This paper deals with the mechanism of inftibition of NHe from urea hydrolysis in silt loam soil with low K content and the prevention of this toxicity by the application of farm manure and K on barley (Hordeum vulgare Jess). The results obtained are summarzed as follows: The pH of soil relating to the concentration of free NH₃ raised about 1.3 units 10 days ofter applying 122kg/ha urea in the soil, and a significant inhibition of NH₃ on barley growth was observed while the concentration of free NH₃ in soil solution was about 4 ppm. The membrave structure of both root and leaf tissue was severely injured by NH₃. The injured barley was low in respiratory rate and activities of photophosphorylation, nitrate reductase and Qlutamine synthetase, but high in the rate of photosynthetic electron transport and the contents of free amino acids.