12 IMPROVEMENT OF SOILS. 



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

 difference 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 compared 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 woody and 

 perennial plants require that their tissue should be solidified and 

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

 weather, the terrestrial temperature remains higher in proportion than 

 that of the atmosphere, the earth parting with its heat very slowly." 

 (Lindley's * 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 culti- 

 vated plants, and pulverization 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. 



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

 the nourishment of the plants ; as it is now found that plants derive 

 a great portion of their carbon and nitrogen from the air penetrating 

 into the soil in which they grow, and being taken up by the spongioles 

 of the roots. The soil also, when loosened, becomes a rapid conductor 

 of water; and, supposing the texture of the soil to be suitable for 

 culture, it will retain a sufficient 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 pulverizing 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 evapo- 

 ration 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 containing iron are in 

 an especial manner improved by exposure to the atmosphere ; the 

 iron being still farther oxidized, and thus acting like sand in sepa- 

 rating the particles, as well as being less likely to be rendered soluble 

 by the addition of saline matters. 



Soils are improved by the modes in which they are cultivated ; as 

 for example, by the order in which crops are made to succeed each 

 other, by fallowing, by resting, and by the manner in which water is 

 applied to growing crops ; but these subjects will come under notice 

 when we are treating of the practice of Horticulture. 



