Layering in Sand Based Putting Greens and Athletic Fields
James Thomas
Soil and Crop Sciences Department
Texas A&M University
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 rootzone 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.
Underlying Principles
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 topdressed 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.
Layers of Thatch and Topdressing
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 topdressed using topdressing materials that are too high in fines such as native sandy loam topsoils 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.
Black Layer
If you probe your greens and notice the presence of a foul-smelling, black colored layer, then you likely have developed black layer conditions in your greens. Black layer forms when soil which is slow to drain and is high in organic matter stays wet (nearly saturated) for a prolonged period of time and becomes anaerobic (without oxygen). When a soil is kept without oxygen for a prolonged time, anaerobic microbes flourish and feed on the organic matter in the soil. Anaerobic microbes are able to obtain oxygen by chemically removing it from sulfate compounds in the soil giving rise to the formation of hydrogen sulfide gas. Hydrogen sulfide gas is toxic to the grass roots and has the characteristic odor of rotten eggs. Oxygen helps give soils their bright orange, tan, and yellow colors while soils without oxygen revert to darker colors and eventually turn black. Since oxygen is necessary for root respiration, roots cannot survive in black layer soils and as a general rule live roots will only be found in soil above the black layer. If prompt action is not taken to get oxygen into the soil, remove the toxic gases and stimulate healthy root growth, the overlying turf may be lost in a short period of time. Irrigation with effluent water having a high amount of suspended biosolids may also contribute to the accumulation of organic matter in the soil and the formation of black layer conditions.
Silt and/or Clay Layers
Superintendents who have experienced severe flooding can attest to the fact that when the flood waters recede, there will often remain a layer of fine silt and clay particles which will cover the flooded area. If not carefully and thoroughly removed, these fines will form a layer near the soil surface which will effectively restrict water and air entry into the soil. Similar layers can occur from the application of an unwashed sod which was grown on native soil to the surface of a high sand content root zone. As the turf is topdressed with new sand, these soil layers become buried and interfere with water and air movement through the soil profile.
Fine silt and clay materials can also be transported and deposited by wind erosion from other areas. Irrigation water from ponds high in suspended solids can also contribute to the accumulation of fines at the soil surface.
Cemented Layers
Under certain conditions, root zone mixtures constructed using calcareous sands may develop a cemented layer which can effectively block drainage through the soil profile. Calcareous soil materials are defined as “Soils containing sufficient free CaCO3 and other carbonates to effervesce visibly or audibly when treated with cold 0.1M HCl.” Depending on the exact form of the carbonates in the soil, local environmental conditions, and management practices; some of the carbonaceous materials may slowly dissolve over time. Typically, the carbonates are moved deeper into the soil along with the drainage water. If the soil conditions are right, the carbonates may reprecipitate at some depth in the soil profile. Most often, this occurs at the bottom of the root zone mixture, just above the gravel layer. The carbonates form coatings around the sand grains which results in two adverse effects. First the carbonates act as weak cementing agents and essentially glue the particles together. Secondly, the coatings fill up the pore space between particles and greatly reduce the saturated hydraulic conductivity and air-filled porosity of the soil profile.
Corrective Actions
When considering ways to overcome the effects of layering of root zone systems, the old adage that “an ounce of prevention is worth a pound of cure” is very applicable. Typically, corrective actions for layered root zone mixtures are expensive and often are only of limited effectiveness. For instance, an aggressive core aerification/sand topdressing program can help address this problem by perforating the layers and providing sand filled channels through which water and air can move. The use of an inorganic amendment or a mixture of sand and inorganic amendment to fill the core aeration holes may further increase the saturated hydraulic conductivity and air filled porosity of the channels. However, while core aeration, sand topdressing, and the use of inorganic amendments may help to improve soil conditions; it will not cure the problem. Areas between aerification holes will probably still remain wet, poorly aerated and will require conservative irrigation to prevent the formation of anaerobic conditions and possible black layer.
The key to dealing with black layer conditions in soils is to get oxygen into the soil. Most commonly this is done by a combination of reducing the amount of irrigation and beginning an aggressive aerification and sand topdressing program. Some additional benefits can be achieved by the application of nitrate containing fertilizers. Nitrate contains three molecules of oxygen for each nitrogen and can serve as a supplemental oxygen source for oxygen deficient soils. Recently, equipment has been developed to force air upwards through the soil via the drain system and to inject air into the soil via specially designed probes; however there is little scientific data available on the effectiveness of these units in alleviating black layer conditions.
If you have greens made with highly calcareous sands, the addition of large amounts of sulfur or other acidifying agents will likely increase the rate of dissolution of the carbonates and increase the probability of developing a cemented layer in the soil profile. Therefore, with carbonaceous sands, it is often best use sulfur, sulfur containing fertilizers, and other acidifying agents sparingly. Occasional foliar applications of micronutrients may be needed to compensate for the reduced availability of micronutrients due to the high pH associated with calcareous soils. Should a cemented layer form, core aerification and sand topdressing may help, however, the cementing action of the carbonates may make the layer so hard that it may damage aerification equipment.
The ideal time to address layered soil profile conditions is during resurfacing of the greens. At this time the sod and the entire upper layer can be removed. The removed soil can be replaced with a compatible sand or root zone mix, tilled and resodded or sprigged. If done correctly, this can nearly return the root zone to its original condition.
A less desirable alternative is to remove the sod and till the upper layer into the underlying layer of original root zone mixture. This results in a deeper root zone but generally reduces the quality of the mixture. It is probable that the finished mixture will be better than the upper Layer was but not as good as the original root zone mix. The other disadvantage is that should you want to improve the greens some years from now, they will likely require complete rebuilding since nearly all the root zone will have been impacted by mixing the upper layer with it.
Summary
Layering of sand based putting greens and athletic fields is a common problem. Most often, it is caused by an accumulation of finer soil and organic materials near the soil surface. Common causes for this type of layering include allowing excessive thatch accumulation, using improper topdressing materials, using unwashed sod, and deposition of fine material from floods, wind storms, and the use of poor quality irrigation water. These layers of fine materials effectively seal off the soil surface and greatly reduce the entrance of water and air into the soil. Layers which are high in organic matter will likely retain large amounts of water and very little to no oxygen which leads to the development of black layer conditions. Cemented layers may form when carbonates are leached from carbonaceous sands and reprecipitate at lower depths.
The major non-destructive corrective action for layered soils is to pursue an aggressive core aerification and sand topdressing program with the goal of perforating the layer and making permanent channels through which water and air may enter and move through the soil. If the area can be taken out of play, then the ideal procedure is to remove the turf and offending layer, replace the removed material with original root zone mix, and replant the area. A less desirable alternative is to remove the turf, roto-till the root zone mixture to 8-10 inches in depth, and replant the area.
