ACTION OF CARBONIC ACID. EXHAUSTED SOILS. 91 



few minerals, perhaps none, being able to resist its long- 

 continued action ; and though its solvent power seems 

 to be slight, in the lapse of time it produces changes of 

 great importance and extent. 



806. Carbonic acid acts in other ways. It unites with 

 the ammonia of the atmosphere, forming carbonate of 

 ammonia, and with the potash and soda in the earth, 

 forming carbonates of potash and of soda. These three 

 alkaline carbonates have the power of dissolving silica. 

 Now it has just been stated that silica enters as an ingre 

 dient into the composition of nearly all the harder rocks. 

 Of the three minerals of which granite is composed, 

 quartz is almost pure silica ; mica is two thirds silica, and 

 felspar is about one half made up of silica. All these 

 minerals and many others are thus gradually disinte 

 grated by the slow action of these carbonates upon the 

 silica in them. 



307. Why does the fertility cease ? The mineral and 

 atmospheric elements of the food of plants arc gradually 

 taken up by successive crops, and carried off with them, 

 the humus grows thin and meagre, and the soil is ex 

 hausted. The crops obtained from the land are, year 

 after year, continually smaller, till at last they are not 

 sufficient to reward the labors of the husbandman. 



308. The obvious remedy is to restore to the soil the 

 elements wanting, as will be shown in the chapter upon 

 manures. 



309. But if a soil be barren for one plant, it is not 

 necessarily so for every other. A field which, for want 

 of soluble silica, will not bear a second crop of Indian 

 corn, may, from having a plenty of potash and lime in it, 

 bear an excellent crop of clover or of beets or carrots. 

 There may not be enough of a particular element for one 



