62 INORGANIC MANURES, CONSIDERED 



198. Marl is carbonate of lime mixed with clay at the rate of from 

 twenty to eighty per cent of carbonate, with alumina, silica, and more or 

 less of the oxide of iron. Its action on the whole is similar to that of chalk, 

 though it is more adapted for sandy and peaty soils than for clays. It is 

 found from experience that it is injurious when spread on soil before being 

 exposed for some months to the action of the atmosphere ; though the reason 

 of this has not yet been explained. 



. 199. Gypsum, which is sulphate of lime, is a calcareous compound which 

 occasionally produces extraordinary effects as manure, though the rationale 

 of its action does not appear to be thoroughly understood. All animal ma- 

 nures contain more or less of sulphate of lime as one of their constituents ; 

 and this mineral compound has also been found in wheat, in clover, saintfoin, 

 lucern, and many other leguminous plants, and in various pasture grasses. 

 Hence it may in part be considered as a specific manure, and it has been so 

 treated by Grisenthwaite in his very ingenious Essay, who contends that no 

 manure that does not contain gypsum is fit for wheat. It is said to have 

 little effect except upon light sandy, gravelly, or chalky soils. 



200. Sea shells are very abundant on some shores, and may be either burned 

 into lime or laid on without burning. Immense quantities are collected on 

 the shore at Whitstable, in Kent, and are laid on the soil without burning 

 between Canterbury and Dover, where the soil is chiefly clayey. They are 

 so much preferred to chalk or lime that they are fetched three times the 

 distance. 



201. The rationale of the action of lime in its different states is thus given 

 by Sir Humphry Davy. " When lime, whether freshly burned or slaked, is 

 mixed with any moist fibrous vegetable matter, there is a strong action be- 

 tween the lime and the vegetable matter, and they form a kind of compost 

 together, of which a part is usually soluble in water. By this kind of ope- 

 ration, lime renders matter which was before comparatively inert nutritive ; 

 and as charcoal and oxygen abound in all vegetable matters, it becomes at 

 the same time converted into carbonate of lime. Mild lime, powdered 

 limestone, marls or chalks, have no action of this kind upon vegetable mat- 

 ter ; by their action they prevent the too rapid decomposition of substances 

 already dissolved ; but they have no tendency to form soluble matters. It 

 is obvious from these circumstances that the operation of quicklime, and 

 marl or chalk, depends upon principles altogether different. Quicklime, in 

 being applied to land, tends to bring any hard vegetable matter that it con- 

 tains into a state of more rapid decomposition and solution, so as to render it 

 a proper food for plants. Chalk, and marl, or carbonate of lime, will only 

 improve the texture of the soil, or its relation to absorption, acting merely 

 as one of its earthy ingredients. Quicklime, when it becomes mild, operates 

 in the same manner as chalk ; but in the act of becoming mild, it prepares 

 soluble out of insoluble matter. It is upon this circumstance that the ope- 

 ration of lime in the preparation for wheat crops depends ; and its efficacy in 

 fertilising peats, and in bringing into a state of cultivation all soils abounding 

 in hard roots, or dry fibres, or inert vegetable matter. The solution of the 

 question, whether quicklime ought to be applied to a soil, depends upon the 

 quantity of inert vegetable matter that it contains. The solution of the 

 question, whether marl, mild lime, or powdered limestone, ought to be ap- 

 plied, depends upon the quantity of calcareous matter already in the soil. 

 All soils are improved by mild lime, and ultimately by quicklime, which do 



