WITH REFERENCE TO HORTICULTURE. 49 



ICO. Peat or bog is composed of partially decayed vegetable matter, soft, 

 light, and spongy to the touch ; and the very reverse of sand with respect to 

 water, holding that element like a sponge, so as, in its natural state, to he 

 totally unfit for the growth of vegetables, except those of the lowest grade. 



161. The organic matter in soils in its solid state may be considered as 

 carbon, which is found pure in the diamond, and tolerably so in the charcoal 

 of wood. In soils it is found in various states of decomposition, from recent 

 woody fibre to humus, which is woody fibre in a state of decay. The 

 proportion of organic matter varies exceedingly in different soils. In ban-en 

 sands there is scarcely a trace of it, while in fertile soils it varies from 10 to 

 SO per cent. ; and peat-bogs which have been drained and cultivated contain 

 often 80 or 90 per cent. Humus, according to Professor Liebig, exercises 

 its influence on vegetation u by being a continued source of carbonic acid, 

 which it emits slowly. An atmosphere of carbonic acid, formed at the 

 expense of the oxygen of the air, surrounds every particle of decaying 

 humus. The cultivation of land, by stirring and loosening the soil, causes 

 a free and unobstructed access of air. An atmosphere of carbonic acid is, 

 therefore, contained in every fertile soil, and is the first and most important 

 food for the young plants which grow in it. The property of humus, or 

 woody fibre, to convert surrounding oxygen gas into carbonic acid, diminishes 

 in proportion as its decay advances ; and at last a certain quantity of a brown 

 coaly-looking substance remains, in which this property is entirely wanting. 

 This substance is called mould (152) ; it is the product of the complete decay 

 of woody fibre, and constitutes the principal part of brown coal and peat." 

 {Organic Chemistry, p. 47.) 



For practical purposes, all the soils ordinarily met with may be reduced 

 to the following : 



162. Loose naked sands or gravels, without either clay or calcareous 

 matter, and almost destitute of vegetation on the surface; exemplified on some 

 parts of the sea- shore, and in Hounslow and other extensive heaths. 



163. Calcareous soils or gravels, containing little or no clay or organic 

 matter, and almost without vegetation on the surface ; found on the sea- 

 shore in some places, and on the surface of chalky districts. 



164. Loams. Rich sandy loams consist of sand, clay, and more or less of 

 calcareous soil, with organic matter ; they never become hard on the surface 

 after rains followed by drought, and never retain water to such an extent 

 as to prove injurious to vegetation. Vegetation commences some weeks 

 earlier in sandy loams than in clayey loams, in the same climate, or even in 

 the same garden ; and during summer plants on such soils will be in ad- 

 vance of those on clays ; so much so, as Mr. Lymburn has observed, as 

 to attain maturity a month earlier. Clayey loams consist of clay with a 

 proportion of sand and organic matter ; they produce large crops, but become 

 hard and baked on the surface after heavy rains followed by drought. Stiff 

 adhesive clays contain in their composition little or no sand or lime, and are 

 almost without organic matter. All clayey loams are later than sandy 

 loams. 



165. Loams are the best soils, and are characterized according to the 

 earths which prevail in them, as a sandy loam, &c. ; according to their 

 degree of friability, as a free loam, a stiff loam, &c. ; or according to both, as 

 a free calcareous loam, &c. These soils, with reference to geology, are gene- 

 rally found on the sides of valleys, along the bases of hills or mountains, or 



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