The present paper deals with the inhibition of free NH₃ formed by urea hydrolysis in calcareous soil and acid soil with low content of potassium.It is showed by experimenu that the toxic symptom of plant leave were similar to K deficiency,the root developments of wheat,corn and rice were inhibited significantly,without any root hairs and new roots grown,appearing yellow-brown colour on root tip sometimes.Plant root was injured clearly while NH₃ concentration was 5 μg/cm³,however the root could recover its normal growth gradually after removing NH₃ treatment.While the root was treated by NH₃ with a concentration up to 30 μg/cm³ for 6 hours,root normal growth could no longer be recovered.Rice had higher resistance ability to free NH₃ than wheat.K⁺ and some amino acids could be released from root tissue injured by NH₃.The amount of K+ released from corn should be up to 4% of root dry weight.It indicates that permeability of cell membrane may be influenced by NH₃.Furthermore,plant root was injured both by top dressing N of 200 ppm or band applying urea of 187,5 kg/ha in calcareous soil.At the 10th day after applying urea at late tillering stage,the dry weight of wheat roots was less than that in check (without urea treatment),while the similar tendency could be observed in acid soil under the condition of potassium deficiency.It indicates that plant root growth injured by free NH₃ is related to potassium disorder in plant.It is suggested that the urea applied should be covered with soil before seedling or the fertilizer should be applied in strip separated from the seeds or urea applied mixed with peat,All these measures are of advantage to prevent plants from the togieity and the loss of NH₃.