I70 THE POTASS IC FERTILISERS [chap. 



specific application of potassic manures is unnecessary. 

 In the earlier years of this experiment only phosphates 

 had been added to the dung and nitrogenous manures, 

 but had produced no increase; we may conclude, there- 

 fore, that in the series quoted it was the potash alone 

 that had been active. This result is the more striking, 

 in that dung itself contains a large proportion of 

 potash, yet the use of 14 tons of dung per acre year 

 after year, beginning in 1856, has still been unable to 

 supply the mangold crop with all the potash it needed. 



Both in this series of plots and that in which the 

 mangolds receive no dung, the value of potassic manur- 

 ings is small where nitrate of soda is the source of 

 nitrogen. This is not only because the sodium can be 

 made to do some of the work usually done by potassium 

 in the plant, but also because it is able to attack the 

 compounds of potash in the soil (the Rothamsted soil 

 contains an enormous reserve of insoluble potash), and 

 bring it into solution so that it becomes available for 

 the plant. 



This can be illustrated by the results obtained on 

 the Rothamsted mangold field in 1900 (a good year); 

 Table XLVII. shows the yield of roots and leaves, and 

 the potash and soda removed from the soil by the crop, 

 both with and without potash, when the nitrogenous 

 manures were ammonium salts and nitrate of soda 

 respectively. 



It will be seen that on Plot 5N, without any potash 

 but where nitrate of soda is used, the yield of roots and 

 leaves is almost as great as that obtained on 6N where 

 potash salts are also added, and is more than double that 

 given by the corresponding plot without potash, 5 A, but 

 which receives its nitrogen as ammonium salts. The 

 amount of potash taken from the soil by the crop on Plot 

 5N is 927 lb. against 596 lb. on $A,the increase repre- 



