Abstract
There has been little written on the penetration of light through soil. In this review, we attempt to collate most of the work that has been published on this topic in order to stimulate further research and to clarify the often‐confused literature.
Light penetration can be measured directly with, for example, a spectroradiometer, or indirectly by using germination of light‐sensitive seeds or the presence of growing algae as bioindicators. Although the penetration of light through soil is greatly affected by factors such as soil moisture content and particle size and colour, it generally appears that physiologically and ecologically significant amounts of light rarely penetrate more than 4–5 mm through the soil, and may often penetrate much less than this. Any penetration beyond 10 mm would generally not be significant, especially as most soils are covered with litter, algae or lichens, or are shaded from direct sunlight. However, for some light‐stimulated geo‐tropic responses of roots, which can be sensitive to very low fluences, the penetration of light to greater depths could well be significant. The role of light in soil in directing root growth is also discussed.