To the fans in the stands and, to a certain extent, the athletes they’re watching, natural grass fields are just that — natural grass fields. But, as they say, it’s what’s inside that counts, and fields are no exception.
In this case, what’s underneath the grass — the type of soil with which the field has been built — is a means to classify the field.
The two types of fields generally discussed are native soil and sand-based:
A native soil field may be a true native field, which uses only the soil found at the site, a modified native soil field, which includes the introduction of amendments such as sand, peat, compost or porous ceramics to provide a better growing medium and/or a more stable base, or a sand cap field, in which the top 2 to 6 inches (typically 2 to 4 inches) of soil is replaced with sand, either during construction or over time.
A sand-based system is a field in which the native soil has been completely removed and replaced with an under drain system, made up of a drainage media layer (principally stone) and root zone material (principally sand) to improve drainage.
If you’re in the beginning stages of field design, and trying to decide upon a system, here are some considerations:
Know your soil type
This may sound overly simplistic, but it’s an inescapable fact: Not all soils will support an athletic field that sees constant, regular use. A soils engineer or a licensed design professional that specializes in outdoor sports facilities can do soil tests at the site and provide recommendations.
Native soil field facts
Despite advances in the construction of natural grass fields, many existing fields and new fields being constructed today are native soil fields. There are multiple advantages to building a native soil field, and not the least of these is the price tag, as it’s by far less expensive to build a native soil field. As such, natural grass fields using native soil may be the only option for some organizations.
Native soil fields can provide a quality playing surface, particularly in climates with moderate rainfall and soils containing a reasonable balance of sand, silt and clay. But remember that soil and weather conditions will vary (often greatly) from the ideal, necessitating more work for the field owner or manager when it comes to maintenance. More maintenance also can mean more investment, so budgeting for such a field may be a fluid process the first year or two. And, while there are many native soil fields that remain playable with less than optimal care, the risk of injury will be greater.
In addition to being less expensive to build, many native soil fields will have high nutrient- holding capacity and offer good water retention, minimizing the need for fertilization and irrigation, as compared with a sand-based field.
However, the most significant problems are also natural processes: compaction and drainage. As the soil becomes packed more tightly, it becomes harder. Most native soils, as a rule, absorb water quite slowly and cannot handle large amounts; therefore, without additional provision for drainage, these fields easily become muddy, worn and unplayable. Compaction makes this problem worse.
Since only a small amount of water can be handled effectively by the soil, the field must be designed to shed water as well. Typically, athletic fields are sloped (meaning tilted in a single plane) or crowned (sloped in more than one direction from a single point or line, generally midfield) in order to move water off the surface. It is recommended a perimeter collector drain be included to move the water away from the field; otherwise, those areas where the slope terminates – often the bench area or the area beyond the goals – are likely to become muddy, worn and/or unusable.
In building the field, a natural inclination might be to create more of a slope in order to move water off the surface and into the perimeter drains more quickly. But this is not the answer. Excessive sloping or crowning may impact performance, particularly in sports involving ball roll such as baseball, soccer or field hockey. Even more important, with a sloped or crowned field, games still may occasionally need to be postponed or canceled, or field use may need to be curtailed because of muddy conditions.
Modified native soil field facts
A modified native soil field is quite a bit like a native soil field except that the native topsoil has materials added to it during construction. Those amendments might include sand, peat, compost, manure, porous ceramics or other materials.
Some of the advantages of a modified native soil field include:
- Water infiltration rate;
- Root development;
- Water storage and movement within the soil;
- Nutrient availability and holding capacity;
- Resistance to compaction;
- Oxygenation and gas exchange.
Remember, even if the soil is perfect at the time of construction, conditions will change over time. The soil will need to be tested on a regular basis and additional amendments may need to be added to keep the soil in condition for the growth of healthy grass. It’s also necessary to check for soil compaction, which, as mentioned previously, can be a problem in any location and on any field.
The use of sand in sports fields
The presence of sand in soil helps it to resist compaction. And ever since the USGA began tinkering with soil formulas in the early 1900s by adding sand to keep golf greens from hardening, the rest of the industry has followed suit. It was the USGA who first recommended a golf green be constructed with a sand-based root zone.
In the sports field industry, this facility is known as a sand-based field. The specific design of the root zone mix for a sports field will depend on several factors including intended use, environmental and site conditions, the availability of materials, budget and owner/manager preferences.
For those who plan to be eco-friendly, sand-based fields are an outstanding option. In addition to their superior drainage, these fields are best able to tolerate the use of reclaimed water for irrigation. The use of non-potable, recycled water may lead to cost savings as well.
But it’s essential to note that, while sand-based fields offer many advantages, they are expensive, calling for large amounts of material to be imported into the site (pea gravel, sand, etc.) at a significant cost not only for the material, but also for transportation.
They also require a substantial amount of maintenance, particularly with regard to irrigation and fertilization.
Sand cap fields
Rainouts are costly to any athletic program. Therefore, owners at all levels often pressure design professionals to ensure fields are playable under all conditions while keeping costs down. One way to increase the ability of any natural grass field to handle water, with or without internal drainage, is by adding sand. However, many programs cannot afford a true sand-based system.
The compromise between a native soil field and a sand-based field, then, becomes the sand cap method. A sand cap can be added at the time of construction or retrofitted into an existing field. It involves replacing 2 to 6 inches of topsoil with a specifically blended layer containing a high level of sand. Once the sand cap has been installed, it can be graded and the grass should then be re-established by the usual methods – sodding, seeding, sprigging or plugging.
It’s fair to say that sand systems require greater expertise and more regular (and therefore more expensive) management than native soil systems. They also require regular testing. But with expert management, their better drainage and resistance to compaction provide an excellent playing surface with more tolerance for play.
Editor’s note: This article was originally published in Nov. 2015.
Photo Courtesy: Rasslava/iStock