Salt Contents in Soils Under Plastic Greenhouse Gardening in China

Field experiments were conducted on three main soils, brown soil, meadow soil and cinnamon soil, of Shandong Province to study the effect of plastic greenhouse gardening on soil salt contents. As compared to the open fields, the soil nutrient contents in the fields under plastic greenhouse gardening all increased significantly. The organic matter, quickly available N and P increases were extremely significant and the quickly available K increase was also significant. Along with the nutrient increases the soil salt contents increased clearly in all the soils investigated not only in the top layer but also in the deeper layers, being extremely significant in the 0～10 cm layer and significant in the 10～40 cm layers. The salt contents in the plastic greenhouses had significant correlations with the soil available nitrogen and phosphorous. Soils with longer plastic greenhouse gardening time tended to have more salt. The plastic greenhouse soils contained less CO-3～(2-) and much more NO-3 than the open soils, which indicated a higher influence of human intervention in plastic greenhouses. Among the constituent ions, Cl～(-), NO-3～(-) , Ca～(2+) and Mg～(2+) had positive while HCO-3～(-) had negative significant or extremely significant correlations with total salt, with correlation coefficients being 0.66*, 0.80**, 0.92**, 0.80** and -0.64* , respectively. Nitrate decreased from the top to deeper layers both in the plastic greenhouses and in the open fields. The plastic greenhouse soils contained much more nitrate than the open fields in every layer and even the nitrate contents of the 80～l00 cm layer were still several times those of the top layer in the open fields. The main reason for the salt increases was considered to be the inappropriate fertilization and selective absorption of nutrients by plants in the plastic greenhouses. The methods recommended to avoid soil salt increase in the plastic greenhouses were to apply fertilizers rationally according to the soils, vegetables and fertilizer properties and to adopt a good intercropping or rotation system.