Maintaining a balanced pH level is very important in producing high yields of crops. While every crop is different, most favor a pH range of 6.5 to 7.0 in the soil type.
Why is soil pH an issue for sustainable agriculture? In natural ecosystems, soil pH is balanced by the varied plant species and natural processes. Mono cropping creates an artificial ecosystem which alters the pH of the field. The only method of stopping a change in pH is adding lime periodically to the field to buffer it. Poorer countries may not always have enough capital to purchase lime to restore fields. Besides the crops,Many beneficial soil organisms require the soil to be kept within a certain range. When the soil pH changes they die, and lower crop yields result.
What causes changing pH in fields? Weathering of the parent rock material can influence pH change. Crop removal of Calcium with crops such as alfalfa can tend to make the soil more acidic. Grassland vegatation with high nutrient recycling can create alkaline conditions. In humid regions water quickly leaches away basic cations. 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.
A good way to slow the salting of soils is to irrigate at night. This reduces the evaporation. Drip watering is also beneficial and uses less water. The problem with drip irrigation systems is they are more expensive to construct.
Agricultural systems must be careful not to allow salt build up in the soil. Once salt becomes a problem, it is very difficult and expensive to get rid of. The pH value of a field is more easy to correct. However, this is a re-ocurring problem in agricultural systems and must continuing be addressed.