74 THE SOIL. 



Oxide of iron exists, as lias been previously stated, 

 usually in the form of the j9($;'oxide (or red oxide). 

 Sometimes, however, it is found in the form of the 

 j>rotox\de (or black oxide), which is soluble and is 

 poisonous to plants, and renders the soil unfertile. 

 By loosening the soil in such a manner as to admit 

 the air, and by removing stagnant water by draining, 

 this compound takes up more oxygen, whicli renders 

 it a peroxide, and makes it insoluble except in the 

 slight degree required for plants. The oxide of 

 manganese is probably of little consequence. 



The usefulness of all of these matters in the soil 

 depends largely on their exposure to the action of 

 roots and of the circulating water in the soil ; if 

 they are in the interior of particles, they cannot be 

 made use of; while, if the particles are so pulverized 

 that their constituents are exposed on their surfaces, 

 they become available, because water can immediate- 

 ly attack to dissolve them and roots can absorb them. 



This is one of the great offices of plowing, harrow- 

 ing, cultivating, and hoeing ; the lamps of soil being 

 thereby more broken up and exposed to the action 

 of atmospheric influences, which are often necessary 

 to produce a fertile condition of soil. 



SUBSOIL. 



The subsoil is usually of a dllferent character from 

 the surface soil, but this difference is more often the 

 result of cultivation and the effect of vegetation than 

 of a different original formation. The surface soil, 



