824 Sterilisation of Soil for Glasshouse Work, [jan., 



well within our limits. As one engine dealt with 40 yards 

 of soil per day, the method has obvious advantages for the 

 large grower. 



In another nursery, where only an old boiler was available, 

 low-pressure steam was blown through a heap of soil 

 covered with sacks.. After some time the soil was thrown 

 into a heap as before ; it was moist, but not over wet. The 

 cost in this case was is. 2d. per ton, but no allowance was 

 made for the boiler (it was not thought worth it), and, of 

 course, the process was slower than the previous one. 



Dry Heat. — Two methods have been suggested : (1) Oven 

 methods; (2) Hot-air methods. Oven methods present some 

 difficulty. Owing to its. bad conductivity soil only allows heat 

 to penetrate slowly, and the temperature of a mass of soil 

 in an oven is for a long time very uneven, the inside remain- 

 ing cold long after the outer layers have got hot. Apart from 

 this loss of economy there is in some forms of steriliser a 

 danger of overheating part of the soil and getting beyond 

 210 0 F., a temperature we see no advantage in exceeding. 



Hot air may be made to pass through the soil, thus obviat- 

 ing the drawbacks to the oven method, but a certain amount 

 of heat is wasted through the evaporation of water from the 

 soil. 



On the other hand, the great advantage of the dry-heat 

 methods is that they can be worked by means of the waste 

 heat from the flues. When this is done there is no expense 

 for firing, and the cost of treatment is reduced simply to 

 labour and outlay on capital, neither of which need be large. 

 Mr. Holmes tells us that he sterilises his soil in this way at 

 $d. per ton, inclusive of everything. The soil is left in the 

 heater for about seven hours, three charges of about a ton 

 each being treated in 24 hours. 



Fuel Requirements and Efficiency. — The amount of water 

 in the soil is a very important factor in determining the cost 

 of the process. Dry soil requires only one-fifth the amount 

 of heat that water requires to raise its temperature. Conse- 

 quently there is a great increase in the fuel expenditure 

 directly one begins to handle wet soil. This fact is brought 

 out by the following results recently obtained by us : — 



