320 



SOILS. 



liable to be washed or leached out of the soil by heavy rains 

 or irrigation, and would be lost in the country drainage. It 

 is therefore clearly desirable that only a relatively small pro- 

 portion of the useful soil-ingredients should be in the water- 

 soluble or physically absorbed condition, but that a larger sup- 

 ply should be present in forms not so easily soluble, yet ac- 

 cessible to the solvent action which the acids of the soil and of 

 the roots of plants are capable of exercising. This virtually 

 available supply we may designate as the reserve food-store. 

 Finally, there is practically in all soils a certain proportion 

 of the soil-minerals in their original form, as they existed in 

 the rock-materials from which the soil was formed. These 

 minerals being usually in a more or less finely divided or pul- 

 verulent condition, they are attacked much more rapidly by 

 the chemically-acting " weathering " agencies, viz., water, 

 oxygen, carbonic and humus acids, than when in solid masses ; 

 and thus, transformation of the inert rock-powder into the 

 other two classes of mineral soil-ingredients progresses in 

 naturally fertile soils with sufficient rapidity to produce, in a 

 single season, sensible and practically important results, known 

 as the effects of fallowing. 



The Reserve. The nature of these processes has been dis- 

 cussed in chapters I to 4; and it will be remembered that two 

 of their most prominent results are the formation of clay, and 

 of zeolitic-compounds, the latter being, as heretofore stated 

 (pp. 36 ff) hydrous silicates of earths and alkalies, easily de- 

 composable by acids, and also capable of exchanging part or 

 the whole of such basic ingredients with solutions of others 

 that may enter the soil. These zeolitic compounds therefore 

 fulfil two important functions in the premises, viz. : a ready 

 yielding-up of part of their ingredients to acid solvents, and 

 a tendency to fix, by exchange, a portion or the whole of the 

 soluble compounds that may be set free in, or brought upon 

 the land. The first-mentioned property is of direct avail in 

 that the soil-humus forms, and the roots of plants exude, acid 

 solvents on their surface, and can thus draw upon the reserve 

 store of food; the second tells in the direction of preventing 

 the waste of water-soluble manurial ingredients supplied to, 

 or formed in the soil. (See above, chapter 3, page 38). 



