246 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



against eelworms, though not against fungi, at M/50 to M/10 ; after it has 

 poisoned them it breaks down apparently to sodium formate and 

 ammonia, the former of which acts as a weak alkali and therefore 

 counteracts acidity, while the latter is a valuable plant nutrient. 



Pyridene also gives rise to ammonia in the soil, though here the 

 change is more obscure. 



The nitro- compounds are of special interest because they happen 

 to be available in large quantities at the present time. Some of them 

 have marked toxic properties to eelworms and fungi, but they are also 

 harmful to the plant. Picric acid is one of the commonest of these ; 

 it is at first very injurious to plants, but after a time it disappears from 

 the soil, for the new vegetation shows little or no sign of harmful 

 effects. After a sufficient interval is allowed to elapse the picric acid 

 acts as a potent fertilizer. Some of the nitro- group may become 

 converted into nitrates in the soil, but the evidence is not yet clear. 

 Experiments on this important subject are in hand. 



Besides these systematic investigations, which, as already indicated, 

 are necessarily slow, progress is also possible by enlightened empirical 

 methods, in particular by testing large numbers of substances direct 

 on the growing plant. M. Truffaut has used this method success- 

 fully at Versailles and has obtained results which he published in his 

 journal, Jardinage (fig. 43). Since the armistice the Chemical Warfare 

 Department has furnished us with one or two interesting substances 

 now under investigation. 



One of the best of these is chlorpicrin, which is of great value 

 as a partial sterilizing agent ; it is fatal to eelworms and wireworms, 

 and harmless to plants ; indeed, it promotes root action to a remark- 

 able degree (figs. 44, 45). Further, it can at present be obtained 

 cheaply and in quantity. It is, indeed, an excellent substance for the 

 horticulturist, and its effects are shown in figs. 41, 44. Unfortunately, 

 it is awkward and dangerous to handle; the worker needs a mask, 

 and he may profoundly incommode unfortunate maskless bystanders. 



The work, however, has gone sufficiently far to show that the main 

 lines are right. The analysis of the soil population and the study of 

 the effects of definite chemical substances on the organisms involve 

 long and difficult problems, but in solving them we shall gain that 

 control over the soil population that we need for the most intensive 

 horticultural and agricultural practice. There is a gap, but not an 

 impassable one, between the science of to-day and the practice of to- 

 morrow. If the science is right the practice will come. 



