Water Use and Fertilizer Response of Azalea Using Several No-leach Irrigation Methods

Abstract

Although research has shown that plants grown with subirrigation systems such as ebb-and-flow and capillary mat require less water and fertilizer inputs than traditional overhead irrigation methods, similar information for capillary wick irrigation has not been available. We compared the growth and water use response of azalea (Rhododendron sp. ‘George Tabor’) grown in 6.5-inch-diameter “azalea” containers with three irrigation methods [overhead (OVR), subirrigation (SUB), and capillary wick (WCK)] and four fertilizer nitrogen (N) rates 0.5 to 2.0 lb/yard3 supplied by an incorporated, resin-coated, controlled-release fertilizer (Nutricote 17N–3.1P–6.7K, 180 d at 77 °F). OVR volume was adjusted to deliver 100% of evapotranspiration (ET) loss. For all irrigation treatments, the lowest N rate resulting in maximum plant growth was 1.0 lb/yard3, which was less than the label recommendation of 1.5 lb/yard3. At the N-limiting N rate of 0.5 lb/yard3, irrigation method had no effect (P < 0.05) on azalea growth. At N rates higher than 1.0 lb/yard3, decreased growth was observed for OVR compared with SUB and WCK. This negative effect on plant growth was attributed to salt injury as indicated by excessive pour-through electrical conductivity (EC) levels in OVR containers. At the end of the experiment, substrate EC was highest in the uppermost layer of SUB and WCK containers, reflecting the upward movement of water associated with these two irrigation methods. Water use efficiency, which ranged from 1.9 to 2.8 g shoot dry weight per liter of water lost through ET, was unaffected (P < 0.05) by irrigation method at the N rate of 1.0 lb/yard3. We concluded that the growth response of azalea to fertilizer N rate was similar for WCK and SUB despite periodic pour-through EC tests indicating higher substrate nutrient levels with WCK.