74 ORGANIC ELEMENTS OF SOIL. 



First. Those which are soluble and precipitable by acid, 

 viz. : ulmic, humic, geic acids. 



Secondly. Those which are not precipitated by acid, viz. : 

 crenic and apocrenic acids. 



In consequence of the ammoniacal combinations with the 

 acids, these are usually found in water drained from soil. 

 Hence, long before the existence of crenic and apocrenic 

 acid was suspected, the substance given up by soil to water 

 was called extract of mould ; the substance dissolved by 

 alkali was called humic acid, from humus, the Latin for soil, 

 or mould. These several substances are convertible. The 

 insoluble becomes soluble, the difficultly soluble in water 

 becomes easily soluble by air and moisture. Ulmic, humic, 

 and geic acids are seldom, and crenic and apocrenic acids are 

 never found free in soil. Hence, in soil, no less than five 

 separate and distinct salts of any one base may be found, 

 but generally the salts are chemical combinations of several 

 bases with each acid, which compound salts are soluble, 

 though singly some may be insoluble in water. These 

 compound salts minister to vegetation in various ways. 



103. Great difference of opinion has prevailed respecting 

 the real constitution and uses of the substances of the class 

 geine. It is practically useful to discuss the question, 

 whether plants draw their carbon, hydrogen, oxygen, nitro- 

 gen, from the air, or from the soil. The nourishment drawn 

 from air, depends on the great physical elements, air, tempe- 

 rature, moisture. Agriculture may not control these. It 

 can palliate them only by controlling that within its power, 

 the state of the soil. With all above ground, the farmer 

 has little concern. If plants are nourished chiefly from the 

 air, it is evident that the farmer is concerned only to pro- 

 duce that state of the development of the organs of plants, 

 best adapted to the aspiration of the aerial elements. This 



