ORGANIC PORTIONS OF SOIL. 217 



The crenic acid has been detected in the sub-soil, and this 

 may account for the utility of sub-soil ploughing. It is also 

 found in river water, and may account for the effects of irri- 

 gation. The compounds of crenic acid, described p. 136, 

 are also found in small quantities in the soil. 



Apocrcnic acid is formed, as its name imports, from the 

 crenic, by simply exposing the latter to the air. It was found 

 in the water of Porla well, in connection with the crenic acid. 

 This acid is found in very small quantities ; in some soils, ac- 

 cording to Berzelius, about two per cent. 



As all the alkalies dissolve these two acids, and as the al- 

 kaline earths render the inert crenates active, we can seethe 

 utility of adding alkaline substances to the soil to act upon 

 these acids, and to bring them into a fit state to enter the vege- 

 table organs. 



" The remarks of De Saussure on soils," says Berzelius, 

 " seem to show, that the three constituents above described, 

 crenic, apocrenic and humic acids, by means of the recipro- 

 cal influence of water and air, become mutually changed. 

 Water in moist soil, changes a part of the insoluble humin 

 into humic acid ; so that, after a sufficient length of time, the 

 greater part of the humin becomes soluble. The atmosphere, 

 on the other hand, re-forms, from the soluble matter, humin. 

 Coal of humus, which in contact with the air changes a por- 

 tion of it into carbonic acid, is itself converted into humin 

 and humic acid, and this appears in fact to be the useful ef- 

 fect of loosening the soil by tillage which exposes it to the 

 influence of the air." 



The extract of humus and glarin are brown matters, mostly 

 composed of carbon, hydrogen, and oxygen ; and, so far as 

 we know, are of little use to vegetation, in their pure state. 

 They may furnish matter for nutriment, after being acted up- 

 on by the air or alkalies. 



Dr. C. T. Jackson states, that the substances which are 

 confounded under the name of soluble humus, soluble geine. 



