special Chemicals for Treatment of Soil 299 



Many bacteria are able to resist these treatments and soon begin to 

 multiply rapitlK after the disinfectant has been removed. Active 

 multiphcation of these bacteria results in extensive decomposition 

 of the soil organic matter, which is accompanied by abundant lib- 

 eration of ammonia. The latter is used by plants, resulting in in- 

 creased plant growth. Partial sterilization of soil may, therefore, be 

 compared to fertilization with nitrogen. The effect of partial sterili- 

 zation of soil with toluol and heat upon the bacterial and fungus 

 popidations, as well as upon the liberation of the nitrogen in an 

 available form, is shown in Fig. 117. 



Several theories have been proposed to explain the effect of par- 

 tial sterilization in increasing the fertility of the soil. It has been 

 asserted by some that disinfectants, when used in small amounts, 

 have a direct stimulating effect upon microorganisms and plant 

 roots; others have assumed that the toxins of the soil are destroyed 

 by such treatments; still others have emphasized the destruction of 

 plant-pathogenic fungi and bacteria in the soil. The theory that has 

 received the greatest consideration, in an attempt to explain the 

 phenomenon of partial sterilization, is the "protozoan theory of soil 

 fertility," discussed previously. 



Use of Special Chemicals for Treatment of Soil 



A number of chemical compounds have been recommended for 

 control of various fungi and nematodes in the soil. The saturation 

 of soil with formaldehyde to prevent spreading of disease-producing 

 organisms has often been practiced. Formaldehyde in concentra- 

 tions of 0.045-0.05 per cent was found to give very good results in 

 combating the sugar-beet nematodes. It is difficult to reach the 

 nematodes at a depth lower than 60 cm, and it is difficult to cause 

 the poison to penetrate the whole mass of soil. The nematodes 

 present in the lower depths of soil and in the form of cysts can be 

 made to develop and come nearer the surface by the use of catch 

 crops and chemical stimulants. In absence of the host plant, the 

 nematode larvae die off. Formalin, sometimes following crude ben- 

 zol treatment for control of potato wart, mercury bichloride, and 

 other disinfectants are recommended for control of various plant 

 diseases. The treatment of soil with a 1:1,000 or 1:1,200 solution 

 of mercuric chloride was found to be effective in controlling root 

 maggot, black rot, club root, and damping-off diseases. 



Arsenic, mixed with ashes for soil dressing, is rather widely em- 

 ployed in China for destruction of worms; a similar practice is used 



