SOILS. 



267 



very finely reduced, it constitutes dust and 

 quicksand. Silica is a most abundant earth, 

 forming a great proportion of primitive rocks, 

 and of the bases of the globe. In fertile sorts 

 it averages 50 to 80 per cent ; in barren lands, 

 40 to 70 ; but these proportions never mark 

 with much precision the quality of the soil, as 

 so very much depends on mixture and combina- 

 tion in a geographical position. Silica absorbs 

 0.25, or one-fourth of its weight of water, with- 

 out dropping, and evaporates twice as fast as 

 chalk, and three times faster than alumen." — 

 Donaldson on Soils and Manures. 



Silica forms the chief constituent in most 

 soils, frequently forming nearly nine-tenths of 

 its whole weight ; " but this is by no means 

 always the case, for in calcareous or limestone 

 countries we frequently find soils containing a 

 very large quantity of lime. These differences 

 in the proportion of the earthy components of 

 the soil give rise to the varieties of light or free, 

 and stiff or clayey soils, which are also modified 

 by a greater or less quantity of organic sub- 

 stances." — Solly's Rural Chemistry. 



Silicious or sandy soils absorb heat from the 

 sun readily, and hence are valuable for the pro- 

 duction of early crops. They are also wrought 

 with much greater ease, and at almost all sea- 

 sons with equal propriety. They are incapable, 

 however, of yielding the greatest weight of pro- 

 duce ; and fruit trees neither live so long in 

 them, nor are so exempt from various diseases. 

 In homely phraseology, they are considered 

 " hungry soils," and speedily decompose manure 

 when applied to them. This is owing to their 

 want of cohesion and incapability of retaining 

 moisture. Air and moisture readily enter into 

 them, and hence the purification of organic 

 matter goes on rapidly in them, and from want 

 of body to retain its parts, it is soon washed 

 down beyond the range of most roots, or its 

 gaseous parts escape into the atmosphere. In 

 the application of manures, therefore, to such a 

 soil, no more should be applied at once than is 

 sufficient for the sustenance of the crop of the 

 current year. Increasing the depth by trench- 

 ing will, to a certain extent, correct the former 

 of these defects ; and adding argillaceous matter 

 to them in due proportions will transform them 

 into a soil nearly approaching in texture and 

 value to the alluvial. 



Calcareous or limey soils. — These readily 

 effervesce upon the application of acids, as both 

 lime and magnesia have a powerful affinity for 

 acids, and, in fact, are never present in the soil 

 unless in combination with some of them, and 

 that is most commonly the carbonic. Lime is 

 also not unfrequently found combined with 

 sulphuric acid, constituting gypsum or sulphate 

 of lime. Calcareous soils are confined to the 

 limestone and chalk formations ; and in the 

 latter case, when mixed with clay, become ex- 

 ceedingly difficult to work. Such soils, also, 

 are late, and unfavourable for the brairding of 

 seeds. However paradoxical it may appear, 

 there are not unfrequent instances in the chalk 

 countries where, although the substratum may 

 be solid chalk to the depth of a hundred feet or 

 more, there may be a great deficiency of it near 



the surface ; and hence it is frequently brought 

 up, by sinking shafts and tunnelling, for the 

 purpose of being laid on as a top-dressing, and 

 to make up what is found to be wanting in the 

 surface- soil. Fruit trees do not long exist in a 

 healthy condition where chalk forms the sub- 

 soil. In such a soil, according to Professor 

 Johnston, there are 56 lb. of lime in 1000 lb. 

 of calcareous soil ; while barren soil contains 

 only 4 lb. That is, if a soil contains more than 

 20 per cent of carbonate of lime, it is called a 

 calcareous soil ; if it contains no more than 5 

 per cent, it is called a marly soil. 



Loamy soils. — This term should be limited to 

 soils containing at least one-third of impalpable 

 earthy matter, and effervescing copiously with 

 acids. Considerable discrepancy, however, ap- 

 pears to exist in regard to what loam really is. 

 " Loam or mould is composed of decomposed 

 animal and vegetable matters, is a principal 

 material in soils, and differs greatly according 

 to the quantity and quality of the matter itself, 

 the state of reduction it has reached, and from 

 the manner in which it is incorporated with the 

 constituents of the soil. Such soils are almost 

 invariably dry, and possess the happy medium 

 of absorbing, retaining, and giving off moisture 

 in the quantity most suitable to the growth of 

 plants. Much of their fertility may be ascribed 

 to the quality of the original subjacent forma- 

 tion being favourable to the growth of vegeta- 

 tion which grew and died upon it, and, along 

 with animal remains, produced the quality and 

 depth of the stratum. The property of depth 

 generally bears a direct ratio to the fertility of 

 the soil, or rather the latter to the former." — 

 Donaldson on Soils and Manures. 



Sir H. Davy defines loam as " the impalpable 

 part of the soil which is usually called clay or 

 loam;" and Reid, in "Chemistry of Nature," 

 offers the following definition : " The term 

 loam is applied to soils which consist of about 

 one-third of finely-divided earthy matter, con- 

 taining much carbonate of lime." Mr Stephens, 

 in ft The Book of the Farm," says — " Loam, in 

 my opinion, consists of any kind of earth con- 

 taining a sensible admixture of decomposed vege- 

 table matter. I say," he remarks, " a sensible ad- 

 mixture, since no soil under cultivation, whether 

 composed chiefly of clay or of sand, but what 

 contains some decomposed vegetable matter. 

 Unless, therefore, the decomposed vegetable 

 matter of the soil so preponderates as to greatly 

 modify the usual properties of the constituent 

 earths, the soil cannot, in truth, be called by 

 any other name than a clayey or sandy soil ; 

 but when it does so prevail, a clay loam or a 

 sandy loam is formed. But if loam is almost 

 synonymous with clay, then a sandy loam must 

 be a contradiction of terms. Again, a soil of 

 purely vegetable origin — such as crude peat or 

 leaf mould — cannot be called loam, as admix- 

 ture of an earth of some sort with vegetable 

 matter is required to make loam, under every 

 recorded definition of that term. Thus, then, 

 all soils have the property of clay and sand ; 

 and a considerable admixture of decomposed 

 vegetable matter converts them into loam. 

 Hence it is possible for husbandry to convert 



