522 SCIENCE OF AGRICULTURE. Part II. 



though they certainly appear to be considerable on clayey soils. Alton {Farmer s Mag. 

 vol. xxii. p. 423.) compares this rage for burning clay, which existed in 1815, to the 

 florin mania of a few years' prior date. In 1822, he found few of the advocates for these 

 improvements disposed to say much on the subject, and saw very few clay-kilns smoking. 

 " To give my ultimatum upon this subject," he says, " I regret that the discoverers of 

 florin grass, and of the effects of burnt clay, have so far overrated their value. Both are 

 useful and proper to be attended to ; the grass to be raised on patches of marshy 

 ground, and used as green food to cattle in winter ; and the burnt earth as a corrector of 

 the mechanical arrangement of a stubborn clay soil ; and I have no doubt, but if they 

 had been only recommended for those valuable purposes, they would have been brought 

 into more general use than they yet are, or will be, till the prejudices against them, 

 arising from the disappointment of expectations raised high by too flattering descriptions, 

 are removed." 



3220. The action of burnt clay on the soil is thus described by the same author : " It must be obvious to 

 every person who has paid attention to the subject, that when clay or other earth is burnt into ashes like 

 brick.dust, it will not (unless acids are applied to it) return again to its former state of clay, but will 

 remain in the granulated state of ashes or friable mould, to which it was reduced by the operation of 

 burning. An admixture of that kind with a strong adhesive clay must evidently operate as a powerful 

 manure, by changing the mechanical arrangement of the latter, and rendering it more friable ; giving 

 greater facility to the percolation of redundant moisture, and to the spreading of the roots of vegetables 

 in quest of food. The application of as much water, sand, or any similar substance, would have exactly 

 the same effect, in opening and keeping open the pores of an adhesive clay soil, and converting it into the 

 quality of loam. Besides this, which would be a permanent improvement upon the staple or texture of 

 every clay soil, burnt clay or torrefied earth may sometimes acquire, in that operation, a small quantity of 

 soot or carbonic matter, that may, in favourable circumstances, operate for one season as a manure, or as 

 a stimulus, to a small extent, to the growth of vegetables. This at least may be the case, if the clay or 

 earth burnt shall abound with vegetable matter, and if the burning is conducted in such a smothered vvay, 

 as to prevent the smoke or vegetable matter from escaping. But as it is the subsoil that is recommended, 

 and seems to be generally used for burning, it is impossible any considerable quantity of vegetable matter 

 can be found in it. 



3221. The calcareous matter in the soil, it is saif', will be calcined and formed into lime by the operation 

 of burning; but I am disposed to consider this argument as far more plausible than solid. Calcareous 

 matter is no doubt found, on chemical analysis, to a certain extent in some soils ; perhaps some per- 

 ceptible portion of it may be found in every soil : but it is seldom or never found in any soil, to such an 

 extent as to be of much use as a manure to other land. Even where the soil is impregnated with a large 

 portion of calcareous matter, if it is not in the form of limestone, but minutely mixed with it, the burn- 

 ing cannot either increase or much alter the lime. If it is in the form of stones, however small, or in 

 what is called limestone gravel, there is little chance of its being calcined in the operation of burning the 

 clay ; it would go through that ordeal unaltered. Any change, therefore, that can be made upon the 

 small portion of calcareous matter in the soil by burning in the manner directed, can scarcely have any 

 perceptible effect, when that matter is applied as manure to other soils. And though it is possible that 

 some qualities in particular soils, unfavourable to vegetation, may be corrected by burning, and that in 

 some other instances the fire may render the clay more nutritive to plants (though I have not been able 

 to trace this, or even to conjecture how it can happen), yet I am much disposed to believe, that its effect 

 as a mechanical mixture in opening the pores of the soil, is the chief improvement that can be derived 

 from the application of burnt clay as a manure. If it has any other effect, it must be from the soot or 

 carbonic matter collected during the operation of burning ; or perhaps it may acquire, by the torrefaction, 

 something of a stimulating quality, that may for a short time promote the growth of particular plants : 

 but these qualities can only be to a small extent, and continue to act for a very limited period." {Far. 

 Mag. xxii. 422.) 



3222. The action of burnt c/ay, according to a writer in The Farmer's Journal, is at least three-fold, and 

 may be manifold. It opens the texture of stubborn clays, gives a drain to the water, spiracles to the air, 

 and affords to the roots facility of penetrating. Clay ashes burnt from turves, containing an admixture 

 of vegetable matter, consist, in some small proportion, of vegetable alkali or potass, a salt which is known 

 to be a good manure. It also, in most cases, happens that a stiff cold clay is impregnated with pyrites, a 

 compound of sulphuric acid and iron. Although the chemical attraction between these two bodies is so 

 strong, that it is one of the most difficult operations in the arts totally to free iron from sulphur, yet a very 

 moderate heat sublimes a large portion of the sulphur. The iron is then left at liberty to re-absorb a 

 portion of the redundant sulphuric acid, which too generally is found in these soils, and thereby sweetens 

 the land ; and it is probable that the bright red or crimson calx of iron, which gives colouring to the 

 ashes when over-burnt, is beneficial to vegetation in the present case, insomuch as it is, of itself, one of 

 the happiest aids to fertility, as exemplified in the red marl strata and red sand strata throughout the 

 kingdom. The evolution and recombination of different gases, no doubt, materially affect the question; 

 but it is reserved for accurate chemical observers to give us an account of the processes which take place 

 in this respect. Curwen notices that clay ashes do no benefit as a top-dressing on grass, which is in part 

 to be explained by reason that the ashes, when spread on the surface of the grass, carmot exert mechanical 

 action on the soil in the ways enumerated. Neither can the calx of iron come so immediately in contact 

 with the particles of the soil, for the production of any chemical effect, as it would do if the ashes were 

 ploughed in. In short, like many other manures which are laid on the surface, unless it contains sojne- 

 thing soluble which may be washed into the ground by rains, it does very little good ; and the feeble 

 proportion of vegetable alkali is probably the only soluble matter the ashes contain. However sanguine 

 may be the admirers of burnt clay, all experience confirms that the most beneficial clay ashes are those 

 which are burnt from the greatest proportion of rich old turf, ancient banks, roots of bushes, and other 

 vegetable matters ; and, I conceive, the value of mere powdered pottery (for such it is) may easily be 

 overrated. (Far. Journ. 1819.) 



3223. The common method of burning clay is to make an oblong enclosure, of the 

 dimensions of a small house (say 15 feet by 10) of green turf sods, raised to the height 

 of 3 or 4 feet. In the inside of this enclosure, air-pipes are drawn diagonally, which 

 communicate with holes left at each corner of the exterior wall. These pipes are formed 

 of sods put on edge, and the space between these as wide only as another sod can easily 

 cover. In each of ^the four spaces left between the air-pipes and the outer wall, a flre is 

 kindled with wood and dry turf, and then the whole of the inside of the enclosure or 

 kiln filled with dry turf, which is very soon on fire ; and on the top of that, when well 

 kindled, is thrown the clay, in small quantities at a time, and repeated as often as nece* 



