Research of long\|term fix\|experiments showed that in the semi\|arid area,the fallow was unfavourable to increase of soil organic matter content.It impelled to drop energy state,increase aromaticity extent and develop forward to aging of humic acid.Fallow made the soil composite aggregate change from G 2 to G\-1,decrease the activity of soil enzyme,increase the intensity of biochemistry action and decrease the level of total fertility.So,agri\|land ecosystem had protective effect of soil fertility.