June 26, 1916 Mineral Phosphates and Plant Nutrition 497 



of clover from the same treatment for three cuttings. For the next 

 higher treatment the comparison is 6,426 pounds of alfalfa to 4,674 pounds 

 of clover. The yields are approximately the same for the third applica- 

 tion, but for the heavy treatment the clover almost doubles the yield 

 from the alfalfa. Special attention is called to Plates LXXIII, LXXIV, 

 and LXXV. 



In drawing conclusions from an investigation of this kind the actual 

 growth of the plant must be regarded as a most significant factor. How- 

 ever, an analytical study of the crops harv^ested can not fail to be of great 

 value. Since phosphorus is the element with which this paper chiefly 

 concerns itself, quantitative determinations were confined to that 

 substance. 



The determinations show that in practically all cases phosphorus is the 

 limiting element in production. In every instance the dry matter 

 increased as the phosphorus content of the pot was increased; also the 

 quantity of phosphorus assimilated increased as the dry matter increased. 

 The percentage of phosphorus in the plant in the majority of cases 

 increased as the application of raw rock grew larger. This is especially 

 noticeable in the hay crop. The most notable exceptions were observed 

 in wheat and oat straw. There is no definite relation in the quantity of 

 phosphorus applied and the percentage assimilated by the crop. There 

 was a slight tendency in the grain for the percentage removed to decrease 

 as the application was increased, but for the legumes this ratio does not 

 hold. As high as 2.49 per cent of the phosphorus supplied in raw rock 

 phosphate was removed in one season's growth of alfalfa. 



COMPARATIVE STUDY OF THE PRODUCTIVE POWERS OF SIX MINERAL 



PHOSPHATES 



The results from Tennessee brown rock phosphate proved so interesting 

 that it was planned to determine the comparative value of mineral phos- 

 phates from the various mines of America, For this purpose Tennessee 

 brown rock phosphate, Tennessee blue rock phosphate. South Carolina 

 land rock phosphate, Utah rock phosphate, Canadian apatite, and Florida 

 soft rock phosphate were selected. 



The materials were ground so that all particles would pass through a 

 sieve with 100 meshes to the inch and were applied in quantities which 

 contained equal amounts of phosphorus for a given set of pots. Clover, 

 oats, and cowpeas were grown with these different phosphates. 



Because of limited space the small battery jars into which could be 

 placed conveniently 4,800 gm. were selected for this rather extensive 

 trial. Without crowding, eight plants per pot could be grown 

 (PI. IvXXVI). Table VH gives the quantity of the phosphate applied 

 and the yields of the crops in question. The planting was done on 

 October 3, 191 4, and the crops of clover were harvested on March 5 

 and April 9, 1915, while the oats were cut on February 5, 191 5. 



