42 PLANT DISEASE 



of the ash, and so through all the constituents 

 of the plant, with a possibility, if the subject 

 were thoroughly investigated, of even greater 

 variations. This appears to be an irregularity 

 owing to a greater or less deficiency, or to an 

 unassimilable state of the plant food. 



I shall now try to show that these variations 

 are the chief governors in the yield of wheat, 

 and other farm products. It is fully recognized 

 that iron and nitrogen in combination with the 

 phosphates are the means by which the plant 

 is enabled to take up the carbon of the air, to 

 form the common carbon compound, starch, 

 and from this starch the various carbon com- 

 pounds of the plant are formed. So we may 

 say the carbohydrates and hydrocarbons of the 

 plant are governed by the percentage of avail- 

 able iron, nitrogen, and phosphates in the soil. 

 I have already shown that these minerals have 

 a very wide variation in the cereal produced, 

 therefore we may argue that there has been a 

 corresponding range of assimilable mineral 

 matter in the soils from which these cereals 

 have been produced. We can now go to the 

 natural deduction that the grain wanting in 

 these mineral constituents will also be wanting 



