COMPOSITION AKD FEETILITY OF THE SOIL. 27 



the more it is manured, the more numerous they are. 

 Their action as soil-fertiHzers consists in swallowing 

 earth, leaves, and organic matter of all kinds, triturating 

 it, converting it, and then ejecting it over the surface of 

 the field. In this way they very soon effect a complete 

 mversion of the soil down to a certain depth, especially 

 on meadow land, which is left undisturbed to their opera- 

 tion. They even make additions to the soil by bringing 

 up fresh matter from the subsoil. Every time a worm is 

 driven by dry weather or any other cause to descend deep, 

 it brings to the surface, when it empties the contents of 

 its body, a few particles of fresh earth. At the same time 

 it fertilizes the subsoil, by opening up passages wiiich en- 

 courage the roots of plants to penetrate deeper, these 

 passages being lined with excreted matter which provides 

 a store of nourishment for the roots. On meadow land, 

 Darw^in found these worm-casts amount annually to 

 eighteen tons per acre, and on good arable land to about 

 ten tons. Dr. Gilbert has analysed worm-casts, and 

 found them to contain .34 per cent, of nitrogen, which 

 is several times more than is found m ordmarv soils. 

 Ten tons of these castings will thus yield eighty pounds 

 of nitrogen, or as much as is contained m two acres of a 

 wheat crop. 



By various artificial means, tillage, drainage, clay- 

 burning, the application of lime, or manures, the pro- 

 cesses of natural soil-renovation may be greatly accel- 

 erated. And these are the subjects of the remaining 

 chapters of this book. But it is plain from the natural 

 influences, as well as the artificial operations, to which 

 soils are amenable, that there is hope for almost any 

 soil — that in few cases can land be so run out as to re- 

 quire the direct supply of all the substances which are 

 needed to create fertility, for many of them are already 

 pa-esent, and it only requires a little skillful management 

 to exhibit them. It is on this principle that we must ex- 



