Abstract
Abstract Excess water retention in the potting medium is a significant problem for plants grown in containers due to the volume of saturated medium which forms above the drainage hole. Adding a layer of coarse material like gravel or sand at the bottom is a common practice among gardeners with the aim of improving drainage, but some researchers have argued that such layers will raise the saturated area and in fact worsen drainage. Two different depths and four different materials of drainage layer were tested with three different potting media to determine the water retention in the container after saturating and draining freely. For loamless organic media, almost all types of drainage layer reduced overall water retention in the container compared to controls. For loam-based media, most drainage layers had no effect on the overall water retention. Two simple models were also used to estimate the water retention in the media alone, excluding the drainage layer itself. All drainage layers reduced water retention of loamless organic media, according to both models. There was disagreement between the two models applied to loam-based media, and further study is required to determine the most accurate. Both models showed that some drainage layers with smaller particle sizes reduced water retention in loam-based media, but disagreed on the effect of drainage layers with larger particle sizes. Overall, any drainage layer was likely to reduce water retention of any medium, and almost never increased it. Thicker drainage layers were more effective than thinner layers, with the most effective substrate depending on the potting media used. A 60 mm layer of coarse sand was the most universally-effective drainage layer with all potting media tested.
Highlights
It is widely recognised that soil in a container behaves differently from soil in the field (Hershey 1990; Landis, 1989; Spomer, 1975)
Soil in a shallower container has a greater percentage water-holding capacity (WHC) than the same soil in a deeper container (Bilderback and Fonteno, 1987; Bunt, 1961; Hershey, 1990; Waters et al, 1970). This effect has led to the concept of container capacity, a measure of the total water retained in a specific container filled with specific soil after saturation and drainage (Bunt, 1988; Heiskanen, 1997; Waters et al 1970)
Adding a layer of rocks or gravel at the bottom of plant pots is a common practise among home gardeners, as well as some researchers (Cavazza et al 2005), and is generally intended to reduce this perched water table and improve drainage
Summary
It is widely recognised that soil in a container behaves differently from soil in the field (Hershey 1990; Landis, 1989; Spomer, 1975). Instead of this reduced water retention being evenly distributed through the volume of the pot (as it would be in a container filled with a more free-draining medium), it is held in alternating layers of saturated and air-filled materials. If the roots do not enter the drainage layer, the plant could be considered to be effectively growing in a smaller container with the total water-holding capacity of the medium itself.
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