EXTRACTION WITH SOLUTION OF CARBON DIOXIDE 275 



adapted to show the amount of easily soluble phosphoric 

 acid and potash in certain soils, but for other soils it has 

 failed to give satisfaction in the hands of a number of 

 analysts. Shorey, for instance, finds that it fails utterly 

 for the highly ferruginous soils of Hawaii. It is, doubt- 

 less, better adapted to soils rich in calcium and low in 

 iron and aluminum. 



The reason urged by Dyer for the superiority of the 

 citric acid over the hydrochloric acid extraction of the 

 soil is that the former gave, in his hands, several times 

 as great a difference in the amounts of soluble phos- 

 phoric acid in soils needing phosphoric manures as com- 

 pared with those not needing them. 



The application of both the hydrochloric and citric 

 acid methods to a soil may, when used to supplement 

 each other, add greatly to a knowledge of the potential 

 and present productiveness of the soil. 



There should be present in a soil for cereals and most 

 other crops at least .01 per cent phosphoric acid, soluble 

 in 1 per cent citric acid. A soil containing less than this 

 amount is deficient in phosphoric acid, unless it exists 

 largely in the form of ferric or aluminum phosphate, 

 which is not readily soluble in citric acid, but is fairly 

 available to the plant. Sod land contains organic com- 

 pounds of phosphorus that are easily soluble in the citric 

 acid, but less readily available to the plant; hence such 

 soil should show by analysis more than .01 per cent 

 phosphoric acid, to indicate sufficiency. 



139. Extraction with an aqueous solution of carbon 

 dioxide. As carbon dioxide is a universal constituent 

 of the water of the soil, and without doubt a potent 



