In order to comprehend the acidity characteristics of mountainous soils planted tobacco for many years

the characteristics of soil acidity index and their relations were studied in Xiangxi Autonomous Prefecture

as well as the effects of planting years

soil types

organic matter and clay on soil acidity were studied. The results were as follows:(1) The effect of tobacco planting years on soil acidity in Wuling mountain area was greater than that of soil types

and with the increasing of planting years

the soil pH

the exchangeable base cations

cation exchange capacity and base saturation of yellow-brown soil decreased

while the potential acidity increased. (2) The relationship between soil pH and exchangeable acid was a power function

and exchangeable Alex was the main body of exchangeable acid in soil

the strength of soil exchangeable acid increased with the increasing of exchangeable Alex and its relative proportion. (3) The relationship between soil pH and exchangeable base cations

cation exchange capacity and base saturation was a quadratic curve

and when the soil pH was less than 6

the exchangeable base cations and cation exchange capacity decreased with the increasing of soil pH

while the base saturation increased; when soil pH was more than 6

the exchangeable base cations and cation exchange capacity increased with the increasing of soil pH

while base saturation fluctuated slightly. (4) Soil exchangeable base ions were mainly exchangeable calcium

followed by exchangeable magnesium

and the exchangeable calcium was the most important factor influenced soil pH. (5) Soil organic matter and clay mainly affected the soil pH

cation exchange capacity

exchangeable base cations and exchangeable calcium.