vii.] ACTION OF DOUBLE SILICATES. 65 



land a greater looseness and friability of character. 

 Land gets a certain amount of firmness during the 

 growth and removal of any crop, and it is for this 

 reason necessary that the land should be broken up. 

 Ploughing also becomes a preparation for other work, 

 which breaks it up still more completely. If land has 

 become hard and firm, it is not in a favourable con- 

 dition for the roots of plants to penetrate and search 

 for food. If, by the mechanical condition of the soil, 

 the plant is prevented from exercising a freedom of 

 growth, the yield from the crop must be thereby 

 diminished. 



127. Not less important than this is the increase in 

 the fertility of a soil caused by its exposure to the 

 sun and air. The soil upon the surface having had 

 the benefit of this action, is in a good condition for 

 being turned down, and the under-soil will be fresh- 

 ened by exposure. The soil becomes " freshened," as 

 it is often termed, or refreshed by the oxidation of its 

 particles, which have in many cases been reduced to 

 a lower condition of oxidation during the time it has 

 been covered up. The oxides of iron are examples 

 of this action; when they are exposed near the surface 

 to the sun and air, they become fully charged with 

 oxygen, but when buried in the soil they give up some 

 portion of their oxygen in the several decompositions 

 which take place in the soil, and thereby they become 

 again reduced to a lower form of oxide. Still they are 

 hereby performing a most important duty, as they 

 really become " carriers of oxygen," and in some cases 

 convey ammonia also. 



128. A still more important source of fertility is 

 obtained by bringing the double silicates of the 

 soil (24) into contact with the atmospheric 

 air. These have the power of absorbing am- 

 monia from the atmosphere. We have already 

 explained (104) how strong is the preference shown 

 for the formation of double silicates containing am- 



c 



