242 CROPS. XI. 



large amounts of nitrogen which they contain, from the air, 

 under certain favourable conditions which have been already 

 described. :;: They contain more than twice as large a propor- 

 tion of nitrogen as the cereals and also much larger amounts 

 of potash and lime, but very little silica. The seeds are re- 

 markable for the large amounts of an albuminoid legumin 

 analogous to casein, which they contain ; there is also present 

 considerable quantities of the fat-like body, lecithin.! 



(2) ROOT CROPS. Turnips, sicedes, mangolds, potatoes. 

 These crops are characterised by the large amounts of nitrogen 

 (much of which, however, is present as amides and often as 

 nitrates) and mineral matter which they contain. They are 

 also remarkable for the high percentage sometimes 90% or 

 over of water which is contained in their " roots" or tubers. 



Turnips and swedes depend chiefly upon the surface soil and 

 have but limited powers of attacking the insoluble phosphates, 

 though able generally to provide themselves with potash. 

 Phosphatic and nitrogenous manures are therefore usually 

 most effective. 



Mangolds are deeper rooted and generally able to provide 

 themselves with food, nitrogen being the constituent most 

 likely to be lacking. Being rich in ash constituents and heavy 

 croppers, mangolds are probably the most exhausting crop 

 grown. A peculiarity of mangolds is the benefit they receive 

 from applications of common salt. 



Beets, which resemble the mangold in many respects, are, 

 on the Continent, largely grown for sugar. Small beets yield a 

 higher percentage of sugar than large ones. 



Indeed, in the case of all roots the large, well-developed 

 ones produced by liberal manuring are found generally to be 

 more watery, to contain less sugar and starch, and altogether 

 to be less nutritious than small ones. Turnips, swedes, and 

 mangolds contain sugar in large quantities (5 to 8 %) . 



Potatoes are chiefly limited to the surface soil for their food. 

 They are rich in starch, for which reason, perhaps, they 

 require considerable quantities of potash to assist in its assimi- 

 lation. They are also benefited by liberal general manuring, 



* v. Chap. IV., p. 80. t v. Chap. IX., p. 206. 



