416 Alfred J. Exvart : 



only, however, be due to the presence of minute traces of an 

 extremely poisonous heavy metal, such as lead, mercury, cop- 

 per, etc., or even of gold^, barium or arsenic compounds. 



Poisons derived from the soil may be naturally present, or 

 accumulated from the continued use of poisonous sprays, or 

 from the use of poisons to destroy weeds. Half-a-pound per 

 acre represents a concentration of more than 1 per 100,000,000 

 per acre of soil one foot deep, and , this is the limit of 

 toxicity of copper sulphate to the pulp cells of some varieties 

 of apples. A single Bordeaux spraying may consume more 

 than six lbs. of copper sulphate per acre, most of which 

 ultimately finds its way to the soil. In the case of 

 arsenate of lead, some growers spray 3 to 10 times per 

 season, which represents a considerable addition of poison to 

 the soil, and is furthermore repeated annually. Fortunately 

 plants never absorb more than a small fraction of the total 

 amount of any one substance present in the soil, and although 

 good humus soils are very retentive to metallic poisons, there 

 must be a certain annual removal of poison by drainage. It 

 would, however, be interesting to know whether copper, lead or 

 arsenic can be detected in the soils of long sprayed orchards in 

 amounts approaching or exceeding the limits of their toxicity to 

 the pulp cells of apples. 



In order to explain the fact that bitter pit is so much more 

 abundant in well-tended and well-sprayed orchards than in 

 neglected, untended ones, many orchardists have suggested that 

 the ploughing may make the roots grow deeper than normal, and 

 so cause interferences with nutrition. This may be the case 

 if the subsoil contains minute traces of poisons, but otherwise it 

 is not a likely explanation. In the same way the disease has 

 been referred to the use of particular stocks or grafts. The 

 nature of the stock or scion may affect the tendency to a 

 particular disease, just as some organisms are resistant to par- 

 ticular poisons or contagious diseases, but this affords no expla- 

 nation of the cause of the disease in question. In the same 

 way, the fact that cancer seems sometimes to run in families 

 may show that the tendency of the disease may be to some ex- 



Gold ia so readily precipitated in contact with organic matter that the possibility of 

 its absorption from soils seems somewhat remote. 



