Runners and fielders sprint, brake, slide and fall on the infield dirt through all nine innings. All of them rely on it for good footing. Fielders expect batted balls to bounce true on it. Each segment of the infield skin, as it's called, has special problems and particular solutions. Maintaining it demands skilled hands and a geotechnical awareness.
Ballfield Dirt Ingredients
Ordinary soils contain organic matter and are too crumbly for sports. Ballfield dirt is blended of water and three grades of sediment—sand, silt and clay—to make possible the best play. Clay is mineral particles smaller than 2 micrometers, or 0.002 mm; it is plastic when wet and solid when dry. Clay furnishes strength and holds moisture. Sand (0.05 to 2 mm) and silt (0.002 to 0.05 mm) soften clay's hardness and allow moisture in and out.
The base layer of an infield skin is 10 to 15 centimeters thick and consists of 60 to 80 percent sand, 10 to 20 percent clay and the remainder silt. Given the right moisture content, this material delivers
- Traction—shoes don't slip or catch
- Playability—balls bounce true
- Resilience—the ground gives when a player's body strikes it
A top layer of loose conditioning material, a centimeter or so thick, keeps cleats from sticking in the clay and allows players to fall safely and slide under control. It also shades the underlying soil and improves drainage in case of rain. Conditioner is made by calcining clay, roasting it at about 600 to 800°C to drive out the water chemically locked in the mineral. The clay expands into a lightweight, hard granular material. Also used is vitrified clay, roasted at a higher temperature and similar to the material in bricks and tiles. Finally there is calcined diatomaceous earth or D.E., which is a nearly pure microscopic silica.
The Pitcher's Mound
The mound and batting areas take a beating from players who dig in with their cleats, so these areas use a stronger mixture with a high clay fraction. Actual unfired bricks, 80 percent clay or more, are commonly used to build up these areas with a thin layer of infield mix on top.
Watering Ballfield Dirt
Daily water is the key to good ballfield dirt. Dirt that is too dry or too wet affects the quality of play and can even lead to injuries. The crew that sprays the infield before the game has already watered it several times that day. They will water it again when the game is over, or first thing the next morning. The ground can never dry out or the infield skin must be rebuilt. Watering has to take into account the region's climate, the weather that day, the presence of clouds or shadows, the wind, and even the style of play the team favors.
Drainage is important for an infield skin, but not the way you might think. The clay content of infield mix does not let water percolate through it quickly; instead the field is built with a slight slope, less than 1°, to direct rainwater off to the side.
Maintaining Ballfield Dirt
Before a game, the grounds crew loosens the upper part of the soil to soften it and prepare it for watering. They also rake and level the infield skin, then add top dressing as needed. They repeat this during the game to maintain consistent playability.
If rain delays affect the game, the crew covers the infield with tarps to keep excess moisture from the skin. Afterward they may need to remove puddles. A fine-grained calcined conditioner works for this purpose. A product made of ground corncobs is also used, but that is raked up before resuming play. The crew may also sometimes need to restore the mound or batting areas with fresh clay.
Groundskeepers test their dirt every season, measuring its grain-size profile. They may have a soil lab do this work, though it's basically a low-tech job involving screens, water and beakers. But observing the soil's behavior under different moisture conditions cannot be outsourced, and good groundskeepers are constantly in touch with the players and coaches as well as the dirt itself.
Let's not forget the umpires. Before every game, they open a bag of regulation baseballs and rub the gloss off them using Major League Baseball's official rubbing mud, a brown, nearly pure silt from a New Jersey streambed. See the photos for my tests on this material.