What’s in the Infield Skin?

When watching a baseball game, we spend a lot of time looking at dirt. It may be in the periphery, its appearance in our consciousness as fleeting as the slight puff of dust surrounding a rogue hop. We notice the dirt when a player slides, creating an unsightly red-brown stain on his pants. We may focus on the actual infield dirt when watching the cathartic, cleansing process of raking the infield skin between innings. But the dirt only really becomes the center of attention when the threat of rain looms, and even then, the grounds crew is tasked with concealing it underneath a tarp.

It’s not as pretty as the cross-cut patterns of the turf, but the infield dirt is the true workhorse of baseball, seeing more playing time than any other piece of equipment, and it’s a deceptively complex material. It’s easy to think of dirt as a fairly uniform, homogeneous mixture, comprised of, well, dirt. But anyone who has taken a look at the dirt in their back yard, local park, or community athletic field will recognize this dirt is probably not what professional baseball players play on.

Indeed, providing dirt for the infield skin of baseball fields has become an industry unto itself. The infield mix used in a major league ballpark can cost anywhere from $80 to $100 per ton. DuraEdge of Slippery Rock, Pennsylvania, provides the infield material for 1,200 to 1,500 infields across the country, ranging in price from $15,000 to $50,000 per infield. At the major league level, DuraEdge provides the infield skin for approximately two-thirds of major league teams. The Philadelphia Phillies were DuraEdge’s first major league customer in 2005, a move that heralded the current increased recognition of, and appreciation for, superior infield clay mixtures. Other vendors include Partac Peat Corporation of Great Meadows, New Jersey; Gail Materials of Corona, California; and Southern Athletic Fields of Columbia, Tennessee.

Some vendors claim their particular geography provides a superior product unparalleled compared to clay sourced from any other location; some vendors just say “clay is clay.” While the ratio of clay, silt, and sand is important, there seems to be a consensus that it’s the amount of sand, and not the source of sand, that is critical. For example, DuraEdge ships its proprietary clay from Western Pennsylvania to its customers and then mixes in local sand obtained closer to the ballpark to reduce shipping costs.

Achieving the proper mixture of components is crucial to achieving a suitable playing surface. There are different considerations for the base paths, the warning track, the batter’s box, and the pitcher’s mound, each of which has its own specific needs. For example, a harder surface is required for the pitcher’s mound. But for most of the infield mixture, the ideal surface will have a “cleat in, cleat out” quality; the dirt will be just soft enough that cleats will leave a clean mark in the dirt that is still visible after the cleat is removed. A dry, sandy soil would backfill the hole, while a composition with too much clay could cause a cleat to stick in the dirt or clumps of dirt to stick to a player’s cleats.

Drainage and moisture content are keys to achieving the ideal mix. We all recognize the perils of a too-wet infield caused by rain, which can lead to a muddy playing surface, but a too-dry surface isn’t desirable, either. Remember, a tarp serves not only to keep the elements out, but to keep moisture in. The mixture needs proper drainage, so as to minimize the potential for standing water on the field, but the mixture must retain some moisture, because a dry field will lead to players kicking up dirt and losing balls in a cloud of dust. Imagine the clouds of dust that would be kicked up by every bounce on a dry field, not to mention a hard, unforgiving dry clay surface. When clay and silt are too dry, they’re hard as a rock and prone to cracking. This will have an effect on the play of the ball – think about how a baseball will hop off a rock-hard surface – and increase the potential for injury.

Sand is loose and drains well, but it’s unstable and unpredictable. Not only will the moisture content affect the way one walks and runs on it, but it will also have a dramatic impact on the way a baseball hops. Clay and silt have poor drainage, leading to pools of water forming on the surface of an infield that is 100 percent clay or silt, which presents challenging and potentially dangerous playing conditions. And it also affects the way a ball would impact on each of those surfaces; a ball will not bounce off an overly sandy infield the way it would bounce off a clay surface.

It’s important to bear in mind that the consistency and quality of the infield skin isn’t just a matter of playability, but of safety concerns. There is the potential for injury not just because of something as simple as dust in one’s eye; a baseball that bounces off the dirt unpredictably could catch a player off guard and hit him. If a cleat catches in the dirt at a bad angle, it could lead to a harmful twist or rotation of the foot or body. Even the way a player’s foot lands on the dirt could pose a risk; dirt that is too hard and compact could be detrimental to joints, while the level of exertion used running through a sandy, too-soft infield could pose an issue as well.

Think of how you feel when you’re running on cement, a packed trail, or a sandy beach; playing in inconsistent conditions presents an increased risk for injury for a baseball player. In a sense, we’re asking the infield skin to provide traction, preventing shoes from slipping or catching on a rogue clump, while also being resilient and forgiving when a player’s body makes an impact or slides across it.

Taking into consideration a major league team’s budget and facilities and the access to water, irrigation, and drainage in the base beneath the infield skin, the ideal major league infield skin mixture is primarily sand, which makes up 50-70 percent of the total, with the rest comprising a mixture of silt and clay. The silt-to-clay ratio is between 0.5 and 1.0, ideally just below 1.0. An infield with too much sand can be dry and irregular, and an infield with too much clay will be too hard and clumpy.

In terms of particle size, sand is the largest, at 0.05 mm – 2.00 mm, followed by silt at 0.002 mm to 0.05 mm, and clay, which has particles less than 0.002 mm in size. The large grain size of sand allows for water to pass through, facilitating drainage in the rain. The smaller particles of clay reside in between the much larger grains of sand, retaining water to maintain consistency in the playing field.

Groundskeepers apply soil conditioners, like calcined clay or vitrified clay, to the infield skin or to the very top layer of the infield to fight compaction and improve drainage. These top dressings on the infield skin also can prevent slick and muddy conditions and can be a quick fix, as the infield dirt sees quite a bit of wear throughout the course of the game. The infield soil can become quite compact during play, and inconsistently so depending on the traffic during the game, thus necessitating a great deal of care at several points during the middle of a game.

Although sand is the majority component of the infield skin, clay draws the attention of vendors, both as a component of the infield skin and as an additive. Clay can be defined by grain size and its mineral components. According to the International Association for the Study of Clays (AIPEA), the term “clay” refers to a naturally occurring material made up of fine-grained minerals, which has plastic properties when wet and hardening properties when dried or fired. There’s no one particulate size, and different scientists use different cutoffs for clay grain size, with sedimentologists accepting a particle size up to 4 μm, soil scientists up to 2 μm, and colloid chemists up to 1 μm.

Not all clays are the same, as clay is the product of the weathering of rocks, which can vary greatly by geography. During the weathering process, feldspar in rocks undergoes hydrolysis to form clays, comprising various minerals. The different types of clays are classified by their lattice structure and the minerals comprising each. The clays of relevance to baseball include kaolin, bentonite/smectites, montmorillite, and illite. Kaolin and montmorillite comprise silicates and aluminates, and clays can also include other compounds comprising magnesium, nickel, potassium, and iron. Silt is a close cousin of clay, formed from quartz and feldspar; it’s commonly observed in nature as a sediment suspension or at the bottom of a body of water. It’s known that clays sourced from different regions will comprise slightly different components; however, it’s not clear at this time which, if any, is best for an infield mix.

Even when we talk about clay in a baseball context, it can encompass many things. We can think of clay in terms of its mechanical properties, such as its plasticity and the way objects (such as a baseball) interact with a clay surface. And there’s no one clay used in baseball; there’s the clay used in the infield, but what about the calcined and vitrified clays used as a top dressing or soil condition for the infield clay? And what about the clays used in drying products?

Calcined clay is obtained through a calcination process. The term “calcination” refers to the process of heating calcium carbonate at extreme temperatures to form calcium oxide. This same process can be applied to clay, although there’s no calcium involved. Rather, the same extreme heating technique is used to drive off water to yield anhydrous clay, where mineral platelets are fused together.

One type of calcined clay is made using montmorillonite clay, which is heated to 1300 F. Calcined clay can absorb quite a bit of moisture. Used as a soil conditioner, it can be used to hold moisture and slowly release it back into the soil. Crushed, powdered calcined clay can be used for absorbing excess water on the infield after rain. Baseball coach Danny Litwhiler and chemical engineer Jack Moore were the inventive minds behind Diamond Dust and Diamond Grit, calcined clay mixtures that absorb water from baseballs and infields. Before the advent of these calcined clay drying products, groundskeepers would use sawdust or gasoline to dry their infields, an environmentally unfriendly and hazardous process.

Another clay that is commonly used in infield top-dressing is vitrified clay. Like calcination, vitrification is also an extreme heating process; the term “vitreous” refers to a glasslike quality, like one might see in fusing silicates. The vitrification process fuses the minerals of the clay together, making the resulting vitrified clay impervious to water. One such vitrified product is heated to 2200 F. Vitrified clay will not absorb moisture the way calcined clay does. Vitrified clays will dry more quickly, because they can’t hold much water themselves, and moisture will drain right through vitrified clay, down to the soil below.

One reason there are so many vendors providing infield clays is the free reign groundskeepers are given in making decisions regarding what’s best for their ballpark. Dimensions and things like mound height are regulated by MLB, but the actual content of the infield dirt is not. Groundskeepers have flexibility in selecting an infield dirt vendor suitable for their own needs, and there is the potential for variability between major league fields. Perhaps park factors also should consider the infield dirt and not just park dimensions, as infield dirt may change from year to year, depending on the organization’s vendor contracts. However, as the infield dirt also varies based upon the day-to-day weather conditions, the effect on everyday play is most likely minimal.

It is worth considering how the infield play affects players as they progress through the minor leagues on their way to the major leagues. The Pittsburgh Pirates reportedly use the same DuraEdge infield material used at PNC Park at their Hi-A through Triple-A affiliates (Bradenton, Altoona, and Indianapolis). It’s worth considering whether that consistency across an organization is important, as a player otherwise would not only have to adjust to a higher caliber of play after a promotion but to changes in the field of play itself. It isn’t clear how much consistency an organization would be able to maintain, particularly when we consider issues of climate and geography. You can truck in DuraEdge for your infield skin no matter where you’re playing, but the weather in Minnesota is not going to be the same as the weather in Florida.

As with any natural resource, we should be mindful of its provenance and not expect an endless supply will be readily available to us. Grant McKnight, president and founder of DuraEdge, claims his clay quarry has 15-20 years of clay remaining; after that, “we know where to look” for more. But like many natural substances, a synthetic substitute may be possible, especially given the lack of formal MLB regulations. Although the compositions and exact sources of these various clays are trade secrets, analytical chemistry could identify the chemical composition of the various clays used in the infield. Using elemental analysis, scanning electron microscopy, and X-ray diffraction analysis, soil scientists could quantify the precise quantities of minerals found in desirable clays and the ideal particle size of grains of sand and silt and create a new infield skin.

Perhaps, as water restriction and conservation becomes more commonplace, synthetic infield clays of the future will be further removed from their natural form. Not everything can be exchanged for a synthetic substitute, though, as evidenced by synthetic turfs. But vendors are already exploring synthetic additives to enhance the infield mix and soil conditioners, such as polymers to improve water retention and release. Just as some athletic fields employ a mixture of natural grass and synthetic turfs, it’s not unreasonable to envision a scenario in which playing surfaces will comprise both natural clays and synthetic materials. We will evolve as necessary to accommodate our surroundings and environment, just as we adapt to changes in baseball overall.

A clean infield is undoubtedly a beautiful sight, and the practice of raking the infield between innings may appeal to the same aesthetic sense one enjoys upon observing a zen garden. But we can appreciate the art of a pristine infield while also appreciating its underlying complexity. Having a better understanding of the science of dirt will pave the path to improvements that can enhance the play of the game while also being mindful of future environmental concerns.

References and Resources

• Lesley Bannatyne, “The dirt on baseball,” Christian Science Monitor, October 18, 2005
• Liz Bloom, “Why Western Pennsylvania dirt is used in the infields of most MLB stadiums,” Pittsburgh Post-Gazette, August 31, 2017
• James Hagerty, “For Opening Day, Baseball Is Racing to Perfect…the Infield Dirt,” Wall Street Journal, March 26, 2018
• Lou Pavlovich, Jr., “Baseball’s Great Inventor Of All Time,” Collegiate Baseball, January 6, 2012
• “Employing conditioners a must for infields without water access,” SportsTurf, February 2013
• Relative size of sand, silt, and clay particles