1912.] Sterilisation of Soil for Glasshouse Work. 825 



Soil. 



Rich cucumber soil. 



Tomato soil. 



Arable soil. 



Moist. 



Drier. 



Air * 

 dry. 



Moist. 



Dry. 



Moist. 



Dry. 



Moisture (per cent. ) ... 



41*6 



25 



5-6 



22*4 





147 



6-o 



bpecinc neat 



0-58 



0-38 



0-23 



0-39 



0*22 



0-30 



0*22 



Thermal units (B.T.U.) re- 

















quired to raise I lb. of soil 





61 





62 









from 52°F. to 212° F. 



92 



37 



35 



48 



35 



No. of lb. of soil heated per 

















lb. of fuel burnt, assum- 

















ing 100 per cent, efficiency 







320 











throughout ... ... ... 



130 



197 



.3 



34o 



250 



340 



From these and -other results we have drawn up the following rule by which the 

 grower can form an approximate estimate of the number of thermal units [B.T.U.] 

 required, if he simply knows the amount of moisture in his soil : — Multiply per cent, 

 of dry soil {i.e., 100 minus percentage of moisture) by 23 if the soil is rich, or 21 if it 

 is poor, add 100 x percentage of water, multiply by 1 '6, and divide by 100— e g. , to find 

 B.T.U. for rich cucumber soil containing 41 "6 per cent, of water, 100 -41 -6 = 58-4 



58-4 x 23=1342 



41 -6 x 100 = 4160 



5502 



1 — = 88. Found by trial, 92. The rule breaks down for dry soils. 



The number of pounds of soil heated per lb. of fuel burnt (100 % efficiency) is obtained 

 by dividing the B.T.U. into 12,000, the calorific power of average quality coal. 



The 100 per cent, efficiency is, of course, unattainable, partly 

 because of the imperfections of boilers and boilermen, but 

 mainly because it involves perfect retention and condensation 

 of the steam, which is undesirable, because it would make 

 the soil too wet. It will be a nice problem for the horti- 

 cultural engineer to see what degree of efficiency can be 

 attained ; the best we have seen as yet is about 30 per cent., 

 i.e., 57 J lb. of soil heated per lb. of fuel, the cost being 

 3§ d. per ton of soil sterilised. 



Chemical Treatment. — In principle the chemical treatment 

 is much the simplest and the cheapest. If the right chemical 

 could be found it would only have to be put into the soil and 

 the work would be done ; there would be no carrying soil to 

 the steriliser and bringing it back again, and no trouble 

 about the efficiency of the heating process. The following 

 requirements have to be fulfilled : the substance must be 

 (1) cheap; (2) volatile or decomposable, so that it disappears 

 when its work is done and causes no subsequent interference 



3 L 



