WITH A VIEW TO HORTICULTURE. 



55 



finement of air in the soil, the heat imparted to it by the sun during 

 the day is retained, and accumulates in all free open soils to such a degree 

 as sensibly to raise their temperature over that of the air, especially 

 during night. From thermomefcrical observations made at different places, 

 it appears that the mean temperature of the soil, at about one foot below the 

 surface, is somewhat higher naturally than the mean temperature of the 

 atmosphere on the same spot ; and hence we may reasonably suppose that, 

 by draining and pulverization, the temperature of the soil may be perma- 

 nently increased as well as that of the atmosphere. From experiments made 

 by Mr. Thompson, in the garden of the Horticultural Society of London, it 

 appears, that " m the valley of the Thames, the maximum mean of terres- 

 trial temperature, at one foot below the surface, has been found to be 64*81° 

 in July, which is the hottest month in the year : but that the greatest differ- 

 ence between the mean temperature of the earth and atmosphere is in the 

 month of October, when it amounted, in the two years during which the 

 observations were made, to between three and four degrees ; and that, in 

 general, the mean temperature of the earth, a foot below the surface, is at 

 least one degree, and more commonly a degree and a half, above the mean 

 of the atmosphere. In these cases, if the terrestrial temperatures be com- 

 pared with those of the atmosphere, it will be found that in the spring, when 

 vegetation is first generally set in motion, the temperature of the earth not 

 only rises monthly, but retains a mean temperature higher than that of the 

 atmosphere by from one to two degrees ; and that in the autumn, when 

 Avoody and perennial plants require that their tissue should be solidified and 

 their secretions condensed, in order to meet the approach of inclement wea- 

 ther, the terrestrial temperature remains higher in proportion than that of 

 the atmosphere, the earth parting with its heat very slowly," (^Lindleys 

 Theory of Hort.^ -p. 97.) In hot countries the sun often heats the soil to 

 such a degree as to be injurious to the roots of cultivated plants, and pulveri- 

 zation is there resorted to to diminish the force of its rays, which, as it is well 

 known, are less effective on a porous and spongy than on a solid substance. 

 This, as Chaptal informs us, is one of the uses of pulverization even in the 

 south of France. 



176. The free admission of atmospheric air to soil is also necessary for the 

 decompositioji of humus, or organic matter, by which carbonic acid is formed ; 

 and atmospheric air is also a great source of nitrogen, which has been lately 

 found in all plants (104), and more especially in the spongioles of the roots. 

 The soil also, when loosened, becomes a rapid conductor of water ; and, sup- 

 posing the texture of the soil to be suitable for culture, it will retain a suffi- 

 cient quantity of moisture for the purpose of vegetation, and allow the escape 

 of what is superfluous by filtration into the subsoil, or into the underground 

 drains which have been formed as a substitute for a porous substratum. 

 The mere act of pulverising any soil has a tendency to improve its texture, 

 more especially if the operation be frequently repeated. In summer, by 

 exposure of a soil to the air, the particles are separated by the evaporation 

 of the water in their interstices by heat ; and by exposing a soil to the frosts 

 of winter, the particles are separated by the expansion of the water in the 

 form of ice. Clayey soils contauiing iron are in an especial manner improved 

 by exposure to the atmosphere ; the iron being still farther oxidised, and 

 thus acting like sand in separating the particles, as well as being less likely 

 to be rendered soluble by the addition of saline matters. 



