342 Journal of the Department of Agriculture, 



Nos. 1 and 3 the percentage of water was found to be higher in the 

 unpeeled than in the peeled tubers. 



With reference to the determinations of specific gravity, attention 

 may be called to Martin's statement* ihat in Irish potatoes there is 

 a connection between the specific gravity and the starch content, so 

 *:hat the percentage of starch in a potato may be calculated by means 

 of a formula, or read off from a set of tables, when the specific gravity 

 is known. In the sweet potatoes from Little Brak a similar relation 

 appears to hold : the common six-months variety, which has the 

 largest percentage of starch, is also highest in specific gravity. On 

 the other hand, the old three-months variety showed the lowest 

 specific gravity, the tubers readily floating in water, and the per- 

 centage of starch is also the lowest of the four. It must, however, be 

 added that the tubers of this variety showed a number of internal air- 

 spaces, the result apparently of excessive stringiness. 



It will be noticed that the analyses show the two three-months 

 varieties to be more watery than the others. 



The sweet potato is, like all root crops rich in starch or sugar, 

 pre-eminently a potash-loving plant, as the following table, giving 

 an average of three analyses of the ash, according to Konig, clearly 

 shows : — 



Potash 50.31 per cent. 



Soda 6.53 ,, 



Lime 9.93 ,, 



Magnesia 3.40 ,, 



Iron oxide .91 



Phosphorus pentoxide 10.60 ,, 



Sulphur trioxide 5.56 ,, 



Silica 3.45 



Chlorine 12.74 ,, 



An analysis of the ash obtained from the red-skinned three- 

 months variety grown on Mr. Robertson's farm gave the following 

 percentage results — 



Potash 46.15 



Lime 7.50 



Phosphorus pentoxide 7.96 



The ash of the skin, peeled oft' to a thickness of one-sixteenth of 

 an inch, from the British East African tubers, gave the following 

 percentages on analysis : — 



Potash 25.49 



Lime 12.30 



Phosphorus pentoxide 7.43 



Potation. 



The foregoing figures show what large proportions of potash, 

 when compared with other inorganic plant food, the sweet potato 

 needs. Hence sweet potatoes, grown continuously on the same lands 

 without a potash fertilizer, naturally exhaust the soil even when 

 manured with farmyard manure, the experience often being that, 



* " Industrial Chemistry : Organic," Third Edition, page 181. 



