43 E. G. Mulder: Copper and Molybdenum 



pot and field experiments with cereals it was found that soils con- 

 taining 0.6 [xg of available copper or less, per gram of air-dried soil, 

 produced cereals with pronounced symptoms of the reclamation 

 disease. With 0.6-1.5 [xg slight symptoms of the disease were seen 

 whereas with 2 [xg and more available copper per gram of soil a nor- 

 mal plant growth was generally obtained. 



From the results of these three series of experiments it was con- 

 cluded that the reclamation disease is brought about by a lack of 

 available copper in the soil. 



Plants growing in copper-deficient soils had to be supplied with 

 amounts of copper more than a hundred times higher than those 

 required by cultures in nutrient solutions. By using a special 



Table 5: Copper content of five weeks old oat plants (^sterns and leaves): — 



Cu, mg/kg 

 Soil no. Condition of plants in previous pot experiments dry weight 



1 Pronounced disease symptoms 1.9 



2 Somewhat less pronounced disease symptoms 3-5 

 1 Provided with 50 kg/ha CUSO4, normal plants 8.1 



3 Pronounced disease symptoms 2.4 



4 Slightly diseased 5-0 



4 Provided with 50 kg/ha CUSO4, normal plants 8.5 



5 Slightly diseased 5.7 

 5 Provided with 50 kg/ha CuS04, normal plants 8.0 



technique it was shown that these differences were due to a 

 fixation of copper by the black humus present in many copper- 

 deficient soils. In this experiment oat plants were grown with 

 part of their roots in a copper-deficient soil and with the other part 

 in a nutrient solution. When the copper was added to the solu- 

 tion, very small amounts resulted in normal plant growth, but 

 much larger quantities were required if added to the soil. 



In using the Asfergillus assay it was shown that copper 

 fixed by the black humus was not available to the fungus. 



In further experiments with Aspergillus niger, as well as with 

 cereals in nutrient solutions, it was shown that copper precipitated 

 by hydrogen sulphide producing bacteria was unavailable to both 

 the fungus and the higher plants. As copper sulphide can be 

 used as a copper source by Aspergillus and cereals the copper pre- 

 cipitated by HoS-producing bacteria apparently was present in a 

 compound other than CuS. 



