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 cut this statement. These investigators first fixed the phos- 
phorie acid of superphosphate by means of the bases occurring in 
soils (CaCO,, Fe(OH), and Al(OH),), 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 20 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 hme, showing that the avail- 
ability as determined by the growing crops was the reverse of that 
shown 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- 
1 Jour. Chem. Soc. Trans., 81, 117-144 (1902). 
2 Jour. Chem. Soc. Trans., 65, 115- (1894). 
3 Bied. Centr. 32, 44-49 (1902). 
4R. I. Agr. Expt. Sta., 18th Ann. Rept., p. 285 (1906). 
5 Va. Polytec. Inst., Agr. Expt. Sta., Ann. Rept. for 1909-10, pp. 44-65 (1911). 
6 Jour. Agr. Research, 6, pp. 485-513 (1916). 
7R. I. Agr. Expt. Sta., Bul. No. 154 (19138). 
