34 PRINCIPLES OF FARM PRACTICE 



rock; some is also supplied by the bones of animals. Bones 

 are rich in phosphorus. Rich deposits of phosphate rock 

 occur in several parts of the country, as in Utah, Tennessee, 

 South Carolina, and Florida. These rocks may be finely 

 ground and sold as rock phosphate; or, after being ground, 

 may be treated with sulphuric acid, and the resulting product 

 sold as acid phosphate. The distinction between ground- 



COMPARATIVE YIELDS ON HEAVY CLAY 50IL5 

 CLOVER HAY 



INCREASE 



43 PER CENT 



INCREASE 

 47 PER CENT 



Chart showing increase in yield of wheat on heavy clay soil by use of 

 manure and rock phosphate as compared with yield when manure alone 

 was used. (Wisconsin Agr. Exp. Station.) 



rock phosphate and acid phosphate must be clearly under- 

 stood. Rock phosphate is insoluble and not immediately 

 available for plant use. Acid phosphate, on the other hand, 

 is soluble and therefore available at once for the plant to 

 use. If quick results are desired from the use of a phosphate 

 fertilizer, acid phosphate should be chosen. 



When and how to use rock phosphate. Ground phosphate 

 rock has a market value of about one-third that of acid 

 phosphate. Owing to this fact, it would seem advisable to 



