162 



very important, if not essential element, in order that the seeds of graia 

 may be brought to perfection. These facts indicate its importance as an 

 ingredient in the soil, and as it is generally found in limited quantities, it 

 is the more important that the farmer should take proper means to secure 

 its presence. Some crops, as will be seen hereafter, require much larger 

 quantities of this element than others, in order that they may be per- 

 fected. 



The principle alkalies with which it is usually found combined in 

 plants, are lime, magnesia, potash, and soda. Many substances upon 

 ■which plants live are derived from the atmosphere, so that a soil impov- 

 erished by their want may be again enriched simply by suffering the land 

 to lie dormant or by fallowing it. The phosphates, however, can never be 

 derived from the atmosphere or from rain water, nor are they furnished 

 by any of the processes of nature. If they become exhausted, direct 

 application must be made of some substance which contains them. Other 

 substances, though they may be important, yet are frequently so abun- 

 dant, that there is really no difficulty in obtaining them. This is the case 

 with sulphuric acid, abundance of which exists in plaster of Paris. But 

 with the phosphates this is not the case.* It is on this account that the 

 " super-phosphate of lime " is of so great value as a manure upon fields 

 exhausted of their phosphates. 



Alumina. — This substance, in its pure or uncombined state, is seldoni 

 found in the soil, but it constitutes a large proportion of all the slaty and 

 shaly rocks, and is the principal ingredient of clays, and clayey soils, 

 giving them their peculiar tenacity. It is insoluble in pure water. 

 Though it exists abundantly in many soils, it probably contributes but 

 little, in a direct manner, to the nourishment of plants. Its function, as 

 a part of the soil, is to give consistency and compactness to it ; thus 

 differing, in its action from silex, which we have said, is to give porosity 

 to the soil. 



These two substances form the basis of all soils, and to them they owe 

 their principal and permanent characters. The presence of the other 

 elements produce comparatively but trifling modifications. The body or 

 basis being thus constituted, the remaining elements, along with soluble 

 silica, may be regarded as food, or as matter out of which the frame 

 "work of the various tissues are formed. A soil, however, composed 



*Emmon8. 



