Rothamsted Symposium on Trace Elements 42 



plants were harvested when the first disease symptoms were ob- 

 served. Tables 4 and 5 show some of the results. 



3) In a number of normal and diseased soils the plant-avail- 

 able copper was estimated by a microbiological assay method. 

 This assay is based on the fact that the fungus Aspergillus niger 

 requires small amounts of copper for the formation of normal 

 black spores. In a nutrient solution purified from copper by char- 

 coal treatment Aspergillus develops a white sterile mycelium, 

 with 0.2 [xg of Cu in 40 ml of nutrient solution yellow spores are 



Table 4 : Cofpr content, mg per kg dry weight, of wheat (j/aiti), 

 grown on different soils. — 



formed, with 0.4 (xg the colour of the spores is yellowish-brown, 

 with 1.0 [xg grey-brown and with 1.5 [xg grey-black. By contrast 

 2.5 [xg and higher amounts of copper induce normal black spores. 

 Thus, the color of the Asfergillus spores provides a measure of 

 the amount of available copper in a nutrient medium. 



For the estimation of the available copper in soils, one gram 

 of air-dried soil is added to 40 ml of a purified nutrient solution in 

 an Erlenmeyer flask. This medium is inoculated with a suspen- 

 sion of Asfergillus spores and after four days incubation at 30° C 

 the colour of the mycelia is compared with the colour scale of a 

 series of standard cultures to which copper in increasing amounts 

 from 0-2.5 [xg per culture has been added. 



By comparing the results of the Asfergillus assay with those of 



