SOILS. 



269 



the soa above it habitually wet ; while a strati- 

 fied one, if the lines of stratification dip down- 

 wards from the soil, wUl preserve even a reten- 

 tive soil above it in a comparative dry state." 

 Much, therefore, in the formation of garden 

 soUs depends on the subsoil, and to this too 

 Httle attention has been in general paid. All 

 efforts to improve, or even to make a new gar- 

 den soil, by the collection of the best materials, 

 wUl be unsatisfactory, if the subsoil is left im- 

 pervious to the free passing downward of water, 

 or if left in its natural state, when containing a 

 superabundance of mineral matter unprofitable 

 or deleterious to the roots of trees or plants to 

 be cultivated over it. Where subsoils are re- 

 tentive of water, they should be thoroughly 

 drained; where they contain mineral matter 

 unfit for the roots of plants, their character 

 should be changed by the application of coun- 

 teracting agents. 



AU soils consist of two parts — organic and 

 inorganic ; the former chiefly owes its origin to 

 the remains of vegetables and animals, " which," 

 as Professor Johnston observes, " have lived 

 or died in or upon the soil, which have been 

 spread over it by rivers and rains, or which 

 have been applied by the hands of man, for the 

 purpose of increasing its natural fertility. This 

 organic part varies very much in quantity in 

 different soils. In some, as in peaty soils, it 

 forms from SO to 70 per cent of their whole 

 weight ; and even in rich long-cultivated soils 

 it has been found, in a few rare cases, to 

 amount to as much as 25 per cent. In general, 

 however, it is present in much smaller propor- 

 tion, even in our best arable lands. The rich 

 alluvial soil of the valley of the Nile contains 

 only 5 per cent of dry organic matter. In stiff 

 and very clayey soils, 10 to 12 per cent is some- 

 times found ; in very old pasture-lands, and in 

 gardens, vegetable matter occasionally accumu- 

 lates so as to overload the upper soil. To this 

 organic matter the name of humus has been 

 given by some writers. It contains, or yields 

 to the plant, the ulmic, humic, and other acids. 

 It suppHes also, by its decay in contact with the 

 air which penetrates the soil, much carbonic 

 acid, which is supposed to enter the roots, and 

 thus to assist the growth of living vegetables. 

 During the same decay, ammonia is likewise 

 produced, and this in large quantity, if animal 

 matter be present in considerable abimdance. 

 Other substances more or less nutritious are 

 also formed from the organic matter in the soil. 

 These enter by the roots, and contribute to 

 nourish the growing plant, though the extent to 

 which it is fed from this source is dependent 

 both upon the abundance with which these sub- 

 stances are supphed, and upon the nature of the 

 plant itself, and of the climate in which it 

 grows. 



" Another influence of this organic portion of 

 the soil whether naturally formed in it, or added 

 to it as manure, is not to be neglected. It con- 

 tains — as all vegetable substances do — a consi- 

 derable quantity of inorganic, that is, of saline 

 and earthy matter, which is liberated as the 

 organic part decays. Thus living plants de- 

 rive from the remains of former races, buried 

 VOL. II. 



beneath the surface, a portion of that inorganic 

 food which can only be obtained from the soil, 

 and which, if not thus directly supplied, must 

 be sought for by the slow extension of their 

 roots through a greater depth and breadth of 

 the earth in which they grow. The addition of 

 manure to the soil, therefore, places within the 

 easy reach of the roots not only organic, but 

 also inorganic food." 



The inorganic part of soils is that which re- 

 mains behind when the organic part is destroyed 

 or abstracted by burning the whole to a red 

 heat in the open air, and consists entirely of 

 earthy and saline substances. The former are 

 insoluble in water, while the latter are soluble. 

 The amount of soluble matter contained in the 

 majority of soils is not great, " and consists of 

 common salt, gypsum, sulphate of soda, sul- 

 phate of magnesia, with traces of the chlorides 

 of calcium, magnesium, and potassium ; and of 

 soda, potash, lime, and magnesia, in combination 

 vrith nitric and phosphoric, and with the humic 

 and other organic acids. It is from these soluble 

 substances that the plants derive the greater 

 portion of the saline ingredients contained in 

 the ash they leave when burned. In hot wea- 

 ther, the evaporation on the surface causes the 

 water to ascend from the porous subsoil ; and 

 as this water always brings with it a quantity of 

 saline matter, which it leaves behind when it 

 rises in vapour, it is evident that the longer the 

 dry weather, and consequent evaporation from 

 the surface, continues, the thicker the incrusta- 

 tions will be, or the greater the accumulations 

 of saline matter on the surface. When rain 

 falls, the saline matter is dissolved, and descends 

 again to the subsoil. In dry weather it re- 

 ascends; hence the surface-soil of any field will 

 contain a larger proportion of soluble inorganic 

 matter in the middle of a hot dry season, than 

 in one of even ordinary rain : hence also the fine 

 dry weather, which, in early summer, hastens 

 the growth of crops, and later in the season 

 favours their ripening, does so probably, among 

 its other modes of action, by bringing up to the 

 roots, from beneath, a more ready supply of 

 those saline compounds which the crop requires 

 for its healthful growth. 



"The earthy or insoluble portion of soils 

 rarely constitutes less than 95 lb. in 100 of the 

 whole weight. It consists chiefly of silica, in 

 the form of sand ; of alumina, mixed or com- 

 bined with silica, in the form of clay ; and of 

 lime, in the form of carbonate of lime. It is 

 rarely free, however, from 2 or 3 per cent of 

 oxide of iron ; and, where the soil is of a red 

 colour, this oxide is often present in still larger 

 proportion. A trace of magnesia also may be 

 almost always detected, and a minute quantity 

 of phosphate of lime. The principal ingredients, 

 however, of the earthy part of all soils are sand, 

 clay, and lime ; and soils are named or classi- 

 fied according to the quantity of each of these 

 three they may happen to contain." 



The following simple rule has been laid down 

 by Mr Johnston, by which the practical culti- 

 vator may determine sufficiently accurately the 

 nature of an ordinary soil : " If an ounce of 

 soil be intimately mixed with a pint of water 



2 M 



