EXPERIMENTAL WORK WITH RAW ROCK PHOSPHATE, ULES 
trial, and therefore their opinions would be less valuable than those 
who had used the material through a term of years. 
Replies were received from 315 farmers, most of them in the 
Middle West where raw rock has been tried out much more exten- 
sively than in the East or South. 
@ Of this number 219 or 69.6 per cent reported favorable results from 
the use of raw rock phosphate, 55 or 17.4 per cent were doubtful 
about its value, and 41 or 13 per cent regarded the material un- 
favorably. 
Many of the responses were very incomplete and in most instances 
no check plots were employed. In no case was a reliable comparison 
made between acid phosphate and raw rock. 
A summary of the data obtained is given in Table LX VII (p. 114). 
SUMMARY. 
Much doubt and difference of opinion exists concerning the fer- 
tilizer value of ground raw rock phosphate, but the use of this ma- 
terial has slowly increased until the annual consumption is now in 
excess of 91,000 tons. 
Numerous experiments have been conducted with this material in 
the field, greenhouse, and laboratory, and while many of these ex- 
- periments are of very little value, others warrant serious considera- 
tion. To avoid confusing the reader by a mass of data of doubtful 
value no field experiments of less than five years’ duration have been 
recorded in detail in this bulletin. 
The use of ground raw rock phosphate as a fertilizer in this coun- 
_ try dates back to the early days of the South Carolina phosphate 
industry, but the Pennsylvania State Experiment Station recorded 
the first field experiment with this material in 1885. Since that date 
» the work has been taken up by practically all of the stations east of 
the Mississippi River and a few of those west. 
_ Since natural phosphate of lime or phosphorite is very resistant to 
weathering influences, it can not ordinarily be expected to yield its 
phosphoric acid readily to the soil solution, but by subjecting it to 
some chemical treatment by which it is rendered water soluble, its 
effectiveness is greatly increased. On the other hand, if the rock 
is ground to an impalpable powder (floats) and applied to the soil 
in such large quantities that an enormous surface is exposed to the 
action of the soil waters, a considerable amount will eventually be 
dissolved. Since carbon dioxide (in solution) and bacteria also affect 
the solubility of raw rock phosphate, it is reasonable to expect the 
latter to be more effective on soils high in organic matter. This 
effect may be expected to be particularly marked on soils of low 
phosphoric acid content. 
56841°—Bull, 699—18—_8 
