The material used for topdressing turf is usually sand, sandy topsoil, compost or a combination of these. Starting a topdressing program can sometimes be a challenge, since labor, mechanical equipment and an annual budget to buy quality material are necessary to do it right. Following are some reasons why topdressing is an important part of any turf management program:
Surface consistency and safety – Topdressing material is used to maintain a consistent, smooth playing surface so there are no ruts or holes that could adversely affect player safety or the quality of the game. The material is applied and dragged or brushed across the turf to ensure any holes or depressions are filled.
Soil improvement – The second role topdressing plays in turf management is to reduce the bulk density of native soil fields or to maintain particle-size distribution on sand-based fields. If you looked at a soil profile that had been deep-tined and topdressed with sand, you’d see a profile with reduced bulk density and open drainage channels.
Sand increases the amount of air spaces in the soil and reduces the tendency for that soil to become compacted. A study at Ohio State University looked at ways to enhance turf recovery between games if the field surface had been severely damaged by traffic and adverse weather. To prepare the soil, researchers used a traffic simulator to cause significant damage to a wet soil surface. Recovery treatments included sand or calcined clay topdressing and the installation of sand or calcined clay slits/bands. The two treatments that retained the most ground cover during traffic conditions were those that had been both topdressed and slit with sand or calcined clay. Sand slits – channels that are created in the top 6 to 12 inches of soil and then backfilled with sand or calcined clay – can provide a drainage channel from the field surface to underlying drainage pipe.
PHOTO BY PURESTOCK/THINKSTOCK.
If slits have been installed, topdressing must be performed regularly to stop the slits from capping over with native soil, since once they are capped off they won’t work. The combination of sand topdressing and slits creates a scenario whereby water can move rapidly off the field surface and down through the soil profile. If there’s not a budget to install sand slits, regular sand topdressing can also significantly improve surface drainage.
Surface organic matter control – Topdressing sand is applied to turf to dilute thatch, which is made up of plant residues like old crowns, roots, stolons and rhizomes. Grass clippings do not contribute to the thatch layer, since they break down quickly. Some thatch on athletic fields is actually a good thing, as it can provide athletes with a soft cushion if they fall. However, once thatch is more than 0.5 inch thick it can hold water and cause problems with shallow rooting. Thatch is naturally broken down or decomposed by earthworms and other organisms in the soil, but if the rate of buildup is quicker than the rate of decomposition and it becomes excessive, then it’s a problem.
On fields with sand root zones that are devoid of earthworms, thatch must be controlled through core aeration and topdressing. For fields with heavy traffic where ground cover gets worn away, thatch accumulation is not a problem. Baseball outfields, which don’t get much traffic, can be prone to excessive thatch accumulation. Certain grasses, such as Kentucky bluegrass and bermudagrass, are prolific thatch producers, so they will need to be managed.
Topdressing helps to control thatch by diluting it, meaning the topdressing material mixes with the thatch. Organisms in the soil can now decompose the thatch.
Renovation success – The fourth reason for topdressing is in conjunction with renovation. The combination of coring, seeding and topdressing provides much greater benefits than any one of those practices performed alone. The topdressing material covers the seed, helping to retain moisture and preventing seed movement.
Topdressing materials – A golden rule of topdressing is to match the material with the underlying soil. This is particularly important on sand root zones. On native soil root zones, topdressing materials can include a quality sandy topsoil, pure sand, a sand/compost combination, a sand/calcined clay mixture, or any combination of these materials. If one of the goals is to improve drainage, then sand is typically the governing material, as it will improve infiltration rates over time.
On sand root zones, topdressing materials are generally composed of pure, clean sand or a combination of sand and calcined clay. The calcined clay is sometimes added to help retain moisture. Not all sands are the same. There are specific criteria for sands used on native soil or sand root zones. Particle size distribution, uniformity, shape and chemical makeup are all important.
Sands are classified by the size of their particles (Figure 1). Fine gravel contains granular particles greater than 2 millimeters in diameter, while fine sands have much smaller diameters of 0.1 to 0.25 millimeter. As a general rule of thumb, medium coarse, uniform sands are used on native soils, with slightly smaller particles introduced on pure sand root zones. Finer particles of sand are used on sand root zones to ensure surface stability.
Basically, the coarser and more uniform the sand, the less stable, or firm, it is. A common topdressing sand used on native soils would adhere to the United States Golf Association (USGA) recommendation. With 60 percent of the sand particles in the medium-coarse size range, a USGA sand is considered a uniform medium-coarse sand. Finding a good, local sand supplier is the key to a successful topdressing program. It’s also important to get to know the materials and what they look like. Quality control of topdressing products is much easier once you know what a uniform medium-coarse sand looks and feels like.
Composts are becoming increasingly popular as topdressing materials either on their own or added to sand or soil. On native soil fields they can help improve the soil structure and add some nutrient value. For example, a school district in central Ohio has been applying an 80:20 sand and compost topdressing mix for many years. The district reports that it has reduced fertilizer input by about 30 percent.
As with sands, not all composts are alike. When adding compost to turf it must contain a bulking agent, such as wood chips. The bulking agent helps to improve soil drainage and porosity. There are numerous criteria for quality compost: It should contain at least 30 percent organic matter and have a carbon-to-nitrogen ratio less than 30-to-1. Composts are also regulated for things like particle size (0.25 to 0.375 inch diameter), pH (ideally 6.0 to 7.0), salt content and nutrient value. Finding a local source of good compost is as important as selecting a sand supplier.
Once a topdressing program is started, it must be done with due diligence and careful planning. The introduction of soil, compost or calcined clay shouldn’t be done without very careful consideration.
In one case, peat moss was applied to the center of a field in the hope that it would soak up the water, smooth out the surface, and provide a place for grass seed to grow. In reality, it created a muddy mess.
Some field managers have applied recycled crumb rubber to high-traffic areas to protect the crown of the grass plant and prevent loss of ground cover. There is some research to suggest that the crumb rubber does improve wear tolerance in high-traffic areas, but again, introducing any type of foreign material onto the field must be done with consideration as to what effect that material may have on soil and turf performance. The recommended rate is a depth no greater than 0.25 inch.
Applying materials – On grass infields and other small areas topdressing material can be applied by hand. The material should be dry so it can be applied swiftly and uniformly. This method is often used on grass baseball infields to keep soil compaction to a minimum. For larger areas, dry materials can be applied with a rotary spreader. To get the greatest benefit from a topdressing application it’s usually done in conjunction with some type of soil cultivation, like coring or tining. Some field managers like to apply topdressing, and then run the cultivator over the top to disperse the topdressing material down into the holes. A recent innovation in cultivation and topdressing is the recycle dresser. This pulls cores, pulverizes the soil, and then reapplies it as a dressing in one pass.
Rates and frequency – The amount of topdressing applied will depend on the amount of damage to the field surface. The mowing height of the grass, the physical texture of the underlying soil and the budget are also important factors. A typical application rate is 0.125 to 0.375 inch, depending upon those factors.
Over an entire season, anywhere from 25 to 100 tons could be applied to a field. How often the field is topdressed depends on the playing season and the factors listed above. Some field managers have the manpower and budget to lightly topdress every few weeks, some make three to four applications per year, and others make one or two heavier applications outside of the playing season. If soil cultivation and topdressing are carried out in the summer, it’s a good idea to have at least 10 days of recovery time before the field is used.
Pamela Sherratt is a sports turf specialist at Ohio State University and served on the Sports Turf Managers Association’s board of directors from 2010-2011.