Salt accumulation and ionic composition change of soil in continuous vegetable growing under structure in Shanghai suburbs

The salt content,ionic composition and pH of soil in different-age vegetable fields under structure were determined in Shanghai suburbs,and the correlations between the NO_3~- and total salt contents of soil and the ionic concentrations of different salts were analyzed.The results indicated that (1) the salt content and electric conductivity(EC) of soil in plastic tunnels after 2 years or more were significantly higher than those in one-year-old plastic tunnels,and except HCO_3~-,7 major ionic concentrations increased significantly;(2) Vegetable fields under structure took on a clear trend of soil acidification, and their soil pH declined to 6.38 after 6 years or more;(3) With time,ratios of NO_3~-. Cl~-,K~+and Mg~(2+)to total salt increased but ratios of SO_4~(2-)and HCO_3~- to total salt decreased,while Ca~(2+)and Na~+changed little;(4) There were significant positive correlations between nitrate nitrogen content,total salt,EC and contents of major ions except HCO_3~-.