By Adam Thoms, PhD and Nick Christians, PhD
Topdressing is the process of applying a thin layer of sand, soil, or some other finely granulated material such as compost to the surface of an established turf area. While topdressing has generally been practiced on golf course greens to provide the best putting surface, it can be a useful practice on sports fields as well.
Topdressing can be an effective way of improving the turf surface, but it is very difficult to do properly. In fact, it is done wrong far more often than it is done right. The biggest problem with topdressing is forming layers in the soil that act as barriers to grass rooting. Once these layers are formed, they can be nearly impossible to remedy, short of reconstruction.
The proper application of topdressing has many positive benefits. It can reduce an existing thatch layer or prevent thatch from building up. Thatch is broken down by microbial activity. For microbes to work on thatch, there has to be enough moisture and oxygen in the layer. Topdressing opens the layer up to moisture and air movement and contributes microbes to the process. It is one of the most effective ways of controlling thatch. Additions of a very light topdressing of sand during the season can help manage the slick layer that forms on the playing field surface. This can be especially useful in overseeded bermudagrass athletic fields, where a slick layer can develop near the soil surface.
Topdressing is a great way to smooth the surface for sports turf managers who want a more uniform playing surface. Sometimes the soil on which the original turf area was constructed is just not suitable. Clay soils make a terrible media for managing sports fields. The addition of a coarser particle’s over the more finely textured particle can help to improve water infiltration and playability during periods of high moisture. It is possible to build up a new rootzone of sand with topdressing, but it has to be done right. Finally, topdressing can be used to cover seed or stolons, or to fill in spaces between sod rolls. It can be a very effective way of growing-in a new turf area.
There are some basic rules to follow when developing a topdressing program. Generally, choose a topdressing that matches the underlying media. This assumes, of course, that the underlying media is suitable for growing grass. If the field is sand-based, or sand-capped and the drainage is good, try to match the sandy media of the field. Never seal off the field with a layer of fine material, particularly a clay-based soil or a smaller, particle-size sand. Layering of fine particles over a coarser material can result in a “perched” water table and water will not drain from the fine material into the coarser layer unless the layer of fines is saturated. This will prevent rooting into the coarser layer.
Some fields with a history of poor topdressing practices can develop a series of layers of fine and coarser material one on top of another. Solving this problem may require reconstruction. If the field is soil based and drainage is good, it would not be wrong to use a soil of similar texture to that in the field.
If the media in the exiting field is unsuitable and the plan is to build up a new sand layer on top of it, be sure to have physical tests performed on the proposed media before the process begins. Also, make sure the sand is not rounded and has good gradation of sizes. First, it is important to have suitable surface drainage. If there is a problem with the grade and standing water in low areas occurs, this process will not work. Some sands work and others do not. Make sure that you start with the right sand and that the sand will be available for many years to come.
Here is the most important rule of topdressing: once you begin, you can never stop. One of the most common errors in topdressing is to start and stop and then start again. This will result in layers that form a barrier to the roots and will cause more harm than good.
Once it has been decided that topdressing is going to be used and the proper material has been chosen, it is critical that the right amount of topdressing is applied per application and that the interval between applications is proper. Applying too much topdressing at a time can result in the burying of an existing thatch layer, which results in a layer of organic matter that acts as a barrier to root growth later. Applying too little at intervals that are excessively long will result in a series of thin layers that again form barriers to rooting.
There are no set rules for how much and how often. It depends on the conditions in the field and is based on the experience of the field manager and observations made of the profile during the process. There is no substitute for an experienced sports field manager who can stay with the process over a number of years. If there is a thick thatch layer, light applications of 1/8 inch or less every few weeks would be appropriate until the thatch layer has broken down. Combining this with core aeration is also a good idea. Be careful not to bury the thatch layer, it has to be broken down slowly to avoid layers. If the field is new and has a sand base, applications of one eight to one quarter inches of topdressing per application, a few times per season should be sufficient. Monitor the sand build up by cutting profiles in the field on a regular basis to observe the build-up of sand.
If there is not a thick thatch layer and the plan is to establish a new rooting media over time, heavier applications can be made more often. Usually the goal will be to build up at least 4 inches of new rooting media on top of the existing soil. Applications of 1/4 inch per application, four times or more per season can be used, but again monitor the process to prevent layering.
Those who have inherited fields with layering due to improper construction or poor topdressing practices will wonder what to do about the problem. Sometimes the problem is so bad that reconstruction is the only solution. More minor problems can usually be improved by core aeration and topdressing. Choose the new material properly, fill aerification holes with the new topdressing and begin to build up a new layer that is connected to the topdressing holes.
Sealing these sand filled holes with a finer material is a bad mistake. It is important to build up a uniform layer of topdressing with the sand-filled holes extending to the new topdressing layer. Again, once the process has begun, you can never quit. A common problem occurs when one sports field manager begins a good, well thought-out program and then leaves for another job. If the new manager begins a totally different program or stops topdressing, the results will not be good. This of course happens all the time and we usually see the results after it is too late.
Topdressing after installing new sod is also a great practice to follow. It will help the seams of the sod grow together quicker, and prevent damage on the sod edges from being exposed to the elements. Ideally the topdressing material, sod rootzone, and existing rootzone will all match, or try to get these to be as close as possible.
Recycling sand study
In one of the studies at Iowa State University, we are looking at is how much sand can we reuse by recycling aerification cores without lowering the performance capabilities of sand-based rootzones. The Wiedenmann Core Recycler has demonstrated the ability to separate the organic matter from a hollow tine core from the sand particles in that core. This means the sand that is in the core that would typically be swept up if the entire core is removed is now returned to the field, and the organic matter can still be removed from the rootzone.
Typically, around 40% of the sand from the core is returned, which can be a real savings to the topdressing budget. Some organic matter is also returned with the sand particles, but early results have demonstrated it is such a small amount that it hasn’t impacted water infiltration or changed the rootzone organic matter to any degree.
Many athletic field managers will use solid tine aerification with a light topdressing several times during the playing season to help maintain the playing surface. Some athletic field managers will perform solid tine aerification weekly during the season, or every time the field has a couple days of downtime. The key is balancing being aggressive enough with solid tine aerification, while not being over aggressive and ruining the stability of the field. Also keep in mind not to open the rootzone up to the elements right before a major change in temperatures, as you will do more harm than good.
Incorporating topdressing sand into the rootzone is typically done only with solid- or hollow-tine aerification; however there are a couple of other machines that have demonstrated some success for athletic field managers. One is the DryJect, which uses a blast of water to create an aeration hole, which is immediately filled with sand supplied from a hopper on the machine. DryJect has a Maximus nozzle to allow for a larger aeration hole than traditionally used on golf greens, which will allow for more sand incorporation. Keep in mind this machine uses a blast of water to incorporate the sand, so the field may have higher soil moisture levels for a day or two after treatment.
Another device that can be used is the Imants ShockWave, which will cut slits into the field, going perpendicular directions will create channels for the sand to fall into when topdressing. This device creates very little surface damage and could be used during the season to help with topdressing incorporation. We are currently conducting studies to investigating how many times during the growing season this device can be used without sacrificing performance of the field. The study is also investigating how much sand topdressing can be incorporated.
Some athletic field managers will use colored sand for topdressing, especially during the season to help it blend in better with the playing surface. The most important key with using a colored sand is that the particle size will still match what is in the rootzone. Many colored sands are made up of smaller particle sizes than traditional topdressing sand, and this causes the field to close off for drainage with time and repeated use of the smaller sand. Also, keep in mind that the colored sand is much more expensive than traditional sand, making one ask if the color is worth the cost?
Topdressing can also dilute out the paint layer left from a logo or yard line markings on a field. These layers can build up very quickly over time in the rootzone, especially in a stadium setting where the field can’t be shifted to move the painting around from one year to the next. Regular topdressing can help dilute these layers out and allow proper water infiltration to continue.
Proper topdressing rates and timing can improve the performance of any athletic field as long a few simple practices are kept in mind. First, once you start never stop. Second, try to topdress with a product that will improve your drainage and not jeopardize stability.
Finally, know your rates and don’t apply too much sand topdressing to bury the turfgrass and cause harm on the playing surface, this is especially true during the season. When done correctly topdressing can improve the performance of an athletic field for use in all weather conditions.
Adam Thoms, PhD, is assistant professor, lecturer, and extension specialist for turfgrass at Iowa State University; Nick Christians, PhD, is University professor of turfgrass management, in Ames.