466 SCIENCE OF AGRICULTURE. Part II. 



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

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

 iiorin 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 tlie 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 bfeen brought 

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

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

 are removed." 



2982. The action of burnt clay on the soilj is thus described by the same author. " It 

 naust be obvious to every person that 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 ad- 

 mixture 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 protection of redundant moisture, and to the spread- 

 ing of the roots of vegetabTSs 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. Be- 

 sides 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 favorable 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 way, 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. 



2983. The calcareous matter in the soil, it is said, 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 Umestone 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, unfavorable 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 mixtwre 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. XX ii. 422.) 



S984. The action of burnt clay, 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 burned from turves, containing an admixture of vegetable matter, con- 

 sist, 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 impreg- 

 nated 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 opera- 

 tions 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 

 coloring 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 is exemplified in the 

 red marl strata, and red sand strata throughout the kingdom. The evolution and recom- 

 bination 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, cannot exert the 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 producing of any chemical effept, as it would do if the ashes were ploughed 



