Abstract: As the nitrogen balance study shows,the crop field soil in Tianjin suburbs,for the most part,presents nitrogen deficiency with a balance coefficient of about 87%.whereas the vegetable soil has an excess with a balance coefficient ofabout 123%.The result of study using 15N stable tracer isotope tells us that for crop field soil,the nitrogen utilization ratio of mam crops averages 27.04%,residue in soil is 24.79% and deficits take 48.17% yet for vegetable soil,the nitrogen utilization ratio of main vegetables averages 29.11%,residue in soil is 22.67%.and deficits take 48.23%.Small differences exist between those of crops and of vegetables.The main deficiency of ammonium nitrogen fertilizen applied to the crop field soil in Tianjin suburbs is due to ammonia volatility,which is mainly influenced by the factors of wind speed,temperature,soil moisturs' soil texture,and fertilizfr types.And the effective way of preventing ammonia from volatilization is deep placement.

Key words: Crop field soil, Nitrogen balance