Water has to go somewhere, right? When it rains or when you irrigate, water runs off, soaks in or evaporates, but it doesn't just disappear. What happens to water is an absolutely critical aspect of turf management. When a high degree of control over drainage is required, as with golf and athletic turf, managers often use sand-based rootzones to maximize internal drainage.
Internal drainage can be loosely defined as the process by which water moves down through and out of a turf system. It is different from surface drainage, which is the process where water is “lost” through runoff.
The “USGA rootzone” is one of the most utilized designs for high-use sports turf and golf courses. This design is based largely on the results of research performed by the U. S. Golf Association (USGA). The USGA has been funding research regarding the physical performance of root zones since the early 1950s and has published guidelines for selecting and evaluating root-zone materials.
The current USGA system (see Figure 1, page 17), which was first released in the 1960s, utilizes a 12-inch, sand-based rootzone layer placed over a 4-inch gravel layer. The gravel layer is on top of a drainage system that is placed in the subgrade. These sand-based turf systems are popular because they provide an excellent medium for sports-turf growth, superior water-management capabilities and because they resist compaction even under high-use situations.
The USGA system provides maximum removal of water during heavy rain, but it also stores water above the gravel during periods when the ground is not saturated. The system is based upon a concept known as the perched water table or inverted filter design. It is referred to as an inverted filter because of the presence of fine sand particles over the more coarse gravel. This design uses water's affinity for more finely textured materials to hold (“perch”) it in the root-zone layer. (This effect occurs because of the capillary effect, whereby water is attracted to the surface of soil particles. Finer-textured materials offer greater surface area for attraction. That's why a fine-textured soil overlaying a coarser material like gravel — such as is found with USGA construction — will tend to hold onto the water rather than allowing it to pass through to the coarser material below.)
Large voids of gravel offer little capillary effect. Thus, at the sand/gravel interface, these larger voids effectively create a barrier to downward water movement as long as the soil has not yet reached the point of saturation. As saturation is approached, additional pressure — from gravity — is applied, allowing water to move into the larger voids of the gravel layer and further down through the sub-surface drainage system.
The expected performance of a USGA green depends not only on having the right amount of sand, but the right kind of sand as well. Thus, when selecting sand for a USGA-type rootzone, you must be careful to ensure that the sand consists mostly of medium-sized sand particles (0.15 to 1.0 mm). You also must make sure that the sand you select consists of minimal amounts of silt, clay, very fine sand and gravel.
One drawback of sand is its relatively poor nutrient- and water-retention capacity. Therefore, USGA specifications call for blending peat into the rootzone layer to aid with water and nutrient retention. The type of peat you choose is not the most critical factor. We have seen excellent rootzones built with Canadian sphagnum, Irish sphagnum and reed sedge peats. However, the amount of peat that you blend in to the sand is critical.
It's worthwhile to prepare and test the various blends of sand and peat that you have chosen to determine if you can achieve the proper water retention and drainage rate from the blend. Once you've determined the best blending ratio, routinely monitor the blend for organic matter and particle size to verify consistency.
We should also note that it is possible for you to build and maintain quality rootzones using sand blended with a variety of other amendments. Compost, calcined clays, zeolites and other amendments all have their proponents. As with a sand/peat mix, the key is to choose the proper materials, create a test blend, check the performance and then monitor the materials and the mixing process throughout the construction of the turf system.
Often, it is a good idea to defer choosing drainage gravel until after you select the rootzone materials. This is because you need to achieve proper “bridging and permeability” between the rootzone material and the gravel. Bridging refers to using rootzone material and gravel of the proper sizes so that the rootzone will stay suspended over the gravel. If the rootzone particles are too small in comparison to the size of the gravel, the potential exists for these materials to migrate down into the gravel over time. Permeability also refers to using rootzone material and gravel of the proper relative sizes. However the goal of proper permeability is to ensure a distinct difference in sizes between the gravel and rootzone layers. This difference in size is necessary to create the perched water table effect.
Straight sand rootzones
In the straight sand rootzone design, a layer of sand rests directly on the subsoil with a drainage system in place (see Figure 2, page 17). Courses often use straight sand rootzones when they need to minimize construction costs. The cost of this type of design is typically moderate and falls somewhere between that of the USGA design and designs using native materials. Straight sand rootzone construction does not typically use amendments, and the gravel blanket is usually eliminated. This can result in a considerable cost savings over the USGA design.
Sand rootzones are typically very efficient at removing water and exhibit high infiltration rates. However, take care when choosing sand for this type of construction. Coarse sands can be too droughty, and very fine sands can hold too much water. If the sand does not have the proper particle-size distribution, your design risks instability of the surface caused by shifting particles. Sand particles that are too similar in size have little natural binding, causing heavy traffic by people or equipment to result in foot-printing. (Athletic fields often reinforce rootzones with synthetic materials to reduce instability resulting from this type of situation.)
Although straight sand greens use gravel over the drainpipes, they do not ordinarily possess a gravel blanket. Thus, these rootzones can be drier than the perched water table design. To increase the water-holding capacity of the rootzone, you can reduce its depth. A shallower rootzone is wetter at the surface than a deeper one. While the typical pure-sand putting green has a rootzone depth of 12 inches, there are quality, high-use, sports fields with rootzone depths constructed as shallow as 6 inches.
As with the USGA design, straight sand greens benefit from sand consisting mostly of particles between 0.1 and 1.0 mm in size. Base your sand selection on the particle size, drainage rate and water-holding capacity. During construction, your quality-control testing should include particle-size analyses to verify consistency of the sand you use.
Native material (silts, loams and clays) rootzones
Many soils found in the United States — especially those exposed to high traffic, such as a sports field or a golf green — do not allow adequate internal drainage. Fields that are primarily silt or clay are unlikely to have any significant internal drainage. Thus, subsurface drainage systems are typically useless in these types of rootzones. The key to water management in such situations is to provide an adequate slope that will facilitate surface drainage. Often, surface drainage systems are placed on the outer perimeters of these types of systems to facilitate water removal.
The turfgrass you use, as well as your method of installation, can affect the internal drainage of a turf system. While the grassing method will not have much effect on a clay field, it can have a profound effect on a sand-based turf system. Also, the amount of thatch that a grass produces (and how you manage it — see next section, “Maintenance considerations”) can be one of the biggest factors affecting the internal drainage of a mature turf system.
From the standpoint of internal drainage, seeding and sprigging are almost always preferable to sodding, because seeding and sprigging add little or no unwanted materials (such as silt or clay) to the rootzone. However, due to time constraints or other factors, sodding is often the method of choice. One way to minimize negative effects is to choose a sod that was grown in soil with a texture as close as possible to the rootzone material that you are sodding over, or to use washed sod.
There are several levels of acceptability for sod used on sand-based systems. The most acceptable is sod grown on material identical to that over which the sod will be laid.
The next-most acceptable level is sod grown in soil that is compatible with the root zone. To be compatible, the fine sand, very fine sand, silt and clay contents are generally lower in the sod than in the root-zone material. The gravel and very coarse sand fractions are acceptable.
Sod grown in sand that meets a similar performance standard (such as USGA recommendations or a certain root-zone specification) as the root-zone medium may be acceptable, though not as much so as the previous alternatives.
The last level of acceptability places the emphasis on the total sand content rather than the sand gradation. Although the other levels of acceptability are more desirable, this level is the most widely used — primarily as a balance of cost and risk. If this level of acceptability is chosen, use sod grown in soil with a minimum sand content of 90 percent.
Thatch is the enemy of internal drainage. Surface buildup can totally shut down a high-quality turf system by not allowing water to pass through the surface layer. Therefore, you should put in place an ongoing maintenance program of core aerification and topdressing. This type of program will help to maintain internal drainage as well as increase oxygen in the root zone.
Be sure to topdress with a material that has a similar particle size to the existing root-zone mix. Topdressing with sand finer than the root-zone sand creates a high risk of layering and increased water retention at the surface. On the other end of the size range, coarse sands tend to dull mower blades, especially at greens mowing heights, and are more difficult to work into the turf surface.
Sam Ferro is president and Duane Otto is vice president of Turf Diagnostics and Design (Olathe, Kan.).
Turf Diagnostics and Design is a physical evaluation laboratory and agronomic consulting company serving the golf-course, sports-field and landscape industries. The laboratory is accredited by the American Association for Laboratory Accreditation for the geo-technical evaluation of sand-based turf systems. Turf Diagnostics and Design has performed testing or consulting on more than 1,000 golf courses and hundreds of sports fields. Services include construction-materials evaluation and selection, quality-control testing during construction, turf-system diagnostics, development of turf-management programs and product development for materials suppliers. You can visit their web site at www.turfdiag.com.
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