MINERAL CONSTITUENTS OF SOIL. 207 



in quantity, and constitutes but about 16 per cent, of all the 

 soils in New England. It varies greatly in different varieties 

 of soil, though it is never absent from any. Pure alumina, 

 however, does not exist in the soil. It is generally combined 

 with silica, and with organic acids, such as crenic and apo- 

 crenic acids, and geic or humic acid. 



The properties of aluminous earth make it a fit associate 

 for silica, in order to give the proper texture and adhesive- 

 ness to the soil. Like silica, it is insoluble in water. But 

 its action upon the roots of vegetables, is just the opposite of 

 silica, giving the roots their basis of action and support, 

 and preventing them from penetrating too far. It retains the 

 water with great force, but yields it to the plant as its wants 

 may require. In consequence of its pow er of absorbing and 

 retaining water, when it constitutes a large proportion of the 

 soil, it is unfriendly to vegetation, forming a soft, ductile paste, 

 which excludes the air in wet weather, and contracts and 

 bakes in seasons of drought. As it contracts by heat, the 

 delicate fibres of the roots are injured in the fissures thus 

 formed, by exposure to the cold, heat and water. 



Aluminous earth is still more nearly allied to vegetables 

 by forming a part of their structure. The ashes of some 

 plants contain very small portions of it. It is also found in the 

 seeds of some grains. It is capable of acting the part, both 

 of an acid and of an alkali, a circumstance which renders it 

 probable, that its chief agency in the soil, is to act upon the 

 vegetable matter, and convert it into veoetable food. Alu- 

 mina is farther serviceable, from its possessing the property 

 of absorbing gaseous bodies, such as ammonia, and of retain- 

 ing in the soil for the use of the plant, what would otherwise 

 escape into the air. The fermentation of manures in the 

 soil, yield several gases, which are retained in this way. 



3. Lime. Lime is also widely disseminated in nature. It 

 forms the basis of extensive mountain ranges, and of a large 

 portion of the cultivated surface of the earth. It exists, how- 



