264 



MANUAL OF AGRICULTURE. 



100 lbs. of 



A. — Bt/ Grain Crops. 



From the.se tables we perceive what a small percentage of the 

 weight of plants is contributed by the inorganic materials of the 

 soil. 



Clay and sand are the two substances which determine the 

 texture of the soil; and to the variety of their admixture, in 

 regard to their respective proportions, are due the different classes 

 of soils. The following classification of soils, as proposed by 

 Johnston, may be selected as being the simplest, and that one, 

 at the same time, by whose standard the classification of any par- 

 ticular soil may most easily be determined. 



The purest clay, such as pipe-clay, found naturally, consists 

 of silica, alumina, and a small quantity of oxide of iron, in chemi- 

 cal combination. No sand can be extracted from it. Tile clay. 



