LETHAL GASES 177 



Fig. 65 shows that flowing 3 cubic feet of Yonkers gas through the soil 

 leaves a residue that kills the root system and the lower part of the tomato 

 plant set in the soil, but that flowing 10 cubic feet of ethylene through the 

 soil does not leave a killing residue. 



It is of interest in replanting trees and shrubs in place of those killed by 

 illuminating gas to know how long the toxic residues remain in the soil and 

 what are the best means of getting rid of them. Fig. 66 shows that gassed 

 soil sealed in a can at 24° C (75° F) loses practically all its toxicity within 



Figure 66. Effect of low temperature storage on the toxicity of soil through which 

 illuminating gas had been previously passed and which was then sealed for one week. 

 Left to right: control soil -15° C (5° F), gassed soil -15°C (5° F), gassed soil 3° C 

 (37° F), gassed soil 24° C (75° F). The tomato plants in gassed soil previously stored 

 at -15° C (5° F) and at 3° C (37° F) were killed, but the soil kept in the laboratory at 

 24° C (75° F) caused only a slight retardation of growth. 



a week, whereas similar soil samples sealed at —3° and —15° C (26° and 

 5° F) do not lose their toxicity. At proper temperatures and other con- 

 ditions for growth the microorganisms of the soil use the hydrocyanic acid 

 — which probably is mainly cyanides because of neutralization by bases 

 of the soil — as a nitrogen source. Leaching the soil with water removes a 

 part of the residual toxic substance. Fig. 67 shows the gro"^i:h of a tomato 

 plant in gassed soil that has been leached ^A-ith water equal to ten times the 

 volume of the soil, in contrast to the gro^^i:h of a similar plant in gassed 

 soil ^\'ithout leaching, and in soil not gassed. It is evident that leaching 

 removed a portion of the poisonous residue, but there are still substances 

 that reduce the growth rate far below that of the check. Leaching is not a 

 very effective way of removing the toxic residues, and it might be even less 

 effective outside where the soil is likely to be packed and poorly drained. 

 In practice, it is probably best to remove the gassed soil and replace it with 

 good soil. If this is not feasible, the soil could be loosened up to the proper 

 depth and limed if it is acid, after which two to three weeks should elapse 

 before planting. In case of acid soils, the lime will neutralize any residual 

 HCN and increase the activity of organisms that consume the cyanides. 



