Efficiency of water and nutrient use in containerised plants irrigated by overhead, drip or capillary irrigation

This paper reports an assessment of the potential of an irrigation system based on capillary matting in comparison to the industry standard, overhead irrigation, and to drip irrigation, for plants growing in pots. The systems were evaluated on the basis of the dry weights of the plants grown, water-use efficiency (volume of runoff) and the environmental impact of the systems. The environmental impact was assessed by the volumes of runoff and the amounts of nitrate nitrogen and phosphate phosphorus in the runoff. Rose and tomato grew on capillary mat as large as or larger than (dry weight) with drip or overhead irrigation when controlled-release fertiliser (Osmocote Plus, 5-6 month release) was mixed through the medium. Lavender and pansy showed no significant difference between the irrigation systems. However, when the fertiliser was applied to the surface or in the dibble hole, pansy, dianthus and anisodontea grew larger with overhead than with capillary irrigation. The volumes of water used by the drip and capillary systems were about equal at 2.6 and 3.3 L/pot. week, respectively, as were the volumes of runoff produced, 0.43 and 0.39 L/pot. week, but both were far lower than the corresponding values for the overhead system, where water use was 7.13 L/pot. week, and the runoff 3.00 L/pot. week. At the end of the trial, plants grown with capillary irrigation had a high electrical conductivity in the surface layers of the pots when controlled-release fertiliser was either mixed evenly through (1997, EC 2.04 dS/m) or was applied to the top of the mix (1998, EC 0.87 dS/m). On the other hand, with overhead watering the EC was more evenly distributed (1997, EC range 0.30-0.36 dS/m) or highest at the bottom of the pot (1998, EC 0.97 dS/m). The concentrations of nutrients in the runoff from the capillary mat system, nitrate nitrogen 6.2 mg/L and phosphate phosphorus 0.12 mg/L, were lower than those for either of the other systems, overhead giving 9.7 and 0.49 mg/L and drip 11.5 and 0.49 mg/L of nitrogen and phosphorus, respectively. Total nutrient leaching per week for nitrate and phosphate, respectively, was 320 and 16.2 mg/m(2) for overhead, 53.9 and 3.1 mg/m(2) for drip and 26.4 and 0.50 mg/m(2) for capillary irrigation. Thus, capillary irrigation gave more efficient water and nutrient use and reduced nutrient contamination of the environment compared with the drip or overhead irrigation systems.