EXPERIMENTAL WORK WITH RAW ROCK PHOSPHATE. 23 
bone meal, 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,O,) 
or 1.98 tons per acre-foot. If such a soil has been given thorough 
cultivation 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 smali 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 P,O,) 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). 
Truog? 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 VARIOUS 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 
171]. Agr. Expt. Sta.. Bul. No. 123 (1908). 
2 Wis. Agr. Expt. Sta., Research Bul. No. 20, p. 42-45 (1912). 
