288 JVIANURE. 



former, the soil itself may frequently be very much 

 improved, and probably some saving effected in the 

 quantity of manure required for the land. 



760. The system of subsoil ploughing, so success- 

 fully practised in many parts of the country, is con- 

 nected with this subject, and depends mainly upon 

 the advantages derived from admixture of the subsoil 

 with the surface. In this operation, a plough is em- 

 ployed to break up and pulverize the soil to a 

 considerable depth below the surface, without bring- 

 ing the soil so disturbed up to the top ; by this 

 process the permeability of the soil to air is greatly 

 increased, and in consequence, the subsoil is soon 

 brought into a fit state to be mixed with the ordinary 

 soil, by the use of a suitable plough. 



761. Part of the benefit derived from this opera- 

 tion is doubtless due to the greater openness conferred 

 upon the soil, which permits a more perfect access of 

 air and moisture, and allows the roots of the growing 

 plants more easily to penetrate through it ; but at the 

 same time, advantage is also derived from the greater 

 supply of saline and soluble earthy matters which 

 are thus made available for the use of the plants. 



762. A very great variety of different substances 

 is included under the general name of manure. 

 Some of the substances added to the soil act princi- 

 pally in a mechanical way, improving its texture ; 

 others are chiefly valuable as sources of carbonic acid 

 and ammonia ; whilst many are useful as supplying 

 saline and earthy matters in which the soil may be 



