CHEMISTRY OF CROP. 



81 



Potash, 22'40 Ibs. 



Soda, 



Lime, 1'34 



Magnesia. "52 



Peroxide of Iron, '18 



Phosphoric acid, 6 '81 



Sulphuric acid, 1'51 



Chloride of Potassium, T96 



Silica, ;fil 



35-33 



This would show that, weight for weight, the crop 

 contains nearly four times as much phosphoric acid and 

 three times as much potash as turnips, mangold, or carrots. 

 The stems no doubt abstract a large proportion of potash 

 also, which renders it the more remarkable that the crop 

 can be cultivated remuneratively on the poorer class of 

 soils, and that continuously, year after year, and without 

 the addition of any large amount of manuring applications. 



The organic composition of the tubers has been exa- 

 mined by Braconnot, 1 who found it to contain 14'80 per 

 cent, of sugar (uncrystallizable), and by Payen, who found 

 even a larger percentage of sugar. Boussingault has also 

 investigated its composition (Economic Rurale, tome i. p. 

 413), which he found on the average to consist of- 

 Water, 76-3 



Solid substances, 237 



100-0 



These solid substances may be taken as follows: 



The proportions of water and of solid matters contained 



1 Annales de Ghirtiie et de Physique, torn. xxv. p. 353. 



