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CHAPTER VIII. 

 OPERATIONS OF HORTICULTURE. 



Ldboufs on the Soil. 



THE objects for which the soil is worked, are pulverization, to render 

 it more readily penetrated by the roots of plants, and by heat, air, 

 moisture, and sometimes by frost ; to allow superfluous moisture to 

 escape into the subsoil ; to mix the upper and lower parts of the upper 

 stratum of soil together ; to deepen the tilth ; to mix the coarser and 

 finer parts together ; to add or mix in earths or manure ; to free the 

 soil from roots or perennial weeds, stones, or other objects unfavour- 

 able to culture ; and to destroy surface or annual weeds. The grand 

 sources of heat to the soil are the sun and the atmosphere, including rain 

 at a higher temperature than the soil ; and the sources of cold, or of 

 the abstraction of heat are, rain at a lower temperature than the soil, 

 frost, snow, ice, and where drainage has been neglected, subterraneous 

 water and evaporation from the surface. The greatest degree of cold 

 produced by these causes, excepting the last, will always be found on 

 the surface of the soil, and the best mode of supplying the heat that has 

 been abstracted will be by leaving the surface to the action of the sun 

 and of the air. By digging or trenching down a cold surface such as 

 ice, or a substance such as snow, heat is abstracted from the soil, 

 the natural temperature of which will in that case be lowered ; and 

 thus a plant grown in a soil so treated, will be, in so far as bottom heat 

 is concerned, worse than if it were in a state of nature, in which heat 

 abstracted by the air is always restored by it. The average tempera- 

 ture :of the surface soil in most countries is believed to be nearly the 

 same #s that of the atmosphere ; but by considering all the causes that 

 contribute to the warmth of a soil, there can be little doubt but in 

 many cases its average temperature might be increased. The colour 

 and texture of some soils is better adapted for absorbing heat than 

 others, and the inclination of the surface of the soil is of as much im- 

 portance in deriving heat from the direct action of the sun's rays as the 

 angles -of glass roofs. Hence the advantage of laying up soil in narrow 

 ridges, which, when in the direction of east and west, very soon become 

 much 'drier and warmer on one side than on the other. Rain, though 

 in the cold season it abstracts heat from the soil, yet in spring and 

 summer, being of the temperature of the atmosphere, it communicates 

 heat more effectually than air, because, under ordinary circumstances, 

 it penetrates deeper, in consequence of its greater specific gravity ; and 

 as it requires two hundred and eighty-nine times as much coal to heat 

 one cubic foot of water as would be required to heat the same bulk of 

 air to the game degree, so is the quantity of heat which water of a 

 given bulk will give out to soil greater than what will be communi- 

 cated by the same bulk of air. Water, in a frozen state, though injurious 

 as abstracting heat, is in many cases favourable by contributing 



