142 AGRICULTURE 



This method of selling, which has now 

 received statutory sanction in this country, 

 has long been practised on the Continent. 



Various theories have been advanced to 

 account for the comparatively easy solubility 

 of phosphate in basic slag as compared with, 

 for instance, phosphate in natural rock. 

 It is not necessary in this place to enter 

 into the details of the various considerations 

 that have been advanced, suffice it to say 

 that in some way or another the phosphoric 

 acid in basic slag is rather loosely united 

 with part of the lime, and in consequence is 

 separable with comparative ease through 

 the action of the weak acids in the soil, or 

 of those exuded by the roots of plants. 



Experiment has shown that the solubility 

 and therefore the value of a basic slag are 

 largely influenced by the degree of fineness 

 of the particles. This has led to the setting 

 up of a standard of fineness, namely, that 

 80 per cent, of the material as delivered to 

 the farmer shall pass through a sieve con- 

 taining ten thousand apertures to the square 

 inch of surface, and this is the grade of 

 fineness that should be insisted upon in the 

 purchase of this manure. 



Superphosphate of lime can be valued in 

 the same way as basic slag, and although on 



