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The relationship between nitrapyrin [2-chloro-6-trichloromethyl)pyridine, a nitrification inhibitor], and the growth and the mineral composition of ‘Cherry Belle’ radish (Raphanus sativus L.) was determined with sewage sludge as the N carrier in a greenhouse study. Plant growth fell sharply as nitrapyrin was increased in concentration from 1 to 10 ppm. The concentrations of Ca, Mg, and NO3− in the plants fell with this range of treatment, whereas that of K rose. The mineral composition of shoots was altered much more than that of roots. Increasing nitrapyrin concentrations to 50 ppm had little added effects on growth or composition relative to the 10-ppm level. Some of the phytotoxicity due to nitrapyrin addition appeared to be ammonium toxicity imparted by the accumulation of ammonium-N from the mineralization of the sewage. Another greenhouse study with nitrapyrin at 10 ppm, but with KNO3, (NH4)2SO4, and sewage sludge as the N carriers, showed that a depression in growth was correlated with the accumulation or retention of ammonium-N in the medium.

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