SPECIAL SOIL IMPROVERS 91 



germicides of a mild character, such as naphthalene, and even 

 copper sulphate, and zinc sulphate, have been used with 

 ultimately satisfactory results. Recent researches have 

 shown that this treatment involves the destruction of all 

 kinds of harmful organisms, from wireworms or millipedes, 

 down to microscopic forms like the amoebae, paramecia, 

 etc., the larger of which directly injure the plant, and the 

 smaller of which destroy the useful nitrifying bacteria. 

 The destruction of pests soon produces an improvement 

 in the crop, whilst the destruction of the enemies of the 

 nitrifying bacteria results in an increased production of 

 nitrate, with a subsequent increased production of plant 

 growth. Heat also produces chemical and physical changes 

 in the soil. The apparent results of heating the soil with 

 steam are very similar to those of the action of frost the 

 soil becomes lighter, easier to work, easier for the plant to 

 establish its roots, richer in soluble mineral matter, and the 

 organic matter is more easily converted into ammonia and 

 nitrates by the organisms in the soil. The application 

 of heat is certainly the most efficient of these methods, but 

 is not very easy to conduct on a large scale. Direct baking 

 is probably one of the best methods, but steam heating 

 is also very satisfactory. The application of germicides is 

 so much easier to carry out that it has attracted a great 

 deal of attention. Gas lime, the waste product from purifi- 

 cation of coal gas, contains sulpho-cyanides, ferro-cyanides, 

 and other poisonous substances. Occasionally, when the 

 gas lime has been applied to pasture, the iron in the green 

 grass is turned to Prussian blue, owing to the action of the 

 cyanogen compounds in the gas lime. The sulphides and 

 sulphites in the gas lime no doubt also play their part in 

 acting upon all forms of soil pests. After the sulphides and 

 sulphites and cyanogen compounds have been oxidized, 

 the residue acts as a fertilizer. Calcium carbide has also 

 been used. Naphthalene is another favourite soil fumigant. 

 Crude naphthalene is a fairly cheap article, and not difficult 

 to distribute. It is mixed with coke dust, gas lime, or ashes, 

 for the production of many patent mixtures, which usually 



