EXPERIMENTAL WORK WITH RAW ROCK PHOSPHATE. 113 



trial, and therefore their opinions would be less valuable than those 

 who had used the material through a term of years. 



Eeplies 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 LXVII (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. 



N'umerous 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 Eiver 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 bs 

 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 m.ay be expected to be particularly marked on soils of low 

 phosphoric acid content. 



568410— Bull. 699—18 8 



