Summing up the facts concerning the water of the sea and of landlocked lakes, with reference to the ingredients of soils needful for the nutrition of plants, it appears that the rock ingredients leached out in the largest amounts (lime alone excepted) are those of which the smallest quantities only are required by most plants; while of those specially needful for plant nutrition, only potash is removed in practically appreciable amounts by the stream drainage. _Result of insufficient Rainfall; Alkali Soils._—When the rainfall is either in total quantity, or in consequence of its distribution in time, insufficient to effect this leaching, the substances that otherwise would have passed into the drainage and the sea are wholly or partially retained in the soil; and when the rainfall deficiency exceeds a certain point, the salts thus retained may become apparent on the surface in the form of saline efflorescences, or as it is usually termed in North America, “alkali.”[13] Their continued presence modifies in various ways the process of soil formation and the nature of the soils as compared with those of regions of abundant rainfall (“humid climates”); one of the most prominent and important results being that, besides the easily soluble salts mentioned above, the _carbonate of lime_ formed in the process of decomposition is also retained, and imparts to the soils of regions of deficient rainfall (“arid climates”) the almost invariable character of _calcareous_ lands. There is thus in the United States a marked and practically very important contrast between the soils of the arid region west of the Rocky Mountains and those of the “humid” region between the immediate valley of the Mississippi and the Atlantic coast. These differences and their practical bearings can be best discussed after first considering more in detail the chemical decomposition of the several soil-forming minerals. [13] In some cases the soluble salts originate in rocks impregnated with salts from marine lagoons or landlocked lakes, or directly from their evaporation residues. But this is the exception rather than the rule. CHAPTER III. THE MAJOR SOIL-FORMING MINERALS.