250 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



kill the vines as well. The treatment cannot be successfully applied on 

 such a soil in its dry condition. On the other hand, diffusion is slowest 

 in heavy wet clay soil ; and when such soil is saturated with water it 

 is almost entirely prevented. Moisture lowers the temperature and 

 decreases the permeability of the soil ; it also prevents the evaporation of 

 the liquid, and thus retards diffusion. Between the two extremes there 

 is a medium condition of moisture which is most favourable for 

 treatment. 



Character of Soil.-— Sandy soil permits an even but too rapid diffusion 

 of the vapour. Rocky soils are not of even texture, and naturally the 

 vapours follow the lines of least resistance. Heavy clay soils, when very 

 dry, are usually much broken by cracks and fissures, which may run from 

 the surface to a considerable depth. Through such fissures the vapour 

 escapes rapidly without permeating the soil to any extent, and its in- 

 secticidal value is therefore slight. But when such a soil is well 

 moistened it is even in texture and very favourable to treatment. 



Depth of Soil. — The depth of the soil is an important factor in 

 determining how much carbon bisulphide must be used for a given 

 area. If the soil is shallow and the subsoil very dense and impel 

 vious, it is evident that much less liquid will be required to produce a 

 death atmosphere than will be needed in a soil of much greater depth. 

 In soils of the same character and condition the amount needed will be 

 proportional to the permeable depth of the soil. In heavy, compact soils 

 increase the number of injections and diminish the dose ; in light, deep, 

 permeable soils decrease the number of holes and increase the dose. 



Amount to use. — In field experiments with the Grape, using plain 

 carbon bisulphide in " quite fresh " soil, vines were found to withstand 

 105 c.c. of carbon bisulphide (4*4 oz. nearly), divided equally among three 

 holes placed about 16 inches from the base of the vine and at a depth of 

 about 20 inches ; but 180 c.c. (1\ oz.) proved fatal to the vines. In a 

 warmer, drier, more shallow soil a dose of 90 c.c. per vine, similarly 

 placed, proved fatal. After considerable rain, when the ground was quite 

 wet, a vine withstood 260 c.c. of carbon bisulphide, and some vines are 

 said to have withstood 400 c.c. 



Conditions favourable to Treatment. — The treatment should never be 

 applied for some time after ploughing or cultivating, as a firm, compact, 

 moist surface is much more favourable to the retention of the vapour. 

 For the same reason about fifteen days should be allowed after treatment 

 before cultivation is resumed. If the soil is either very wet or very dry, 

 treatment should be withheld. To be in the most favourable condition 

 for treatment the soil should be quite moist and moderately permeable, 

 with a firm even surface, well compacted by rain, and having a depth of 

 at least eight inches. 



Extent of Diffusion. — The extent of diffusion of the vapour determines 

 the distance apart at which the injections must be made in order to reach 

 all parts of the soil evenly and effectively. This varies considerably 

 with the amount of the dose, the temperature and humidity of the 

 soil, and other conditions. It has been found more satisfactory to 

 employ small and frequent doses than a few large ones. A dose of 

 five or six grams (\ to ] oz.) is believed to be thoroughly effective 



