Calcium Requirement for Penetration of Subsoils by Primary Cotton Roots

Short‐term, split‐root experiments were conducted with cotton seedlings ( Gossypium hirsutum ) in which the upper portion of the root medium was a sandy loam surface soil and the lower portion was either a nutrient solution or a subsoil material at various Ca levels. The Ca required in subsurface media for penetration was dependent upon the Ca/total‐cation ratio rather than the Ca concentration per se . The Ca requirement was apparently the same in soil solutions in situ as in nutrient solutions, namely, between Ca/total‐cation ratios of 0.10 and 0.15 in all cases. Critical levels of exchangeable Ca were equal in Norfolk and Dickson subsoils when Ca was expressed as a ratio of Ca to total exchangeable cations even though the clay fraction of Norfolk is kaolinite while the clay fraction of Dickson is vermiculite with some montmorillonite. Norfolk subsoil at pH 5.0 and Dickson subsoil at pH 4.6, as obtained from the field, contained adequate Ca for normal growth of primary cotton roots.