16 



SCIENCE OF GARDENIXG. 



lime, and flint-earth in the form of sand and gravel of 

 various degrees of fineness,, together ^vitb, sometimes, 

 magnesia, iron, and a few other metals, contributes little 

 or nothing to the nourishment of plants. These portions 

 of the soil appear to be chiefly used mechanically or che- 

 mically, in improving the texture and distributing the 

 more nutritive parts, or in mixing vith other things, or 

 operating upon them, to produce nourishing compounds. 



On these principles, we may easily account for the 

 barrenness of stiff clays, dry sands, and, more particularly, 

 soils chiefiy consisting of granite sand, as tho^fe in Arran, 

 and near Plymouth : vbile in the instance of sand or clay 

 from basalt or whinstone, as well as from, limestone and 

 chalk, w^hen mixed with other soil, the carbonic acid gas 

 tends to promote greater fertility, as in the Lothian s, 

 Ayrshire, and Kent. Volcanic rocks, as in the Cam^jagna 

 of Rome, are very fertile for the same reason. Xo 

 mixture, then, of clay and sand vdll be productive, with- 

 out limestone, chalk, cr basalt, (that is, whinstone,) and, 

 more particularly, without decayed plants and manures. 



Some mineral substances, such as iron, are injurious to 

 soils, and, perhaps, all the metals are so when combined 

 with oxygen gas or acids. Many good soils, it is true, 

 contain iron, known by the redidish rusty colour i't iin- 

 2?arts : but their fertility appears not to be ovang to the 

 iron, but to exist in spite of it. 



8. — Manures. 



There is no branch of plant-culture in which a more 

 thorough change has been effected of late years than in the 

 application of manures. The old-fashioned, substantial, 

 simple manures have now very much given way Ijefore 

 the use of such as are highly concentrated, or are com- 

 pounded chemically, or are administered in a liquid state, 

 or contain some single ingredient, which the particular 

 crop to be grown appears most to require. 



Two or three ver\^ important results ha^'e followed from 

 this alteration in the sjstem of manuring. First, the new 

 kinds of manure are generally of easy aiq lication. They 

 travel in a small compass, and may often be put on by the 



