MINERAL CONSTITUENTS OF SOIL. 213 



The protoxide of iron is generally unfavorable to vegeta- 

 tion, but the peroxide seems to act the part of an alkali, con- 

 verting the vegetable substances into the proper state to be 

 absorbed by the roots of plants, while the protoxide does not. 

 Dr. Dana says, that if" iron peroxidates itself in contact with 

 vegetable fhre, the texture of the vegetable fibre is weakened, 

 and geine is produced, and that in a few hours. It is during 

 the passage from protoxide to peroxide, that the 'saponifying^ 

 action takes place, geine is produced, and then combines with 

 the peroxide.''^ 



Oxide of iron is also found in the vegetable substance, and 

 must be carried there in some of its combinations with acids, 

 as the oxide is insoluble. It probably combines for this pur- 

 pose, with some of the organic acids in vegetable mould, 

 such as crenic or humic acids, or both, and thus acts the part 

 of a base to those acids, which are the products of the living 

 principle. 



Crenate of iron and of alumina are deposited in iron tanks 

 where river water runs. Liebig regards the oxide as perform- 

 ing the office of absorbing and retaining ammonia. 



Oxide of manganese. But very small quantities of this ox- 

 ide are found in soils, and still smaller quantities in plants. 

 It probably acts in a manner similar to oxide of iron, forming 

 a base for the combination of the humic or crenic acids. 



3. Salts and Urets. Under this head, are included sev- 

 eral compounds which analysis has detected in soils. Such 

 as common salt, sub-phosphate of alumina, phosphate of lime, 

 nitrate of potash and of soda, sulphate of lime, sulphate of 

 iron (copperas), sulphuret of iron, etc. Some of these sub- 

 stances have received attention in other places, and but a 

 few remarks are required to show what is most important to 

 be noticed respecting them. 



Common salt, or chloride of sodium, constitutes about 2 J 

 per cent, of sea-water. It also exists in the rocks, especially 

 in the new red sand-stone. It seems to act as an alkali, by 

 , 18* 



