Within three years of deforestation in rain forest areas, the forest becomes highly acidic. Only with the application of lime will the soils recover and become useful again without waiting generations for the soils to naturally recover. Those interested in developing sustainable agriculture also worry about acid rain. Acid rain speeds up the process of nutrient leaching and creates even more pressure to use lime to rejuvenate soils. Salinity is also important to control.
Fields can naturally salt up over time.
In general, irrigation and the subsequent evaporation of water leaves salt on the soil. This process along with desertification is what lead to the decline of the ancient civilizations lining the Tigris and Euphrates rivers. The salt built up on the fields, and crop yields decreased. If the salt content is high, plants have a more difficult time sucking up water. They wilt and are more vulnerable to pests.