The Physical Character of Soils 33 



tains potash. When we burn wood in the fire, especially 

 the harder woods, such as oak and hickory, we find that 

 these plants had a large supply of potash, which is left 

 in the ashes. Wood ashes, therefore, are useful as 

 appHcations to the soil for restoring potash, Hme, and 

 phosphorus in a form very readily available for the use 

 of plants. 



We have in a former lesson mentioned the great 

 abundance of limestones, and have said something of 



their formation in past ages. Carbonic 

 Lime, Sodium, acid, united with the metal calcium, formed 

 p ^'d ^V these limestones, in many of which there is 



Factors a-^so found a varying percentage of another 



metaUic element, magnesium. Both calcium 

 and magnesium are essential as plant food, and are found 

 abundantly in all the seeds of our cereal grains and of 

 other plants. The carbonates of these elements are 

 found so plentifully in nature that they are practically 

 inexhaustible so far as mere plant food is concerned. 

 But Hme, or the carbonate of calcium, in the form of 

 calcined limestone or common quicklime, plays a very 

 important part in the development of the productive 

 capacity of our soils when used in an intelligent way. 

 Of its use and action we will have a lesson further on. 



The metal sodium is important as the base of our com- 

 mon salt; but, as we have said, though closely resembling 

 potash it cannot be used as a substitute for that material 

 in a fertilizer mixture. When combined with nitric acid, 

 however, forming the nitrate of soda, it becomes one of 

 the most readily available and important carriers of 

 nitrogen. 



