SOIL COMPOSITION 



59 



amount, but nevertheless the total supply of potassium in nearly all 

 soils is exceedingly large compared with crop requirements; and, 

 while it has a money value in commercial fertilizers and is quite 

 extensively used, there is much evidence to show that on many 

 soils the influence which it produces is due in part at least to in- 

 direct effects, as in the liberation of other more deficient plant- 

 food elements. 



Sulfur and phosphorus are not in the same class with the eight 

 abundant elements composing the silicates; and between these two 

 elements there are also marked differences, since sulfur is brought 

 to the earth in rain in considerable amounts and is also about as 

 abundant as phosphorus in the earth's crust, while crops require 

 from three to ten times as much phosphorus as sulfur. 



If we disregard the three elements which agricultural plants ob- 

 tain from the air and water (in CO 2 and H 2 O), as being in large 

 measure beyond our control, we may secure a clear conception of 

 the relative abundance of the remaining essential plant-food ele- 

 ments, based both upon the most original natural supplies and upon 

 crop requirements, by a study of Table 8. 



TABLE 8. RELATIVE "SUPPLY AND DEMAND" OF SEVEN ELEMENTS 



Two million pounds correspond to the weight of the plowed soil 

 of an acre of average land to a depth of - 6| inches (counting 300,000 

 pounds per acre inch), so that the supply of the plant-food elements 

 given in Table 8 is simply what would be contained in an acre of 



