BOOK II. CHANGING THE CONDITION OF LANDS. 231 



in a soil composed of one fifth carbonate of iron, and four fifths carbonate of lime. 

 Carbonate of iron abounds in some of the most fertile soils in England, particularly the 

 red hop soil. And there is no theoretical ground for supposing that carbonic acid, 

 which is an essential food of plants, should, in any of its combinations, be poisonous to 

 them ; and it is known that lime and magnesia are both noxious to vegetation, unless 

 combined with this principle. 



1092. The soils improved by burning are all such as contain too much dead vegetable 

 fibre, and which consequently lose from one third to one half of their weight by inciner- 

 ation ; and all such as contain their earthy constituents in an impalpable state of division, 

 i. e. the stiff clays and marls, are improved by burning : but coarse sands, or rich 

 soils containing a just mixture of the earths ; and in all cases in which the texture is 

 sufficiently loose, or the organisable matter sufficiently soluble, the process of torrefaction 

 cannot be useful. 



1093. All poor siliceous sands are injured by burning. Young in his Essay on Ma- 

 nures, states, " that he found burning injure sand ; and the operation is never performed 

 by good cultivators upon siliceous sandy soils, after they have once been brought into 

 cultivation." 



SUBSBCT. 5. Changing the Condition of Lands, in respect to Water. 



1094. The water of the soil ivhere' superabundant may be withdrawn, and when deficient 

 supplied : these operations with water are independent of its supply as a manure, or as 

 affording the stimulus of heat or cold. 



1095. Stagnant water may be considered as injurious to all the useful classes of plants, 

 by obstructing perspiration and intro-susception, and thus diseasing their roots and sub- 

 merged parts. Where the surface-soil is properly constituted, and rests on a sub-soil mo- 

 derately porous, both will hold water by capillary attraction, and what is not so retained 

 will sink into the interior strata by its gravity; but where the sub-soil is retentive, it will 

 resist, or not admit with sufficient rapidity, the percolation of water to the strata below, 

 which accumulating in the surface-soil, till its proportion becomes excessive as a com- 

 ponent part, not only carries oft* the extractive matter, but diseases the plants. Hence 

 the origin of surface-draining, that is, laying land in ridges or beds, or intersecting it 

 with small open gutters. 



1096. Springs. Where the upper stratum is porous in some places,, and retentive in 

 others, and on a retentive base, the water, in its progress along the porous bed or layer, 

 will be interrupted by the retentive places in a great variety of ways, and there accumu- 

 lating will burst through the upper surface in the form of springs, which are more in- 

 jurious than surface-water, as being colder, and generally permanent in their operation. 

 Hence the origin of under-draining in all its varieties of collecting, extracting, and con- 

 veying water. 



1097. The water of rivers may become injurious to lands on their banks, by too fre- 

 quently overflowing their surface. In this case the stream may be included by mounds 

 of earth, or other materials impervious to water : and thus aquatic soils rendered dry and 

 fit for useful herbage and aration. The same may be said of lands occasionally over- 

 flown by the sea. Hence the origin of embanking, an art carried to a great extent in 

 Holland and Italy. (See Smeaton's Posthumous Works ; Sigismondi, Agr. Tosc. ; and our 

 article Embankment, in Supp. Encyc. Brit. 1819.) 



1098. Irrigation. Plants cannot live without water, any more than they can prosper 

 in soils where it is superabundant ; and it is therefore supplied by art on a large scale, 

 either by surface or subterraneous irrigation. In both practices important points are to 

 imitate nature in producing motion, and in applying the water in the mornings or even- 

 ings, or under a clouded sky, and also at moderate intervals. The effects of water con- 

 stantly employed, would, in most cases, be such as attend stagnated water, aquatic soils, 

 or land-springs ; and employed in hot sunshine, or after violent heats, it may check eva- 

 poration and destroy life, exactly as happens to those who may have bathed in cold spring 

 water after long and violent exercise in a hot day. (Phytologia, xv. 3. 5.) 



1099. In surface irrigation the water is conveyed in a system of open channels, which 

 require to be most numerous in such grounds as are under drilled annual crops, and 

 least so in such as are sown in breadths, beds, or ridges, under perennial crops. This 

 mode of watering has existed from time immemorial. The children of Israel are repre- 

 sented as sowing their seed and " watering it with their foot ;" that is, as Calmet explains 

 it, raising the water from the Nile by a machine worked by the feet, from which it was 

 conducted in such channels as we have been describing. It is general in the south of 

 France and Italy ; but less required in Britain. 



1 100. Subterraneous irrigation may be effected by a system of drains or covered gutters 

 in the sub-soil, which, proceeding from a main conduit, or other supply, can be charged 

 with water at pleasure. For grounds under the culture of annual plants, this mode would 

 be more convenient, and for all others more economical as to the use of water, than sur- 



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