214 GEOLOGY AND CHEMISTRY OF SOILS. 



furnishing soda to plants. The quantity in the soil is ex- 

 ceedingly small, but only small quantities are wanted. The 

 chlorine, which plants sometimes exhale, must come from this 

 substance. It is highly useful on some soils. 



Sub-phosphate of alumina and phosphate of lime may be 

 noticed here in connection, because they have lately been 

 shown to be present in all fertile soils. Phosphate of lime is 

 the most common form in which both the lime and the phos- 

 phoric acid exist. 



It is from these substances, that animals obtain their phos- 

 phorus. About 50 per cent, of bones is phosphate of lime. 

 Almost all the vegetable products contain it, whether the 

 land has been cultivated or not. It even exists in the pollen 

 of the pine in forests. 



Nitrate of potash and nitrate of soda are sometimes de- 

 tected in soils. 



Sulphate of iron is also detected in a few soils, and is 

 highly poisonous in its effects. Lime converts it into gyp- 

 sum and oxide of iron, thus rendering it a valuable saline 

 manure. 



Sulphuret of iron is found in considerable quantities, but 

 by exposure to air and water, it changes, first to the sulphate 

 of iron, and then, by the action of lime, to the sulphate of 

 lime, as above. 



Carbonic acid in a free state is also found in soils, the 

 quantity varying with circumstances. The action of this acid 

 has been fully discussed. One fact of a highly practical value 

 is, that the urcts are constantly becoming salts, so that the 

 soil is often found to contain a larger quantity of salts than the 

 rocks. The process of disintegration produces changes in 

 the arrangement of the simple elements. 



Thus we have enumerated all the inorganic bodies, which 

 are found in the soil, as ascertained by analysis, and their 

 general relations to growing plants. From this examination 

 soils are composed, generally, of the 



