FieldScience
Figure 4
The sand component of the mix should ideally be
medium-coarse in size and uniform. In many situations,
the sand component of the mix does not meet this specification and it is not unusual to see sand mixes that
contain large amounts of silt and clay or gravel. Silt and
clay particles are very fine and they clog a soil system.
Air spaces are blocked and the soil becomes prone to
compaction. When dry, silt and clay soils are rock-hard,
but they turn to a quagmire when wet. For these reasons, very fine sand, silt and clay are generally restricted
in mixes to less than 15% of the total mix.
There are no set guidelines for the amount of gravel
allowed on a sports field but there is a landscape recommendation. ASTM D 5268-92 "Standard Specification
for Topsoil Used for Landscaping Purposes" suggests that
no more than 5% deleterious material (rock, gravel etc.)
be included in a topsoil mix. Gravel is not a suitable material to improve soil physical or chemical properties and
on a playing surface it can disrupt play and possibly cause
player injury. In addition, gravel on the surface could
damage mower blades and be very difficult to grow grass
or seed in. For a whole multitude of reasons then, gravel
should not exceed 3-10% of the total mix.
Unfortunately, this has not been the case in many situations. Site visits to sports fields over the years have
shown that many topdressing mixes contain far too
much gravel. In one notable instance, a college soccer
field was constructed with a material that contained
44% gravel. That particular field had also been graded
and then leveled with a vibratory roller, making it as
hard and impenetrable as a parking lot. The ultimate
Figure 6: The textural triangle. To plot sand, find the percent sand along the bottom and follow the line diagonally left.
To plot clay, find the percent clay along the left edge and follow the line horizontally across from left to right. To plot silt,
find the percent silt along the right edge and follow the line diagonally left. Example: Soil containing 30% sand, 30% clay
and 40% silt would be a "Clay Loam."
Figure 5: Hand-texture test. The flowchart is easy to follow and results in a better
understanding of what a silty, clay or sandy soil feels like.
14 SportsTurf | July 2013
www.sportsturfonline.com
