124 EXPERIMENTS UPON ROOT-CROPS 



with mixed mineral manure, including potash, 58 '2 cwt. Thus, 

 purely nitrogenous manures yielded less than purely mineral 

 manures, indicating that the potato finds a difficulty in 

 obtaining ash constituents rather than nitrogen from an 

 impoverished soil. 



By superphosphate of lime alone the produce was raised 

 from an average of 27 '4 to 54 '4 cwt. ; and by a mixed mineral 

 manure containing besides superphosphate of lime, salts of 

 potash, soda, and magnesia, to 58*2 cwt. ; that is to very little 

 more than by the superphosphate alone. It is clear that as 

 regards the small crops in question the land is still able to 

 supply sufficient available potash when it has become 

 comparatively exhausted of the phosphoric acid which can 

 reach the crop. 



In reference to this increase of produce of potatoes by 

 mineral manures alone, it may be observed that the result is 

 quite consistent with that obtained with other root-crops 

 having comparatively shallow root-development ; and in such 

 cases the source of the nitrogen is chiefly the store of it in the 

 surface soil. The beneficial effects of mineral manures, and 

 especially of phosphates, are indeed observed generally with 

 all crops which are spring sown and have but a short period 

 of growth, so that they possess a comparatively superficial 

 root system, and are therefore forced to rely much on the 

 stores of food in the surface soil only. 



It is remarkable that there is much less increase of produce 

 of potatoes by nitrogenous manures alone than by mineral 

 manures alone. Thus by ammonium-salts alone there is an 

 average produce of 34 cwt., or only between 6 and 7 cwt. more 

 than without manure ; and with nitrate of soda alone there is 

 an average of only 42 '5 cwt. per acre. 



With the mixed mineral manure and ammonium-salts 

 together, the average produce of total tubers was 105*7 cwt., 

 and with the mixed mineral manure and nitrate of soda, 1087 

 cwt. per acre. The better result from the nitrate of soda is 

 doubtless due to its nitrogen being more immediately available 



