S^o SCIENCE OF AGRICULTURE. Tart II. 



bonatcs of lime and magnesia : these differences may, however, possibly depend upon the 

 differences in their state of division, and upon the surface exposed. 



2153. The power of soil to absorb water from air is much connected with fertility. When 

 this power is great, the plant is supplied with moisture in dry seasons; and the effect of 

 evaporation in the day is counteracted by the absorption of aqueous vapour from the atmo- 

 sphere, by the interior parts of the soil during the day, and by both the exterior and in- 

 terior during the night. The stiff clays approaching to pipe-clays in their nature, which 

 take up the greatest quantity of water when it is poured upon them in a fluid form, are 

 not the soils which absorb most moisture from the atmosphere in dry weather. They 

 cake, and present only a small surface to the air; and the vegetation on them is gene- 

 rally burnt up almost as readily as on sands. The soils most efficient in supplying the 

 plant with water by atmospheric absorption are those in which there is a due mixture 

 of sand, finely divided clay, and carbonate of lime, with some animal or vegetable 

 matter, and which are so loose and light as to be freely permeable by the atmosphere. 

 With respect to this quality, carbonate of lime, and animal and vegetable matter, are of 

 great use in soils ; they give absorbent power to the soil, without giving it likewise 

 tenacity ; sand, which also destroys tenacity, on the contrary, gives little absorbent 

 power. The absorbent power of soils, with respect to atmospheric moisture, is always 

 greatest in the most fertile ; so that it affords one method of judging of the productive- 

 ness of land. 



21.54. Examples of the absorbent poivers of soils. 1000 parts of a celebrated soil from 

 Ormiston, in East Lothian, which contained more than half its weight of finely divided 

 matter, of which 1 1 parts were carbonate of lime, and 9 parts vegetable matter, when 

 dried at 212°, gained in an hour, by exposure to air saturated with moisture, at a 

 temperature of 62°, 18 grains. 1000 parts of a very fertile soil from the banks of the 

 river Parret, in Somersetshire, under the same circumstances, gained 16 grains. 1000 

 parts of a soil from Mersea, in Essex, gained 13 grains. 1000 grains of a fine sand, 

 from Essex, gained 11 grains. 1000 of a coarse sand gained only 8 grains. 1000 of a 

 soil from Bagshot Heath gained only 3 grains. 



2155. The absorbent powers of soils ought to vary with the climate in which they are 

 situated. 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 

 bottoms of valleys. Their productiveness likewise 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 flourishing and green. There is a considerable difference between the sandy 

 soils of the east and west coasts of Scotland. All along the west coast from the Solway 

 Frith to the Clyde, such soils are more productive than soils of a similar quality on the 

 east coast, under the same circumstances of management. The extensive culture of 

 potatoes for instance, and the succession of corn crops in Dumfriesshire and Galloway, 

 would soon reduce to a state of sterility much of the best sandy soils of Roxburghshire 

 and the Lothians. 



2156 In a moist climate where the quantity of rain which falls annually equals from 4(> 

 to 60 inches, as in Lancashire, Cornwall, and some parts of Ireland, a silicious 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 flourish 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 sufficient 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. 



2157. 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 



