Book III. USES OF SOIL TO VEGETABLES. 317 



parts of a soil from Mersea, in Essex, gained 13 grains. 1<XX) grains of a fine sand, 

 from Essex, gained 1 1 grains. 1000 of a coarse sand gained only 8 grains. 1003 of a 

 soil of Bagshot Heath gained only 3 grains. 



2105. The absorbent powers of soils ought to vary with the climate in which they are si- 

 tuated. The absorption of moisture ought to be much greater in warm or dry countries, 

 than in cold and moist ones ; and the quantity of clay, or vegetable, or animal matter in 

 soils greater. Soils also on declivities ought to be more absorbent than in plains or in the 

 bottom of valleys. Their productiveness liicewise is influenced by the nature of the sub- 

 soil, or the stratum on which they rest. When soils are immediately situated upon a bed 

 of rock or stone, they are much sooner rendered dry by evaporation than where the sub- 

 soil is of clay or marl ; and a prime cause of the great fertility of the land in the moist 

 climate of Ireland, is the proximity of the rocky strata to the soil. A clayey sub-soil 

 will sometimes be of material advantage to a sandy soil ; and in this case it will retain 

 moisture in such a manner as to be capable of supplying that lost by the earth above, in 

 consequence of evaporation or the consumption of it by plants. A sandy or gravelly 

 sub-soil often corrects the imperfections of too great a degree of absorbent power in the 

 true soil. In calcareous countries, where the surface is a species of marl, the soil is often 

 found only a few inches above the limestone ; and its fertility is not impaired by the 

 proximity of the rock; though in a less absorbent soil, this situation would occasion 

 barrenness ; and the sandstone and limestone hills in Derbyshire and North Wales may 

 be easily distinguished at a distance, in summer, by the different tints of the vegetation. 

 The grass on the sandstone-hills usually appears brown and burnt up ; that on the lime- 

 stone-hills florishing and green. 



2106. In a moist climate, where the quantity of rain that falls annually equals from 40 

 to 60 inches, as in Lancashire, Cornwall, and some parts of Ireland, a siliceous sandy soil 

 is much more productive than in dry districts ; and in such situations wheat and beans 

 will require a less coherent and absorbent soil than in drier situations ; and plants having 

 bulbous roots will florish in a soil containing as much as 14 parts out of 15 of sand. 

 Even the exhausting powers of crops will be influenced by like circumstances. In cases 

 where plants cannot absorb suflicient moisture,, they must take up more manure. And 

 in Ireland, Cornwall, and the western Highlands of Scotland, corn will exhaust less 

 than in dry inland situations. Oats, particularly in dry climates, are impoverishing in a 

 much higher degree than in moist ones. 



2107. Many soils are popularly distinguished as cold or hot ; and the distinction, though 

 at first view it may appear to be founded on prejudice, is really just. Some soils are 

 much more heated by the rays of the sun, all other circumstances being equal, than others ; 

 and soils brought to the same degree of heat, cool in different times, i. e. some cool much 

 faster than others. This property has been very little attended to in a philosophical point 

 of view ; yet it is of the highest importance in culture. In general, soils that consist 

 principally of a stiff white clay are difficultly heated ; and being usually very moist, they 

 retain their heat only for a short time. Chalks are similar in one respect, that they are 

 difficultly heated ; but being drier they retain their heat longer, less being consumed in 

 causing the evaporation of their moisture. A black soil, containing much soft vegetable 

 matter, is most heated by the sun and air ; and the colored soils, and the soils containing 

 much carbonaceous matter, or ferruginous matter, exposed under equal circumstances to 

 the sun, acquire a much higher temperature than pale-colored soils. 



2108. When soils are i^erfectly dry, those that most readily become heated by the solar rays, 

 likeivise cool most rapidly ; but the darkest-colored dry soil, (that which contains abund- 

 ance of animal or vegetable matter ; substances which most facilitate the diminution of 

 temperature,) when heated to the same degree, provided it be within the common limits 

 of the effect of solar heat, will cool more slowly than a wet, pale soil, entirely composed 

 of earthy matter. Sir H. Davy " found that a rich black mould, which contained nearly 

 one fourth of vegetable matter, had its temperature increased in an hour from 65 to 88 

 by exposure to sunshine ; whilst a chalk soil was heated only to 69" under the same cir- 

 cumstances. But the mould removed into the shade, where the temperature was 62, 

 lost, in half an hour, 15 ; whereas the chalk, under the same circumstances, had lost 

 only 4. We may also refer to the influence of black earth in melting snow, as prac- 

 tised empirically on the Alps, and tried philosophically by Franklin and Saussure. 

 The latter placed on the top of the high Alpine mountain Cramont, a box lined with 

 black cloth with the side next the sun, closed by three panes of glass at a little distance 

 apart the one from the other, and found the thermometer rise thirty degrees in two hours 

 from the concentration of the sun's rays. [Jgriculture ajrplique, J^c. torn. i. 82.) A 

 brown fertile soil and a cold barren clay were each artificially heated to 88, having been 

 previously dried ; they were then exposed in a temperature of 57 ; in half an hour the 

 dark soil was found to have lost 9 of heat ; the clay had lost only 6. An equal portion 

 of the clay containing moisture, after being heated to 88, was exposed in a temperature 



