200 A PRACTICAL TREATISE 



effects have followed. Train-oil, blubber, and similar refuse, 

 should therefore be made into composts with a large body of 

 earth.* Rape and malt dust, requiring no mixture, are very 

 commonly laid upon the land as top-dressings — the difference 

 between which and manure ploughed into the ground, is, that 

 the former are applied chiefly with a view to the sole benefit 

 of the immediate crop, without regard to the further improve- 

 ment of the soil ; though there can be no doubt that if the 

 crop be increased, the soil will also feel their good effects. In 

 this manner soot is also almost invariably used ; but its fer- 

 tilizing properties are solely referable to the ammonia con- 

 tained in it, which is an active stimulant of vegetation. The 

 practice of laying it upon land which has been limed, or of 

 mixing it, as sometimes done, in composts with lime, is tliere- 

 fore injudicious. 



It has long been a disputed question, whether all plants 

 extract the same nutritive juices from the soil, and convert 

 them into the kind of sap adapted to their peculiar qualities, or 

 whether each is nourished by a different substance. It would 

 at first appear improbable that plants differing from each other 

 in form, smell, taste, and properties as food, should be produced 

 by the same matter; yet, when we reflect that different plants 

 deprive each other of nourishment, by extending their roots 

 into the same soil in which various kinds are planted, we can- 

 not but conclude that their first nutriment must be of the same 

 nature, though the sap probably acquires different properties 

 in its progress towards perfection. This, however, is one of 

 the secrets of vegetation with which we are unacquainted; 

 but as we also see that some soils are better adapted than 

 others for the growth of particular kinds of grain and vege- 

 tables, and that those crops to which they appear the most 

 favourable yet become deteriorated if repeated, even though 

 regularly dressed with one species of manure, it seems evident 

 that there must be some advantage in the change of manures, 

 as well as in the system of cropping tillage land. This will 

 be gained by every farmer who has at his command manures 

 of an unusual kind, and who understands their use, for he 

 may then adopt many plans of cropping which are out of the 

 power of others not similarly situated, and vary his rotations 



♦'Tallow and oils received in a crude state by the roots may clog the 

 pores of the plant, repel the aqueous fluid, and obstruct the free communi- 

 cation of the leaves with the atmosphere.'— Davy on Agr. Chemistry. 



