Gontrol A:3 



the film of vrater and the nematode cuticle. 3) It must kill or at 

 least disable the nematodes, h) Since the ultimate object of treating 

 the soil is the growing of crop plants, no residue toxic to plants must 

 be left in the soil after a reasonable waiting period. 



The first requirement is best met by gases. These are difficult to 

 confine in large scale work, though this can be done on a small scale. 

 The next most favorable material is vapors or fumes of volatile liquids. 

 If very large amounts of water are available, it is possible to dis- 

 tribute a water soluble chemical through the upper layers of the soil. 

 Thorough mixing of a solid (powder or granule) with the soil is possible 

 only on a small scale or to a depth of not more than 6 or 8 inches id-th 

 ordinary farm machinery. 



The water film around the nematode will be penetrated by any substance 

 even slightly soluble in water. The lipoidal cuticle is best penetrated 

 by fat solvents. 



The fourth requirement (lack of phytotoxicity) may be inherent in the 

 nematocide used. However, if the substaiice is at all toxic, it must 

 either: 1) be dissipated into the atmosphere] 2) decoii5)ose, forming 

 harmless compounds^ 3) be leached out of the soil. 



Materials which are gases at ordinary temperatures must be confined by 

 an impervious cover over the soil. Methyl bromide (boiling point k° C) 

 is used in this way for seedbeds and other comparatively small areas. 



The most successful nematocides are volatile materials. They may be 

 applied as solutions, emulsions, powders, or granules, but the active 

 ingredient spreads in the soil as a volatile agent. Distributed at 

 intervals through the soil, the chemicals diffuse evenly in all 

 directions — downward, laterally, and upward. As they diffuse, the 

 vapors pass over soil particles and soil organisms, each surrounded 

 by a thin film of water. As conditions are ideal for solution of the 

 chemical vapors in the water, they make a nearly saturated solution 

 until equilibrium is reached. 



It is evident that this simple and automatic process is superior to 

 anything that can be done with a non-volstile nematocide. It would be 

 difficult to get as thorough a distribution of such a chemical even if 

 it were water soluble. Obviously, the mixing of a non-volatile solid 

 nematocide as thoroughly in the soil would be far more laborious, even 

 if machinery capable of doing such a job were available. 



Procedure in application of liquid and solid-carrier soil fumigsnts is 

 designed to make the best possible use of the fumes. For large scale 

 wor'-, tractor applicators which deliver the fumigant in continuous 

 streaias spaced at intervals in the soil are used. Generallj'-, linos of 

 fujninant are f^paced at about 12-inch intervals, thougli 10-inch inter- 

 vals ore usnd in some soils. 



In e:>"norimontal work, spacin;- and application rates are varied Kyste- 



