Rhizoliths are organosedimentary structures formed in soils or fossil soils (paleosols) by plant roots. They include root moulds, casts, and tubules, root petrifactions, and rhizocretions. Rhizoliths, and other distinctive modifications of carbonate soil texture by plant roots, are important for identifying paleosols in the post-Silurian geologic record. Rock units whose structure and fabric were established largely by the activity of plant roots are called rhizolites.

Colin F. Klappa first proposed the term rhizolith for various organosedimentary structures produced by the activity of plant roots in 1980, and his terminology has since been widely adopted with some extensions.

Root moulds are tubular voids that preserve the shape of a root that has subsequently decayed away. Such voids will collapse unless the root penetrated soil that was already at least partially lithified. Closely packed, very thin root moulds give the sediments an alveolar texture.

Sediments or minerals that fill a root mould and become cemented produce a root cast.

Root tubules are cemented cylinders around a root mould. The cement is typically calcite and is responsible for the preservation of root morphology in otherwise poorly consolidated sediments. Root tubules can form while the root is still alive or during its decay, and often take the form of fine, needle-like calcite crystals that preserve the root tubule after the root has completely decayed.

Root petrifactions are similar to petrified wood and are formed when minerals encrust, impregnate, or replace the organic matter of a plant root, sometimes preserving it in great detail. The replacement mineral is typically calcite. Cell walls are most commonly preserved, perhaps because calcium pectate is already present in the walls.

Rhizocretion is distinguished from petrifaction by the manner of formation. Petrifaction is defined as 'a process of fossilization whereby organic matter is converted into a stony substance by the infiltration of water containing dissolved inorganic matter, such as calcium carbonate and silica, which replaces the original organic material, sometimes retaining the original structure'. Thus root petrifaction is a process which involves replacement, impregnation, encrustation and void-filling of organic matter by mineral matter without total loss of root anatomical features. By contrast, rhizocretions which include rhizoliths, are created by the accumulation of mineral matter around roots. Accumulation, usually accompanied by cementation, may occur during life or death of plant roots.

Rhizohaloes are zones of chemical reduction around decomposed plant roots. These typically appear as elongated gray mottles with reddish rims. They form when iron and manganese are reduced close to the root and the soluble reduced metals diffuse outwards. The metals are then oxidized again and deposited as hematite or goethite.