SOILS. 23 



large pebbles, and even stones and rocks, are continually 

 broken up by the combined action of the vital roots and the 

 constituents of soil?;, by which new elements of vegetable 

 food are developed and become available, and in a form so 

 minute* as to be imbibed by the spongioles of the roots, and 

 by the absorbent vessels, they are afterwards distributed 

 in their appropriate places in the plant. Where this action 

 has been going on for a long period, a manifestly beneficial 

 oiicct has immediately followed from bringing up and mixing 

 with the superficial earth, portions of the subsoil which have 

 never before been subject to cultivation. 



A subsoil which is permeable to water, is sometimes 

 imperceptibly beneficial to vegetation, not only by allowing 

 the latent moisture to ascend and yield a necessary supply 

 to the plants, but a moisture frequently charged with lime 

 and various saline matters, which the capillary attraction 

 brings from remote depths below the surface. It is probably 

 from this cause, that some soils produce crops far beyond t he- 

 yield which might be reasonably looked for from the fertili- 

 zing materials actually contained in them. This operation 

 is rapidly going forward during the heat of summer. The 

 water thus charged with saline matters ascends and evaporates 

 at and below the surface, leaving them diffused throughout 

 the soil. After long continued dry weather, a thin white 

 coating of these salts is frequently discernible on the ground. 



Where rain seldom or never falls, this result is noticeable in 

 numerous and sometimes extensive beds of quiescent (not 

 shifting) sand. Deposits ofttimes occur several inches in 

 thickness. Such are the extensive beds of impure muriate 

 of soda and other salts in the arid deserts of California ; in 

 the southern parts of Oregon; the nitrates found in India, 

 Egypt, Peru, and various other parts of the world. 



ADDITIONAL PROPERTIES OF SOILS. 







Besides the qualities of soils already noticed, there are 

 several physical conditions which affect their value. They 

 should be of sufficient depth, friable, or easily pulverized, 

 they should possess the right color, and be susceptible of the 

 proper admission and escape of heat, air, and moisture. 



Jethro Tull, who wrote more than a. century ago on the sub- 

 ject of agriculture, maintained that if a soil be worked to a 

 proper depth and perfectly well pulverized, nothing more is 

 necessary to insure an indefinite succession of the most 

 luxuriant cropg without the aid of manuros; and it must be 



