INFLUENCE OF AMMONIA ON QUALITY 99 



the use of 3 cwts. sulphate of ammonia, 6 cwts. superphosphate, 

 and 2 cwts. sulphate of potash, produced an increase of 3^ tons 

 of saleable potatoes. These are profitable results. 



3. Influence of Ammoniacal Manures on Quality. 

 The quality of a potato crop depends on the size and shape 

 of the tubers, their freedom from disease, and on their charac- 

 ter when cooked. When potatoes are grown, as they frequently 

 are on the continent, for distillery purposes, their value depends 

 on the percentage of starch which they contain. This par- 

 ticular of their composition is also intimately connected with 

 their general value as food. The potato richest in starch will 

 be the one containing the smallest proportion of water, and 

 the one which \\ill yield a dry, floury vegetable when boiled. 



Many particulars as to the quality and composition of the 

 potatoes grown with various manures have been regularly 

 determined at Rothamsted. We take as our examples the 

 average results for twelve years, 1876-87, given by Sir J. H. 

 Gilbert in the Agricultural Students' Gazette, 1888, 29. 



Any manure which furnishes a good supply of ash con- 

 stituents, and thus allows the tuber thoroughly to mature, will 

 give a greater proportion of large, saleable potatoes, than a 

 manure in which the supply of potash and phosphate is de- 

 ficient. The average results of twelve years at Rothamsted 

 show that 90 per cent of the crop on plot 10, with superphos- 

 phate and alkalies, were good, saleable potatoes ; nearly 89 per 

 cent on plot 3, receiving farmyard manure, with superphos- 

 phate in the earlier years ; 88i per cent on plot 7, manured 

 with ammonia salts, and a full supply of ash constituents ; 84 

 per cent on plot i, without manure ; and 82 per cent on plot 5, 

 receiving ammonia salts without ash constituents. 



The proportion of diseased potatoes seems generally to follow 

 pretty closely the bulk of the crop, the heaviest crop suffering 

 most. On an average of 12 years, 3-2 per cent of the potatoes 

 on the unmanured plot were diseased ; 3-5 per cent on plot 10, 

 with cinereals only ; 4-9 per cent on plot 3, with farmyard 

 manure ; and 6*3 per cent on plot 7, with ammonia salts and 

 cinereal manure. 



Xo determinations of starch have been made at Rothamsted 



