244 CROPS. XI. 



leguminous plants are remarkable for the large amounts of 

 nitrogenous matter, lime, and potash which they contain. 

 The lime is mainly contained in the leaves. Silica is almost 

 absent. Clover, lucerne, &c., are also grown as crops upon 

 arable land with valuable after-effects. The land is thereby 

 actually enriched in nitrogen, notwithstanding the fact that a 

 very large amount of nitrogen is removed in the crop. The 

 nitrogen is obtained from the air in the manner already de- 

 scribed. The beneficial effect of the growth of clover upon the 

 soil has long been known and utilised in agriculture; but it 

 was not until after many laborious researches had been made 

 that the explanation of the fact was discovered by-Hellriegel 

 and Willfarth about 1888.* 



By repeated cropping of land with clover a condition known 

 as " clover sickness" is often set up. The seed in the summer 

 germinates and grows well until the autumn or winter, when 

 the plants die off rapidly, and, in many cases, a minute eel- 

 worm attacks the root and stem. Whether these nematoids 

 (Tylenchus) are the cause or a consequence of the disease 

 appears to be uncertain. Clover sickness has also been 

 ascribed to certain fungi. It is said that deficiency of the soil 

 in potash and lime is a predisposing cause of this disease. 



Other plants are sometimes grown for fodder, e.g., rye, 

 vetches, oats, &c. 



In some cases the fodder crop is eaten green by animals ; but 

 in most cases it is preserved for future use, either by being 

 made into hay or silage. 



Hay-making consists in drying the plants by exposure to sun 

 and air to such an extent as to greatly check fermentation, 

 which, in the presence of moisture, soon occurs in vegetable 

 matter. This fermentation is due to the action of micro-orga- 

 nisms and is accompanied by absorption of oxygen from the 

 air and consequent evolution of heat. During hay-making the 

 most important change is the loss of water ; this naturally 

 varies considerably with the nature of the crop, its ripeness, 

 &c. Ordinary meadow grass will usually contain about 

 75 % of water, while the hay from it, in the stack, may contain 



* v. p. so. 



