By Dr. Andrew McNitt
There seems to be some confusion in the industry regarding the proper topdressing practices for sports fields. The confusion stems from taking a management practice designed for a high-sand soil and applying it to native silty soils or vice versa.
Let’s review. If you have a soil that is high in silt and clay, you should be managing the soil to improve soil structure. Structure is the aggregation or binding of soil particles together to form larger particles. These larger particles can then stack against each other, almost like sand, and gaps or air spaces (pores) are formed between the particles.
So when you are managing for increased soil structure, you are trying to create/increase soil particle aggregation. The way you do that is to add glue and stir.
Organic matter is the glue that binds soil particles together. Thus if you are adding high quality organic matter to your soil, you are adding the glue and working to create stronger aggregates that in turn provide air to the root system. In a previous article I discussed how to select a quality compost to use as topdressing just before aeration. That information can be found at http://plantscience.psu.edu/research/centers/turf/extension/factsheets/composts and composts can be tested for quality at www.aasl.psu.edu.
I keep running across high schools that are topdressing with a combination of sand and compost. I don’t understand why. Adding a small amount of sand to a soil high in silt and clay (most soils in Pennsylvania) will not help with soil structure. In fact it can hurt. Sand does not aggregate to an appreciable degree and just takes up space until you add so much sand that you make the soil into a loamy sand.
Let’s look at the textural triangle. If your soil is a silt loam soil, you probably have between 20 and 30 percent sand in there now, maybe less. What happens if you add just a little bit of sand and move the percent sand from say 22 to 25%? If you were trying to amend the top 3 inches that would translate into about 30 tons of straight sand per acre. You’ve now moved your soil texture from Point A in Figure 2 to Point B.
Do you think this helps? The answer is no and it may even hurt, here’s why. Imagine that the rectangle represents a soup can filled with flour. There have been some holes poked in the bottom to allow water to drain. Imagine that the marbles aren’t there yet. Let’s percolate the flour in the soup can. It has some percolation rate, I don’t know what it is but it percolates at some rate. Now let’s add those marbles. After we added the marbles, did the percolation rate go up or go down?
Think about it for a second.
If you said no change, you are, for all practical purposes correct. Likely we wouldn’t be able to measure much change, but if we could, we would find that the percolation rate would go down. The reason is that before water would move through the areas where the marbles were, now the water much move around those areas.
As you add more and more marbles the percolation rate keeps going down until you have enough marbles, so that they are all touching each other, and there isn’t quite enough flour to fill in all the gaps. At that point the percolation rate, and thus macroporosity, increases. The textbooks tell us that this happens at around 60% sand and I believe if you have the perfect sand and only the perfect sand, then this may be true. My experience is that it typically happens around 75% sand. If a 3% increase equals 30 tons per acre, what does a 50% increase mean? It means 500 ton per acre to amend the top 3 inches. And this is the minimum that is likely to be required.
So if you are topdressing a native soil field, use straight compost. Could you cut that compost with a little bit of topsoil? I guess, but why? You have all the sand silt and clay you need, all you need for good quality topsoil is the addition of organic matter. Remember, adding a little bit of sand is sometimes worse than adding no sand at all. Bricks have a lot of sand in them. Typically they are about 60% sand.
Now if you have a native soil field and want to change to managing a sand field or at least a sand cap field that is a different story. Now you want to follow what the golf courses typically do to their push-up greens, that is, build a layer of almost pure sand on the surface. You’re not mixing in the sand; you are layering it on the surface.
Here is a really good guide on one method to accomplish building a ‘push-up’ sports field, or as it is typically referred to: a sand cap system. http://turf.msu.edu/assets/ArticlePDFs/Built-Up-Sand-Capped-System.pdf
Several words of caution: For a sand cap system to work well you’ll need a couple things. 1. A good in-ground irrigation system. 2. A core harvester, because every time you aerate you MUST pick up the cores. If you don’t, you’ll be mixing the sand and soil and you’ll be back to making bricks and sealing off the sand surface. Also, you should plan on topdressing with sand regularly.
Something else to consider: Turf growing on sand cap systems doesn’t wear better under moderate soil moisture conditions; however, the sand cap will help prevent that Friday night mud bowl that can ruin a field. It will help prevent damage from use during wet soil conditions.
If you’ve never managed sand, it is sometimes less forgiving than soil in that there is less room for error managing water, fertilizer, topdressing etc. It takes a higher level of care but can offer a very nearly all-weather playing surface.
Dr. Andrew McNitt is Director of Penn State’s Center for Sports Surface Research (ssrc.psu.edu) and also the Program Coordinator for the 4-year turfgrass science major and the Basic & Advanced Certificate as well as the Associate, Bachelors, and Masters of Professional Studies Programs offered through Penn State’s World Campus Online Learning. Dr. McNitt is currently the technical adviser to the NFL Groundskeepers Organization.