Manures and Manuring 161 



Gas Lime obtained from the gasworks is often used for garden pur- 

 poses. In a fresh state it contains many compounds fatal or poisonous 

 to plant life; but in this state it is a valuable dressing for soil infested 

 with clubroot (Plasmodiophora). It must not, however, be applied in 

 a fresh state to land already carrying a crop. After exposure to the 

 weather for about three months gas lime loses its poisonous properties 

 and then becomes a very useful manure. In composition it may contain 

 as much as 40 per cent of chalk (calcium carbonate) and 15 per cent of 

 slaked lime, but the amount of these varies considerably. About 5 tons 

 to the acre is a fair dressing. 



Other lime manures are shells of various descriptions when ground and 

 obtainable in sufficient quantity. They are valuable for their carbonate 

 of lime and a certain amount of organic matter. 



Since basic slag (see p. 157) has become prominent it is often used 

 instead of lime, and an excellent substitute it is, as it contains large 

 quantities of lime in a mild and useful form. 



It has been found by experiment at Rothamsted that the application 

 of sulphate of ammonia to the soil causes a loss of carbonate of lime 

 (chalk), and growers would do well to bear this fact in mind. About 

 800 Ib. of lime per annum is naturally dissipated from the top 9 in. of 

 the soil by the action of the weather and cultivation, but the application 

 of 400 Ib. of ammonium salts raised the loss to 1045 Ib. The loss of lime 

 was still further increased to 1429 Ib. per acre by the application of 400 Ib. 

 of ammonium salts and superphosphate. It is therefore a simple matter 

 to rob a soil of lime simply by the careless or injudicious application of 

 sulphate of ammonia, superphosphate, and other manures. 



Gypsum (Calcium Sulphate). This is well known as the source of 

 plaster of Paris. As a manure it is rarely used by itself, but it is largely 

 applied in the form of superphosphate. It is thought that the presence 

 of gypsum in the soil not only increases the solubility of the potash, 

 but also prevents the loss of nitrogen (in the form of ammonia) from 

 stable manure. Some authorities doubt this; but in any case gypsum 

 would scarcely pay for special application. It is favoured for light sandy 

 or gravelly soils, from 2 to 3 cwt. per acre being considered a reasonable 

 dressing. 



8. MISCELLANEOUS MANURES 



There are few substances beyond those already mentioned used as 

 manures, simply because there is very little to be obtained from them, 

 or because the foods they yield are generally present in superabundance 

 in the soil. 



Magnesium Salts are sometimes applied as a potato manure, as mag- 

 nesium carbonate or magnesium sulphate (otherwise Epsom salts). Mag- 

 nesium occurs in the ash of all plants (see Tables at p. 109), and is returned 



to the soil in farmyard and other natural manures. 



VOL I. 11 



