Types of Fields
These include fields constructed from the natural soil existing at the site, or in part from topsoil hauled to the site; fields constructed from modified soil; and fields constructed from a soilless medium (essentially 100 percent sand).
Natural soil fields
The majority of athletic fields constructed today, at least at the high school, club, and small community levels, are natural soil fields. This is because natural soil fields are the least costly to establish initially. Although some athletic field sites in Pennsylvania may have sandy loam soils, most sites will have much heavier soils. These heavier soils, due to their high base exchange capacity, hold adequate nutrients resulting in relatively simple maintenance fertilization programs. They also have high water holding capacity, eliminating the absolute necessity but not the desirability of having an irrigation system.
Natural soil fields often are plagued with internal drainage problems.
These fields must be crowned with a turtle-back crown to ensure adequate surface drainage. Because these soils compact readily, especially in the surface inch, internal drain tile within the playing area normally is not recommended; but perimeter tile lines with open catch basins are required to rapidly remove the concentration of surface water coming off the field crown. Crowned fields are not suitable for multiple sports because of the difficulty they cause in controlling the ball in such sports as soccer and field hockey.
As turf thins during the playing season natural soil fields may become quite muddy during periods of heavy rain. Although it is not unusual to incorporate a small amount of organic matter into the surface of a natural soil field, such soil is not considered to be modified soil construction.
Modified soil fields
Those fields that have had a coarse physical amendment, such as sand, mixed uniformly with the existing soil are modified soil fields. Most Pennsylvania soils require 50 percent or more coarse amendment be added to make a marked physical improvement of these soils. Properly modified soils will have better internal drainage and less susceptibility to compaction, compared to unmodified soils. Nevertheless, modified soil fields require crowning of the field, as well as perimeter tile lines with open catch basins. In addition, the subgrade also should be crowned and a 6-inch stone blanket placed between the subsoil and the modified soil to facilitate water movement from internal drainage to the perimeter tile lines.
The particle size range of the coarse physical amendment is important and it may be difficult to obtain the desired size range without paying a premium for specially screened material. Also, it is essential to obtain a uniform mixture of the soil and physical amendment. This may be difficult if on-site rather than off-site mixing is used.
Modified soil fields require irrigation facilities and increased attention to the maintenance fertilizer program. Like natural soil fields the crowning of the field restricts multiple use and fields may still become quite muddy under adverse weather conditions. The cost of modified fields is quite high, especially if the physical amendment must be custom screened. However, this increased cost is compensated, in part, by better playing conditions and easier maintenance.
Although quite expensive initially, soilless fields offer a number of advantages over natural or modified soil fields. If the proper sand size is used these fields will have excellent internal drainage, eliminating the need to crown the field and to have open perimeter catch basins. Soilless fields do require internal drainage tile within the playing area to remove the water that rapidly percolates through the sand.
As with modified soil fields the sand particle size range is extremely important and it may be difficult to obtain the desired size range without custom screening. Irrigation facilities are an absolute necessity because of the low water-holding capacity of the sand. Sand fields require intensive management. Although aeration requirements normally are reduced, an intensive fertilizer program as well as frequent irrigation are required. Establishment from seed is more difficult than with a soil medium due to the drying problem, but seeding is recommended in preference to sodding when commercial sod produced on sand is unavailable. The use of sod grown on mineral soil may negate the infiltration advantage of the sand. There also have been some complaints of reduced footing on sand fields where the turf has thinned badly.
A number of variations are used in constructing sand fields. The Prescription Athletic Field (PAT) system developed at Purdue University is a highly modified system involving the installation of a plastic film barrier between the underlying soil and the sand. This film prevents the vertical movement of water out of the system. Tile drainage lines are trenched into the underlying soil at depths of 4 to 18 inches, depending on the drainage tile size. The plastic barrier must follow the contour of the trenches resulting in a closed system. The sand layer over the plastic barrier varies from 12 to 30 inches thick. Moisture content above the barrier is controlled by pumping water into or out of a grid sytem of 2-inch porous pipe buried in the sand and connected to the 4-, 6-, and 8-inch drainage lines. The system rapidly draws water down through the sand by using vacuum diaphragm type pumps, capable of handling both water and air contained in the system. Operating in reverse, the pumps serve as an irrigation system by pumping water into the system. Some PAT fields incorporate electrical cables near the surface to extend the growing season of the grass.