232 SOILS AND MANURES 



suitable form of phosphatic manure for other leguminous 

 crops. 



Comparison of Basic Slag and Superphosphate. As a 

 purely phosphatic manure, Thomas' phosphate naturally 

 lends itself to comparison with superphosphate. In 

 making such a comparison, however, it should be re- 

 membered that, owing to the difference in their character, 

 the same conditions are not equally suitable for both. 



From any point of view there is little to be gained by 

 comparing the manurial effects of equal weights of 

 Thomas' phosphate and superphosphate, unless they con- 

 tain nearly the same proportion of phosphates. A ton 

 of basic slag (35 per cent.) contains approximately the 

 same quantity of phosphoric acid as 28 cwt. of super- 

 phosphate (25 per cent.), and it is a matter of consider- 

 able scientific interest to determine in which of these 

 two forms, i.e., as acid phosphate or as basic phosphate, 

 a given quantity of phosphoric acid will produce the best 

 result. Numerous experiments have been carried out in 

 order to settle this point, but have given contradictory 

 results, probably because it is very difficult to secure 

 conditions equally favourable to the action of both 

 manures. It is generally considered, however, that 'if 

 such conditions could be secured, the acid phosphates, 

 being more readily soluble, would prove more readily 

 available to the plants, and that 28 cwts. of super- 

 phosphate would produce a larger increase than 20 cwts. 

 of basic slag. 



From the purely practical point of view it is more 

 important to determine which of the two manures will 

 give the larger return for a similar expenditure. At 

 the ordinary market rates, basic slag generally costs 

 rather less per ton than superphosphate, and the differ- 

 ence in price per unit of phosphate is still greater. 

 For a given sum of money, a considerably larger quan- 



