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| The white circle is sand and the orange colored space represents air space where roots will grow. |
A USGA spec green is simply a method of construction that creates a well drained rootzone that supports turfgrass growth in high maintenance scenarios. The two most important factors when selecting sands, depending on the application, are:
1. Particle Size
2. Shape of the sand particle
Sand is nothing more than a tiny rock- sand that originates from windy or water environments are typically rounded (like a tiny river rock) and angular sands are mined or in some cases, man made where the sand is "cracked" to create angular sand (picture a giant mortar and pestle!). While my education background consists of soil chemistry and soil physics, talking to Mr. Jameson of HCC, is really interesting. Being in the business, he can really open your eyes to the business and logistics of sand; where it originates and how it's created. The State of Georgia has a lot of great sand sources, including the bunker sand we installed last year.
Particle size is what will ultimately affects drainage. The distribution of sizes must be relatively similar. Otherwise, the small pieces of sand will clog up the air pores (where roots grow) of the root zone mix. Ever wonder why tree roots grow in septic lines? It's not water, it's actually air they are after. Roots ONLY grow in air space; so a soil with no air porosity will result in minimal rooting which results in poor turf. This is the case of golf courses that don't aerate greens enough...the organic matter clogs pore space resulting in very few roots resulting in lacking turf! Next, think about concrete. When you buy a 40 or 80lb bag of cement mix, is there really anything magical about it?? Is there an adhesive in that mix that allows it to "set up?" The answer; No. Concrete is a particle size analysis that is blended so that when water is added, the particles settle out and the result is a sealed off surface. There are some soils that behave this way as well. This is precisely why you don't want to over rototill a garden. Doing so would result in destroying the soil structure by creating too many small particles. This is also why you have the physical properties of the sand you select tested by an independent lab to ensure you are getting what you desire. Different particle sizes settle, and a shift to more coarse or more fine sand can be the difference in a green draining or holding water. Many are familiar with 'river sand.' This is an inexpensive sand dredged locally in many areas. The problem with it is, there is no baseline or standards. Every load could be different and that is driven by things like weather. For example, the more rain in a season may influence the size of the sand and sediment that is deposited in the dredging area. I've gotten loads in the past that were beautiful and other loads that drained worse than a solid clay soil. Most times it isn't worth the risk because you can't predict what's coming. It's horrible when used as a fairway topdressing material because it often contains small rocks and pebbles that will wreak havoc on the mowers if they are not manually picked up.
A USGA green also has a gravel blanket below the 12" rootzone mix. Now, do you think this is for drainage? Answer: Not really. When a finer textured soil overlies a courser textured soil (sand on pea gravel, for example) a perched water table is created. This means the sand won't drain into the gravel until it is completely saturated. This is exactly why you should never put gravel in the bottom of a planted pot in your home. One would think this helps the plant by offering a better draining environment, but it actually causes the soil to hold water. A USGA green is exactly that- a manmade perched water table. It allows the sand to stay moist for grass growth but not too wet to play on.
On the picture above, you see another layer of "soil" under the root zone. This 2" layer is called a choker layer, which is used to prevent sand from washing into the drainpipe. All soils have a "bridging coefficient" which is used to determine if a soil will support another soil. If the sand "bridges" the soil below, a choker layer isn't needed. Augusta National greens were constructed with a choker layer. As an employee, we would water them heavily on a weekly basis in the summer. I knew I was done when I could hear the water break through the choker layer. Remember, a fine texture soil will only drain into a courser textured soil when the root zone is completely saturated. The sound is somewhat of a suction. When I heard that, I knew it was time to head to the next green. In my case, I was responsible for #1, 8, 17 and the nursery green.
Sand shape is mostly important with Bunkers. Round sand is not desirable because it's not stable. Think about how your calves hurt when walking through dry beach sand, which is very round. It gives way, causing you to work harder! In bunkers, fried egg lies are the norm with round sand. Round sand also blows around a lot more, making it a maintenance headache.
These diagrams show up close what round and angular sand looks like.
