114 BELL SYSTEM TECHNICAL JOURNAL 



of the wood. If decay is to be used as a criterion of toxic effectiveness, 

 the importance of eliminating variations in the water content of the block 

 can be fully realized. The wood soil technique offers an excellent means 

 of controlling moisture for studies of wood decay. 



Sand, cotton, sawdust, wood flour, and soils with varying moisture con- 

 tents were also used as supporting substrates, but in no case^was the amount 

 of decay as great as that with the same technique using soil of 20-25% 

 moisture described above. When soils with water contents of 5%, 10%, 

 20-25% and 30% were compared, the moisture contents of blocks in contact 

 with them were 12.8%, 23.9%, 27-30% and 73.9%, respectively. Decay 

 of the blocks was adversely affected by lack of moisture in the first two 

 cases and by full cell saturation of the wood in the last instance. However, 

 if moisture were the only controlling factor the amount of decay of wood in 

 contact with sand should be comparable to that in soil, but this was not 

 the case. 



The Influence of Soil Nutrient or Niitrilites 



Nitrogen in the form of asparagine has been shown by Schmitz and 

 Kaufert'- (1936) to cause an increase in the amount of decay of Pinus 

 resinosa by Lenzites trabea. Since wood contains only about 0.1% to 0.3% 

 of nitrogen, any additional nitrogen received from the soil should promote 

 decay. It might be expected that the soil supplies nitrogenous and other 

 nutrients, nutrilites, vitamins, etc., that accelerate decay. Evidence for 

 this was obtained by comparing the decay of blocks in contact with (1) 

 top soil, (2) top soil that had been leached for several days with hot water, 

 and (3) three artificial soils composed of washed sand and fuller's earth. 

 In this experiment Poria incrassata was used as the inoculum for a test 

 period of 12 weeks. The moisture content of the uninoculated control 

 blocks was 27-30 percent and of the substrate for each series 22 percent; 

 the temperature and time were constant. The average weight loss for the 

 blocks in contact with top soil was 54%, with water extracted top soil 45.4% 

 and the three mixtures of sand-fuller's earth 24.7%. 



It is apparent from these data that the top soil promotes decay to a far 

 greater extent than the sand-fuller's earth mixtures and slightly more than 

 the water extracted top soil, despite the same moisture (fiber saturation) 

 content of the blocks. The actual rate of decay of blocks in contact with 

 the top soil was more than double that of the other mixtures. Therefore, 

 the conclusion may be drawn that nutrients or nutrilites are present in the 

 top soil which stimulate growth of the fungus and promote decay. 



Since the water-extracted soil proved not so favorable for decay as the 

 original top soil, some of the growth-promoting substances must have been 

 soluble in water. The greater decay of wood in contact with the extracted 



