XII. 



CROPS 



249 



phate or slag, and sulphate of potash are the artificial manures generally 

 used. Potash manures containing chlorides, e.g., muriate of potash and 

 kainite, are said to render potatoes waxy. 



The berry of the potato, as well as the leaves or haulms and, to a 

 much less extent, the tubers, contain a poisonous substance solanine, 

 G 52 H 93 NO 18 , which, by the action of acids, splits up into sugar and 

 solanidine, C 40 H C1 NO.,. 



The amount of solanine in potatoes is not sufficient to do harm, 

 but, in the shoots from stored potatoes, the amount of this substance 

 so concentrates itself, that poisoning of animals eating the shoots may 

 occur. Still more dangerous are the berries and even the haulms. 



Potatoes are liable to considerable fluctuation in composition, the 

 proportion of water being especially variable. Wet seasons, liberal 

 supplies of nitrogen, or large quantities of potassium chloride (e.g., 

 kainite) tend to make the tubers watery and poor in starch. Of the total 

 nitrogen in potatoes, about two-fifths is present in non-proteid forms. 



The following figures (Kellner) give the average composition of 

 potatoes : 



The amount of dry matter and of starch in potatoes can be estimated 

 with sufficient accuracy for commercial purposes by a determination of 

 the specific gravity of the tubers. 



A considerable quantity (8 or 10 Ib.) of the well-cleaned potatoes is 

 weighed as accurately as possible in a wire basket. The latter is then 

 wholly immersed in water and the weight again determined. The 

 weight of the potatoes in air, divided by the loss of weight when 

 weighed in water, gives the specific gravity. The following table, com- 

 piled by German investigators, gives approximately the relation between 

 specific gravity, total solid matter and starch : 



