SOILS OF THE ARID AND HUMID REGIONS. 379 



the contrast. This shows that for instance, in Washington 

 highly calcareous soils are directly derived from the black 

 basaltic rocks; while similarly, calcareous lands are found in 

 California to be the outcome of the decomposition of granites, 

 diorites, lavas, clay-shales and sandstones. 



It is not easy to overrate the importance of this feature of 

 the soils of the arid region, as it is intimately connected with 

 other theoretically and practically important facts, in part al- 

 ready mentioned. 



Summary of Effects of Lime Carbonate in Soils. It is best 

 to summarize, briefly, at this point, the advantages (and possi- 

 ble disadvantages), resulting from the presence of a proper 

 amount of lime carbonate in soils, so far as these are at present 

 understood. 



Physically, even a small amount of lime carbonate, by its 

 solubility in the carbonated soil-water, will act most beneficially 

 in causing the flocculation of clay and in the subsequent con- 

 servation of the flocculent or tilth condition, by acting as a 

 light cement holding the soil-crumbs together when the capil- 

 lary water has evaporated ; thus favoring the penetration of 

 both water and air, and of the roots themselves. It should be 

 added that according to the experience of the writer, amounts 

 of lime carbonate in excess of 2% do not add to the favorable 

 effects, except as would so much sand. 



As to chemical effects, among the most important are : 



1. The maintenance of the neutrality of the soil, by the 

 neutralization of acids formed by the decay of organic matter, 

 or otherwise. 



2. The maintenance, in connection with the proper degrees 

 of moisture and warmth, of the conditions of abundant bac- 

 terial life (see above, chapter o,. p. 146) ; more especially those 

 of nitrification, thus supplying the readily assimilable form of 

 nitrogen. Also in favoring the development and activity of 

 the root bacteria of legumes, and of the other nitrogen-gather- 

 ing bacteria, such as Azotobacter (ibid. p. 156). 



3. The rendering available, directly or indirectly, of re- 

 latively small percentages of plant-food, notably phosphoric 

 acid and potash ; as shown in the preceding pages. 



4. The prompt conversion of vegetable matter into black, 



