SOILS. 



275 



are, the more injurious they are to the soil over 

 them. 



The effects of colour on soils are various, and 

 have a considerable influence in regulating the 

 quantities of heat absorbed from the sun's rays. 

 The darker-coloured soils absorb most heat, 

 while the lighter-coloured ones reflect most of 

 that element. Much, however, of the absorbent 

 property depends on the nature of the soil, 

 whether it be free of superabundant water or 

 the reverse ; and hence a great difference exists 

 in this respect between water-logged black peat 

 and dry black soil containing abundance of 

 organic matter, although the latter facilitates 

 the preservation of temperature, in consequence 

 of the vegetable and animal matter it contains, 

 when heated to the same degree, cooling more 

 slowly than a wet pale soil entirely composed of 

 earthy matter ; while stiff clay is with difficulty 

 heated, and soon parts with the heat it acquires. 

 Hence it may be considered the coldest of all 

 soils. Chalks are heated with difficulty, but, 

 being drier, when once heated they retain it 

 longer, less being consumed in causing the eva- 

 poration of their moisture. Soils containing 

 much carbonaceous or ferruginous matter ac- 

 quire a much higher temperature from the sun 

 than pale soils. From the experiments made by 

 Schiibler with the thermometer in the shade, 

 sand of a natural colour indicated a temperature 

 of 1124°, black sand, 1234, and wnite saud > 110 °> 

 giving a difference of 13° in favour of a black 

 colour. 



Sir H. Davy ascertained that a black-coloured 

 soil containing one- fourth of vegetable matter 

 increased in temperature in one hour 23° by ex- 

 posure to the sunshine ; while a white chalk soil 

 only gained 4° in the same time, and under the 

 same circumstances. But the black soil, when re- 

 moved into a shaded place, cooled 15° in half an 

 hour; while the chalky soil only lost 4° in the 

 same time, and under the same circumstances. 

 . Most physiologists agree that the pure soil, 

 without organised matter and water, is of no 

 other use to plants than merely affording them 

 a fixed abode, and a medium by which they may 

 fix themselves to the globe. Soil is also neces- 

 sary to plants by enabling them to obey those 

 laws of nature by which their roots are kept 

 below the surface, and their leaves exposed to 

 the free atmosphere. 



" It is commonly supposed that plants derive 

 the whole of their food from the soil, but this 

 is a great error. It is a fact well ascertained by 

 chemical experiments, that plants derive the 

 greater part of their nourishment from the air, 

 although the soil is equally essential to their 

 growth."— Solly in Rural Chemistry. 



In general, much more depends on the tex- 

 ture of a soil, and its capacity for retaining or 

 parting with water and heat, than on its chemi- 

 cal composition. 



Subsoils are of vast importance, because on 

 them depends, in a great measure, the capacity 

 of the surface-soil for retaining or parting with 

 water and heat. Of these, the worst is clay, 

 kept wet with subterraneous water, and the 

 best are those of clay resting on gravel or por- 

 ous rock. The latter of these is the best of all 



for a garden, because, while the water finds a 

 ready means of escape through the fissures in 

 the rock, the roots of the trees are at the same 

 time prevented from extending too deep, and 

 made to take a horizontal direction nearer the 

 surface, and also prevented from extending into 

 a cold and uncongenial soil, causing late excite- 

 ment of the sap in spring, immature ripening 

 of the wood and buds in autumn, and conse- 

 quent paucity of fruit and premature decay in 

 the trees. The soil, for reasons already stated, 

 is, under such circumstances, much increased in 

 temperature, producing earlier crops of culinary 

 vegetables, and those of much better quality, on 

 account of the greater rapidity of their growth. 

 Gravelly subsoils are not always to be depended 

 upon, for these are sometimes placed over basins 

 of impervious clay, from which water cannot 

 escape until it rises as high as the margin of the 

 basin itself, leaving the gravel thus saturated 

 with water. Again, gravel abounding in oxide 

 of iron is a bad subsoil, and often occurs. Next, 

 therefore, to a rocky bottom, that of clean allu- 

 vial gravel is to be preferred ; and in such all 

 the productions of a garden will prosper, and 

 that often when the climate is indifferent. Not- 

 withstanding these advantages, how seldom, in 

 practice, is this attended to. Much labour and 

 expense are gone into in the improvement of 

 the upper soil, while the lower and more im- 

 portant is left in its natural state. Artificial 

 subsoils are seldom thought of, although of 

 their advantages no one can doubt ; and as nature 

 has pointed out to us that an open porous one 

 is the best, our first attempt at garden-making 

 should be to imitate her in this respect. We 

 have shown in vol. i, p. 29, 31, how an artificial 

 subsoil can be formed for fruit-tree borders; 

 and as experience has proved the great utility 

 of these, the enlightened horticulturist will at 

 once see the feasibility of extending the same 

 process under the whole of his garden. We 

 may here only state, that where sufficient drain- 

 age material may be difficult to procure, re- 

 course may be had to concreting, taking care 

 that a sufficient number of drain-tiles be laid 

 underneath for the free escape of the water, 

 which should find its way into them through 

 apertures left in the concrete floor. It must, 

 however, be admitted that such excessive drain- 

 age is not applicable to all situations alike, so 

 far as the removal of water alone is concerned ; 

 but so far as keeping the roots near the surface, 

 and preventing their extending into a bad subsoil, 

 the rule is absolute. Along the eastern coast of 

 Britain, where the annual fall of rain is less than 

 half of what it is on the western, this excessive 

 drainage is of much less importance, unless 

 in low situations where subterranean water 

 abounds, and where it cannot be conveniently 

 drained off. 



Mineralogical quality of soil. — The temperature 

 and proper amount of moisture in a soil is of 

 far greater importance to the successful culture 

 of plants than its mineralogical structure, and 

 this has reference to hardy as well as to tender 

 plants. " I entertain little doubt," Dr Lindley 

 observes (in " Theory of Horticulture"), " that 

 the time is at hand when it will be considered 



