4 SOILS AND FERTILIZERS 



them sometimes leads to a deficiency in the available 

 supply. This is either because they are not present in 

 sufficient quantity, or because they are not readily dissolved 

 by the liquids with which they come in contact. Many 

 things tend to influence the quantity of these substances 

 that plants may obtain. Among these are tillage, decaying 

 vegetation, drainage and the kind of plant grown. It is 

 the nitrogen, phosphoric acid, potash and possibly sulfur 

 that are most likely to be deficient in the solution to which 

 plants have access, and commercial fertilizers usually con- 

 tain one or more of these substances. 



The kind of fertilizer that it will be desirable to apply 

 depends, in part, on the so-called availability of each of 

 the nutrient substances contained in the soil, availability in 

 this case meaning the readiness with which the plant can 

 appropriate these food materials. But some plants require 

 more of certain of these substances than they do of others. 

 Hence the needs of the plant must also be taken into con- 

 sideration in deciding what fertilizer to use on a given soil. 



5. Quantities of plant-food materials in the earth's crust. 

 — As all of the food materials that plants draw from soil, 

 with the exception of nitrogen, came originally from rocks, 

 it is of some interest to know what the proportions of these 

 substances are in the entire crust of the earth. As stated 

 by Clarke they are present in the following percentages : 



Oxygen 47.17 Potash . 3.00 



Iron ........ 4.44 Sulfur 0.11 



Lime 4.79 Phosphoric acid .... 0.25 



Magnesia 3.76 



Nitrogen does not appear in this list because it does not 

 occur as a constituent of the rocks forming the earth's crust. 

 The nitrogen that soil contains is derived from the atmos- 

 phere by processes that will be described later. Most of 

 the constituents of soil have, however, been formed from 



