pH variation and the driving factors of farmlands in Yangzhou for 30 years

Objectives Soil pH is an important index to measure quality of cultivated lands. The influence of cropping system, cultivation system, rainfall and fertilization practices on soil acidification over the last 30 years was investigated.Methods This study investigated soil pH, soil parent material, soil type, soil organic matter content in 1984, 1994, 2005 and 2014, and the cultivation system, cropping system, rainfall, types of fertilizer application and their inputs in these years. The data came from 16996 sampling points in Yangzhou City, including 4107 agricultural chemical soil samples of the second general soil survey in 1984, 2862 general soil survey points in 1994, 4018 soil nutrients survey points in 2005, and 6009 soil nutrients survey points in 2014. Referencing to Chinese Soils, we classified the soil pH into five grades:Ⅰ ( 7.5), Ⅱ (6.5-7.5), Ⅲ (5.5-6.5), Ⅳ (4.5-5.5), and Ⅴ ( 4.5). The method of Kriging and relevant statistical methods in geostatistics were used in this study, and ArcGIS 10.1 and SPSS19 were used as tools for analyzing data.Results The results showed that the average soil pH was 7.51, 7.07, 6.83 and 6.74 respectively in 1984, 1994, 2005 and 2014. The spatial distribution of pH in the four periods was relatively constant, i.e. Lixiahe area Yangtze River diked area Sandy soil area in Tongnan Hilly region. In 1984, 1994 and 2005,the soil pH mainly belonged to Grade I and Ⅱ, which accounted for more than 90% of total area in 1984 and decreased to 75% in 1994 and 2005; in 2004, the soil pH mainly belonged to Grade Ⅱ and Ⅲ, which accounted for 65.7% of total area. In 30 years, soil pH was significantly decreased, 47.2% areas decreased by 0-1 unit and 39.3% areas decreased by more than 1 unit. In the first 20 years, soil pH was obviously declined and over 80% area soil pH declined by 0-2 units. The overall soil pH of different soil parent materials and soil varieties decreased by 0.9 unit and 0.8 unit, respectively. There was a negative correlation between changes of soil organic and pH. The average content of soil organic matter was increased by 6.01g/kg, it was one of the reasons for the decrease of soil pH as a whole. The pH value of the rainfall in this city decreased, especially rapid for the hills and areas alongside the river. This was consistent with the view of rapid decline of soil pH in hills and areas alongside rivers. The input of chemical fertilizer was negatively correlated with the change of soil pH. In 2005, the input of chemical fertilizer was about 505300 tons, increased by 2.42 times comparing with the input in 1984. From 2005 to 2014, the input of chemical fertilizer showed a steady trend, which was consistent with the trend of soil pH in these 30 years. The average soil pH in vegetable protected house was lower than that in the surrounding wheat field by 1.5-2 units, which indicated that the types of land use also affected soil pH. In general, the influencing factors were land use types and soil parent materials, and the factors of influencing temporal distribution of pH were fertilization and acid rain. Both acid rain and fertilization were the important driving factors of soil acidification.Conclusions The arable land soil pH in the study area showed an acidification tendency during 1984-2014. The spatial distribution of soil pH in the study area was mainly influenced by the parent materials of soil, soil types, content of soil organic matter, and the temporal distribution of soil pH in the study area was influenced by acid rain, fertilization and land use type. Acid rain and fertilization were the main factors causing soil acidification.