Ronald Strahan, Koske, Thomas J.
Layering in soils is something easy to get into and difficult to mitigate. A well-regarded agronomist from the mid-Atlantic once professed that, "Layering. . . it is desirable in cakes but not in turf."
This phrase is especially true for sand-based sports areas but applies to any soil profile as well: heavy soil with sand layers or sands with clay layers. Remember, the primary reason that sand is used as a root zone medium is that the particles naturally resist compaction. The second reason is that (assuming the sand is properly sized and selected) it should drain well. This in turn means we are able to bring sand-based root zones back into play after heavy rainfall or irrigation much more quickly than if we had to wait for surface runoff and slower internal percolation.
Ideally, sandbagged root zones are established using seed, clean sprigs or washed sod; the textbook answer for this is that turf establishment via seed or sprigs minimizes potential layering. So why is layering undesirable? Two answers come to mind.
First, soil layering affects the internal drainage properties of the root-zone surface. Layers of fine-textured soil particles (a heavy soil) and/or organic matter (soil-based sod on a sand-based field) will restrict water infiltration and percolation because of a discontinuity in the medium’s texture. Studies show soil-water movement does not like a discontinuity and pauses or stops completely. It might be like putting a swimming pool liner between the base soil and the pool’s water or like putting coarse sand in a tea cup. We make good use of this movement principle in a USGA spec green with its coarse "choker layer" placed between the root-zone mix and the drain tile system; it makes the doughy medium hold more water. Most soil layers, however, cause problems.
Second, because the textural properties of this surface zone are dramatically different than the underlying root zone, turfgrass roots naturally want to remain in the more favorable zone. Native soils have much more water and nutrient retention than the sand below. You may also notice the formation of what was probably a "perched water table" and the formation of an anaerobic or oxygen-deprived layer as evidenced by a thin, toxic "black layer." This, too, is not a healthy condition for turfgrass root systems. Take the sand and tea cup example; water poured in certainly percs in, but will sit there and drown the roots.
So what is a turf or lawn manager to do? If you can, always avoid sodding a sand-based root-zone area with native-soil-based sod. If you cannot avoid sodding, then implement an aggressive cultural program consisting of hollow-tine core cultivation with core removal. This should be followed by moderate sand topdressing and sweeping to fill the holes. Your ultimate goal should be to dilute the odd layer over time. This process will require substantial efforts in terms of resources and time, but it will improve turf performance, especially rooting. Additionally, a deep-tine operation also would be appropriate (with or without topdressing) once the turf is well-rooted.
When topdressing, use a similar-textured mix or use only light dressings of sand. This approach should not create that sand-layer "teacup" and its associated shallow root zone. If much dressing material is needed, it should be blended into the soil surface so as to soften any distinct boundary between heavy and light soils. Obviously, this must be done before planting the area.