BOOK II. PULVERISATION OF SOILS. 227 



but the strength of the vegetable, in consequence of this multiplication of fibres, must 

 depend a good deal on the quantity of food or of extract within their reach. The root of 

 a willuw-tree, as we have seen (782.), has the fibres prodigiously increased by coming in 

 contact with the water in a river, and so have various other aquatic trees and plants, as 

 alder, mint, lysimachia thyrsiflora, calla palustris, cenanthe fistulosa, &c. ; but their herbs 

 or trunks are not proportionally increased unless the water be impregnated with organised 

 remains. 



1071. Pulverisation increases the capillary attraction, or sponge-like property of soils, 

 by which their humidity is rendered more uniform. It is evident this capillary at- 

 traction must be greatest where the particles of the earth are finely divided ; for 

 gravels and sands hardly retain water at all, while clays, not opened by pulverisation or 

 other means, either do not absorb water, or when, by long action it is absorbed, they re- 

 tain too much. Water is not only necessary to the growth of plants as such, but it is 

 essential to the production of extract from the vegetable matters which they contain ; and 

 unless the soil, by pulverisation or otherwise, is so constituted as to retain the quantity 

 of water requisite to produce this extract, the addition of manures will be in vain. 

 Manure is useless to vegetation till it becomes soluble in water, and it would remain 

 useless in a state of solution, if it so abounded as wholly to exclude air, for then the fibres 

 or mouths, unable to perform their functions, would soon decay and rot off. 



1072. The temperature of a soil is greatly promoted by pulverisation. Earths, Grisenthwaite 

 observes, are also amongst the worst conductors of heat with which we are acquainted, 

 and consequently, it would be a considerable time before the gradually increasing tem- 

 perature of spring could communicate its genial warmth to the roots of vegetables, if 

 their lower strata were not heated by some other means. To remove this defect, which 

 always belongs to a close compact soil, it is necessary to have the land open, that there 

 may be a free ingress of the warm air and tepid rains of spring. 



1073. Pulverisation contributes to the increase of vegetable food. Water is known to be 

 a condenser and solvent of carbonic acid gas, which, when the land is open, can be im- 

 mediately carried to the roots of vegetables, and contribute to their growth ; but if the 

 land be close, and the water lie on or near its surface, then the carbonic acid gas, which 

 always exists in the atmosphere and is carried down by rains, will soon be dissipated. 

 An open soil is also almost suitable for effecting those changes in the manure itself, which 

 are equally necessary to the preparation of such food. Animal and vegetable substances, 

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

 decompositions, which would not otherwise take place. 



1074. By means of pulverisation a portion of atmosjrfieric air is buried in the soil. This 

 air, so confined, is decomposed by the moisture retained in the earthy matters. Am- 

 monia is formed by the union of the hydrogen of the water with the nitrogen of the at- 

 mosphere ; 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 hydro- 

 gen. Heat is given out during these processes, and " hence," as Dr. Darwin remarks 

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

 had been comminuted and turned over ; and this the 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 compressed 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." 



1075. The great advantages of pulverisation deceived Tull, 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 which 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. 



1076. The depth of pulverisation, Sir H. Davy observes, " must depend upon the nature 

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

 in sands, unless the sub-soil contains some principles noxious to vegetables, deep commi- 

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

 either by excess of 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." 



1077. 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 



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