328 SCIENCE OF AGRICULTURE. Pakt II. 



burning were examined by chemical analysis. The first was from a chalk soil, and 200 

 grains contained 80 of carbonate of lime, 1 1 gypsum, 9 charcoal, 1 5 oxide of iron, 

 3 saline matter, sulphate of potash, muriate of magnesia, with a minute quantity of 

 vegetable alkali ; the remainder alumina and silica. Suppose 2660 bushels to be the 

 common produce of an acre of ground, then, according to this calculation, they would 

 give 172,900 lbs., containing carbonate of lime 69,160 lbs., gypsum 9509'5., oxide of 

 iron 12,967*5., saline matter 2593'5., charcoal 7780*5. In this instance there was un- 

 doubtedly a very considerable quantity of matter capable of being active as manure 

 produced 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 

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

 Leicestershire, containing only 4 per cent of carbonate of lime, and consisting of three 

 fourths light silicious sand, and about one fourth clay. This had been turf before burn- 

 ing, 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 had been brought into cultivation from a 

 heath, by burning, about ten years before : but having been neglected, furze was spring- 

 ing up in different parts of it, which gave rise to the second paring and burning. 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 remainder 

 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 unprofitable 

 living vegetable matter. 



2196. Causes of the effects of burning soil. Many obscure causes have been referred 

 to for the purpose of explaining the eflEects of paring and burning ; but they may be 

 referred 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 his Phytologia, has supposed that clay, during torrefaction, may absorb 

 some nutritive principles from the atmosphere which 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 which they may contain in combin- 

 ation. 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 colour of cjay 

 changes to red. The oxide of iron, containing its full proportion of oxygen, has less 

 attraction for acids than any 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 

 torrefaction 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. 



2197. 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 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. 



2198. All poor silicious 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 silicious sandy soils, after they have once been brought into 

 cultivation." 



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



2199. The water of the soil where 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. 



2200. Stagnant water maybe considered as injurious to all the useful classes of plants, 



