No. 4.] CATCH-CROPS. 323 



the means of warming it. The darker color resulting from 

 the presence of humus favors the absorption of heat from the 

 sun, and the process of decay, being in its final effect pre- 

 cisely like combustion by fire, helps to raise the temperature 

 of the soil, though not to an important degree. 



Many of the most valuable green-manuring crops arc dis- 

 tinguished by the fact that they send their roots into the soil 

 to an enormous depth. Such crops are highly beneficial in 

 two ways : first, by means of their deep roots they bring up 

 from the subsoil food not available to ordinary crops ; but 

 haviug been thus pumped up, as it were, it becomes available 

 to succeeding shallow-rooted crops ; second, it has been 

 found that crops which are ordinarily shallow-rooted send 

 their roots much deeper than usual when they are made to 

 follow a deep-rooted green-manuring crop. Thus, for exam- 

 ple, Schultz-Lupitz has noticed that the roots of the potato, 

 which ordinarily develop almost entirely near the surface, 

 penetrate deeply into the ground after a crop of lupines, fol- 

 lowing the furrows left by the decay of the roots of the lu- 

 pines. This renders the potato less liable to injury from 

 drought, increases enormously the store of food within its 

 reach and so makes the crop more certain and larger. 



Green-manuring may be made the means of cleansing 

 the field from weeds, for which purpose, of course, as has 

 been pointed out, only the crops of the most rapid growth 

 are useful. Green-manuring increases the store of organic 

 matter in the soil, and so furnishes the conditions favorable 

 for the multiplication of earth worms, and these, as Darwin 

 has pointed out, by their activities improve the soil in many 

 ways ; most important among which are better aeration, 

 bringing of the finer materials to the top, pulverization and 

 increased solubility of its constituents. 



Finally, green-manuring may be made to protect the soil 

 from the injurious action of violent winds and from surface 

 washing, for which purposes, as has been pointed out, those 

 crops which occupy the land in winter are by far the most 

 useful, although those which are killed by frosts, if allowed 

 to remain upon the surface during the winter, will prove use- 

 ful in the same direction. 



The list of possible benefits from green-manuring, it will 



