CHAPTER XX.

SOILS OF THE ARID AND HUMID REGIONS, 371.—Composition of Good Medium Soils; Table, 371.—Criteria of Lands of the Two Regions, 371.—Tables of Soil-Composition in Both Regions, 372.—Soils of the Humid Region governed by Time, 374.—Soils of the Arid Region Governed by Moisture, 374.—Lime and Magnesia Uniformly High in Arid Soils, Despite Scarcity of Limestone Formations; Potash also High, 374.—_General Comparison of the Soils of the Arid and Temperate Humid Regions_, 375.—Basis of Same, 376.—New Mexico and Analysis of Soil, 376.—General Table, 377.—Discussion of the Table, 378.—_Lime_; Summary of Physical and Chemical Effects of Lime Carbonate in Soils, 378.— Discussion of Summary, 379.—_Magnesia_: Its role in Plant Nutrition, 381.—_Manganese_: Its Stimulant Action, 383.—The “_Insoluble Residue_” or Silicates, 384.—_Soluble Silica and Alumina_, 384.—Analysis of Clay from Soil, 385.—Difference in Sand of Arid and Humid Regions. Table, 386.—Soluble Silica or Hydrous Silicates more Abundant in Arid than in Humid Soils, 388.—_Aluminic Hydrate._ Table, 389.—Retention of Soluble Silica in Alkali Soils, 391.—_Ferric Hydrate_, 392.—_Phosphoric Acid_, 392.—Sulfuric Acid, 394.—_Potash_ and _Soda_, Retained more in Arid Soils, 394.—Arid Soils Rich in Potash, 395.—_Humus_, Low in Arid Soils, but Rich in Nitrogen, 396.—The Transition Region, 397.