Oxygen in soils

The soil is at the heart of many transformations involving oxygen. The amounts of oxygen in the soil influence many processes, such as oxidation, reduction, acidification, that take place in the soil. There are huge numbers of organisms living in the soil. These drive many of soil processes and depending on the soil environment will govern the form in which oxygen and elements containing oxygen, occur in the soil. Like the atmosphere, soil air consists mainly of nitrogen and oxygen although it has a lower concentration of oxygen than is present in the atmosphere. This is because respiration by the many organisms in the soil and the plant roots tends to reduce the amount of oxygen in the air that fills the soil pores.

The level of oxygen in the soil governs to a large extent of many of the important processes such as microbial activity and weathering of minerals, and influences the state of many of the inorganic elements that occur in soils. For example, in the presence of plenty of oxygen, nitrogen is in the form of nitrates and in this form is available to plants. When oxygen levels fall, as they do in waterlogged soils, reduced forms of carbon such as methane, and reduced forms of sulphur such as hydrogen sulphide can develop, both of which can be harmful to plant life.

Soils differ very much from one another in their ability to allow the movement of gases such as oxygen within the soil body. Sandy soils, with their large pore space, generally allow fairly free movement of gaseous oxygen whereas in clay soils it will depend on their water content which varies seasonally. Wet clay soils are generally starved of oxygen and become anaerobic and various processes, such as intense acidification and formation of undesirable compounds can then occur which are harmful to many plants. The more water there is in a soil, the smaller will be the volume of soil air and the faster will be the loss of oxygen by respiration.