Variations of pH value, enzyme activity and nitrogen phosphorus content in protected vegetable soils

In order to know the basic physical and chemical properties of protected vegetable soils, pH and content of organic matter, nitrogen, phosphorous, enzyme activity in the fluvo-aquic vegetable soil and Heijiang vegetable soils were reported. The results showed that the soil pH value was decreasing at a rate of 0.05-0.06 units per year on average. The fluvo-aquic vegetable soil had been switched from alkaline to neutral soil within 15 years, while the Heijiang vegetable soils had been transformed from neutral to acidic soil. The content of soil organic matter and total nitrogen in vegetable soils was 11.6%～62.8% higher than that of cereal crop soils. NO3-1-N was accumulated in vegetable soils, and the content of nitrate in old vegetable soils was 21.91～49.52 mg/kg and was 13-18 times higher than that in cereal crop soils. Phosphorous especially inorganic phosphorous was accumulated in plow layer. The total P content in 10-year old vegetable soils was 1000 mg/kg higher than that of cereal crop soils. The accumulation rate of P was nearly 100 mg/kg per year. Inorganic P (IP) was accumulated 1335.7 mg/kg in the top soil of 15-year old fluvo-aquic vegetable soil, while organic P was accumulated only 67.9 mg/kg, 20 times less than IP. The content of available P increased with the increase of cultivation years, and in 10-year vegetable soils, the content of available P is above 200 mg/kg, the form and content of IP was varying. The content of Ca8-P and Ca2-P was higher in fluvo-aquic vegetable soils, while Al-P and O-P contents were higher in Heijiang vegetable soils. The content of Ca8-P attained its highest value at 10th year in fluvo-aquic vegetable soil. Enzyme activities of soil urease and alkaline phosphatase were 2 times higher in fluvo-aquic vegetable soils than that in Heijiang vegetable soils. Activities of neutral phosphatase and acidic phosphatase in Heijiang vegetable soils were 4 times higher than that in fluvo-aquic vegetable soils. Catalase activity had nothing to do with soil types. Soil urease, acidic phosphatase, neutral phosphatase and catalase activity in fluvo-aquic vegetable soils were higher than that in cereal crop soils. Soil urease and neutral phosphatase activity in Heijiang vegetable soils were higher than that in cereal crop soil. Soil urease activity was sensitive to vegetable soil age and type. So after a few years of protected cultivation, soil basic properties would change. Measures should be taken to prevent soil acidification, nitrate and phosphorous accumulation