In the southwestern U. S. A., layers of calcareous hardpan, or "caliche," are abundant over extensive areas. Its formation is due primarily to the interrupted penetration of rain water under arid conditions. It occurs in hard layers and in amorphous masses, in both of which forms it has an important influence on the physical and chemical properties of the soil and on the development of the root system. The maximum water content is 3.2-6.5% for the hard layers but 12.9-17.3% for the softer masses. Even the thinnest layers greatly retard the upward or downward movement of water in the soil. Evaporation from caliche which is underlaid by water is less than from an equal surface of a porous cup atmometer. Considerable differences of moisture content may exist in bodies of soil separated by a layer of caliche. Caliche will convey water more rapidly from soil beneath it to soil above it than it will from soil to the atmosphere. Roots are unable to penetrate the silicified hard layers. The abundance of caliche in the soil of bajadas appears to be an important condition in determining the low and open character of the vegetation, chiefly restricted in such localities to the creosote bush, Larrea tridentata. Cultures of Larrea were made in 18 types of soil, varying in texture and % of soft caliche. The poorest growth was made in pure caliche; the best growth in soils containing equal amounts of caliche and sand, loam, or clay respectively, or these soils with lower % of caliche. The dry wt. of the plants decreased with increasing % of caliche. The CaO in the ash of the cultured plants was 20.24-39.19% and MgO 2.06-6.02%. The varying amounts of caliche in the soils were without influence on the amounts of CaO and MgO in the ash. The fact that the Ca/Mg ratio of the young cultured plants is consistently higher than that of mature plants growing in their natural habitats is due to the higher CaO, and may indicate greater drought resistance on the part of the young plants. The chemical properties of highly calcareous soils appear to be of less importance than the physical in relation to the growth of Larrea.