5o8 Journal of Agricultural Research vci. vi. no. 13 



Pots 5 to 10, inclusive, received the ground rock phosphate as it was 

 obtained from the mills. The degree of fineness varied from that passing 

 a sieve 80 to 100 meshes to the inch to that which would go through a 

 sieve of 200 meshes to the inch. Pots 17 to 22, inclusive, received the 

 ground phosphate which was shipped in the lump form and afterward 

 ground to the same degree of fineness as that ground at the mill. 



There is a tendency for the dry matter to increase as the degree of 

 fineness increases. The phosphate received from the mill in lump form 

 was sHghtly better than that sent to us in a ground condition. 



DISCUSSION 



Under the conditions of these experiments a fairly large portion of the 

 phosphorus in brown rock phosphate was available for plant growth. 

 The quantity was variable, depending upon the crops and the circum- 

 stances attending the full development of the plant. The data show 

 only a very small amount of phosphorus soluble in water and plant food 

 solutions. It is clear that other factors which might bring about avail- 

 ability must be considered. The sand cultures contained very little 

 organic matter; hence, these sHght fermentable substances should not be 

 considered. There is nothing left but the plant for our examination and 

 there is abundant proof that the plant itself is a significant item. Since 

 plants excrete large quantities of carbonic acid, there is but little question 

 that this substance plays the primary roll in the liberation of phosphorus. 



The reactions with carbon dioxid which occur when tricalcium phos- 

 phate is put into sand cultures of the kind described in these pages may 

 be shown in the following manner: 



(B) Ca3(P04)2^Ca3(P04)2^3Ca++ + 2PO, 

 Solid In solution. 



When A and B are mixed, the following equilibria develop: 



(6) 



(C) PO4+H+ HPO4 (The ion of reverted phosphate) 

 ■ (7) 



HP04 4-H+<=^H2P04 (The ion of soluble phosphate) 

 (8) 



HjPO^-hH+^^HgPO^ 



