Indirect photocatalysis may occur via the production of a reactive species which then participates in another reaction. For example, photodegradation of certain compounds has been observed in the presence of kaolinite and montmorillonite, and this may proceed via the formation of reactive oxygen species at the surface of these clay minerals. Indeed, reactive oxygen species have been observed when soil surfaces are exposed to sunlight. The ability of irradiated soil to generate singlet oxygen was found to be independent of the organic matter content, and both the mineral and organic components of soil appear to contribute to this process. Indirect photolysis in soil has been observed to occur at depths of up to 2 mm due to migration of reactive species; in contrast, direct photolysis (in which the degraded compound itself absorbs light) was restricted to a "photic depth" of 0.2 to 0.4 mm. Like certain minerals, organic matter in solution, as well as particulate organic matter, may act as an indirect catalyst via formation of singlet oxygen which then reacts with other compounds.