Once a dense turf cover is established, soil conditions begin to
be altered by the presence of the growing turf. The grass produces a
large root mass in the soil along with a network of stolons, rhizomes
and stem tissue. In addition, any clippings not caught by the mower's
basket are returned to the soil surface. As some of these plant parts
die, they begin to be degraded by the soil microbes and form what is
referred to as the thatch layer. Technically, thatch is defined as "A
tightly intermingled layer of living and dead roots, crowns, rhizomes,
stolons, and organic debris that accumulates between the zone of green
vegetation and the soil surface". To promote rapid degradation and
prevent excessive accumulation of organic matter near the soil
surface, most superintendents make frequent light applications of
topdressing sand. Ideally, the topdressing sand should have the same
particle size distribution as that of the existing root zone mix. When
done correctly, the topdressing sand mixes with the organic matter and
promotes better air and water movement into the surface soil and by
doing so, it also promotes improved microbial degradation of the
organic materials.
Even though adding topdressing sand promotes organic matter
degradation in soil, some recalcitrant organic matter will remain.
Over a ten year period of time, it is possible to accumulate a 2-4
inch thick layer of topdressing sand and residual organic matter on
top of the original root zone mixture. Because of the high organic
matter content of such a layer, it has vastly different physical
properties than the underlying root zone mixture. In general, these
layers have a reduced saturated hydraulic conductivity, an increased
amount of capillary porosity and a reduced amount of air-filled
porosity. Thus, they are slow to drain, retain large amounts of water
and are poorly aerated. Because of the high water content and poor
aeration, the turf roots will concentrate in the upper portion of this
layer. This shallow rooting of the turf in turn leads to additional
management problems.
If greens are top dressed using topdressing materials that are too
high in fines such as native sandy loam topsoil's or processed sands
which are much finer than the existing root zone mixture, this will
lead to the development of layered greens. Occasionally,
superintendents will want to use a native topsoil or mixture of sand
and native topsoil for topdressing sand greens to improve the nutrient
retention of the greens. These finer textured materials do not mix
with the underlying soil but merely lay on top of it where they
essentially form a barrier that limits the entrance of water and air
into the soil.
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