Soil acidity is very important. Factors which may change soil pH: * Use of ammonium fertiliers --> More acid soil * Acid rain --> More acid soil * Vegetation rotting --> More acid soil This acidity can be removed by adding crushed limestone (crushed to improve surface area, and therefore rate of reaction) * Calcium hydroxide and calcium oxide are also used to make soil more basic, however as this is soluble, it will make the soil basic