GENERAL RULES FOR PLOUGHING. 443 



establish themselves, and stretch themselves out in search of 

 food j and, by bringing the stony portions of the soil under the 

 influence of external agents, to produce a chemical decomposi 

 tion, and supply of those mineral ingredients, a portion of which 

 is indispensable to the healthy growth and productiveness of the 

 plants which are cultivated. It is important, therefore, to reduce 

 the soil to as fine a tilth as possible. It is important to do this, 

 likewise, that the manures which are applied may be thoroughly 

 intermixed with the soil. In gardens, and in small plats, this is 

 done by the spade, which in fields is attempted by the plough ; 

 the object in both cases being to render the soil loose, fine, and 

 friable. The more care is exercised in the ploughing, the more 

 certainly will these ends be accomplished. 



6. TRENCH-PLOUGHING. I come next to speak of what is 

 called trench-ploughing. This term is applied to a deeper 

 ploughing than usual, or to a double ploughing, where one 

 plough follows directly in the furrow left by a preceding plough. 

 In trenching land with the spade, which I have before described, 

 the object is completely to invert the soil, laying the surface soil 

 underneath, and covering it with that stratum of soil upon which 

 it had previously rested. The object obviously is, to deepen the 

 cultivatable soil, if I may coin a word which will be very well 

 understood, and. by bringing the lower stratum to the surface, 

 expose it to influences by which it may gradually become 

 enriched. Soil taken from almost any depth, after lying upon 

 the surface for a length of time, will ordinarily of itself acquire 

 a productive power, and may be cultivated with success. I have 

 known this to be the case with earth taken from the bottom of 

 a deep well, which, after a length of time, became productive. 

 There is always, in such cases, an accumulation or accession of 

 extraneous matters, which come one hardly knows whence, 

 how, or when. The surface of the coral reefs, of which the 

 islands in the Pacific are examples, after being raised above the 

 water, are gradually decomposed and enriched ; seeds of plants, 

 floating in the air, or brought by birds, or cast ashore by the 

 waves, gradually establish themselves. The lichens, or mosses, 

 and an humble class of vegetation, present themselves, until pres 

 ently, from their decay, and the deposits of animal life in various 

 forms, a rich mould is formed, and this barren rock becomes, in 



