512 Journal of AgriciiIturQl Research voi. vi, ko. ij 



have any direct relation to the amount of phosphorus which they con- 

 tained. Brown rock, which had the smallest amount of phosphorus, 

 produced the most satisfactory yields. The differences must be attrib- 

 uted to modes of formation and weathering since the minerals were laid 

 down. 



SUMMARY 



(i) Phosphorus in rock phosphate can be assimilated by farm crops 

 in sand cultures under greenhouse conditions, even in the absence of 

 decaying residues. 



(2) Crop residues, when employed in conjunction with brown rock 

 phosphates, were beneficial. 



(3) Tennessee brown rock phosphate, Florida soft rock phosphate, and 

 Tennessee blue rock phosphate in the heavier applications proved superior 

 to South Carolina land rock phosphate, Utah rock phosphate, and 

 Canadian apatite, for oats, clover, and cowpeas when grown in sand. 



(4) The phosphorus in brown rock phosphate and Florida soft rock 

 phosphate was more soluble in water and in plant-food solutions than 

 the phosphorus in other mineral phosphates. The superiority of these 

 two phosphates over the others tested is shown chiefly by the first crop. 



(5) Chemical analysis showed that the plant-food solutions applied 

 did not appreciably modify the results. 



(6) The cereals produced as satisfactory yields as the legumes. 



(7) The crop yields tended to increase as the application of rock 

 phosphate increased up to a point where the size of the pots seemed to 

 be a limiting factor, apatite being the only exception. 



(8) The plants obtained their calcium, as well as their phosphorus, 

 from brown rock phosphates. No better results were secured when 

 calcium carbonate was applied than when rock phosphate alone was 

 used. 



(9) There was no particular relation between the citric-acid-soluble 

 phosphorus and the availability of these phosphates for plants. 



(10) Dextrose, when used as a fermentable substance, was harmful. 



(11) The degree of fineness is a factor which determines to some 

 extent the availability of rock phosphate, as indicated by the brown 

 rock. 



(12) These investigations extended over a period of 3^^ years, and 

 embrace results from 700 pot cultures and 400 phosphorus determinations. 



