Commercial amendments for sand-based root zones: Review and interpretation

Abstract

Amendments used in sand-based root zones are typically native peats, composts, or inorganic materials of diverse mineralogy. Literature-based guidelines for amendment inclusion include root zone capillary porosity (CP) values exceeding 0.2 m3•m-3 or soil surface water contents exceeding 0.1 m3•m-3. It is also suggested that the cation exchange capacity (CEC) need not exceed 4 cmolc•kg-1. Summarizing published data, native peat added to a U.S. Golf Association (USGA) guideline sand yielding organic matter (OM) content values of ≈15 g•kg-1 would typically yield benchmark values of CP = 0.2 m3•m-3, surface water content = 0.1 m3•m-3, and CEC values between 3 and 4 cmolc•kg-1. This OM content is roughly equivalent to 15% by volume of a fibric sphagnum or 7.5% by volume of a hemic reed-sedge. By comparison 15% by volume of an inorganic amendment would likely result in CP and surface water content values less than the benchmark values, and CEC values between 1.5 and 4 cmolc•kg-1. However, the CP benchmark measured at 30 cm of soil water suction should be re-evaluated because it may bias against the use of biosolids compost and inorganic amendments that reduce the thickness of the capillary fringe and create a broader pore size distribution. Also a weak relationship has been observed between longer-term turfgrass quality and 30-cm CP values. Measurement of CP at 40-cm suction would be a better indication of water retention in sand-based root zones for all amendment types. Lastly, inorganic amendments produce a drier root zone that may be preferred for both playability and long-term agronomic concerns.