Drainage is an essential part of maintaining healthy and playable high-profile turfgrass facilities. The soil particle size distribution of a sports field influences pore size distribution, and therefore influences drainage. Pore space can be separated into two categories: micropores (capillary pore space) and macropores (noncapillary pore space) (McCarty et al., 2016). Macropores are large (diameter, 0.08–5.0 mm) and drain gravitational water readily through the soil profile. Micropores are small (diameter, <0.08 mm) and, as a result of capillarity, they retain water and are not involved actively in soil drainage (Brady and Weil, 2002). As a result of the presence of free-draining macropores, sand-based soils drain more rapidly than soils containing large quantities of silt and clay, yet retain less water as a result of fewer micropores. Soils with high concentrations of clay particles tend to be dominated by micropores, which retain soil water and do not drain as freely as sandier soils. The requirement for rapid drainage that allows play to resume after heavy rainfall means many modern sports fields and golf greens are constructed using sand-based root zones containing a large number of macropores.
In USGA-style golf greens, the profile is 30 cm of sand over a gravel layer, creating a textural discontinuity. This design holds water in the bottom of the green through capillary tension, preventing it from draining until enough pressure potential is applied above to break this tension. USGA greens are designed to provide sufficient water for turf growth in the profile, but still have adequate drainage. Air-filled pore space is more likely to be dependent on the soil water content–soil matric potential relationship than interactions between macropores and micropores for profiles such as this.
Despite the desired drainage advantages of sands, limitations exist. Sands are inert, thus provide little nutrient holding capacity and, as a result of fewer micropores, have limited water holding capacity, which causes problems with rooting depths, organic material accumulation, fertility, and water retention (McCarty et al., 2016). To improve this, turf facilities often combine sand with native soil containing some clay to take advantage of sand’s rapid drainage and clay’s greater cation exchange capacity. Clay has the added effect of moderating excessive drainage. To reduce construction costs, mixing of sand and native soil often occurs onsite, where native soil is applied to the sand’s surface and rototilled to a depth of 10 to 15 cm (4–6 inches). This method rarely provides uniform mixing, creating chlorotic turfgrass because of soil composition variability creating pockets of high-nutrient and low-nutrient soil (McCarty, 2018). When combining sand with native soil and vice versa, pore spaces in both soil sources can become compromised. Macropores in sand can be filled by finer soil particles, reducing porosity, increasing bulk density, and reducing necessary water movement through the profile.
Two methods to ascertain water distribution and movement in soil are SWC curves and Ksat. SWC curves describe soil water retention at different matric potentials or depths (Rowell, 1994). As the matric potential increases, a corresponding increase in water extracted from the soil usually occurs. The point where the matric potential exceeds the soil’s ability to retain water at saturation is the air entry point (Hillel, 1982). Ksat is a constant that describes the relationship between the flux and hydraulic gradient of a soil under saturated conditions, and is influenced by soil composition, bulk density, and porosity.
Several studies have been performed using various amendments to alter soil characteristics, most notably the work by Waddington et al. (1974). In their research, three sands; blast furnace slag; perlite; calcined clay; and heat-treated composted soil, sand, and organic matter, were mixed with various per volume ratios of reed-sedge peat and Hagerstown silt loam soil. Mixtures were designed to be used as root zone material and were assessed for bulk density, porosity, and air porosity, among other variables. Mixtures were monitored in field plots under compaction and aerification regimes. The authors concluded at least 50% to 60% of the coarse amendments were required to positively modify the Hagerstown soil effectively. The importance of aerification to maintain acceptable high-traffic turf areas was noted.
Our study addressed the situation when sand is added to clay to increase drainage, or soil is added to a sand root zone to reduce drainage and/or improve nutrient holding capacity. Specifically, it investigated the influence of the addition of silt and clay to a USGA-specified sand on soil physical properties by evaluating SWCs, Ksat, porosity, and bulk density.
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