450 



THE DICTIONARY OF GARDENING, 



Soil continued. 



varies much in Soils, according to their composition and 

 the size of the particles of which they are made up. 

 Sandy Soils rapidly absorb the water ; but they allow it 

 to drain away almost as quickly; BO that plants growing 

 in sand are liable to suffer from drought. Fine sand 

 is about twice as retentive of water as coarse. Clay re- 

 tains twice or thrice as much as sand. Vegetable earth, 

 or humus, absorbs and retains about twice or thrice as 

 much as clay, and becomes about two and a-half or three 

 times as heavy when soaked in water as when dry. The 

 more retentive Soils lose less by evaporation than do the 

 others. Almost all Soils are more or less full of water, 

 in the liquid form, at a few feet below the surface, the 

 depth varying with the nature of the Soil and Subsoil 

 and with the season of the year and the climate. This 

 subterranean water is brought within reach of the roots 

 of plants by what is known as capillary attraction, the 

 water rising in the fine crevices between and in the 

 particles of the Soil. If the subterranean water is 

 stagnant near the surface, substances are apt to be 

 formed in the Soil that are injurious to plants, and that 

 thus diminish its fertility. Drainage is usually necessary 

 in clays and peat Soils, to prevent water from accumu- 

 lating and doing harm in this way. 



Certain Soils also have the property of absorbing 

 a good deal of vapour from the atmosphere, and con- 

 densing it in their particles, the amount increasing in 

 proportion to the moistness of the air. Experiments 

 have shown that humus can absorb about half its own 

 weight of water from air saturated with vapour; and 

 clays absorb one-tenth to one-fifth of their own weight ; 

 but quartz sand absorbs little, if any, moisture in that 

 way. This source of moisture becomes less productive 

 as the temperature rises. During the night, and in cold 

 weather, it is probable that Soils absorb a good deal of 

 moisture from the atmosphere ; but when the air is 

 warmer and drier than the soil, the latter loses water 

 by evaporation, and what is lost in this way is replaced 

 from below by capillary attraction. Evaporation cools 

 the surface from which it is going on ; hence, wet land 

 is colder than that which is well-drained. Humus gives 

 up least by evaporation, and quartz sand most, of all 

 Soils under similar conditions. 



The Power of Absorbing and Retaining Chemical Com- 

 pounds is one possessed to a greater or less extent by 

 all Soils, and is of the utmost importance in the nutri- 

 tion of plants. If solutions of various kinds (e.g., of 

 Potassic Nitrate or Ammonium Nitrate, or of Phos- 

 phates of Potassium, Calcium, &c.) are allowed to trickle 

 through a moderately thick layer of Soil, it is found 

 that the water flowing off contains little of these sub- 

 stances for a time; but at last the Soil becomes satu- 

 rated with them, and allows the solution to pass through 

 unchanged. By this property of retaining certain com- 

 pounds, Soils are enabled to store up soluble manures, 

 as well as Ammonia and Nitrates from the atmosphere, 

 and various substances formed in the changes due to 

 weathering of the rocks and Soils ; and from this store 

 plants can draw, as they need these substances in their 

 food. Owing to this process, filtering impure water 

 through, or over, a sufficient extent of earth is a very 

 efficient mode of removing impurities, and is frequently 

 resorted to for purifying sewage-water before discharging 

 it into streams. 



The Capacity for Absorbing and Retaining Heat varies 

 with colour and texture, with the amount of moisture in, 

 and of evaporation from, the Soil, and with the angle of 

 exposure to the rays of the sun, direct or reflected. Of 

 course, the actual temperature depends also upon the 

 amount of protection afforded by buildings or other ob- 

 jects in the neighbourhood. 



Bottom-heat is of great use in stimulating the action 

 of roots, but in Great Britain it can be given only in 



Soil continued. 



hotbeds or hothouses. Exposure to the pun's rays is, 

 therefore, the only source of warmth that need be dis- 

 cussed here. The more directly the rays fall, the greater 

 is their effect. Evaporation keeps down the temperature, 

 and it has been found that wet Soils are usually from 

 lOdeg. to 15deg. Fahr. colder than dry ones of the same 

 composition. Hence, draining wet Soils renders them 

 warmer, and hastens the ripening of the crops on them. 

 Dark grey Soils absorb most heat ; next come black Soils, 

 then brown and dark red. Pale sands, marls, and clays 

 absorb least. The temperature of the Soil exercises a 

 marked influence on the growth of plants, since they 

 suffer if the buds and leaves are stimulated by a warm 

 atmosphere while the soil is too cold to permit of the 

 roots supplying the necessary sap to the other organs. 



The Density and Power of Cohesion of the particles of 

 Soils, and the Mode of Shrinkage, are of interest and 

 importance chiefly on account of their influence on the 

 capacities of Soils for moisture, chemical substances, and 

 heat. Pure sands show little cohesion, and change 

 little in bulk or form during dry weather. Clays are 

 very coherent, and may lose as much as one-fifth of 

 their bulk by shrinkage during droughts ; and peaty 

 Soils shrink even more than clays. Cracks form in 

 these Soils to a considerable depth, and allow evapora- 

 tion to continue ; and roots are broken across, and 

 exposed to the air in the cracks, or are crushed by the 

 shrinkage. 



Soils can frequently be much improved by a judicious 

 mixture with others : e.g., sand should be added to clay ; 

 clay or peat to sand ; lime, clay, and sand to peat. 



ORIGIN. All Soils are formed in one or other of two 

 ways, either from the weathering and breaking down 

 of rocks, or from the decay of plants or animals, and 

 most of them are produced more or less in both ways ; 

 though the former has been the source of by far the 

 greater bulk of all Soils except peat. In some localities, 

 it is not difficult to recognise that the Soils are of the 

 same composition as the rocks on which they lie, and 

 that they are formed by the action of the weather in 

 breaking up the rocks and reducing them to fragments. 

 In course of time, also, the decay of successive genera- 

 tions of plants gives origin to an admixture of humus 

 in almost all Soils. But, in most parts of Great Britain, 

 the Soils are different in composition from the rocks 

 on which they lie, and must have been brought into 

 their present situations from a greater or less distance. 

 That this should be the case in valleys, is easily under- 

 stood, for the Soil on the slopes is constantly being 

 carried down by showers and streams, and spread 

 over the lower ground. This also occurs on fields 

 or meadows along rivers in the lowlands liable to 

 be overflowed, as the finer particles of mud in the 

 water are deposited on the flooded ground, where free 

 from the action of currents. Frost splits pieces of stone 

 from exposed rocks and cliffs, and causes them to roll 

 down the slopes, and it is also a powerful agent in re- 

 ducing rocks and stones into the finer particles of which 

 Soils are composed. But a layer of a few inches of 

 earth will protect the rocks very greatly, if not 

 entirely, from the action of the air, rain, and frost; 

 and these agents are not sufficient to account for the 

 amount and depth of soil, or for the relations of the soil 

 to the subjacent rocks, now prevalent over the country, 

 especially in Scotland and in the northern half of England. 

 A far more powerful force than any now existing in 

 Britain was, however, at work during a comparatively 

 recent geological period. This was ice, which, for a long 

 time, covered the land (much as Greenland is covered 

 now) with a continuous sheet. Formed and renewed on 

 the higher tracts of country, it spread down the valleys, 

 and over all but the highest peaks, extended over the 

 lowlands of Scotland and Northern England, and met ice 



