June 26. 1916 Mineral Phosphates and Plant Nutrition 511 



added raw rock phosphate. When the calcium bicarbonate and mono- 

 calcium phosphate are both removed from the medium of growth by 

 plants, the reaction is driven rapidly to the right. Mass relationship in 

 a mixture of this kind confirms such an interpretation as the one presented 

 above. Our first assumption, that plants should get their calcium from 

 rock phosphate in the same manner that they get their phosphorus, is 

 supported at several points in this work. This must be so, since the 

 calcium is furnished by the calcium salt of phosphoric acid or by the 

 bicarbonate. There was no greater growth when calcium carbonate was 

 added than where raw rock alone was used. In fact, the growth might 

 be even less, since calcium carbonate might furnish a greater concentra- 

 tion of Ca(HC03)2 or HCO3, which might decrease the concentration of 

 H from equation i , thus decreasing the rate of the availability of rock 

 phosphate. 



The most marked feature of the investigation is the difference of the 

 availability of the various minerals. The fact that the crop yields 

 increase as the application of the brown rock phosphate was increased 

 indicates that a portion of the phosphorus was readily assimilated while 

 the plants were young, and that by the time these plants became well 

 established they were able to utilize the more insoluble form. If we are 

 to assume that a part of the phosphorus is of animal origin, this position 

 probably is more tenable, or on the other hand, through long years of 

 weathering the compound had been so changed that a portion was more 

 easily taken up by plants than before weathering began. 



There is an indication that the crops grown first took up the more 

 available phosphorus and that the second crop made very slow growth 

 because the more soluble phosphorus was removed by the first crop and 

 nothing left but the rather insoluble for later crops. These points have 

 proof from the cowpeas on the large application series and the clover on 

 the crop residue series. 



Brown rock phosphate and Florida soft rock phosphate lead the others 

 in supplying available phosphorus for plant nutrition, especially for clover. 

 The brown rock phosphate leads for all the crops. These two phos- 

 phates gave the largest quantity of phosphorus soluble in water and 

 plant-food solutions. The results indicate a relation in solubility in 

 plant-food solution and the availability for plants. 



The difference in the assimilation of these phosphates can not be 

 attributed to the degree of fineness of the particles, since they were all 

 ground, so that the entire sample passed through a sieve of 100 meshes 

 to the inch. If the degree of fineness influenced the results, the differ- 

 ences then come from the size of particles, which were smaller than those 

 found in commercial phosphates. 



The variation in the agricultural value of the six mineral phosphates 

 studied is difficult to explain. Their productive powers seemed not to 



