EXPERIMENTAL WORK WITH RAW ROCK PHOSPHATE. 9 



that it would be N/200 to methyl orange at the end of the digestion. 

 This method, it is said, however, did not prove satisfactory where 

 the soil had been fertilized. Hall and Plyman,^ in an exhaustive 

 article on the subject of "Available Plant Food in Soils," concluded, 

 after trying numerous solvents on 19 different soils, that a 1 per cent 

 solution of citric acid as recommended by Dyer^ gives results more 

 nearly showing the available plant food in soils. Dyer sought to 

 determine the average acidity of the root sap, with a view to finding a 

 natural solvent for plant food elements in the soil, and decided that 

 a 1 per cent solution of citric acid most nearly approximated the 

 average solvent power of the root sap. The theory, however, that 

 the roots of plants excrete an acid (other than carbonic) which aids 

 them in dissolving and securing their food has been practically aban- 

 doned. Kossowitch ^ concluded that the solvent action of plants on 

 relatively insoluble phosphates is due to the carbon dioxide given off 

 by the roots, but that plants are also able to supply themselves with 

 phosphoric acid from extremely dilute solutions. 



Hartwell * states that it is doubtful if any solvent will extract from 

 all soils amounts of phosphorus bearing definite relations to those 

 removed by even a given crop. The experiments of Ellett and Hill ° 

 bear out this statement. These investigators first fixed the phos- 

 phoric acid of superphosphate by means of the bases occurring in 

 soils (CaCOg, Fe(0H)3 and A1(0H)3), and then tested the solu- 

 bility of the resulting phosphates in N/5 nitric acid, neutral ammo- 

 nium citrate, and 1 per cent citric acid (Dyer's method). These 

 solutions dissolved practically all of the phosphate of lime but only 

 from 30 to 46 per cent of the iron and aluminum phosphates. When 

 these phosphate compounds were mixed in pots with nearly pure 

 sand, however, and wheat, oats, and corn grown therein, the pots 

 treated with phosphates of iron and aluminum gave greater yields 

 than those treated with phosphate of lime, showing that the avail- 

 ability as determined by the growing crops was the reverse of that 

 :;hown by the conventional solvents. 



The results obtained by Burlison® in a recent investigation agree 

 in a general way with those of Ellett and Hill, since he found that 

 the solubility of a mineral phosphate in a 0.2 per cent solution of 

 citric acid bore no particular relation to its availability as determined 

 by pot tests. 



Hartwell ^ suggests that a better index of the available phosphoric 

 acid in a soil might be gained by growing turnips therein and deter- 



iJour. Chem. Soc. Trans., 81, 117-144 (1902). 



2, Jour. Chem. Soc. Trans., 65, 115 (1894). 



sBied. Centr. 32, 44-49 (1902). 



*R. I. Agr. Expt. Sta., 18th Ann. Kept, p. 285 (1906). 



»Va. Polytec. Inst., Agr. Expt. Sta., Ann. Kept, for 1909-10, pp. 44-65 (1911). 



•Jour. Agr. Research, 6, pp. 485-513 (1916). 



■'K. I. Agr. Expt. Sta., Bui. No. 154 (1913). 



