126 EXPERIMENTS UPON ROOT-CKOPS 



a sufficient quantity of potash from the soil, as may be seen 

 from the fact that while the unmanured crop withdrew about 

 21 Ib. of potash per acre per annum from the soil, the addition 

 of superphosphate on Plot 9 raised this to 51 Ib., and the 

 further addition of potash salts and other alkaline salts to Plot 

 10 only increased the amount annually withdrawn to 54 Ib. 

 Clearly the soil, which is known to have been originally well 

 stocked with potash and also to contain considerable residues 

 from potash manurings previous to this experiment, could 

 from its own resources supply ample potash for the require- 

 ments of a crop averaging no more than 2 to 3 tons per acre. 

 Where, however, big crops of potatoes are grown with the 

 aid of dung and artificial manures, it is well known that an 

 abundant supply of potash salts is essential both to the yield 

 and the quality of the potatoes. 



It is well known that season has much to do with the 

 development of potato disease ; and there was on the average 

 much more disease in the wetter seasons. As regards the 

 influence of manure, the proportion of diseased tubers was 

 the least where there was no supply of nitrogen ; that is, where 

 there was the least luxuriance, the most restricted growth, and 

 where the ripening was early developed. On the other hand, 

 with liberal supply of nitrogen and luxuriant growth there 

 was the greatest proportion of diseased tubers ; these being 

 the conditions resulting in a juice relatively rich in nitro- 

 genous and mineral matters. 



PRACTICAL CONCLUSIONS 



1. The best basis for the growth of potatoes is a supply of 

 well-rotted farmyard manure, 12 to 15 tons per acre. In the 

 absence of farmyard manure it should be replaced by some 

 manure containing organic nitrogen, e.g., by 5 cwt. per acre of 

 a good Peruvian guano, or by a meat or fish manure, or by 

 10 cwt. per acre of shoddy. 



2. A free supply of ash- constituents is essential to the 

 successful growth of the potato ; 3 cwt. of superphosphate 



