GREEN MANUEE. 191 



because in that case annual cultivation, and all the 

 seed after the first, will be saved. Such plants 

 cannot, as a matter of course, be ploughed in on 

 the spot ; but they must be cut down and carted on 

 the field most in need of manure. For this purpose 

 plants may be used that have not yet been much 

 thought of. Fifth: those plants more especially 

 are to be used for green manure which have many 

 broad leaves, because they most absorb aeriform 

 substances (carbonic acid, hydrogen, and nitrogen). 

 Spurry is an exception to this rule. Sixth : those 

 plants must be selected which absorb from the sub- 

 soil those mineral substances in which the surface 

 is deficient, and which are required for the nour- 

 ishment of the future crop of corn ; but whether 

 those substances on which the success of deep- 

 rooted plants depend are to be found in the subsoil, 

 can only be decided by a chemical analysis. Seventh: 

 in order that the plants sown may yield the great- 

 est amount of herbage, seed must be used abun- 

 dantly. It may, therefore, be useful to sow different 

 sorts together, so that if one does not succeed the 

 other may. In all cases it is essential that the field 

 should be clear of grass and other weeds ; because, 

 as the land cannot be ploughed more than once, 

 weeds may increase subsequently to such an extent 

 that the failure of the corn crop may be appre- 

 hended. Deep-rooted weeds are best destroyed by 

 the closeness and shade of the green crop, and this 



