Study on Evolution of Soil Fertility in Yunlianghu Farm

Based on the datum of soil fertility obtained by investigation, an evolution of soil fertility in Yunlianghu Farm was analyzed. The results showed that soil Organic Matter (OM) declined continuously from 1981 to 2007, in which it reduced quickly from 1981 to 1998 and slowly from 1998 to 2007, and the rate was 1.1365 g/kg and 0.58889 g/kg per year, respectively, the content of soil available phosphorus increased all the time because of a great deal of phosphorus accumulation in soil after long-term application of phosphorus fertilizer, and the rate was 0.4692 mg/kg per year. Meanwhile, available nitrogen decreased by 4.806 mg/kg per year from 1981 to 1998 and increased by 7.311 mg/kg per year from 1998 to 2007 which was below the content of 1981. Available potassium was in V-shaped fluctuation that decreased by 1.417 mg/kg per year from 1981 to 1998 and increased by 6.778 mg/kg per year from 1998 to 2007 that was the highest content. The reason of the variation of the soil fertility was as follows: a) an ecological environment for soil microorganisms had changed due to groundwater decline by drainage, which caused that OM to be decomposed was dominant, simultaneously, the availability of soil potassium was reduced because of lower soil moisture, b) the change of fertilization and cropping patterns, especially after 2000 stalk/straw of crop (rice wheat and cotton) returning to field, slowed down the decrease of OM and increased the available nutrient of the soil.