Soil Secondary Salinization And Principle Of Nitrate Accumulation In Open Vegetable Fields Under Continous Cropping Of Vegetables

Based on the investigation of open vegetable fields of NanHai district in Foshan city,Guangdong province, the status, influencing factors, characteristics, and seasonalvariation of secondary salinization of vegetable soils were discussed, respectively. Andwe had further studied the effects of fertilization on soil salt accumulation and growth offlowering Chinese cabbage (Brassica campestris L. ssp. chinensis var. utilis Tsen et Lee)by pot experiment. The main results were obtained as follows:1. The total salt contents of surface soils for 136 open vegetable fields ranged from0.02 g/kg to 2.7 g/kg, the average content was 1.2 g/kg, the total salt contents of 58.1%of the samples were higher than 1.0 g/kg, and reached the low grade standard of soilsalinization. At the same time, the contents of nitrate in soils were ranged from 1.070mg/kg to 854.4 mg/kg, the average content was 194.3 mg/kg, and the nitrate contents of19.9% of the samples were higher than 300 mg/kg, which were very high. The formationof soil secondary salinization at open vegetable fields was related to over-fertilization,climate, soil texture, groundwater lever, irrigation methods, and so on.2. Some special characters of spatial distribution of soil salt were showed in openvegetable fields. The electrical conductivity of topsoil was the highest, at lower layers, theelectrical conductivity declined. Soil salt was accumulated at topsoil obviously. Views oncomposing of salt ions, the main cation was Ca²⁺, was about 13.4% to 20.7% to total salt;the main anions were NO₃^- and SO₄^2- , NO₃^- was about 15.8% to 31.0% to total salt,SO₄^2- was about 10.9% to 45.7% to total salt. One of ions with most contents was NO₃^-,the NO₃^- content was high when the total salt content was high, there was closerelationship between NO₃^- content and total salt content, statistical analysis showed that,the relationship was significant. Views on seasonal variation, the electrical conductivityand the contectsof most salt ions of every layer soil in summer were lower than that inspring, indicated that the soils were in desalinization condition in this period, and that wasrelated to frequent rainfall in the period. The electrical conductivity of three samplingpoints in autumn and winter was lower than that in summer, and other two samplingpoints was higher than that in summer, it was related to different fertilizer application rateand soil texture. The status of different layer soils and different kinds of ions was different.The electrical conductivity was significantly correlated to total salt content, and topsoilsalt ions contents were correlated to total salt content in different extent.3. In the pot experiment, under cropping of flowering Chinese cabbage andun-cropping, the salt were both obviously accumulated at topsoil, the electrical conductivity of 0～2 cm layer was obviously higher than other layers. The electricalconductivity of 0～2 cm and 2～5 cm layers was higher than that before experiment, andthat of 5～10 cm layer was lower than that before experiment. When planting floweringChinese cabbage, the electrical conductivity of 0～2 cm layer soil nearby the root washighest, salt moved to root surface and accumulated. The electrical conductivity of everylayer under cropping was all lower than that under un-cropping. Fertilization can increasethe electrical conductivity of soil obviously, the electrical conductivity of every layerincreased with fertilization rate increase under cropping and un-cropping, and it was mostobvious at 0～2 cm layer. With the soil electrical conductivity going up, the content ofproline, chlorophyll a, chlorophyll b, total chlorophyll and nitrate in flowering Chinesecabbage increased; the value of chlorophyll a/b and plant height decreased; the yieldbegan to decrease from nitrogen application rate of 0.1 g/kg.