CHAPTER XVIII. PHYSICO-CHEMICAL INVESTIGATION OF SOILS IN RELATION TO CROP PRODUCTION, 313.—_Historical Review of Soil Investigation_, 313.—Popular Forecasts of Soil Values, 313.—Cogency of Conclusions Based upon Native Growth, 314.—Ecological Studies, 315.—Early Soil Surveys of Kentucky, Arkansas and Mississippi, 316.—Investigation of Cultivated Soils, 316.—Change of Views, 317.—Advantages for Soil Study offered by Virgin Lands, 318.—Practical Utility of Soil Analysis; Permanent Value _vs._ Immediate Productiveness, 319.—_Physical and Chemical Conditions of Plant Growth_, 319.—Condition of Plant-food Ingredients, in the Soil, 319.—Water-soluble, Reserve, and Insoluble Part, 320.—Hydrous or “Zeolitic” Silicates, 321.—Recognition of the Prominent Chemical Character of Soils, 322.—Acidity, Neutrality and Alkalinity, 322.—Chemical Analysis, 323.—Water-Soluble and Acid-Soluble Portions most Important, 324.—We cannot Imitate Plant-root Action, 324 Cultural Experience the Final Test, 324.—Analysis of Cultivated Soils, 325.—Methods of Analysis, 325.—_The Solvent Action of Water upon Soils_, 327.—Extraction of Soils with Pure Water, 327.—Continuous Solubility of Soil Ingredients. Tables, 328.—King’s Results. Table, 329.—Composition and Analysis of Janesville Loam, 331.—Solubility of Soil Phosphates in Water, 332.—Practical Conclusions from Water Extraction, 332.—_Ascertainment of the Immediate Plant-food Requirements of Cultivated Soils by Physiological Tests_, 333.—Plot Tests; their uncertainties. Diagram, 334.—Crop Analysis as a Test of Soil Character, 337.—Chemical Tests of immediately Available Plant Food; Dyer’s Method, 338.