Effect of crop rotation on ion accumulation in secondary salinization soil of vegetable field in greenhouse

Soil samples from ten plastic greenhouse vegetable fields (PGVF) and another ten soil samples from open vegetable fields (OVF) were collected in Aug. 2007 in Yixing City, Jiangsu Province. The soils were analyzed and compared for soil ion accumulation and soil electrical conductivity (EC). Two PGVFs with secondary salinization soil were selected to compare two crop rotation systems (CCC: castor-Chinese cabbage-castor; PCP: pepper-Chinese cabbage-pepper) for EC and ion accumulation. The study showed higher soil EC under PGVF (average of 433 μS·cm^-1) compared with OVF (178 μS·cm^-1). About 20% of the samples was detected with secondary salinization for EC >500 μS·cm^-1 under PGVF. SO₄^2- and NO₃^- anions were statistically higher in PGVF than in OVF. This implied that SO₄^2- and NO₃^- were the main ions controlling soil salinization in the region. Field trial results showed that soil EC dropped by 5% and 33% after three consecutive crops in two years of CCC and PCP rotations, respectively. PCP rotation more effectively alleviated secondary soil salinization in PGVF than CCC rotation. SO₄^2- ion accumulation was different in the two rotation systems, which dropped in PCP and rose in CCC. It was concluded that suitable rotation system should be adopted based on the main ions of secondary salinized soils and plant selectivity of ion uptake.