SOIL. 



foils ; as the vegetable nourifliment is long preferved in 

 them, unlefs when taken up by the organs of plants. Sih- 

 ceous fands, on the contrary, are thought to merit the term 

 hungry, which is commonly applied to them ; as the vege- 

 table and animal matter they contain, not being attracted by 

 the earthy conftituent parts of the foil, are more hable to 

 be decompofed by the aftion of the atmofphere, or carried 

 off or away from them by water in different ways. In mod 

 of the black and brown rich vegetable moulds, the earths 

 feem, it is fuppofed, to be in combination with a pecuhar 

 extraftive matter, afforded during the decompofition of vege- 

 tables. This is flowly taken up, or attrafted, it is believed, 

 from the earths by water, and appears to conftitute a prime 

 caufe of the fertility of the foil. The (landard of fertility 

 of foils for different plants muft vary, it is fuppofed, with 

 the climate, and mult be particularly influenced by the 

 quantity of rain. The power of foils to abforb moifture 

 ought, it is faid, to be much greater in warm or dry coun- 

 ties than in cold and moift ones, and the quantity of clay 

 or vegetable or animal matter they contain greater. Such 

 foils as are on declivities, alfo, fhould be more abforbent 

 than thofe on plains, or in the bottoms of valhes. Their 

 produftivenefs is likewife influenced by the nature of the 

 fub-foil, or the ftratum on which they reft. When foils 

 are immediately fituated upon a bed of rock or ftone, 

 they are much fooner, it is faid, rendered dry by eva- 

 poration, than when the fub-foil is of clay or marie ; and 

 a principal caufe of the great fertility of the land in the 

 moilt climate of Ireland is, it is imagined, the proximity 

 of the rocky ftrata to the foil. A clayey fub-foil will fome- 

 tiraes, it is thought, be of material advantage to a fandy 

 foil ; and in this cafe it will retain moifture, in fuch a man- 

 ner as to be capable of fupplying that loft by the earth 

 above, in confequence of evaporation, or the confumption 

 of it by plants as crops. A fandy or gravelly fub-foil often 

 corrects the imperfeftions of too great a degree of abforbent 

 power in the true foil. In calcareous countries, where the 

 furface is a kind of marie, the foil is often found, it is ob- 

 ferved, only a few inches above the limeftone, yet its fer- 

 tility continues unimpaired by the proximity of the rock ; 

 though in a lefs abforbent foil, this fituation would occafion 

 barrennefs : and thus the fandftone and limeftone hilly foils, 

 in fome weftern diftrifts, may be readily diftinguiflied at a 

 diftance, in the fummer feafon, by the different tints or cafts 

 of colour in the vegetation. On the fandftone kind it will 

 commonly appear brown and burnt up ; while on the lime- 

 itone fort, it will be green and flourilhing in a confiderable 

 degree. 



It not unfrequently happens that foils are met with in a 

 quite unaltered ftate, upon the rocks from which they were 

 produced. The manner in which they were formed may 

 eafily be conceived, it is fuppofed by fir Humphrey Davy, 

 by referring to the cafe of J'oft or porcelain granite, which 

 confiils of three ingredients, quartz, felfpar, and mica. 

 The quartz he confiders as almoft pure filiceous earth, in a 

 cryttalline form. The felfpar and mica are very compounded 

 fubftances : both contain filica, alumina, and oxyd of iron ; 

 in the former there is commonly lime and potafl'a ; in the 

 latter, lime and magnefia. Therefore, when a granite rock 

 of this fort has been long expofed to the influence of air and 

 water, he fuppofes the lime and potaffa, contained in its 

 conftituent parts, are afted upon by water or carbonic acid ; 

 and the oxyd of iron, which is moftly in the leaft oxyded 

 ilate, tends to combine with more oxygen ; in confequence 

 of which the felfpar decompofcs, as well as the mica, but 

 the firft in the moft rapid manner. The felfpar, which is 

 as it were the cement of the ftone, forms a fine clay ; the 



partially decompofed mica blends and intermixes with it at 

 fand ; and the undecompofed quartz appears as gravel, or 

 fand of different degrees of finenefs. On the fmalleft layer 

 of earth being formed on the furface of the rock, the feeds 

 of moffes, and other imperfeft plants, which are moftly 

 floating in the furrounding atmofphere, and which have made 

 it their refting place, begin, it is faid, to vegetate ; and 

 their decay, death, and decompofition afterwards, afford a 

 certain quantity of organizable matter, which mixes with 

 the earthy materials of the rock. In this improved foil, 

 more perfeft plants are capable of fubfifting : thefe, in their 

 turn, abforb nourifliment from the water of the atmofphere ; 

 and, after perifliing, afford new materials to thofe already 

 provided. The decompofition of the rock ftill continues ; 

 and at length, by fuch flow and gradual proceffes, a foil is 

 formed, in which even foreft trees can, it is fuppofed, fix 

 their roots, and which is fitted to reward the labours of 

 thofe undertaking its cultivation. 



It is likewife noticed, that where fucceflive generations 

 of vegetables have grown upon a foil, unlefs part of their 

 produce has been carried away by man, or confumed by 

 animals, the vegetable matter increafes in fuch proportion, 

 that the foil approaches to a peat m its nature ; and if it be 

 fo placed as that it becomes impregnated with water from a 

 higher fituation, it gets fpongy, and is gradually rendered 

 incapable of fupporting better plants. A great number of 

 peat moffes appear, however, it is faid, to have been formed 

 by the deftruftion of forefts, by the imprudent ufe of the 

 hatchet, at fome early former period. It is fuppofed, that 

 when the trees in the outer parts of the woods were cut 

 down, thofe in the interior were expofed to the influence of 

 the winds ; and, having been accuftomed to ftielter, became 

 unhealthy, and died in their new fituations ; the leaves and 

 branches of which, gradually decompofing, produced a 

 ftratum of vegetable matter. In feveral of the great peat 

 bogs in the north of this and the neighbouring country, the 

 larger trees met with on the outer parts of them, it is faid, 

 bear the marks of having been felled ; while in the interior, 

 few entire trees are found : and the caufe is, probably, it is 

 fuppofed, that they fell by gradual decay ; and that the 

 fermentation and decompofition of the vegetable matter 

 were moit rapid, where it was in the greateft quantity or 

 proportion. 



Other lefs healthy forts of fpurious peat bogs are formed 

 in watery fituations, by the accumulation of the remains of 

 aquatic plants. There is here a fermentation of an appa- 

 rently different kind, and a much greater evolution of 

 gafeous matter. 



It is found that the earthy matter of peats is invariably 

 analogous to that of the ftratum on which they repofe : the 

 plants which have formed them muft have derived the earths 

 that they contained from this ftratum. Thus, the above 

 writer obferves, that in the counties of Wilts and Berks, 

 where the ftratum below the peat is chalk, calcareous earth 

 abounds in the afties, and very little alumina and fihca.' 

 They likewife contain much oxyd of iron and gypfum, both 

 of which may be derived from the decompofition of pyrites, 

 which is fo abundant in chalk. The different fpecimens of 

 peat that have been burnt by this experimenter, from the 

 granitic and fchittofe foils of different parts of thefe iflands, 

 have conftantly given him aflies principally of a filiceous and 

 aluminous nature; but one from the county of Antrim, in 

 Ireland, yielded afties, which afforded very nearly the fame 

 conftituent parts as the great bafaltic ftratum of the 

 county. 



It is fuggefted that poor and hungry foils, fuch as are 

 produced from the decompofition of granitic and fandftone 



rocks, 



