2 BULLETIN 699, U. S. DEPARTMENT OF AGRICULTURE. 
lined methods of treatment and application which it claims will re- 
sult in the maximum benefit from its use. But ground raw rock phos- 
phate has been used to a small extent in many other States and would 
undoubtedly have been more widely employed were it not for con- 
flicting conclusions concerning its agricultural value, conclusions 
which in many instances have been drawn from insufficient data. © 
Numerous experiments have been conducted, not only in the field 
but in the greenhouse and laboratory as well, to test the fertilizer 
value of ground raw rock phosphate. While some of the field ex- 
periments have been carried on in a careful, systematic Way over a 
term of years, others have been conducted for such a brief period and 
apparently with so little regard for the numerous factors influencing 
crop yields that it is not only unwise to draw any definite conclusions 
from the results but unfair to class them with those obtained from 
carefully conducted long-time experiments. After a careful analysis 
of a number of long-time fertilizer experiments, Prof. Whitney, of 
this bureau, states that “a period of not less than 15 years of observa- 
tion is required to draw any safe conclusions from fertilizer plot 
tests.” If the results of experiments so far published, therefore, be 
taken without carefully weighing their relative merits, the only con- 
clusion one could possibly reach is that ground raw rock phosphate 
is of very questionable value. 
In giving the results of field tests with ground raw rock phosphate, 
the writers found it difficult to draw the line between experiments 
warranting discussion and those which were not sufficiently im- 
portant to justify consideration in detail. Finally the plan was 
adopted to discuss only those field experiments which were con- 
ducted for five years or more. In some instances this plan may 
appear not quite fair, but almost any other treatment of the sub- 
ject would result in a manuscript so bulky and so filled with data of 
relatively little value, that it would only serve to confuse the reader. 
HISTORY. 
The use in this country of ground raw rock phosphate as a fer- 
tilizer dates back to the early days of the South Carolina phosphate 
industry. 
Holmes? recommended finely ground South Carolina rock for 
direct application to the field as early as 1870, and several of the 
phosphate mining concerns then operating at Charleston advertised 
the material for sale. Chazal* states that some correspondence of 
Prof. Charles U. Shepard shows that the latter advised the use of 
De 
1 Unpublished work. 
2 Phosphate Rocks of South Carolina, pp. 45-46 (1870). 
8A Sketch of the South Carolina Phosphate Industry (1904). 
