94 



SOILS AND FERTILIZERS 



tains. This is because there are so many conditions, to 

 which soils are subject, that interfere with the ability of 

 plants to obtain the nutrients or that, in other ways, inter- 

 fere with plant growth. It is, however, possible for the 

 quantity of some substance required by plants to be so small 

 that it is not sufficient to furnish enough nutriment for prof- 

 itable crop production. Probably all of the soils, whose 



Fig. 20. — Relative quantities of potash, lime, phosphoric acid and nitro- 

 gen in a sack containing 200 pounds of dry soil, when the percentages present 

 are respectively 1.98, 1.64, 0.19 and 0.165. 



analysis is stated in Table 17, would be benefited by the 

 application of some fertilizers, with the possible exception 

 of the rich prairie soils. This is not because there is not 

 actually enough plant-food material in the soil, but because 

 it is not in a form that is available to plants. 



110. Available and unavailable plant-food materials. — 

 The available plant-food materials in soils consist of those 

 portions of the total supply that plants are able to secure 

 in their growth. We have seen that it is necessary for all 



