5IO Journal of Agricultural Research voi. yi, No. 13 



tion. For the purpose cc is taken equal to 90 per cent, from which the 

 concentration of H is calculated thus: 



Ka^^° = , where (C) = Ka^^ ^ = concentration of H. 



I — oc oc 



r TT*/W8WT+x i.iXio~2(o.i) i.iXio"^ ^ _3 

 . • . concentration of H+(C(^>H+) — -^ — -' = — — =1X10 ^ = 



o.ooi. 



It is seen that only the first H of H3PO4 can furnish a. greater concen- 

 tration of H+ than H2CO3 for equivalent concentrations. In the actual 

 experiment the concentration of H3PO4 is much less than that of H2CO3. 

 However, the availability of the rock phosphate by means of H2CO3 is 

 not conditioned by the liberation of free H3PO4 according to equation 8. 

 Equation 6 or 7 is driven in the direction to remove H+, would render 

 the tricalcium phosphate more available, but a reaction between ions pro- 

 ceeds if a lesser ionized product be formed. Calculations of the H+ con- 

 centration for equations i , 6, and 7 shows that for equivalent concentra- 

 tions the H+ from carbonic acid is greatly in excess of the H+ concen- 

 tration for equ ations 6 and 7. So if equations i, 6, and 7 are present 

 simultaneously HPO4 and H2PO4 of equations 6 and 7 would be formed 



by the union of H+ of H2CO3 with PO4 and HPO4, respectively, thus 

 causing more Ca3(P04)2 to dissolve to reestablish the equilibria for 

 equations 6 and 7. It is a fact, however, that a greater concentration 

 of H2CO3 is present than any of the ionizing substances, as HPO4, HjPO^, 

 or H3PO4. This would increase the rate of availability of the trical- 

 cium phosphate. 



These calculations are borne out by the fact that more Ca3(P04)2 

 dissolved in water containing H2CO3 than in pure water. Seidel's 

 solubility tables state that i liter of water saturated with H2CO3 dis- 

 solves 0.15 to 0.30 gm. of Ca3(P04)2 at 25°, while i liter of pure water 

 dissolved only o.oi to o.io gm. of Ca3(P04)2 at 25°. 



Reactions 6 and 7 may be shown in the nonionic form as follows: 



Ca3(P04)2-f 2H2C03t;Ca2H2(P04)2 + CaH2(C03)2 or 

 Ca,(P04)2+2HX03!i;Ca2(HP04(2 + Ca(HC03)2 

 Ca2H2(P04)2 4-2H2C03;i!CaH4(P04)2+CaH2(C03)2 



i i 



or or 



Ca(H2P04)2 Ca(HC03)2 



In the first equation calcium is found in a form readily assimilated by 

 plants, and in the second the monocalcium phosphate is in a very assimil- 

 able form. On this equation we have based our belief that there is no 

 necessity for applying lime to sand cultures to which had previously been 



