and percolation. Values during the study remained close to the upper range of the USGA recommended rate of 8 to 25 cm per hour. By the end of the study, infiltration and percolation rates for all ten geotextiles were in the range of 22 to 34 cm per hour (see Table 2, page Golf 32). A sand layer and no intermediate layer resulted in infiltration/percolation rates that were excessively high; much higher than the USGA-recommended 25 cm per hour.

  • Water retention

    From our studies and from other published research, we determined 25 to 35 percent by volume to be the optimal range of rootzone water retention (WR). The Terrabond treatments ranged from 32 to 37 percent WR. The Typars ranged from 42 to 54 percent. Pro 5 and Duon, 38 and 39 percent, respectively. Sand averaged 20 percent and no intermediate layer exhibited a mean of 14 percent WR; both too low from an agronomic perspective.

  • Particle migration

    Particle migration was first observed as dirty water leachate coming out of all 48 chamber drains in September 1987. Dirty leachate continued to be observed throughout the 7 years of this study. The primary source of the dirty leachate was windblown dust of which silt comprised about 78 percent. Silt totally migrated out of the rootzones by May 1988 but began building up again by April 1989 and gradually increased thereafter. Clay totally migrated out of the rootzones by March 1988 but began a slight buildup again by April 1990, maintaining a very low level thereafter. The buildup of silt and clay in the rootzones appeared to be influenced by progressive rootzone settling. Dust reentry into the rootzones during the 7 years was an unpreventable phenomenon.

The 7-year evolution of infiltration/percolation rates, water retention, particle migration, rootzone composition alteration and several other phenomena were influenced by each of the twelve study treatments. Detrimental root massing and soil caking occurred on the surface of Typars, Pro 5 and Duon. No root massing or soil caking occurred with the sand or no intermediate layers, or the Terrabonds. The researchers judged the seriousness of these problems sufficient by themselves to eliminate the Typars, Pro 5 and Duon from consideration for use as an intermediate layer.

The trouble with sand

The only significant amount of very coarse sand migration into the pea gravel came from the very coarse sand intermediate layer. The sand intermediate layer (Treatment 1) shrank from 3.8 cm to 2.5 cm in thickness over the 7 years, losing 33 percent of the very coarse sand into the pea gravel and eventually almost that much out of the drain tubes under the rhizospheres. The size differential between the coarse sand and the gravel (6 to 9 mm pea gravel vs. 1 to 2 mm very coarse sand) failed to provide a reliable bridging over the gaps between pea gravel to prevent the 1 to 2 mm intermediate layer sand from readily moving into the pea gravel. This problem alone served to disqualify the 1 to 2 mm diameter of very coarse sand from use as an intermediate layer.

Treatment 2 (no intermediate layer) allowed the largest amount of coarse, medium, fine and very fine sand, and silt, to be lost out of the rootzones into the pea gravel. The sand layer (Treatment 1) allowed the next greatest loss of coarse sand migration into the pea gravel. However, nearly all of the geotextile trts (4 to12) resisted the loss of coarse sand very well.

As rootzones were dismantled at the end of the study, the no intermediate layer treatment revealed whole rootzone soil totally filling all gaps in the top 5 centimeters of the pea gravel and lesser amounts deeper down in the gravel. The instability and degeneration with time of the very coarse sand intermediate layer, and the failure of the 6 to 9 mm pea gravel to perch the whole rootzone soil for the no intermediate layer (Treatment 2), disqualified these two treatments for use in USGA-type greens.

Integrity of the geotextiles

The thinnest Typar and the Duon developed a large number of tiny (1 to 4 mm) holes scattered across the fabrics. The holes occurred directly over the points of contact of rounded pea gravel. The source of the holes was theorized to be caused by vibrations transmitted from surface activity down through the rootzones. The potential for these two fabrics to develop this problem served to disqualify them for use as intermediate layers. Pro 5 was a basket weave-type of unfused ribbon-pattern fabric in which the ribbon weave could easily separate (and did upon stretch testing in the laboratory). This problem also served to disqualify Pro 5 for use as an intermediate layer material.

The intermediate layers exhibiting the best performance, greatest durability and that were found at the end of the study to be in excellent condition with no functional alterations were Treatments 8, 9, and 10 (Terrabond). These stayed where they were installed with no problems of degenerating or migrating, and they maintained optimal infiltration/percolation and water-retention characteristics. Further, they permitted no sediment buildup on their surface and allowed a steady release of sediment very small particles through them with no problem of plug-up. They also maintained an optimum and essential saturated wetzone.

A fixed geotextile clearly can function as a reliable intermediate layer, as exhibited by most of the Terrabond geotextiles. These sustained good infiltration and percolation rates, and an optimum essential water reservoir or saturated wetzone.

Editor's Note: The UT study considered a variety of geotextile alternatives. The Terrabonds were manufactured as intermediate layer geotextiles, so perhaps it is not surprising that they performed best in this study. However, the other geotextiles used in this study were not necessarily intended for this particular use, and their performance here should not be misconstrued to mean they are inferior products for their intended purposes..

This article is adapted from Experiment Station Bulletin 699, Geotextiles as an Intermediate Layer in USGA and USGA-type greens, by L.M. Callahan, R.S. Freeland, J.M. Parham, A.M. Saxton, R.D. von Bernuth, D.P. Shepard and J.M. Garrison. For a free copy of the complete bulletin (67 pages), write: Dr. Lloyd Callahan
Dept. of Ornamental Horticulture and Landscape Design
259 Ellington Plant Science Building
University of Tennessee Knoxville, TN 37996-4500; Or call (865) 974-7324.

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