PHOSPHATIC MANURES 141 



the weak acids in the soil, or by the sub- 

 stances exuded by plant roots. It is believed 

 that a certain proportion of the phosphate 

 of slag is much more readily available than 

 the remainder, in fact it is probable that 

 a considerable residue is so insoluble as to 

 have no practical value. An attempt has 

 therefore been made to differentiate between 

 what is easy of absorption by plants and 

 what is not ; the standard agent now uni- 

 versally employed for this purpose being a 

 2 per cent, solution of citric acid, in which 

 the basic slag is left for half an hour, during 

 which it is subjected to continuous shaking. 

 The phosphate removed from the slag by 

 this weak solvent is supposed to be a fair 

 indication of what farm crops are able, 

 within a reasonable time, to make use of ; 

 a common basis of sale being a total citric 

 solubility of 80 per cent., that is to say, 

 the vendor guarantees 80 per cent, of the 

 total phosphate to be soluble in the said 

 solution, when manipulated in the authorized 

 manner. On this basis a guarantee of 32 per 

 cent, of citric-soluble phosphate is equivalent 

 to a guarantee of 40 per cent, of total phos- 

 phate, and many vendors now sell basic 

 slag only on such a basis of solubility, though 

 the total phosphate must also be mentioned. 



