Manured loessial soil (Lou soil) is one of the mam cultivated soils in the loess region of Shaanzi and Shanzi provinces of Northwestern China. This paper deals with the seasonal variation of soil NO₃-N and the effect of summer fallow on the fertility of "Lou" soil.Field survey,field experiments and periodical tests of permanent plots were carried out,the results obtained are as follows:1.In the cropping system of summer fallow-winter wheat,the seasonal regime of soil NO₃-N might be divided into five periods annually,i.e.,the periods of accumulation,leaching,equilibrium of supply and requirement,supply-requirement discrepancy,and that of sharp discrepancy of supply-requirement of nitrogen.2.Soil NO₂-N accumulated about 1.18-3.30 kg per mu after fallow,and leaching depth of soil NO₃-N ranged from 1-2 m below topsoil.The regression equation of leaching depth of NO₃-N (Y mm.) and rainfall (X mm.) during the period of summer fallow was Y=3.86 X.3.Soil NO₃-N accumulated after summer fallow could meet the need of nitrogen for the early growing stage of wheat.Thus the accumulated nitrogen provided a foundation for high yield of crop.Yield of winter wheat after summer fallow was 57.5-124.0% (average 85.3 %)higher than the yield of winter wheat after autumn crop.4.In order to promote the favorable effect of summer fallow on soil fertility,attention should be paid on the time,depth and quality of summer ploughing.In addition,it is necessary to apply adequate organic manure or plant quickly growing green manure crops.5.Under the condition of dry farming,if the rainfall is less than 300mm in the summer fallow period and the amount of manure is less than 2500 kg per mu,rotation system of wheat and summer fallow is appropriate;if the rainfall is sufficient (more than 400 mm) and the amount of manure is also sufficient (5000-7500 kg per mu),the cropping indea may be increased relatively.