There are two ways to look at this and so there is no single answer.
On the one hand, the PCD element has pore sizes similar to a medium sand. So if one were concerned with perfectly matching the pore sizes in the sand with the pore sizes in PCD then one would choose a medium sand. Then with a medium sand adjacent to the PCD element there would be a perfect continuity of pores. But there is not really a need to perfectly match the pore sizes between the sand and the PCD. This is because the PCD is also installed with a hanging water column that applies a suction to the element so there is a hydraulic gradient driving water flow from the sand to the PCD, and less worry about pore size differences, or a capillary break between the sand and the PCD. Also, because the PCD element has a greater porosity compared to a typical sand, it has a hydraulic conductivity compared to a fine gravel. Thus, as fast as water can flow through most root zone sands, it can flow more rapidly through the capillary space of the PCD element.
So PCD has the capillary attraction for water comparable to a medium sand but the hydraulic conductivity of a fine gravel. This combination works perfectly in drawing water from the root zone and conducting water out of the root zone.
But, perhaps the more important consideration of sand size is the pore size distribution of the backfill sand in comparison to the pore sizes of the existing root zone. This may come to bear in the situation of a push-up green where the surface layer has a sandy texture due to decades of applying topdressing. And it’s important within this surface layer and not adjacent to the PCD element. Consider a very coarse sand (or fine gravel) used as a backfill when the top layer of the green consisted of, say, a medium sand. This large contrast in particle sizes of adjacent materials would result in a large contrast of pore sizes between these adjacent materials. The result could be an incomplete drainage of the top layer of the green because after gravity flow has ceased, water held in the finer pores of the top layer would not enter the larger pores of the backfill sand. It is like what happens in a USGA green where after gravity drainage has ceased, perched water is retained in the root zone because there is a capillary break between the root zone and the gravel. But again, this capillary break only occurs when there is a large differences in texture between adjacent materials. As the difference in particle sizes between these adjacent layers becomes less, then the capillary break disappears and the top layer will become more completely drained. So if the top layer was a medium sand texture but the backfill sand was medium coarse sand texture, these are close enough to not cause a problem in draining the top layer.
So provided the superintendent has used some reasonably textured topdressing sand, such as a medium coarse sand or even a medium sand, then the best backfill sand is precisely the same sand as was used in topdressing.
Dr. Ed McCoy, Developer of PC Drainage
Ohio State University