Morphology of a Paralithic Contact in a Soil Over Soft Sandstone

Abstract Root distribution was studied in a soil having a paralithic contact in southeastern Montana. An abrupt change in root abundance was the most useful criterion for locating the paralithic contact. The soft rock below the contact is part of the Tongue River member of the Ft. Union formation. The soft rock is indistinguishable from the overlying soil in texture, color, structure, and water‐holding capacity, but has a bulk density of 1.50 g/cm 3 and is hard whereas the soil has a bulk density of only 1.35 g/cm 3 and is soft when dry. The soil, which has a paralithic contact within 50 cm, supplied 86% as much water to plants during the 1976 growing season as a nearby, texturally similar, deep soil. The difference was less than expected. Removal of water from the soft rock apparently occurred through a few roots which follow widely spaced, vertical pockets of soil deep into the soft rock. Roots from the pockets proliferate horizontally along natural laminations and planes of weakness in the soft rock. Water is removed from as deep as 30 cm below the paralithic contact during the growing season. Present interpretation of paralithic contacts is discussed as it relates to this research. Proposed revisions to the definition are considered and further research is suggested.