PLANT FOOD AND SOIL FERTILITY 



37 



have been imported from Germany. During the Great War 

 when importation was cut off, an increasing amount of po- 

 tassium was supplied from a number of sources: among 



ROTHAMSTED 



AGR.EXP.STA. 



ENGLAND 



PENNSYLVANIA 

 AGR.EXP.STA. 



30YRS. 



OHIO AGR. 

 EXP. STATION 



18 YRS. 

 TEST 



INDIANA AGR 

 XR STATION 



Diagram showing increase in yield of wheat where fertilizers were used 

 as contrasted with yields on similar areas where no fertilizers were used. 

 (Ohio State Agr. College.) 



them, sea weed or kelp, found in great quantities in the 

 Pacific Ocean; mineral deposits in old lake beds, as in Searles 

 Lake, California; wood ashes; and the by-products of such 

 industrial processes as the making of cement and washing of 

 wool. It seems probable that, in time, a means will be found 



