154 AGRICULTURE 



in excess, and especially chloride of potash, 

 has a tendency to depress the percentage of 

 starch in potatoes, but in ordinary practice 

 such a result will seldom be likely to arise. 



Of all the root crops, mangolds are most 

 dependent upon a supply of available potash 

 in the soil. One of the most interesting and 

 important results of the long-continued ex- 

 periments at Rothamsted has been obtained 

 in the continuous growth of mangolds. There 

 it was found, on the average of a long series 

 of years, that superphosphate and ammonium 

 salts, without potash, produced little more 

 than 7| tons of roots per acre, whereas the 

 addition of a fair dressing of potash pro- 

 duced an average crop of 14 tons per acre. 

 Where superphosphate and nitrate of soda 

 were used, without potash, the crop was 

 nearly 15^ tons per acre, and when potash 

 was added to this dressing the yield was not 

 increased. It is evident, therefore, that 

 potash has proved very effective in the 

 presence of ammonium salts, but has been 

 inoperative in the presence of nitrate of 

 soda. 



Various reasons may be advanced to 

 account for these results. Nitrate of soda 

 is known to sink deep into the soil, and 

 this deeper diffusion of the plant food in- 



