20 INORGANIC MANURES, CONSIDERED 



ingredient in almost all the plants which have hitherto been analysed 

 by chemists. 



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. 



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

 occasionally produces extraordinary effects as a manure, though the 

 rationale of its action does not appear to be thoroughly understood. 

 All animal manures 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. 



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. 



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 between 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 operation, 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 matter ; 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. Quick- 

 lime, in being applied to land, tends to bring any hard vegetable 

 matter that it contains 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, w r hen it becomes mild, operates in the same manner as 

 chalk ; but in the act of becoming mild, it prepares soluble out of in- 

 Boluble matter. It is upon this circumstance that the operation of 



