228 



HORTICULTURAL LABOURS ON THE SOIL. 



same as that of the atmosphere; bat by considering all the causes that con- 

 tribute 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 soil is of as much importance in deriving heat from the direct 

 action of the sun's rays, as we have just seen (482) the surface of glass roofs 

 to be. 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 warm.er 

 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 289 times as much coal to heat one cubic 

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

 degree, so is the quantity of heat which water of a given bulk will give out 

 to soil greater than what will be communicated by the same bulk of air. 

 Water, in a frozen state, though injurious as abstracting heat, is in many 

 cases favourable by contributing to the pulverization of stiff soils, which are 

 laid up in a rough state, in order to expose as large a surface as possible to be 

 cooled and frozen during winter, and to be thawed and heated during spring. 

 The retention of moisture by pulverization is an important object of labouring 

 the soil. All properly cultivated soils hold water like a sponge, while in un- 

 laboured soils the rains either never penetrate the surface, or they sink into 

 the subsoil and are lost, or are retained by it and prove injurious. Wind, 

 like rain, will communicate heat or abstract it from soil, according to its 

 temperature and the rapidity of its motion ; but as in either case it carries off 

 moisture in proportion to its dryness and velocity, it is in general in cold 

 climates much more favourable than hurtful for soils, considered apart from 

 the plants which grow in them. If possible no operation should be performed 

 on the soil excepting when it is in a dry state, and when the weather is also 

 dry. Moist soil cannot be dug mthout first treading on it, and thus making 

 it into a kind of paste or mortar, which renders it unfit for being pierced by 

 the fibres of plants, and prevents it from being penetrated either by moisture 

 or air ; and water in the form of ice or snow, if dug in, abstracts that heat 

 from the soil which, as we have already seen, it ought to derive direct from 

 the atmosphere. A pound of snow (newly fallen) requires an equal weight 

 of water heated to 172° to melt it, and then the dissolved mixture is only of 

 the temperature of 32°. Ice requires the water to be a few degrees warmer 

 to produce the same result. A\^hen ice or snow is allowed to remain on the 

 surface, the quantity of heat necessary to reduce it to a fluid state is obtained 

 chiefly from the atmosphere ; but when buried so that the atmospheric heat 

 cannot act directly upon it, the thawing must be very slowly effected by the 

 abstraction of heat from the soil by which the frozen mass is surrounded. In- 

 stances have occurred of frozen soil not being completely thawed at midsum- 

 mer when so buried. But this is not the whole of the evil : — the moisture of 

 the air which fills the interstices of the soil will be continually undergoing 

 condensation as it comes in contact with the cold portions, and these will be 

 found in a very saturated condition, even after they have become thawed." — 

 (Robert Thompson in Gard. Chron. Feb. 6, 1841, p. 89.) All these and similar 

 facts ought to be kept constantly in mind while performing the operations of 

 digging, trenching, forking, hoeing, raking, and rolling. 



