6o8 POPULAR SCIENCE MONTHLY. 



and even in contiguous localities, as a rule the useless ingre- 

 dients 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 iden- 

 tical 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 re- 

 tained in the soil for a length of time, they are given the oppor- 

 tunity of entering into the insoluble combinations already men- 

 tioned 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 water- 

 soluble 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 cer- 

 tain 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 consid- 

 erable 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 Moun- 

 tains (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 under- 

 stand this it is only necessary to remind the reader of the com- 

 mon saying that "a limestone country is a rich country'' abun- 

 dantly illustrated in the Atlantic States by the blue-grass region 



