| In order to understand the reason of secondary salinization of soil under protected cultivation, the soil and groundwater samples of different section in three different protected facilities with different greenhouse age were tested and analysed in Nanhui district of Shanghai city in this paper.The main results are as follows:The top layer soil was dry and with high EC value (the highest value reached 1.84 mS/cm),while the EC value of button layer soil was normal.The main cation of soil salt in Nanhui was Ca2+ (occupied 60% to 80% of total cation content) and the main anion was NO3- (occupied 60% of total anion content), meanwhile, the content of NO3- was increased with the increasing of total salt content. There was also another accidental factor, which was the seawater back irrigation, caused the high content of NaCl. In general, it was realized that the secondary salinization of protected soil in Nanhui was caused by accumulation of Ca(NO3)2 duo to unreasonable fertilization.The salt content of groundwater was not high and varied from 1.4 to 1.7g/L. In composition of salt in groundwater, NaCl occupied 17.11%～24.51% and Ca(NO3)2 occupied 26.31%～43.48%. The salt of groundwater was transferred upward with the elevation of groundwater level but it had less chance to shape soil secondary salinization.Finally, the key practices to prevent and alleviation secondary salinization of soil under protected cultivation, such as breeding good and salt-tolerant crop variety, rational and balanced fertilization, wash salt by fresh water and reasonable irrigation, phytoremediation and so on, were put forward according to above results.
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