92 



SOILS AND FERTILIZERS 



106. The total supply of plant-food materials. — The 

 statement of analyses in Table 17 shows the quantities of 

 plant nutrients in 2,000,000 pounds, which represents the 

 weight of an acre of soil to a depth of only six to eight inches. 

 There is below this a considerable volume of soil through 

 which roots ramify, and from which some nutriment is 

 drawn. The roots of ordinary crops extend to a depth of 

 three or four feet into the soil, depending on different condi- 

 tions of soil and climate. In semi-arid and arid regions 

 roots extend deeper than they do in humid regions, and in 

 well-drained soils they penetrate deeper than they do in 

 poorly drained ones. It is, however, from the furrow slice 

 that plants derive most of their nourishment. 



Subsoils sometimes contain more and sometimes less 

 plant-food materials than do the surface soils. Nitrogen 

 is almost always present in greater quantity in the surface 

 soil, because it is a constituent of material that has been 

 plowed into the furrow slice. Table 18 contains a statement 

 of the analyses, expressed in percentage composition, of two 

 soils to a depth of four feet, each foot of which was analyzed 

 separately. 



Table 18. — Ultimate Analyses of Two Soils to a Depth of 

 Four Feet, Expressed in Percentage Composition 



