Book III. IMPROVEMENT OF SOILS. 323 



exposed to the alternate action of heat, moisture, light, and air, undergo spontaneous 

 decompositions, which would not otherwise take place. 



2168. By means of pulverisation a portion of atmospheric air is buried in the soil. This 

 air, SO' confined, is decomposed by the moisture retained in the earthy matters. 

 Ammonia is formed by the union of the hydrogen of the water with the nitrogen of the 

 atmosphere ; and nitre, by the union of oxygen and nitrogen ; the oxygen may also unite 

 with the carbon contained in the soil, and form carbonic acid gas, and carburetted . 

 hydrogen. Heat is given o\it during these processes, and " hence," as Dr. Darwin remarks 

 {Phytologia, sect. xii. 1.), " the great propriety of cropping lands immediately after they 

 have been comminuted and turned over ; and this tlie more especially, if manure has 

 been added at the same time, as the process of fermentation will go on faster when the 

 soil is loose, and the interstices filled with air, than afterwards, when it becomes com- 

 pressed with its own gravity, the relaxing influence of rains, and the repletion of the 

 partial vacuums formed by the decomposition of the enclosed air. The advantage of the 

 heat thus obtained in exciting vegetation, whether in a seed or root, especially in spring, 

 when the soil is cold, must be very considerable." 



2169. The great advantages of pulvciisation deceived Ttdl, who fancied that no other 

 assistances were required in the well-management of the business of husbandry. A 

 knowledge of chemistry, in its present improved state, would have enabled him to discover 

 that the pulverisation of the soil was of no other benefit to the plants that grow in it than 

 as it " increased the number of their fibrous roots or mouths by viiiich they imbibe their 

 food, facilitated the more speedy and perfect preparation of this food, and conducted the 

 food so prepared more regularly to their roots." Of this food itself it did not produce 

 one particle. 



2170. The depth of pulverisation, Sir H. Davy observes, " must depend upon the nature 

 of the soil, and of the subsoil. In rich clayey soils it can scarcely be too deep ; and even 

 in sands, unless the subsoil contains some principles noxious to vegetables, deep 

 comminution should be practised. When the roots are deep, they are less liable to be 

 injured either by excessive rain or drought ; the radicles are shot forth into every part of 

 the soil ; and the space from which the nourishment is derived is more considerable than 

 when the seed is superficially inserted in the soil." 



2171. Pulverisation should, in all cases, be accompanied with the admixture of the parts 

 of soils by turning them over. It is difficult, indeed, to pulverise without effecting this 

 end, at least by the implements in common use ; but, if it could be effected, it would be 

 injurious, because the difference of gravity between the organised matters and the earths 

 has a constant tendency to separate them, and stirring a soil only with forks or pronged 

 implements, such as cultivators, would, in a short time, leave the surface of the soil too 

 light and spongy, and the lower part too compact and earthy. 



SuBSECT. 2. Of the Improvement of Soils by Compression. 



2172. Mechanical consolidation will improve some soils, such as spongy peats and light 

 dusty sands. It is but a limited source of improvement, but still it deserves to be 

 noticed. 



2173. Tlie proper degree of adhesiveness is best given to loose soils by the addition of 

 earthy matters ; but mere rolling and treading are not to be altogether rejected. To be 

 benefited by rolling a soil must be dry, and the operation must not be carried too far. 

 A peat-bog drained and rolled will sooner become covered with grasses than one equally 

 well drained and left to itself. Drifting sands may be well rolled when wet, and by 

 repeating the process after rains they will in time acquire a surface of grass or herbage. 

 Every agriculturist knows the advantages of rolling light soils after sowing, or even 

 treading them with sheep. Gardeners also tread in seeds on certain soils, 



SuBSECT. 3. Of the Improvement of Soils by Aeration or Fallowing. 



2174. Soils are benefited by the free admission of the weather to their interior parts. This 

 is generally considered as one of the advantages of fallowing, and its use in gardening is 

 experienced in compost heaps, and in winter and summer ridging. The precise advantages, 

 however, of exposure to the air, independently of the concurrent in-fluence of water, 

 heat, and the other effects mentioned as attendant on pulverisation, do not seem at present 

 to be correctly ascertained. It is allowed that carbonic acid gas may be absorbed by 

 calcareous earths, and Dr. Thomson considers that the earths alone may thus probably 

 administer food to plants; but Sir H. Davy seems to consider mere exposure to the 

 atmosphere of no benefit to soils whatever. " It has been supposed by some writers," he 

 says, " that certain principles necessary to fertility are derived from the atmosphere, which 

 are exhausted by a succession of crops, and that these are again supplied during tlie 

 repose of the land, and the exposure of the pulverised soil to tihe influence of the air ; but 

 this in truth is not the case. The earths commonly found in soils cannot be combined 



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