RAPID ASSESSMENT OF THE RATE OF NITROGEN IMMOBILIZATION IN ORGANIC-COMPONENTS OF POTTING MEDIA .2. NITROGEN DRAWDOWN INDEX AND PLANT-GROWTH

In several experiments, cabbage seedlings were grown in soilless potting media produced using a range of aged and composted pinebarks, peat and sawdusts. Their growth was compared with the Nitrogen Drawdown Indexes (NDI) of the media, measured by a standard method previously reported. Growth was highly correlated with NDI when N was applied via pre-plant additions and/or liquid drenches. Aged pinebarks and composted hardwood sawdusts, with high N immobilisation potential, have NDI values near zero and consume N at about 300 mg/L each week. Adequate plant growth is difficult to obtain in these media. Media with NDI values of around 0.5 consume about 40 mg/L N each week. Pre-plant amendment with N equivalent to that in 0.5-1.0 g/L NH4NO3 is desirable and weekly feeding with a solution containing 200-400 mg/L N will give maximum growth rate. Weekly feeding with solutions containing 100-150 mg/L N provides sufficient N for excellent plant growth in peat-based media, whose NDI values are 0.95-1.0.