PLANT FOOD IN THE SOIL 



73 



is in an insoluble form, it is necessary that this food be gradually 

 made soluble if the plants are to secure enough for proper growth. 

 There are various processes of making these materials soluble 

 in the soil, some of which are under the control of the farmer. 

 It is his business to handle the soil so as to bring enough of 

 these plant food-materials into solution to supply crop needs. 



78. The amount of food used by plants. The following table 

 shows the amount of nitrogen, phosphorus, and potassium re- 

 moved from an acre of land by good yields of crops : 



These materials are required in approximately these propor- 

 tions whatever the yield. If the soil is in such condition that 

 only two thirds of the phosphorus, for instance, is available for 

 a seventy-five-bushel corn crop, approximately two thirds of this 



1 The composition of fertilizers is usually expressed in per cent of phos- 

 phorus pentoxide (P 2 O 5 ) (a compound of phosphorus, incorrectly called 

 phosphoric acid), and in per cent of so-called potash (K 2 O) (a compound of 

 potassium). Neither of these compounds actually exist as such in fertilizers. 

 They are simply means of expressing the quantities of phosphorus and potas- 

 sium present. It is much simpler to speak of these plant foods as phosphorus 

 and potassium, and there is an increasing tendency to use these simpler terms. 



2 The corn plant without the ear. 



