EXPEEIMENTAL WORK WITH RAW ROCK PHOSPHATE. 23 



bone i^eal, and ground rock phosphates, the distribution must be 

 brought about by mechanical means. 



A brief consideration of the facts will show how important it is 

 that applications of raw. rock phosphate should be heavy in order 

 that the material may have a fair trial. 



An average soil contains 0.113 per cent of phosphoric acid (P^Og) 

 or 1.98 tons per acre-foot. If such a soil has been given thorough 

 culjtivation for a number of years the phosphoric acid contained 

 therein should be fairly well distributed, at least much better than 

 it is possible to distribute a relatively small application of raw rock 

 phosphate in a limited time by mechanical means. It hardly seems 

 possible that such a soil could respond to applications of rock phos- 

 phate supplying an amount of phosphoric acid equivalent to only 

 1 or 2 per cent of that which it already contains in a form nearly if 

 not quite as available. An application of 1,300 pounds per acre 

 of average raw rock phosphate (30 per cent P2O5) would increase 

 the quantity of phosphoric acid already present in an average soil 

 approximately 10 per cent. 



Hopkins^ in his recommendations for soil treatment advises the 

 use of raw rock phosphate on soils containing phosphoric acid in 

 subnormal amounts at the rate of 1,000 pounds per acre every five or 

 six years (preferably in connection with farm manure). 



Truog2 of the Wisconsin Experiment Station conducted a pot 

 experiment in which he showed that where raw rock phosphate was 

 very thoroughly mixed with ground quartz much better yields of 

 corn and oats were^obtained than where only ordinary care in mixing 

 was practiced. 



The use of applications of from 1,000 to 2,000 pounds of very finely 

 ground raw rock phosphate every few years and the mixing of the 

 same intimately with the soil by thorough cultivation should bring 

 about the desired distribution. In this way a large surface of the 

 relatively insoluble material is exposed to the action of the soil 

 water, and much more of the phosphate can therefore be dissolved 

 within a limited time. 



METHODS OF COMPARING VAEIOUS PHOSPHATES. 



In nearly all of the field experiments so far conducted to compare 

 the relative fertilizer values of ground raw rock phosphate and the 

 more soluble phosphates, three general methods of application have 

 been followed : (1) Equal weights of the various phosphatic materi- 

 als have been applied, usually at a medium rate per acre. (2) The 

 various phosphates have been added in such quantities as to furnish 



»m. Agr. Bxpt. Sta.. Bui. No. 123 (1908). 



*Wis. Agr. Expt. Sta., Research Bui. No. 20, p. 42-45 (1912). 



