258 SOIL CONDITIONS AND PLANT GROWTH 



mately the numbers of various fungi in the soil. When used 

 for fungi, the plating methods devised for bacteria and for 

 protozoa suffer from the drawback that they involve the break- 

 ing up of pieces of mycelium and the scattering of clusters of 

 spores, with the result that a single fragment of fungus in the 

 soil might appear as hundreds or even thousands of individuals 

 on the plates. 



Finally, there is great difficulty in ascertaining the physio- 

 logical effects of the soil fungi. Because a fungus brings 

 about a particular reaction in a culture medium it by no means 

 follows that the same reaction will occur in the soil where the 

 conditions and the compounds are wholly different. 



In these circumstances the only safe method is to ac- 

 cumulate observations and interpret them cautiously. The 

 existence of a fungus flora is undoubted, and it is largely con- 

 fined to the top 6 inches of soil. Formerly it was supposed 

 (and the view was emphasised by Ramann, 2330) that fungi 

 predominated in acid soils, and bacteria in neutral soils, but 

 the evidence is not beyond criticism : fungi can, however, 

 certainly produce and tolerate acidity. Little can be stated 

 as to relative numbers in grass and arable land, or in poor and 

 rich soils. The species most frequently found in temperate 

 regions, and therefore presumed to be commonest, are the 

 Penicillia and Mucors : in addition Fusarium, Aspergillus, 

 Trichoderma and Cladosporium are common. In the warmer 

 Southern States, however (e.g. Texas), Waksman (292^) found 

 Aspergillus more frequently than Penicillium. In the absence 

 of quantitative methods of estimating numbers or of physio- 

 logical methods of estimating activity it is impossible to say 

 how fungal activity varies with soil conditions. 



Like all other living organisms fungi require sources of 

 energy and of nitrogen. All species can obtain their energy 

 supply .from various carbohydrates, and most species can 

 obtain it from cellulose, in which respect fungi differ from 

 many bacteria. McBeth and Scales (185^) found the most 

 active cellulose decomposers to be the Penicillia, Aspergilli, 

 Trichodermae and Verticillia : on the other hand, the Mucorales 



