An evaluation of conventional and subirrigated planters for urban agriculture: Supporting evidence

Although interest in integrating agriculture into the urban landscape in the USA is increasing rapidly, there is a shortage of guidance for agricultural production in this context as well as a unique set of significant biophysical constraints. A common constraint is not being able to grow directly in the soil, making raised-bed gardening a necessity. Subirrigated planters (SIPs) are a style of raised bed with a subsoil reservoir that provides aeration and allows growers to irrigate below the soil where water is pulled up via capillary action. This bed design has vocal advocates; anecdotally, growers find them to be high yielding, water efficient and easier to maintain than standard raised beds. Given their apparent promise, there is interest in promoting SIPs and in utilizing them in larger-scale urban gardening operations but no rigorous tests compare these beds relative to standard raised beds. At one location and for one season, we compared yields for three crops: cayenne pepper (Capsicum annuum), sungold cherry tomatoes (Solanum lycopersicum) and lacinato kale (Brassica oleracea), crop quality and labor input for two styles of SIPs, as well as a sack garden, a variation of a SIP that does not require lightweight soil, with two conventional raised beds (one with a compost and topsoil mix and one with the soilless growing medium ideal for container gardening). Results from our first year of data indicate that both the SIP beds and the conventional beds with the soilless growing medium were more productive overall than conventional raised beds with topsoil and compost (P<0.01). Tomato production in the SIP without the root barrier was greater than both the conventional bed with the compost and topsoil mix (P<0.01) and the conventional bed with the soilless growing medium (P<0.05). The majority of the higher-cost beds had a positive revenue stream in the first summer season; given these results, investing in SIPs or in soil appropriate for raised beds appears to be worth the higher initial cost.