Boor III. CHANGING THE CONDITION OF LANDS. 323 



duced in the operation of burning. The charcoal very finely divided, and exposed on a 

 large surface, must be gradually converted into carbonic acid. And gypsum and oxide 

 of iron seem to produce the very best effects when applied to lands containing an ex- 

 cess of carbonate of lime. The second specimen was from a soil near Coleorton, in 

 Leicestershire, containing only four per cent, of carbonate of lime, and consisting of 

 three fourths light siliceous sand, and about one fourth clay. This had been turf before 

 burning, and 100 parts of the ashes gave 6 parts charcoal, 3 muriate of soda and sulphate 

 of potash, with a trace of vegetable alkali, 9 oxide of iron, and the remainder the earths. 

 In this instance, as in the other, finely divided charcoal was found, the solubility of 

 which would be increased by the presence of the alkali. The third instance was that 

 of a stiff clay, from Mount's Bay, Cornwall. This land has been brought into cultiva- 

 tion from a heath, by burning, about ten years before : but having been neglected, furze 

 was springing up in different parts of it, which gave rise to the second paring and burn- 

 ing, 100 parts of the ashes contained 8 parts of charcoal, 2 of saline matter, principally 

 common salt, with a little vegetable alkali, 7 oxide of iron, 2 carbonate of lime, the re- 

 mainder alumina and silica. Here the quantity of charcoal was greater than in the other 

 instances. The salt was probably owing to the vicinity of the sea, it being but two miles 

 off. In this land there was certainly an excess of dead vegetable fibre, as well as un- 

 profitable living vegetable matter. 



21 38. Causes of the effects of burning soil. Many obscure causes have been referred to 

 for the purpose of explaining the effects of paring and burning ; but they may be re- 

 ferred entirely to the diminution of the coherence and tenacity of clays, and to the 

 destruction of inert and useless vegetable matter, and its conversion into a manure. 

 Dr. Darwin, in iiis Ph?/tologla, has supposed that clay, during torrefaction, may absorb 

 some nutritive principles from the atmosphere that afterwards may be supplied to plants ; 

 but the earths are pure metallic oxides, saturated with oxygen ; and the tendency of 

 burning is to expel any other volatile principles that they may contain in combination. 

 If the oxide of iron in soils is not saturated with oxygen, torrefaction tends to produce 

 its further union with this principle ; and hence, in burning, the color of clay changes to 

 red. The oxide of iron, containing its full proportion of oxygen, has less attraction for 

 acids than the other oxide, and is consequently less likely to be dissolved by any fluid 

 acids in the soil ; and it appears in this state to act in the same manner as the earths. 

 A very ingenious author, Naismith (Elements of Agr.)^ supposes that the oxide of 

 iron, when combined with carbonic acid, is poisonous to plants ; and that one use of tor- 

 refaction is to expel the carbonic acid from it ; but the carbonate of iron is not soluble 

 in water, and is a very inert substance ; and a luxuriant crop of cresses has been raised 

 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. 



2139. The soils improved hy burning are all such as contain too much dead vegetable 

 fibre, and which consequently lose from one third to one half 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 in 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. 



2140. All poor siliceous sands are injured by burning. Young in his Essay on Ma- 

 nures, states, " tliat 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." 



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



2141. The water of the soil luhere superabundant may be withdrawn, and when deficient 

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

 affording the stiiuulus of heat or cold. 



2142. 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 subsoil 

 moderately 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 subsoil is reten- 

 tive 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 exces- 

 sive as a component part, not only carries off the extractive matter, but diseases the 



Y 2 



