June 26, 1916 



Mineral Phosphates and Plant Nutrition 



501 



Soon after the clover was harvested in series 3, these pots v\^ere seeded 

 to cowpeas. Cowpeas were planted to determine the ability of this 

 legume to utilize the phosphorus contained in mineral phosphates. The 

 cultures were seeded January 24, 191 4, and harvested April 5, 1914 

 (Table IX). 



The results secured for series 4 are in accord with those from the 

 clover and oats grown on the pots with large applications. The pots to 

 which had been added brown rock phosphate produced a good return of 

 cowpea hay after having given satisfactory yields of clover. 



The data presented in the previous tables show conclusively that cer- 

 tain species of plants have the power to obtain phosphorus from brown 

 rock phosphate, but how they acquire this element is the problem of 

 vital concern. Do they secure their phosphorus without indirect aid 

 and what influence do other plant foods applied in a soluble form exert 

 on the phosphorus compounds? 



It will be remembered that the sand cultures were maintained at a 

 moisture content of 14 per cent. The plant food application, the infusion, 

 and the water added when the seeds were planted constituted the first 

 moisture supply; or, in other words, all these solutions brought the 

 water content up to 14 per cent. In most of the cases five applications 

 of plant food were sufficient to produce a crop of clover or oats. 



To estimate the influence of water and plant-food solutions on the 

 solubility of the phosphates, quantities of raw rock which correspond to 

 the smallest application (1.81 gm.), soluble plant food equivalent to 

 five applications, and water sufficient to bring the supply of the solu- 

 tion to the same amount that was necessary to bring the moisture con- 

 tent to 14 per cent, or 672 c. c, were placed in a i -liter flask and shaken 

 each day for three months. The soluble phosphorus was then deter- 

 mined with the results shown in Table X. 



Table X. 



-The influence of soluble plant foods on the solubility of the phosphorus in 

 mineral phosphates 



Material applied 

 and pot No. 



Water only: 



I 



Water and solu- 

 ble plant food : 



Kind of phosphate. 



Tennessee brown rock , 



.do. 



Tennessee blue rock , 



Canadian apatite 



South Carolina land rock . 



Utah rock 



Florida soft rock 



37770' 



-16—3 



