6 BULLETIN 699, U. S. DEPARTMENT OF AGRICULTURE. 
therein is combined with lime in the proportion to form tricalcium 
phosphate (Ca,;(PO,),). Since this material in its natural state is 
rather sparingly soluble in water and very weak acid solutions, there 
may be little difference in the amount of phosphoric acid which the 
various grades of rock will yield to a definite quantity of such sol-, 
vents within a limited time, provided the reck samples are ground to 
the same degree of fineness. 
There has accumulated, however, considerable data, indicating that 
the availability of the amorphous phosphates is greater than that of 
apatite, and in a recent investigation Burlison? found that there was 
apparently quite a difference in the availability of rock phosphate 
from different sources when measured by crops grown in pots of 
small size. 
The presence of such an impurity as limestone may have a con- 
siderable effect on the solubility of the phosphate in certain conven- 
tional solvents, since carbonate of lime is much more readily attacked 
by weak or dilute acid solutions and therefore tends to reduce their 
solvent power either by neutralization of the acid or by furnishing a 
common ion (Ca). A determination of the amount of free lime 
present may be of considerable importance if the availability of the 
phosphate is to be measured by its solubility in a conventional solvent. 
Phosphates of iron and aluminum, while less soluble than the 
phosphates of lime, are not usually present in sufficient quantities 
materially to reduce the solubility of the rock. Unless, therefore, 
there is reason to suspect that these phosphates are present in abnor- 
mal amounts their determination is hardly deemed necessary. 
The most important and often the only essential determination to 
be made in an analysis of phosphate rock for direct application to the 
field is its phosphoric-acid content. Nearly all rock is sold on the 
basis of the phosphoric acid which it contains and the degree of 
fineness to which it is ground. 
METHODS OF DETERMINING AVAILABILITY OF PHOSPHATES, 
The solubility of natural and manufactured phosphates in water 
and various media has been the subject of numerous investigations. 
practically all of which have been carried on with a view to obtain- 
ing an index of the availability of phosphates under soil conditions. 
Deherain,? in attempting to determine the available plant food in 
soils, used a dilute solution of acetic acid. Gerlach * and Schloesing * 
employed aqueous solutions of carbon dioxide, on the theory that the 
natural soil waters owe their solvent action to this gas. The results 
1 Mineral Phosphates and Plant Nutrition, Jour. Agr. Research, 6, pp. 485-514 (1916). 
2Ann. Agron., 17, 445 (1891). 
8 Landw. Vers.-Stat., 46, 201 (1896). 
Compt. Rend., 131, 149 (1900). 
