Page:Popular Science Monthly Volume 48.djvu/680

608 and even in contiguous localities, as a rule the useless ingredients are present in larger proportions than is the case in the original material from which the soil was formed, or in the latter itself.

The question as to what has become of the useful mineral plant food representing this difference is categorically answered by the analysis of the soils themselves. If we analyze, by identical methods, series of the soils of the arid and humid regions respectively, we find constant differences in their composition, that are manifestly due to the conditions under which they have been formed. They show in those of the arid regions, on the average, a markedly greater proportion of certain elements of plant food than in the soils that, under the influence of copious rainfall throughout the year, have been currently leached of whatever soluble matters were set free by weathering.

The explanation is, that when these soluble matters are retained in the soil for a length of time, they are given the opportunity of entering into the insoluble combinations already mentioned as repositories of "reserve" plant food—i. e., such as may be gradually drawn upon by plants, either by the direct solvent action of their acid root-sap, or by being again rendered watersoluble by a repetition of the weathering process.

Thus the soils of the arid region, whether containing a natural surplus of water-soluble salts in the objectionable guise of alkali or not, are found to be greatly superior, in the native stock of certain ingredients of plant food, to the average soils of the regions of abundant rainfall; their average being, in fact, equal to the most highly productive (usually alluvial) soils of the humid region.

The chief substances of which the arid soils thus retain considerable amounts that run to waste in the countries of abundant rainfall, are potash, lime, and magnesia. The average ratios of these as found in the United States, for the region east of the Mississippi, when compared with that west of the Rocky Mountains (or of the hundredth meridian), by the comparison of over a thousand analyses, are as one to three, one to fourteen, and one to six respectively.

But these numerical ratios do not adequately express some of the chief advantages enjoyed by the soils of arid regions. While the large amount of potash they contain relieves the farmer for a long time from supplying to his fields the potash fertilizers that prove so effectual and necessary in the East and in Europe, yet the almost universal presence of a surplus of lime (in the form of carbonate) is perhaps of even higher importance. To understand this it is only necessary to remind the reader of the common saying that "a limestone country is a rich country"—abundantly illustrated in the Atlantic States by the blue-grass region