142 



PHOSPHATIC FERTILISERS 



[chap. 



nitrogen, but then the starch also migrates in an equally 

 constant ratio to the nitrogen compounds, though no 

 such association is claimed between nitrogen and starch. 

 Both actually and relatively the nitrogen is highest in 

 the grain from Plot lo, where the plant is phosphoric 

 acid starved. 



The phosphatic manures are practically all com- 

 pounds of phosphoric acid with lime, and, as is well 

 known, four distinct combinations exist and are found 

 in commerce. Only one, the dihydrogen calcium 

 phosphate, the characteristic constituent of super- 

 phosphate, is to any degree soluble in water ; the 

 others give rise to extremely dilute solutions of phos- 

 phoric acid, too dilute, as has been shown by experiment 

 to nourish a plant properly, with however large a volume 



