238 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



partial sterilization was discussed in the Masters Lectures for 191 5 

 (see Journal R.H.S., vol. xli. p. 173). It seemed at first to be of 

 academic rather than of practical interest, and the few efforts made 

 at Rothamsted to apply the method on a large scale promised so 

 little success that they were not continued. While productiveness 

 could be increased by this method the result was no better than 

 could be obtained considerably more cheaply by the use of a quick- 

 acting nitrogenous manure. 



But the case is quite different when disease organisms are present. 

 Here some of the soil population seriously interfere with the growth of 

 plants and cannot be put out of action by the simple device of adding 

 manure. Partial sterilization is found to be an effective, and in some 

 cases the only, remedy. The best prospects for partial sterilization 

 in practice are therefore afforded when disease organisms, pests, or 

 some form of soil sickness, have to be dealt with. 



The case first studied was the sick soil of glasshouses. The older 

 method of throwing away this soil is extremely wasteful* In many 

 of the soils examined specific disease organisms occurred ; in others 

 there was no recognized disease, but only the condition vaguely known 

 as sickness. Experiments showed that the killing of some of the soil 

 organisms, whether by heat or by poisons, was followed by an improve- 

 ment in the health of the plant and an increase in the crop. 



The earliest experiments were made with tomatos grown in sick 

 soils supplied by growers in the Lea Valley. The experiments were 

 conducted in pots and gave the following results : — 





Soil B. 



Soil M.C. 



Plant weight. 



Fruit. 



Plant weight. 



Fruit. 



Untreated 



roo 



IOO 



IOO 



IOO 



Heated to 200 0 F. 



163 



204 



188 



255 



Treated with Toluene . 



139 



119 



144 



200 



Carbon disulphide 



1 



Il6 



! 



124 



138 



179 



Much smaller differences, however, were obtained in borders in a 

 commercial nursery where there was no striking amount of disease, 

 and the plants were grown in the usual way, supplied with the " base 

 manure " dug into the soil before planting, and the usual liberal top 

 dressings. 



* A recent examination of cucumber soils thrown out from the houses is as 

 follows : 



Per cent, of dry matter. 

 A B 



Nitrogen 130 08S 



Phosphoric acid (P a 0 6 ) . .103 0 82 

 Equivalent to tricalcic phosphate 2-24 179 

 Potash (K a O) . . . . 0 81 0 69 

 Carbonates (as CaCO s ) . . 5" 2 3 4 29 



A was fairly recently thrown out ; B had been thrown out three years before. 



