Study on characteristics and comprehensive evaluation of the fertility of tobacco cultivated soil in Fengdu karst area,Chongqing

The fertility of limestone soil cultivated with tobacco in karst areas of Fengdu county, Chongqing was studied through field sampling and lab analysis. Methods of principal component analysis and membership function of fuzzy mathematics were used to estimate soil fertility by means of estimating SFI(Soil Feasibility Index). The results showed that: (1) The soil in the area was weakly acidic to neutral. 23.08 % of soil samples had pH less than 5.00, thus there are obvious soil acidification problems in the region. (2) The soil was rich in organic matter and nitrogen. 78.85 % of the soil samples had OM content in the optimum range for planting tobacco (15～30 g/kg). All soil samples had available nitrogen content in a suitable range for planting tobacco (65～100 mg/kg) with a coefficient of variation of only 0.92 %. (3) Total phosphorus content was medium to high (>0.6 g/kg), but there was insufficient supply of available phosphorus in the soil. Phosphorus was absent (<10 mg/kg) in 20.93 % of the soil samples. (4) The soil had an adequate supply of potassium. All samples had rich or extremely rich total potassium content (> 25 g/kg). 81.84 % of the soil samples had available potassium content in the proper range for planting tobacco (>150 mg/kg); (5) The area's overall soil fertility level was not high. Only 15.38 % of samples had medium-high soil fertility (0.70>SFI≥0.60), 57.69 % had medium-low fertility (0.60>SFI≥ 0.40), and 26.93 % had low fertility. Obviously the karst areas are affected by factors including soil erosion and severe degradation,and irrational fertilization. Therefore, in karst areas attention should be given to scientific farming, reduction of soil erosion and degradation, the application of lime to regulate soil pH, and proper increase of the ratio of phosphorous in fertilizer.