OF* 1 AGRICULTURE. 207 



do the elements of the soil act ? The answer involves 

 two important considerations. First, the mutual 

 chemical action of the elements of the soil, the vege- 

 table matter and salts upon each other; and, second, 

 this action, as influenced by the living, growing 

 plants. 



The elements of the soil are silicates, salts, and 

 humus, or vegetable mold. The silicates are such as 

 have no tendency to act upon each other; these are 

 gradually decomposed by the action of the air. The 

 great agent in this action is carbonic acid, which 

 gradually combines with the alkaline base of the 

 silicates, and the potash and soda in them are con- 

 verted into soluble salts, while the sand and clay 

 remain. The result of this action is that the plants 

 are enabled to take up by their roots the soluble 

 salts which have been set free, and the soil becomes 

 more clayey. 



If lime be applied to land, either air-slacked, shell 

 or marl, the result is, slowly but surely chemical 

 action takes place, resulting in rendering soluble an 

 additional supply of nutritive ingredients, such as 

 potash, soda, phosphoric and silicic acids, from the 

 previously undecomposed minerals in the soil. Thus, 

 clay soils are benefitted by liming, and also sandy 

 land, if not limed too frequently and in too large 

 doses. It may be laid down as a principle that car- 

 bonic acid and ammonia decompose the earthy, 

 alkaline and metallic silicates in the soil. The result 

 of this action is that lime, potash, soda, magnesia, 

 and metallic oxydes are rendered available as food for 

 plartts. 



Experiments have shown conclusively that the 



