1922.] 



Phosphatic Fertilisers. 



(2) The present-day open-hearth slag of high solubility con- 

 taining phosphorus equivalent to 15 to B5 per cent, of fcricalcic 

 phosphate, and therefore poorer than the old Bessemer -la^ r . 

 There is no reason to suppose that anything except the phosphate 

 has fertiliser value, and in comparing these slags with the old 

 it is usual and probably sound to do so on the basis of ^qual 

 phosphate content. Experiments made on these lines indicate 

 thai the phosphate in the high soluble slag has on the whole the 

 same agricultural value as that in the Bessemer; in other words, 

 a dressing of 10 cwt. per acre of a 20 per cent, high soluble slag 

 could be expected to have approximately the same effect on 

 grass land as 5 cwt. per acre of a 40 per cent. Bessemer slag. 



(9J Borne of the open-hearth slag, while containing the same 

 amount of total phosphorus as the preceding material, has a 

 much lower solubility according to the official test. It is not yet 

 clear that there is any great difference between slags of some 

 n0 per cent, and those of 80 per cent, solubility, but it does 

 appear that those of 30 per cent, and still more so those of 20 per 

 cent, solubility are less effective. In the Essex experiments the 

 low soluble slags were less reliable than those of high solubility; 

 sometimes they acted well and sometimes they did not. There 

 is evidence that they are slower in action than the high soluble 

 slags, and hence they should be given time and not expected to 

 work in a hurry : they should be applied in autumn and used 

 preferably in districts of sufficient rainfall. Out of 41 experi- 

 ments with both types of slag coming under review during the 

 dry season of 1921, 15 of the high soluble were effective, but 

 only 9 of the low soluble. 



The comparison of prices of slag and superphosphate is ren- 

 dered easy by the circumstance that both are sold on the basis 

 of their phosphate content, and although the actual compounds 

 differ in the two fertilisers, they are each expressed in terms of 

 the standard tri-calcic phosphate. Thus, as already explained 

 k< 80 per cent, superphosphate " means that 100 lb. of the super- 

 phosphate contains as much phosphorus as is present in 80 lb 

 of the standard tri-calcic rmosphate. So a " 20 per cent, slag " 

 contains in 100 lb. as much phosphorus as does 20 lb. of the 

 standard tri-calcic phosphate. If therefore 30 per cent. Btiper- 

 phosphate is offered at .1*4 5s. each unit costs 2s. 10d.: while if 

 20 per cent, slag is offered at L'B each unit costs 8s. The price 

 should be worked out to include delivery at the farmer's Btation. 



Solubility of Slag. — There is a further description of basic slag 

 which often causes some confusion — the solubility. This figure 



