THE PARTIAL STERILIZATION OF SOILS. 



241 



gallons an acre of water at 200 0 F. in November, and a further 160,000 

 to 240,000 gallons an acre later ; in all equivalent to 10-15 inches of 

 rain. Pumping costs 2d. for 1,000 gallons, heating by a steam injector 

 costs 2s. to 2s. 6d. for 1,000 gallons ; the cost is therefore £4 10s. a 

 house ; or £36 * an acre. The method is not as effective as steam, and 

 plants sometimes take on a curiously mottled appearance indicating 

 some obscure physiological effect. 



However the heating is done, it is costly and inconvenient, and 

 there is no possibility of reducing the cost indefinitely ; the number of 

 thermal units needed to heat a given weight of soil can readily be 

 calculated, and however good the method it can never exceed 100 per 

 cent, efficiency. Even in theory one ton of fuel cannot heat more than 

 130 to 250 tons of moist soil, or 340 tons of dry soil, from 52 0 F. to 

 212 0 F., no matter how the process is done. In practice 1 ton of 

 coke may bake 40 tons of soil for 24 hours, while 12 tons or more 

 are needed to steam 1 acre of soil. 



Chemical methods of partial sterilization are much more convenient 

 and potentially much cheaper, since there is no limit to the reduc- 

 tion in cost as in the case of heat. Many efforts have therefore been 

 made to find suitable agents. A difficulty arises, however, which 

 heat does not present. After a soil has been heated it cools, and the 

 deterring effect of heat is not exercised on the plant. But chemicals 

 may persist ; they may indeed poison the crop. It is essential 

 that the substance disappear from the soil after its work is done, 

 either by evaporation or by one of those remarkable oxidations 

 or decompositions brought about by living or other agencies in the 

 soil. 



In the first experiments at Wye and at Rothamsted volatile 

 antiseptics only were used because they readily evaporate from the 

 soil and present no danger of after-effects. Volatile antiseptics were 

 effective in throwing out of action some of the soil population that 

 impede the action of the ammonia-producing organisms ; they there- 

 fore brought about a larger production of ammonia in the soil and so 

 increased productiveness. 



They were, however, difficult to handle in practice, and the two 

 with which we had most experience — carbon disulphide and toluene — 

 had the disadvantage of being inflammable. The railway companies 

 refused to carry them in ordinary goods trains, and when they finally 

 reached the experimental centres the workmen did not treat them 

 with proper respect. These difficulties might have been overcome — 

 indeed, the ingenious Monsieur Truffaut has overcome them, so far 

 as carbon disulphide is concerned, by making an emulsion ; but a more 

 serious difficulty presented itself. The antiseptics were not always 

 equally effective ; in some cases they were very useful, in others they 

 were not, toluene especially being liable to fail in moist soils. They 

 were nothing like as reliable as heat, for while heat could be depended 

 uponjwith a great degree of certainty, they could not. 



* In this case there are eight houses to the acre ; the more usual number in 

 the district is seven. 



