8 BULLETIN 699, U. S. DEPARTMENT OF AGRICULTURE. 
It will be noted that the solubility of the amorphous phosphate 
(South Carolina rock) is considerably greater than the crystalline 
variety (apatite). Also that fine grinding increased the amount of 
the various materials dissolved. 
Liebig,’ Fleisher and Kissling,? Cameron and Hurst,? and Greaves* 
found that the presence of most fertilizer salts increased the solu- 
bility of phosphates of lime, and Cameron and Bell® showed that 
certain aqueous solutions increased the solubility of the various phos- 
phates occurring in the soil. 
Peterman ° advocated an ammoniacal solution of ammonium citrate 
to determine the available phosphoric acid in soils. .Emmerling? 
recommended a 1 per cent solution of oxalic acid to distinguish be- 
tween lime phosphate and the phosphates of iron and aluminum. 
Hoffmeister * suggested an ammoniacal solution of “ humic acid” for 
determining the various forms of phosphoric acid in soils. Fraps® 
determined the solubility of a number of soil phosphates in various 
strengths of hydrochloric and nitric acids as well asin 1 per cent citric - 
acid, and tried to establish a relation between the chemically available 
phosphoric acid and that shown to be available by pot tests. While 
he found in several experiments that the order of availability as meas- 
ured by N/5 nitric acid was the same as in pot tests, he also found 
that the difference in the feeding power of various crops and the 
nature of the soil influenced the degree of availability of soil phos- 
phates. This experiment did not establish any standard for chemi- 
cally available phosphoric acid in the soil. Stoddart *° found that 
the digestion of 25 grams of soil with 250 cubic centimeters of N/5 
nitric acid for five hours dissolved practically all of the lime phos- 
phate but very little of the phosphates of iron and aluminum, and 
concluded that this test serves as an excellent indication of the amount 
of available phosphoric acid present in a soil. In the hght of other 
experiments, however, the assumption that the phosphates of iron 
and aluminum are unavailable to crops is hardly justified. 
Moore*™ attempted to find a strength of hydrochloric acid which 
would remove approximately the same amount of phosphoric acid 
from a soil as a crop of oats grown in pots. The conditions finally 
adopted for the extraction were to digest 200 grams of soil for five 
hours at a temperature of 40° C. with 1 liter of acid of such strength 
1 Ann. Chem. Phar. 106, 185 (1858). 
2 Bied. Centr., 12, 155-161 (1883). 
$s Jour. Am. Chem. Soc., 24, 885 (1904). 
4 Jour. Biol. Chem., 7, 287-319 (1910). 
’ Bureau of Soils, U. S. Dept. of Agr., Bul. No. 41 (1907). 
© Recherches de Chem. et Physiol., 8, 50 (1898). 
7 Bied. Centr., 29, 75 (1900). 
§Landw. Vers.-Stat., 50. 363 (1898). 
®Jour, Am. Chem. Soe., 28, 823-834 (1906). 
10 Wis. Agr. Expt. Sta., Research Bui. No. 2, pp. 50-60 (1909). 
u Jour. Am. Chem. Soc., 24, 79 (1902). 
