v.] SOLUBTTJTY OF PHOSPHATES 143 



of the solution it may be in contact. Yet insoluble as di-, 

 tri-, and tetra-basic phosphate of lime are, when they are 

 sufficiently finely divided and well incorporated with the 

 soil so as to be in contact with the roots, they are all 

 effective in supplying the plant with phosphoric acid. 

 It will be shown later (p. 290) that the carbonic acid 

 excreted by the roots of the plants is the chief agent 

 in producing a solution in the soil water capable of 

 attacking these insoluble phosphates ; acid humus and 

 ammonium sulphate, which gives rise to free acid in 

 soils, also assist in rendering them available to the 

 plant. 



Since only one of the commercial phosphates is 

 freely soluble in water, yet all of them have to enter 

 into solution before they can be utilised by the plant, 

 the question of their relative availability is not easy to 

 settle, and a variety of solvents have been proposed for 

 its determination in the laboratory. In Germany, for 

 example, basic slag was formerly valued, not on the total 

 amount of phosphoric acid it contains, but on the 

 amount that is soluble in a strong ammoniacal solution 

 of ammonium citrate, the idea being that this reagent 

 discriminates between the tri-calcium phosphate, which 

 is insoluble, and the di- and tetra-calcium phosphates, 

 which will dissolve in the medium. Instead of the 

 ammonium citrate, a 2 per cent, solution of citric acid 

 is now employed, and i per cent, and o-i per cent. 

 solutions have also been proposed by various chemists 

 for the valuation of phosphatic fertilisers. None of 

 these solvents, however, really draws a sharp distinc- 

 tion between the different phosphates, which are all 

 soluble up to a certain point, when an equilibrium is 

 established between the phosphoric acid in solution and 

 that remaining undissolved. If the first solution formed 

 is replaced by a fresh portion of the solvent, more 



