USE OF THE NITROGEN DRAWDOWN INDEX TO PREDICT FERTILIZER NITROGEN REQUIREMENTS IN SOILLESS POTTING MEDIA

Philadelphus mexicanus, Cuphea hyssopifolium 'Violet' and Viola wittrockiana 'Blue Angel' (pansy) were grown in mixtures of peat and Pinus radiata bark with Nitrogen Drawdown Index (NDI75) values ranging from 1.1 to 0 (NDI150 value of 0.36). Nutrients were provided via liquid feeds containing 150-700 mg/L N or via coated fertilizers added at 3-9 g/L medium. Growth responses covered the full spectrum from N deficiency to growth reduction through excessive salinity. Some of the controlled-release fertilizers used released nutrients at considerably higher rates than that nominated by the manufacturer. At the lowest levels of N application, growth generally graded down from a maximum in medium with the highest NDI value to a minimum in medium with the lowest NDI value. Calculations based on growth response indicated that N immobilisation was at a weekly rate of 1.5 to 2.9 mg/L for each 0.1 NDI unit fall below 1, with a mean of about 2.5 mg/L.