MANURES SUITED FOR SPECIAL CROPS 219 



Potatoes. Potatoes are grown under such a great 

 variety of circumstances that it is not possible to lay down 

 any general rules for manuring. Potatoes sometimes 

 form a portion of the general rotation, replacing turnips or 

 mangolds. In other cases, where there is a good market, 

 they are grown more frequently. In Dunbar little farmyard 

 manure is used, but potatoes are grown after clover, which 

 has been grazed with cake and corn. By the use of more 

 phosphatic fertilizers on the clover, probably much of the 

 expense of cake and corn could be avoided. With any 

 large excess of nitrogen, potato plants become more suscep- 

 tible to disease and the tubers turn a bad colour on boiling. 



Remarks are often made about manures which set up an 

 alkaline reaction, and facilitate attacks of potato scab. The 

 author has, however, grown potatoes on experimental plots 

 with as much as 20 tons per acre of lime without any exhibi- 

 tion whatever of potato scab. Similarly, very large dressings 

 of basic slag have failed to produce any signs of potato scab. 

 Although the general opinion of the inadvisability of using 

 lime for potatoes must be respected, potato scab is probably 

 not a primary but a secondary result. Injuries may be 

 done to the potato by wire-worms, millipedes, slugs, and other 

 soil creatures ; these wounds, doubtless, render the potato 

 plant susceptible to bacterial attack. The potato is tolerant 

 to a very wide range of soil conditions, and it is quite con- 

 ceivable that an improvement in the soil conditions might 

 assist the growth of the enemies of the potato more than that 

 of the potato plant. The very large dressing of 20 tons per 

 acre of lime would probably not have made the soil a more 

 convenient resort for insect pests, and may therefore have 

 left the potato more immune from attack. In general, 

 it may be said that very large dressings of farmyard manure 

 are unnecessary, excepting for garden cultivation, where, 

 owing to the much greater depth of soil worked, and the 

 much greater amount of labour put on the land, it is practi- 

 cable to utilize these larger dressings. Where manure is 

 somewhat short, dressings of 4-5 cwt. to the acre of super- 

 phosphate or basic slag, 1-2 cwt. of potash and 1-2 cwt. 



