EARTHS. 



EARTHS. 



declivities ought to be more absorbent than in ' 

 plains, or in the bottom of valleys. Their pro- 

 ductiveness, likewise, is influenced by the na- 

 ture of the subsoil, or the stratum on which 

 they rest. When soils are immediately situ- 

 ated upon a bed of rock or stone, they are 

 much sooner rendered dry by evaporation than 

 where the subsoil is of clay or marl ; and a 

 prime cause of the great fertility of land in the 

 moist climate of Ireland is the proximity of 

 the rocky strata to the soil. A clayey subsoil 

 will sometimes be of material advantage to a 

 sandy soil ; and, in this case, it will retain 

 moisture in such a manner as to be capable 

 of supplying that lost by the earth above, in 

 consequence of evaporation or the consump- 

 tion of plants." (Davy's Lectures, p. 186.) 



It has been shown by the experiments of M. 

 Saussure, with some sprigs of peppermint, that 

 when supported by pure water only, and allow- 

 ed to vegetate for some time in the light, they 

 nearly doubled the portion of carbon which 

 they originally contained. (Recherches sur la 

 Veg. 51.) This they could have procured only 

 from the atmosphere; and, under these circum- 

 stances, there is now little doubt of the cor- 

 rectness of the conclusion of M. Berthollet, that 

 Slants, by means of their roots and leaves, 

 ave the power of decomposing the water as 

 well as the carbonic acid of the atmosphere, 

 and furnishing, with these elements, new com- 

 binations. How essential a free access of the 

 atmosphere is to the roots of plants was long 

 since shown by M. Saussure, who found that 

 oxygen gas is absorbed by the roots of plants 

 as well as 'by their leaves, and that it is at the 

 roots united with carbon, and transmitted to 

 the leaves to be decomposed. Even the branches 

 absorb oxygen ; in its absence flowers will not 

 even expand. (Thomson's Chcm. vol. iv. p. 353.) 

 It has been proved that their vegetation is 

 greatly increased by nourishing them with 

 water impregnated with oxygen gas ; hence, 

 too, the superiority of rain-water. Some re- 

 markable experiments were made by Mr. Hill, 

 demonstrative of the great benefits plants de- 

 rive from oxygen gas being applied to their 

 roots : hyacinths, melons, Indian corn, &c., 

 were the subjects of the experiments. The 

 first were greatly improved in beauty, the se- 

 cond in flavour, the last in size, and all in vi- 

 gour. This, too, is another use of increasing 

 the moisture of the soil, by deep and complete 

 ploughings, for M. Humboldt and M. Schubler 

 have clearly shown that a dry soil is quite in- 

 capable of absorbing oxygen gas. Thus, it 

 must be evident to the most listless observer, 

 that the more deeply and finely a soil is pul- 

 verized, and its earths rendered permeable, the 

 greater will be the absorption by them of both 

 oxygen and watery vapour from the surround- 

 ing atmosphere. 



It is perhaps needless to prove that the roots 

 of commonly cultivated plants will penetrate, 

 under favourable circumstances, much greater 

 depths into the soil in search of moisture than 

 they can, from the resistance of the case-hard- 

 ened subsoil, commonly attain. Thus, the 

 roots of the wheat plant, in loose deep soils 

 have been found to descend to a depth of two 

 or three feet, or even more: and it is evident 

 434 



that if plants are principally sustained in dry 

 weather by the atmospheric aqueous vapour 

 absorbed by the soil, that then that supply of 

 water must be necessarily increased, by enabl- 

 ing the atmospheric vapour and gases, as well 

 as the roots of plants, to attain to a greater 

 depth ; for the earth, &c., of the interior of a 

 well pulverized soil, be it remembered, con- 

 tinues steadily to absorb this essential food of 

 vegetables, even when the surface of the earth 

 is drying in the sun. 



By facilitating the admission of air to the 

 soil another advantage is obtained, that of in- 

 creasing its temperature. The earths are na- 

 turally bad conductors of heat, especially down- 

 wards : thus, it is a well-known fact, that at the 

 siege of Gibraltar, the red hot balls employed 

 by the garrison were readily carried from the 

 furnaces to the batteries in wooden barrows, 

 whose bottoms were merely covered with earth. 

 Davy proved the superior rapidity with which 

 a loose black soil was heated, compared with 

 a chalky soil, by placing equal portions of each 

 in the sunshine ; the first was heated in an 

 hour from 65 to 88, while the chalk was only 

 heated 69. This trial, however, must not be 

 regarded as absolutely conclusive, since the 

 surface of the black soils naturally increases 

 more rapidly in temperature' when exposed to 

 the direct rays of the sun than those of a 

 lighter colour. A free access of the air to the 

 soil also adds to their fertility, by promoting 

 the decomposition of the excretory matters of 

 plants and other organic substances of the soil. 



In the truth of these conclusions and labori- 

 ous experimental researches of the chemist, 

 does not the practical testimony of the ablest 

 cultivators of all ages and in all countries con- 

 cur? Thus, in enforcing the advantages of 

 rendering the soil more completely permeable 

 by the atmosphere, nearly two thousand years 

 since, Cato asked the Italian farmers, "What 

 is good tillage 7" To plough. "What is the 

 second?" To plough. The third is to ma- 

 nure. Cato, however, mistook the cause of the 

 benefit, for he says, "He who stirs his olive 

 ground oftenest and deepest will plough up the 

 very slender roots ; if he ploughs ill, the roots 

 will become thicker, and the strength of the 

 olive' will go to the root." (Lib. 61.) Virgil, 

 when giving an erroneous explanation of the 

 advantages of paring and burning, says, "The 

 heat opens more ways and hidden vents for the 

 air, through which the dews penetrate to the 

 embryo plant." (Georg. i. 90, 91.) 



And at this very period do not the best of 

 England's agriculturists find the greatest ad- 

 vantage from stirring the ground between their 

 rows of drilled turnips, which only operates so 

 .beneficially to the plants, by promoting the 

 access of the air to their roots ; and that, too, 

 on soils where a weed is hardly to be seen ? Is 

 not one great object of fallowing to produce by 

 pulverizing and deepening the soil the same 

 result ? Did not Jethro Tull labour long, and ( 

 sometimes too sanguinely, in illustrating the " 

 same position ? And does he not support 

 almost all the observations of the chemist, as 

 to the attraction of the earth for the gasses and 

 aqueous vapour of the atmosphere, when he 

 says " I have had the experience of a multi- 



