Rapid assessment of the rate of nitrogen immobilisation in organic components of potting media: II. Nitrogen drawdown index and plant growth

Abstract

Abstract 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.