134 BELL SYSTEM TECHNICAL JOURNAL 



Since these results indicate that the soil-contact provides optimum conditions 

 for decay, the laboratory method serves as a means of quickly eliminating 

 inferior preservatives and minimizing the number for field studies. 



Summary 



The soil-contact method described in this paper has been shown to be a 

 valuable laboratory tool for the study of fungus destruction of cellulose and 

 wood and for the determination of the value of wood and cellulose preserva- 

 tives. 



Top soil containing 20 to 25% moisture on a dry-weight basis, when used 

 as a supporting substrate for decaying wood, proved to be an excellent means 

 controlling the moisture content of wood during the decay process. In- 

 vestigation showed that the optimum moisture content for initiating the 

 decay of wood was fiber saturation. It also was found that during decay 

 the initial water content of the wood remained constant, through main- 

 tenance of a constant volume of the wood structure despite loss of wood sub- 

 stance. 



Experiments with various combinations of nutrients and nutrilites added 

 to artificial soil showed the importance of these materials in decay studies. 

 The need for nitrogen in the destruction of cellulose by fungi was confirmed. 

 Lack of wood decay in the presence of a sugar when there is also a deficiency 

 of nitrogen presents an interesting problem the explanation of which may 

 throw considerable light on discrepancies in many test procedures. Com- 

 parison of results with nutrient artificial soils and an average top soil in- 

 dicates the possibihty of employing a standard artificial soil in the contact 

 test method. 



The optimum temperature for most wood-destroying fungi tested was 

 found to be 26°-28°C. Decay occurred over a wider range of temperature 

 in soil-contact tests than in petri dish tests. 



It was found that decay was much more uniform and more rapid in the 

 soil-contact method than in other laboratory methods or in field trials. 

 There is a large, single, vigorous inoculum in the soil-contact laboratory 

 method, while in the field antagonism between wood-destroying organisms 

 and the other flora and fauna of the soil frequently checks the decay process. 



Toxicity studies based on petri dish assays showed that the amount of a 

 compound tolerated by several fungi varies considerably. Petri dish assays 

 of toxic materials are often misleading. Generally, higher retentions of the 

 preservatives are needed to prevent decay than are indicated by petri dish 

 assay. Occasionally, a material which performs poorly in the petri dish 

 test will, however, act as a satisfactory preservative of cellulosic derivatives 

 in both soil-contact and field tests. 



Field trials of preservatives, though in general less rapid, confirm the 

 results of the soil-contact method and in addition determine the degree of 



