180 



THE SOIL: INORGANIC MATTER 



These figures have been calculated from data published 

 by the Bureau of Soils. They show that at any one time 

 phosphorous compounds and nitrates are present in con- 

 siderably less quantity than are potash and lime. It is true 

 that these two elements, nitrogen and phosphorus, are usually 

 the limiting essential elements in crop production, as this 



Fig. 38.— Waste water. Outlet of drain tile. (Elliott.) 



might indicate; but on the other hand, if these concentra- 

 tions are maintained throughout the growing season there 

 will be enough of these foods supplied to nourish the crop. 

 It is not at all probable that this same concentration is 

 maintained in all soils and at all times. In fact quite the 

 contrary. The amount of material in the film water must 



