Abstract: With the increase of soil organic matter，the mineralizable nitrogen，which is about 95 percent of soil total nitrogen，Was also increased，thus soil can provide more nitrogen．In the treatments with application of nitrogen alone or combination with phosphorous an d potassium，it Was dificult for the N sink to be enlarged，SO the available nitrogen Was decreased quickly．Long-term fertilization trial in protected field showed that the soil was rich in total phosphorus an d lack of potassium．Excessive and long-term application of inorganic nitrogen alone accelerated the progress of soil acidization, decreased the exchangeable bases and their proportion was in disorder．Such problems can be prevented by application of organic matter，protected cultivation and comb ination tll phosphorus an d potassium．Soil physical degradation was a long-term progress from quantity change to quality change and it Was destructive to soil productivity．

Key words: Long-term fixed fertilization, Protected vegetable cultivation, Soil mineral nutrients, Soil acidization, Soil exchangeable base