Total salt content and ion composition in tillage layer of soils in the main vegetable production regions of China

【Objectives】 The investigation on the total salt content and the main ions will provide a scientific basis for prevention and remediation of secondary soil salinization caused by fertilization in vegetable production. 【Methods】 In this study, 501 representative soil samples in plow layers were collected from the main vegetable production regions in the Northeastern, North, Northwestern, Southwestern, Central, East and South China. The total salt content and the ratio of main salt ions NO-3, SO2-4, HCO-3, Cl-, Na+, K+, Ca2+ and Mg2+ in the samples were analyzed.【Results】 1) The contents of total salt and individual salt ions (NO-3, SO2-4, Cl-, Na+, K+, Ca2+ and Mg2+) in the vegetable soils of greenhouse (with back wall and without back wall) were 69.3%, 36.2% to 170% higher than those in open fields (P0.05), respectively. 2) The percentage of soil samples with total salt contents beyond the critical value of secondary salinization (soil total salt content ≥ 2 g/kg) in the greenhouse vegetable fields was distinctly higher than in the open fields. The greenhouse vegetable fields were still in generally light degree of salinity, 38.2% of them belong to light salinity (2-5 g/kg), and 4.7% belong to medium and above medium salinity (≥ 5 g/kg). 3) The total salt contents and main ion composition (NO-3, SO2-4 and Ca2+) in greenhouse vegetable soils tended to accumulate fast at first, and reached peaks in the 5th to 6th year of vegetable production, then slowed down and kept stable. 4) In greenhouse vegetable fields, NO-3 and SO2-4 were the dominant ions, averagely accounting for 27.9% and 26.9% of the total salt content, respectively. The contribution of NO-3 and SO2-4 to the total salt content varied in different greenhouse vegetable production regions. Ca2+ was the second main contributor to the total salt contents, averagely accounting for 15.3%. In the open fields, the major salt ions were SO2-4, NO-3, Ca2+ and HCO-3, their contribution to the total salt contents was generally in order of: SO2-4 NO-3 Ca2+ and HCO-3, the SO2-4, NO-3, Ca2+ and HCO- 3contents accounted for 29.0%, 21.1%, 16.1% and 14.1% of the total salt content, respectively. In Northwestern China, HCO-3 was the major salt ion in open vegetable soils, followed by SO2-4, NO-3 and Ca2+. 【Conclusions】 Although the percentage of soil samples with total salt contents beyond the critical value of secondary salinization was distinctly higher in greenhouse than in the open vegetable fields, the greenhouse vegetable fields were still in generally light degree of salinity. The major salt ions contribution to the total salt contents was generally in order of: NO-3, SO2-4Ca2+ for the greenhouse vegetable fields, and SO2-4 NO-3 Ca2+ and HCO-3 for the open vegetable fields.