21 



The new method does not, however, enable the slag to be 

 completely characterised and there are still differences in effective- 

 ness which cannot be explained. Slags No. 1, 6, 7 and 8 were 

 compared in the sheep grazing trials at Rothamsted over a period 

 of four years. The gains in live weight of sheep over those 

 obtained on the unmanured plots have been : — 



It is obvious that No. 7 is by far the most effective of these 

 slags, being better even than No. 1 which was known to act 

 well on other soils, but no chemical test so far tried would show 

 this superiority to a prospective purchaser. At the time we 

 obtained the slag neither the makers nor ourselves knew or even 

 suspected that it would prove any better than No. 8 or as good 

 as No. 1, nor can we yet explain why it should be so. It seems 

 clear that somewhere in its history this slag received some treat- 

 ment which, if it could be repeated on other slags, might greatly 

 enhance their agricultural value. A possible clue has been fur- 

 nished by the manufacturers and an observation has been made in 

 the chemical laboratory which may furnish the solution of a very 

 attractive problem. 



A third important chemical factor has been discovered during 

 the past season by Dr. Brenchley and Mr. Page. Some of the 

 slags examined were found to contain substances harmful to the 

 plant. This does not, of course, mean that they actually damaged 

 the crop : what happened was that in these particular slags the 

 beneficial effect of the phosphate present was in part counteracted 

 by the harmful substance. All these problems are being followed 

 up and the co-operation of the slag makers is secured through 

 the Permanent Basic Slag Committee of the Ministry of Agri- 

 culture. In the meantime farmers who have applied slag to their 

 grass and obtained disappointing results are requested to com- 

 municate the facts to the Director. 



POTATOES. 



The experiments with the different potash manures begun in 

 1921 have been continued (p. 120). The muriate and the sulphate 

 of potash behave nearly but not quite alike, the muriate giving 

 sometimes a slightly better and sometimes a slightly less yield than 

 the sulphate. The determining factor is partly rainfall, the 

 sulphate tending to give the higher yield in drier conditions and 

 the muriate in wetter, but there is something beside this, for in 

 1924 the sulphate came out the better in spite of the wetness of 

 the season. 



Addition of other chlorides (e.g., salt) to the muriate, is, 

 however, injurious; neither kainit nor sylvinite gave the full 

 benefit expected from the potash because of the harmful effect of 



