EXPERIMENTAL WORK WITH RAW ROCK PHOSPHATE. 5 
under with a green crop. Either of these treatments increases both 
the bacterial growth and the amount of carbon dioxide in the soil. 
On the other hand, highly productive soils not particularly rich in 
organic matter and which have never been fertilized contain much of 
their phosphoric acid as apatite or in some relatively insoluble form. 
‘Limestone soils high in phosphate, such as those in the Bluegrass 
Regions of Kentucky or in the Central Basin of Tennessee, continue 
to yield large crops without the addition of any soluble phosphate, 
and frequently do not respond to such applications.* 
Burlison,? in a series of pot tests in pure sand, showed that farm 
crops were able to utilize the phosphoric acid of rock phosphate with- 
out the aid of organic material and that the yields were greater when 
the applications of rock phosphate were increased. 
Other things being favorable, therefore, it seems reasonable to 
expect a soil low in phosphoric acid to respond to the addition of 
ground raw rock phosphate provided that the material is very finely 
ground, applied lberally, and thoroughly distributed through the 
soil by cultivation. 
METHODS OF STUDYING THE SUBJECT. 
In efforts to determine the fertilizer value of raw rock phosphate 
three general lines of investigation have been followed, namely, 
laboratory work, pot or greenhouse tests, and field experiments. The 
‘last named of these methods is by far the most important and the 
only one of much value, taken by itself. The other lines of work, 
however, are useful supplements to field experiments, and their 
importance should not be minimized. 
LABORATORY WORK. 
Laboratory work on ground raw rock phosphate divides itself into 
the following investigations: 
1. Determination of composition of rock. 
_ 9. Methods of determining availability of phosphate. 
8. Effect of degree of fineness of the phosphate on its solubility or 
availability. 
4. Effect of organic fermentation on the solubility or availability 
of the phosphate. 
COMPOSITION OF ROCK. 
While phosphate rock from various sources differs considerably in 
composition and grade,’ the bulk of the phosphoric acid contained 
1Mooers, C. A., Tenn. Agr. Expt. Sta., Bul. 86, pp. 48-44, and p. 86 (1909). 
2Mineral Phosphates and Plant Nutrition, Jour. Agr. Research, 6, No. 138, 485-514 
(1916). 
3 Bureau of Soils, Buls. Nos. 69. 76, and 81: U. S. Dept. of Agriculture Buls. Nos. 14 
and 312. i 
