Soil improvement work is mainly targeted at neutralizing acid soil.
Acid soil contains much hydrogen or aluminum ions.
These ions obstruct normal growth of plants by fixing the roots of plants and solidifying plants' albumin. They also give a negative effect on the functions of bacteria necessary for plants. To resolve these effects, acid soil is neutralized with alkalis such as lime. However, appropriate acidity differs according to plants. For example, slightly acid soil is favorable for rice, while barley prefers alkali soil.