by James C. Thomas C.P.Ag.
Many high performance putting greens and athletic fields are designed and built based on the USGA recommendations which call for placement of a 12 inch deep uniform root zone mixture over a gravel drainage blanket. The root zone mixtures are carefully chosen mixtures of sand and organic amendment designed to have a specific particle size distribution, saturated hydraulic conductivity, and pore space distribution. These properties are chosen to provide resistance to compaction plus excellent drainage and aeration to promote optimal turf growth. For these systems to perform correctly, it is important that the root zone mixture be as uniform as possible from top to bottom since the presence of layers of differing materials will interfere with the desired water and air movement through the root zone.
The flow of water through soil is interrupted each time there is a change between soils of different particle size. First, let us consider the case of a finer textured soil layered over a coarser textured soil as would occur if a green were top dressed with too fine of a sand. In this case, water will enter the upper, finer textured layer and pass through it at a rate equal to its saturated hydraulic conductivity. Once the percolating water reaches the bottom of the upper layer, the water will not enter the underlying coarser textured soil until the upper layer becomes nearly saturated and there is sufficient gravitational force to overcome the capillary attraction of the fine pores for water. When this occurs, water will enter the larger pores of the underlying coarser soil and flow through it until it reaches the bottom of that layer. Here again, the water will not enter the underlying gravel until the sandy root zone becomes nearly saturated and there is sufficient gravitational force to overcome the capillary attraction of the sand for water. In a system of this nature, many times the water applied during routine irrigation is only retained in the upper layer of finer textured soil and it does not move into the underlying soil. As a result, the upper layer remains quite wet and very poorly aerated. These conditions are ideal for promoting disease, shallow rooting and potential black layer conditions.
Secondly, let us consider the case of a coarser textured material layered over a finer soil. In this case water can rapidly enter the coarse textured upper layer and move to the bottom of this layer. Here its movement is slowed by the lower saturated hydraulic conductivity of the underlying layer. If the rate of rainfall or irrigation exceeds the saturated hydraulic conductivity of the underlying layer, free water will begin to accumulate in the upper layer until it becomes totally saturated at which time any further applied water will runoff. After rainfall or irrigation ceases, the standing water in the upper zone will slowly seep into the underlying soil. If the rate of water movement into the underlying finer soil is too slow, then the upper layer may not have sufficient time to dry out and get oxygen into the pores before the next rainfall or irrigation event occurs. This type of continuously saturated and nearly saturated soil leads to the development of anaerobic soil conditions and may eventually result in black layer formation.