THE PARTIAL STERILIZATION OF SOILS. 245 





Toxic Dose for 



Eelworms. 



Fungi. 



Protozoa . 



Cresol .... 

 Monochlorcresol 

 Dichlorcresol . 

 Chlordinitrobenzene 

 Sodium cyanide 



M/io 

 M/50 

 M/50 



M/200-M/500 

 M/50 



M/2-M/IO 



M/io 

 M/lO-M/50 

 M/20O-M/500 



M/IO 

 M/io 

 M/50 



M/io 



The work is necessarily slow but it has the merit of certainty. 



When promising substances are found they are examined more 

 extensively. Pot experiments are made. These are slower and 

 require much more space than laboratory experiments ; indeed, they 

 can only be done when the preliminary laboratory experiments have 

 allowed a selection of substances to be made. 



Of the numerous pot and plot experiments now in hand at 

 Rothamsted and the Lea Valley, figs. 41 and 42 illustrate both the 

 possibilities and the dangers. At the commencement of the investiga- 

 tion the so-called liquid carbolic acid (really cresylic acid) was the most 

 convenient of all chemical substances for large-scale use, and it was, 

 and still is, widely adopted. The laboratory experiments, however, 

 have shown that its effectiveness against eelworms and fungi is much 

 improved by introducing chlorine atoms. Pot experiments confirm 

 this result. The dichlorcresylic acid is seen to be much more effective 

 than the cresylic acid in helping tomatos in a soil infested by eelworms 

 or fungi. It also causes greater production of ammonia. It happens, 

 too, to be more convenient to use, and need not necessarily be much 

 more expensive. 



Figure 42, however, also illustrates the dangers of the method. 

 Laboratory experiments indicated that even more striking effects 

 would be produced by adding a nitro group to the chlor- derivative. 

 Chlordinitrobenzene is toxic both to eelworms and fungi in the very 

 small dose lying between M/500 and M/200 ; it is, indeed, the most 

 potent poison we have yet found. Unfortunately it is also highly 

 toxic to plants. In this it resembles most of the other antiseptics ; 

 its distinguishing feature is that it does not easily decompose or dis- 

 appear from the soil, so that for a long period it persists and is liable 

 to injure the plant. 



The question of disappearance from the soil is of very great import- 

 ance. Phenol, cresol, and the chlorcresols quickly decompose and 

 become innocuous ; the nitro- compounds, however, do not. From 

 the chemical point of view, interesting possibilities are presente d by 

 these and certain other compounds which, having acted in the first 

 instance as partial sterilizing agents or soil insecticides, may proceed 

 to decompose and give rise to valuable plant nutrients. Calcium 

 sulphide is an instance ; it is an effective sterilizer and it soon gives 

 rise to valuable calcium carbonate. Sodium cyanide is effective 



