230 



SCIENCE OF GARDENING. 



Part II, 



stones is made the subject of experiment, and two equal portions of basalt ground into im- 

 palpable powder, of which one half had been strongly ignited, and the other exposed only 

 to a temperature equal to that of boiling water, gained very different weights in the same 

 time when exposed to air. In four hours the one had gained only tw o grains, whilst tlie 

 other had gained seven grains. When clay or tenacious soils are burnt, the effect is of 

 the same kind ; they are brought nearer to a state analogous to that of sands. In the 

 manufacture of bricks the general principle is well illustrated ; if a piece of dried brick 

 eaith be applied to the tongue, it will adhere to it very strongly, in consequence of its 

 power to absorb water ; but after it has been burnt, there will be scarcely a sensible ad- 

 hesion. 



1088. The advantages of burning are that it renders the soil less compact, less tenacious 

 and retentive of moisture ; and when properly applied, may convert a matter that was 

 stiff", damp, and in consequence cold, into one powdery, dry, and warm, and much 

 more proper as a bed for vegetable life. 



1089. The great objection made by speculative chemists to paring and burning, is, that 

 it destroys vegetable and animal matter, or the manure in the soil ; but in cases in which 

 the texture of its earthy ingredients is permanently improved, there is more than a com- 

 pensation for this temporary disadvantage. And in some soils where there is an excess 

 of inert vegetable matter, the destruction of it must be beneficial ; and the carbonaceous 

 matter remaining in the ashes may be more useful to the crop than the vegetable fibre 

 from which it was produced. 



1090. Three specimens of ashes from different lands that had undergone paring and 

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

 grains contained 80 of carbonate of lime, 11 gypsum, 9 charcoal, 15 oxide of iron, 

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

 getable alkali. The remainder alumina and silica. Suppose 2660 bushels to be tlie 

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

 give 172,900 lbs., containing carbonate of lime 691,60 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 pro- 

 duced in the operation of burning. The charcoal vei*y 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 eflfects 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. Tliis had been turf before 

 burning, and 100 parts of the ashes gave 6 parts charcoal, 3 mm-iate 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 stiflP 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 

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

 profitable living vegetable matter. 



1091. 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 ; btit 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 his Phytologia, 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 



