An age-old practice

TOPDRESSING MATERIALS AND METHODS EVOLVE, EXPAND

One of the oldest cultural practices performed on golf courses is the art and science of topdressing. Not only is topdressing one of the oldest practices, it might be one of the most important. Topdressing can be defined as the application of a thin layer of material, such as soil or sand, to the surface of existing turf.

Topdressing has evolved throughout time. Initially a practice performed only on greens, it's now performed on greens, tees, approaches and even fairways. Years ago, one might have received a strange look from someone in the golf course business if he said, "Someday we will be topdressing fairways on a consistent basis." The fact that this practice is being adopted for use over almost the entire golf course speaks volumes about its benefits.

But the key to topdressing can be summed up in one word: sand.

The use of sand as a component has been around for years. It has even been written that Old Tom Morris used sand for topdressing in the late 180Os to smooth and firm greens. However, it wasn't until the early 1970s when sand started to dominate the mixture used to blend a topdressing material. In fact, it also was during this period that straight sand made major inroads as a topdressing material by itself. This immediately created much controversy. The thought of growing turfgrass on straight sand, with no soil or organic matter mixed in, was out of the question to many. Even today, the debate hasn't subsided completely. One fact is certain, though: Straight sand is the dominant material used by most for this practice.

The major scientific benefit of topdressing is the dilution of the upper organic matter fraction of the turfgrass surface. As the grass plant grows, senescence of roots and above-ground plant parts contribute to organic matter build-up. This organic matter build-up is diluted through sand topdressing.

The key word is dilution. At one time, many thought the major benefit to topdressing was the breakdown of organic matter. This was true in the old days of the 1-1-1 mixtures but isn't the case with the present day use of straight sand. Sand is inert basically and has no microbiology that can contribute to the decomposition of organic matter. So with topdressing, sand is used to mix with, or dilute, the organic matter at the surface and to maintain or increase sufficient pore space.

Greens

So how much should superintendents be topdressing their greens? This is possibly one of the most popular questions asked about this practice. In recent years, topdressing practices have become more of a calendar event rather than a scientific event. This shouldn't be the case. However, with the help of research by Robert Carrow, Ph.D., of the University of Georgia, the question can be answered with scientific data.

Carrow conducted extensive research about the organic dynamics in the upper surface of sand-based greens. He found that an organic matter content of greater than 4 percent by weight in the upper 2-inch surface should become a red flag for greens performance. So with this in mind, the goal of less than 4 percent organic matter becomes the all-important target as superintendents plan their aerification and topdressing strategies.

To keep the organic-matter content below the 4-percent threshold, suggestions have been made to apply sand at about 50 cubic feet per 1,000 square feet per year. This said, there are two important factors that should be used in the equation before adopting 50 cubic feet of sand per 1,000 as the all-important amount.

First is the length of the growing season. Some parts of the country have a growing season as short as three or four months, and other parts have a 12-month season. Should courses in both these areas of the country be on a 50-cubic-feet-per-1,000-square-foot rule? Probably not.

Secondly, and maybe the most important part of a topdressing program, is plant growth rate. Remembering the most important fundamental aspect about topdressing, applications should be in direct proportion to plant growth. Plant growth directly influences the amount of thatch development. Therefore, to dilute the organic matter with best results, topdressing volumes and frequencies should be increased as the growth rate increases. The same applies to the plant when growth decreases - topdressing volumes and frequencies should be decreased.

Another factor that should be considered is the amount of nitrogen applied in the fertility program. For example, there are two golf courses with similar conditions: golf course A applied 2 pounds of N/M per year, and golf course B applied 5 pounds of N/M per year. Should both of these courses adhere to the 50-cubic-foot rule? Probably not, because even though the suggested 50-cubic-foot amount might be a good guideline, it's not a steadfast rule.

There also have been suggestions that applying 50 cubic feet of sand per year would dilute the organic matter sufficiently to eliminate aerification. And one researcher wouldn't go as far as saying superintendents don't need to aerify, but says it's something to look at.