240 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



sition, thus greatly facilitating the work of the soil organisms. A 

 steamed soil is therefore not only a healthier but also a richer medium 

 for plant growth than a corresponding untreated soil. There is 

 a tendency to retardation of germination and to some rankness of 

 growth in heated soil, but this can be overcome by modifying the 

 manuring and by growing easily setting varieties such as ' Ailsa Craig.' 

 But so great are the advantages that many growers of cucumbers and 

 tomatos in the Lea Valley now heat their soil as a regular part of 

 their procedure, either by steam or hot water (figs. 39, 40). 



Unfortunately heating the soil is not only inconvenient but also 

 costly. In the palmy days before the War the lowest cost at which it 

 had been done so far as I know was £24 an acre. Present costs are 

 very considerably above this. Two statements taken from the books 

 of large commercial nurseries showing the cost for a house in winter 

 191 8-19 was as follows : — 



» 



Nursery No. I. 



Nursery No. 2. 







£ s. d. 



Labour* 



1200 



4 10 0 



Cokef ...... 



1980 



410 



Boxes, boilers, &c. .... 



500 



400 



Total for one house 



36 8 0 



12 11 0 



Total for an acre 



300 0 0 



87 17 0 



Time taken for a house (about } acre) 



16 days 



3 days 



Nursery No. I was badly infested so that work could proceed 

 only very slowly. Nursery No. 2 was much more lightly affected ; 

 the crop was 35 tons an acre, which is quite good. After steaming, it 

 was raised to 40 tons an acre, about as good as a commercial grower 

 hopes to attain. It is doubtful whether steaming would be worth 

 while in a house where the yield is already 40 tons an acre, as in these 

 circumstances there can be no serious number of disease organisms or 

 pests. 



Three general methods of heating the soil have been adopted : — 

 X. By steam, using the tray or box method. \ This is applicable 

 for large-scale work ; the tray method being suitable for soils heated 

 in situ such as tomato borders, and the box method for soils that have 

 to be moved, such as cucumber borders. 



2. By baking, using waste heat from flues or boiler, or in a special 

 oven heated by coke. This is most convenient for small scale work 

 such as the purification of soil needed for the raising of seedlings in 

 nurseries where the damping-off fungus causes trouble, and the treat- 

 ment of composts in private glasshouses. 



3. By hot water, using water from the pipes, heated if necessary 

 in a supplementary cylinder. 



One grower, who successfully adopts this method, applies 64,000 



* 2 \ men each 36 hrs. @ 1$. | 1 5 CWt. a day @ 365. a ton. 



X See Jour. Board of Agriculture, 1913, vol. xix. p. 809 and 1914, vol. xxi. 

 p. io2, for the essential features ; there are, however, modifications in detail 



introduced since these papers were written. 



