MICRO-NUTRIENT PROBLEMS 23 



tages where work on fungi is concerned. It does not involve the 

 introduction into the solution of calcium, an element of which the 

 necessity for fungus nutrition is, as we have already seen, 

 doubtful. Indeed, it appears to effect a pretty complete re- 

 moval of any calcium present as impurity in the nutrient solu- 

 tion. Also the use of an autoclave is unnecessary, heating for 

 20 min. at 100° C. being sufficient to precipitate the heavy 

 metals. Care has, however, to be taken to avoid excess of the 

 basic magnesium carbonate, as otherwise more or less complete 

 removal of phosphate may result. 



Bortels (1927) similarly purified the nutrient solution by pre- 

 cipitating the traces of the heavy metal contaminants with a 

 small quantity of ammonium sulphide and adsorbing the 

 precipitate on charcoal. Actually the ammonium sulphide 

 appears to be unnecessary, according to Roberg (1928), who also 

 purified the charcoal from mineral ash constituents by a pre- 

 liminary treatment with acid. However, Steinberg pointed out 

 that removal of the ash constituents appears to reduce the 

 adsorbing power of charcoal, and he concluded that the use of 

 charcoal is only advisable when for some reason it is essential 

 to avoid the use of an alkaline earth compound. 



In 1927, Hopkins and Wann made use of the adsorptive pro- 

 perty of calcium phosphate for the removal of iron from culture 

 solutions for the green alga Chlorella and later, for work with 

 green algae and Lemna, Hopkins (1934) again employed calcium 

 phosphate as an adsorbent for removal of traces of manganese 

 from the nutrient solution. Sakamura, who had previously 

 (1933, 1934) used charcoal for removal of traces of heavy metals 

 from the nutrient medium of Aspergillus spp., concluded (1936), 

 by polarographic examination 1 of nutrient solutions after treat- 

 ment with charcoal and calcium phosphate respectively, that 

 the latter effected a much more complete removal of the heavy 

 metal contaminants, a conclusion which was confirmed by growth 

 experiments. His procedure was as follows. Calcium phosphate 

 was first purified by washing with water distilled in a glass still, 

 a suspension of 50 g. calcium phosphate in a litre of distilled 

 water being shaken for 5 hr., during which time the water was 



1 See this chapter, p. 27. 



