i9 1 2.] Sterilisation of Soil for Glasshouse Work. 823 



because by so doing- he would recover some of the manurial 

 residues it contains. We can therefore seriously consider 

 methods costing 2s. a ton, and, of course, the lower we can 

 get the price the better. 



Sterilisation by Heat. — For a variety of reasons we consider 

 that partial sterilisation by heat is the most effective method 

 at present available. We say at present advisedly, because 

 we have not yet examined all possible methods of chemical 

 treatment, the advantages of which are obvious. The soil 

 should reach a temperature of 200 0 F., but need not pass 

 210 0 F.; it should be maintained for an hour or more at a 

 temperature exceeding 180 0 F. It is immaterial, so far as 

 our present results go, whether dry heat or moist heat is 

 used, i.e., whether the soil is baked or steamed, but if steam 

 is used the soil must not be allowed to get too wet, but the 

 steam must blow through at a sufficient rate to prevent com- 

 plete condensation. After sterilisation the heap should not be 

 exposed more than necessary to the rain, as plant food rapidly 

 begins to form, and is very liable to be washed out. 



Steam Heat. — Growers in this country and in the United 

 States have for some time known that steaming the soil killed 

 certain pests, and they have devised various methods of 

 carrying out the operation. Probably the best plan is to blow 

 high-pressure steam through a harrow-like implement with 

 jets corresponding to tynes buried in the soil. The most 

 economical pressure has yet to be determined; we saw 

 the method worked at some large nurseries with steam 

 pressure at 80-90 lb., and found that the temperature of the 

 soil rose in two or three minutes to 212 0 F., and the soil 

 remained nearly as dry as before. In this case the heating 

 was continued for 15 minutes, and the soil then thrown into 

 a heap, but so slowly did it cool (soil being a very bad 

 conductor of heat) that even after five hours the temperature 

 remained at 160 0 F., while at the end of a week it was still 

 75 0 F. Account was kept of the cost of the operation, in- 

 cluding the hire of the engine and all other charges, and 

 it worked out to is. 6d. a cubic yard, which at the 

 modest reckoning of only 20 cwt. to the cubic yard,* is still 



* Rich cucumber soil is lighter than ordinary arable soil which would be nearer 

 20 cwt. per cubic yard. 



