THE POTATO 211 



other conditions, but that they may be used as a 

 guide for experimenting. 



There is removed in a 600-bushel crop of pota- 

 toes approximately: 



160 pounds of nitrogen 

 60 " " phosphoric acid 

 160 " " potash 



An acre of soil eight inches deep weighs about 

 2,375,000 pounds. (This varies somewhat and 

 this estimate is for soil on the Twin Falls North 

 Side Irrigation Project, Jerome, Idaho.) The ele- 

 ments of fertility vary, but, as an example, the soil 

 at Jerome, Idaho, contains: 



.47 per cent, of potash 



.11 per cent, of phosphoric acid 



.06 per cent, of nitrogen 



This would be a total of 



Nitrogen 1,425 pounds 



Phosphoric acid.. 2,612.5 pounds 

 Potash 17,692.5 pounds 



Even though it is not possible for plants to use 

 all of this, if proper cultivation methods are used 

 there is food enough to last for a great many years. 



The three elements of fertility that are called 

 the "essential elements" are nitrogen, phosphoric 

 acid and potash. At least ten other elements 

 enter into the plant and are important and neces- 

 sary for the production of plants. All but these 

 three are used in small quantities and it is generally 

 considered by students of the soil that the seven — 

 magnesium, sodium, chlorin, sulphur, iron, sili- 

 con and calcium — are present in most soils in 

 suflBcient quantities to supply the needs of crops 



