892

THEORY AND PRACTICE OF ENGINEERING.

Book II.

if the soil contain lime an effervescence will take place, by the briskness and durationof which the proportion of lime contained in it may be ascertained; but where it isnecessary to know the exact quantity of lime in an earth, this may be found by puttinghalf an ounce of the soil into a glass, and pouring upon it dilute muriatic acid, letting itstand for four-and-twenty hours; after which pour off the clear liquid, adding water andstirring it: when the earthy matter has subsided, pour off the clear water, and suffer theresiduum to dry till it acquire the same consistence it had previous to pouring on thedilute acid ; when weighed, whatever it has lost in weight will be the proportion of limecontained in the soil. For practical purposes, however, the valuator may divide the ar-gillaceous soils into clayey, clayey loam, and argillaceous- alluvial : the siliceous soilsinto sandy, gravelly, slaty, rocky, &c. ; the calcareous into limestone, limestone gravel,chalk, and marl: the peat into moor, peat, and bog.

Clay, when either blue or yellow, and in its quality tenacious or lying upon aretentive subsoil, is nearly unfit for tillage; while on an open subsoil it is capable ofimprovement, and when clays contain a due admixture of sand, lime, and vegetablematter they are well suited to the growth of wheat, and may be classed among the mostproductive: such may degenerate into a cold stiff clay when there is little or no sandor vegetable matter, and their value is diminished accordingly.

Loamy soils are a compound of argillaceous and siliceous earths, and frequently containsome proportion of 1 ime and vegetable as well as animal matters; from being friable they aresoon reduced to a finely pulverised state: when a fine loam is mixed with sand, gravel,clay, or peat, it is to be so designated, and the term loam is only to be understood as com-prehending soils of a fine tilth, not forming clods when ploughed in wet weather.

A stiff clay, with either sand, peat, lime, chalk, or stable manure added to it, inthe course of time will become a rich loam, but it is only by numerous ploughings andexposure to frost that a due mixture can take place.

A strong clayey loam contains about one-third part and sometimes more of clay, the otheringredients consisting of sand, gravel, lime, &c., the sand predominating.

A friable clayey loam contains less clay and more sand: such a soil is easily cultivatedin wet weather.

Sandy or gravelly loams are those where either predominates ; both render the soil openand free, though not sufficiently retentive of moisture.

Argillaceous alluvial soils, usually on the banks of great rivers or near the sea-shore,are evidently derived from the deposits of water : they are composed of fine argillaceousloam, or other earths arranged in layers, with alternate beds of clay, shells, sand, &c.The value of these soils depends in a great measure on the quantity of lime they contain.

Rich alluvial soils are the most productive of any when not subject to floods.

Siliceous soils comprise sands of all gradations from open sandy loam to pure sand :white shelly sands, when near the sea, are very productive, although there is not a greatportion of earthy matter found in them.

Gravelly soils are those where coarse sand or gravel abounds ; a due admixture of loamrenders them sufficiently retentive of moisture ; such produce excellent crops.

Slaty soils occur on the sides of mountains, and arc composed of the debris of slate rockcither coarse or fine-grained.

Itocky soils are found abounding with fragments of rock intermixed with mould, wherethe substratum is rock.

Calcareous or limestone soils contain an unusual quantity of finely pulverised limestone ;these form excellent grazing lands.

Limestone gravel soils occur in limestone districts; these are sometimes calcareous andoften composed of calcareous sand.

Marly soils consist of two kinds; that which contains clayey marl or calcareous mattercombined with clay and white marl, which is a deposit from water, usually found at thebottom of lakes and rivers.

Moory, peaty, or boggy soils vary in their quality ; some contain very little earthymatter, and when burnt the ashes which remain scarcely amount to a tenth or twelfth ofthe original weight; the value of peat soils is dependant upon their production of red oryellow ashes when burnt: in deep bogs these are light and white, and do not amount tomore than an eightieth part of the original weight, and are of little use as a manure.

Where peat earth yields a small quantity of white ashes its value is trifling, unlesscovered with a heavy coat of loamy earth or clay; a solidity is then given to the soil, andcrops may be produced upon it.

It is of the utmost importance that due attention should be paid to the nature of thesoil, and of all the ingredients of which it is composed; and the person who has to fix avalue should thoroughly understand what is meant by the following terms, and should soarrange his field-book that the whole of the lands surveyed should come under one or otherof these qualities.

Stiff, when the soil contains above one-half of tenacious clay, and which in dry weather