18 



(1) Where the crop yield is limited to the supply of nitrogenous 

 plant food, and where therefore an increased production of ammonia 

 in the soil is desirable. 



(2) Where disease organisms and other detrimental forms are 

 present, and the micro-organic population of the soil has lost much 

 of its effectiveness in producing ammonia from the nitrogen com- 

 pounds of the soil. Such soils are known as "sick" soils and are 

 fairly prevalent in certain types of high farming and market 

 gardening. To some extent also sewage sick soils come into this 

 category. 



The first case is the simplest in principle, but the most difficult 

 in practice, from the circumstance that it is already provided for by 

 the various nitrogenous manures on the market. Until the antisep- 

 tic treatment can be made to cost less than a dressing of a nitro- 

 genous manure, it will have no chance against these competitors. 



The second case is more difficult in principle but easier in 

 practice because it is not provided for, and there is a clear field here 

 for the application of antiseptics in practice. 



The following is found to be roughly the order of effectiveness 

 of a number of typical antiseptics : — 



Class 1. Most effective. Formaldehyde, pyridine. 



Class 2. Cresol, phenol, calcium sulphide, carbon disulphide, 

 toluene, benzene, petrol. 



Class 3. Least effective. Higher homologues of benzene 

 (e.g., heavy solvent naphtha), napthalene and certain of its 

 derivatives. 



None of these antiseptics is as good as steam, either in increas- 

 ing the amount of ammonia in the soil, in killing insect and fungoid 

 pests, or in inducing a good fibrous root development. In all trials, 

 therefore, a steamed soil is included to set the standard of excellence 

 previously unattained by antiseptics. 



The following experimental methods have proved useful in our 

 laboratories and may be adopted by the works chemist in sorting out 

 possible antiseptics for practical purposes :• — Some rejected glass- 

 house "sick" soil — the worse its character the better for the experi- 

 ment — is divided into three lots, one is left untreated while the 

 other two are treated respectively with 0*1 and 0*25 per cent, of the 

 antiseptic, care being taken that the admixture is as far as possible 

 perfect. Five experiments are then carried out : — 



(1) Chemical analyses are made at periodical intervals extend- 

 ing over a month, to ascertain the rate at which ammonia and nitrates 

 accumulate in the treated and untreated soils. 



(2) At the same time, bacteriological counts are made by the 

 gelatine plate method to ascertain the rate of development of 

 bacteria. 



(3) Some of each lot is inoculated into test tubes containing a 

 one per cent, hay infusion, and after six days' incubation at 25°C. 

 drops of the infusions are examined under the low power of the 

 microscope for protozoa. If these organisms are killed by the treat- 

 ment, it commonly happens that other harmful organisms are killed 

 also. 



(4) Seeds are sown in the soils and the young plants are care- 

 fully watched to observe the development of "damping off" root, 

 knots, or other diseases. 



