1922.] Ground Mineral Phosphates as Manures. 707 



phosphates appeared to be finely ground, that less than 50 per 

 cent, of each would pass through a sieve of 120 wires to the 

 linear inch. He therefore had all these mineral phosphates 

 sifted, and tested each of them as 



(1) Sifted — all passing through a No. 120 sieve, and 



(2) Unsifted — from 40-50 per cent, passing through a 

 • No. 120 sieve. 



The same amount of phosphoric acid (about 100 lb. an acre) 

 was applied of each, so that the quantities per acre were : 

 Superphosphate. 800 lb. ; Basic Slag, 560 lb. ; Curacao, 260 lb. ; 

 -Canadian, 360 lb.; Carolina, 400 lb.; and Belgian, 560 lb. 



On the average the weights per acre of the turnips were : — 



Tons. Cwt. 



No phosphates ... ... ... ... ... 9 9 



Superphosphate ... ... ... ... ... 11 9 



Basic slag ... ... ... ... ... ... 11 13 



Mineral phosphates sifted ... ... ... ... 11 17 



Mineral phosphates unsifted ... ... ... 10 18 



He summed up the results thus : — 



"1. Ground mineral phosphates are the more active the more finely they 

 are ground. 



"2. When ground as finely as to pass through a sieve of 120 wires per 

 linear inch, they are nearly as active as superphosphates. 



"3. The nature of the phosphate is of much less importance than the fine- 

 ness to which it is ground. 



" 4. Basi • slag is at pr. sent 188G) the most finely ground and the cheapest 

 phosphate on the market." 



Dr. Aitken conducted such trials for many years, not only 

 at Pumpherston, but on farms in all parts of Scotland. Many 

 of his trials with ground mineral phosphates were disappoint- 

 ing and he believed that the lack of fineness of grinding caused 

 a number of the poorer results. The advent of basic slag, the 

 good results it gave, the fact that it could "be readily obtained 

 ground to a standard fineness, and also that phosphates could 

 then be obtained at a cheaper cost per unit in this than in 

 ground mineral phosphates, all tended to discourage further 

 experimental work with ground mineral phosphates at that 

 time. 



In 1889 Dr. Aitken stated that a unit of phosphate of lime 

 cost Is. Id. in mineral phosphates and only lid. in basic slag. 

 In 1896 he put their relative costs at 8Jd. and Is. 2Jd. Since 

 that time the phosphates have usually been at a lower cost per 

 unit in mineral phosphates than in basic slag. There were 

 :for some time, however, abundant supplies of basic slag, and 



c 2 



