GROWTH, NUTRIENT CONTENT, AND GROWTH MEDIUM ELECTRICAL CONDUCTIVITY OF POINSETTIA IRRIGATED BY SUBIRRIGATION OR FROM OVERHEAD

Abstract

The effects of subirrigation versus conventional overhead irrigation on the growth and nutrition of containerized poinsettia (Euphorbia pulcherrima Willd. Ex Klotz) were studied by growing ‘Red Sails’ at different fertilizer levels. Plants were grown in 1.6-L pots of soilless growth medium and fertilized twice a week with 20N-4.3P-16.6K water-soluble fertilizer at 100, 175, 250, or 325 mg NL−1. Fertilizer solutions and extra irrigation water were applied by subirrigation from saucers placed below the pots or by conventional overhead watering. Regardless of irrigation method plants received 200–250mL of fertilize solution or tapwater at each irrigation. All treatments resulted in commercially acceptable plants at the end of the study, however overhead-irrigated plants were larger in diameter and had more shoot dry weight than subirrigated plants. There were no differences in plant height or bract diameter due to irrigation method and plant growth parameters were not affected by fertilizer level. The recently mature leaves of overhead-irrigated plants contained more nitrogen (N), phosphorus (P), boron (B), and molybdenum (Mo), but less magnesium (Mg) than subirrigated plants. Regardless of irrigation method or fertilizer level, there were no major differences in the nutrient content of plants in this study. At the end of the experiment the top one-third and bottom two-thirds and a composite sample of all of the growth medium were analyzed for electrical conductivity (EC). Overall there was no difference in EC between sample types with overhead irrigation, but the top and composite samples had higher EC than the bottom sample with subirrigation. Except at 100 mg NL−1 and all fertilizer levels in the case of the bottom sample, subirrigation resulted in greater salt accumulation than overhead irrigation. The highest EC levels were found in the top and composite samples of subirrigated growth medium; some of these ECs exceeded the satisfactory range for poinsettia but no visible symptoms of excess salinity were apparent. Results of this study indicate that there is little difference in the growth and nutrient status of subirrigated and overhead-irrigated poinsettias over a representative range of fertilizer levels and receiving about the same amount of water. Interpretation of EC analyses of subirrigated growth medium would be affected by sampling technique.