42 CHEMISTRY OF FARM PRACTICE 



wood ashes contain potash. Potassium has a tendency to 

 lengthen the growing season of some crops. 



36. Calcium. Compounds of calcium are widely dis- 

 tributed, but the element does not occur free in nature; 

 it may be prepared by electrolysis. The most abundant 

 compound of calcium is the carbonate. Calcium carbonate 

 or " ground limestone rock " (CaCOs) is of much agri- 

 cultural importance, due to the fact that it corrects acidity 

 in the soil. Calcium carbonate is a compound that is 

 quite readily decomposed by other acids. Even the dilute 

 acetic acid contained in vinegar will replace the carbonic 

 acid of the calcium acetate. 



No plant growth can take place without the presence 

 of calcium. It has been shown that even the rather insol- 

 uble acid silicate of calcium may serve to furnish calcium 

 to the plant. All normal soils have a supply of calcium 

 compounds sufficient to furnish the calcium necessary for 

 plant growth; but many soils are acid and, therefore, are 

 benefited by applications of ground limestone to correct 

 the acidity. These uses will be taken up later. 



37. Magnesium. Magnesium ranks a little below cal- 

 cium in its abundance in nature. It, too, never occurs 

 in the free state in nature. Its compounds are quite abun- 

 dant in the earth's crust, in rocks, in sea water, and in 

 mineral water. It is also widely distributed in both animal 

 and vegetable life. It exists in nature largely in the car- 

 bonate form, having the power to correct soil acidity, 

 and being even more effective for this purpose per unit 

 of weight than is calcium carbonate. If, however, mag- 

 nesium is present in the soil in excess of about l-fo per cent, 

 it produces undesirable effects on vegetation. 



38. Iron. Compounds of iron are widely distributed 

 in nature in the form of brown or yellow oxides giving 

 characteristic color to soils and as carbonate. These com- 

 pounds form valuable deposits of iron ore. Iron sulphide 

 in the form of pyrites or " fool's gold " is frequently found 



