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A Wood Soil Contact Culture Technique for Laboratory Study 
of Wood-Destroying Fungi, Wood Decay and Wood Preservation 


Limitations imposed by other biological test methods have largely been over- 
come by using autoclaved top soil for the substrate and pure cultures of the decay 
organisms. The use of soil was the direct result of observations on the rapid 
decay of wood in contact with soil in laboratory termite colonies. 

Development of a wood-soil contact culture technique as a result of these 
observations furnished an excellent laboratory tool for further research on the 
biological factors promoting and the preservative compounds proposed for pre- 
venting decay. Research on the factors promoting decay showed not only that 
the average top soil furnishes nutrients and nutrilites in the quantity and propor- 
tion highly favorable to many decay organisms but also an effective means of 
regulating the water content of wood or cellulose during the decay period. 

Comparisons between laboratory and field results showed the amount of decay 
obtained by the wood soil contact technique to be more rapid and uniform than 
decay in the field. The severity of the exposure in the laboratory ensures imme- 
diate eliminations of compounds unworthy of further more expensive field 
studies and evaluates compounds in the same order of effectiveness. 

Comparisons and evaluation of wcod and cellulose preservatives plus artificial 
weathering cycles followed by exposure to the method will provide valuable infor- 
mation on initial toxicity and permanence thereby affording a sound basis for the 
engineering selection of preservatives for a variety of purposes. 

LABORATORY tests for evaluating fungicides are often used as a means 
of predicting field results and for investigating the action of cellulose 
and wood-destroying fungi. Of the several laboratory procedures hitherto 
devised for these purposes, however, none has been entirely adequate. 
This has led to incorrect interpretation of laboratory assays of fungicidal 
compounds, with attendant misapplication of preservatives. The con- 
fusion and misunderstanding concerning the use of preservatives have been 
further increased by the misapplication of the laboratory procedures them- 
selves. A brief review and explanation of some procedures and their applica- 
tion will clarify these statements. 

Minute quantities of toxic agents and growth-promoting substances 
which are not readily detected by known chemical analyses may be deter- 
mined by bio assay methods, the value of which depends upon a prior 
determination of the reaction of one or more organisms to known quantities 
of these substances. Another bio assay is the so-called "acceptance test" 
for fungicides, by which the fungus resistant qualities of materials impreg- 
nated with fungicides may be determined. Since fungus resistant qualities 
are the primary concern in such a test, the identity and quantity of the 
preservative are of only incidental interest. However, the identity, fungus- 
proof qualities and quantity of fungicidal compounds are important when 



laboratory procedures are devised for comparing effectiveness in the develop- 
ment of different preservatives. In addition, the chemical and physical 
properties of the different preservatives must be considered for the deter- 
mination of their subsequent behavior when exposed to a variety of en- 
vironmental conditions. Bio assays may thus be used for quantitative, 
qualitative, comparative, or predictive purposes. 

In order to survey existing tests, it may be helpful to classify them. 
There are three groups of rather ill-defined laboratory methods based on the 
nutrient and physical properties of the substrate. The first group is com- 
prised of those methods in which an agar or similar base is used. Various 
nutrients or nutrilites* may be added to this base 1 , and prior to inoculation 
with one or more fungi the preservative may also be added. This group 
includes the standard petri dish test described by Richards 2 , 1923, which 
has had extensive use in the field of wood preservation. The carbohydrate 
source in the standard petri dish method was malt sugar. Later, in response 
to the requests by industry, Richards attempted to substitute wood flour 
as the nutrient. However, the radial fungus growth used as the criterion 
of toxicity was very sparse and thin and the substitution of wood flour for 
sugar was discarded. It is of interest to record here that Richards also 
summarized the previous work on toximetric tests of wood preservatives. 

The second group includes those methods in which the preservative is 
added directly to a cellulose material before exposure to organisms. The 
preserved material may be the only source of nutrient for the fungi, or a 
piece of similar untreated material may be provided. Such a method is 
described in a paper by Waterman, Leutritz and Hill 3 , 1938. No agar is 
used, and the untreated wood is supported over water by mechanical means. 
When agar is used to support the preserved material and to supply water, 
nutrients, nutrilites or combinations of each of these may be added to the 
agar. This may be done in several ways, among which are the kolle flask 
method for wood preservatives described by Falck 4 , 1927, the standard 
method of the American Society for Testing Materials for testing fabrics 5 , 
1942, and the present Signal Corps test of fungicidal coatings 6 , 1943. Of 
these, the first two methods are used chiefly as "acceptance" tests by de- 
termining the fungus-proof qualities of fungicidally treated wood and fabrics. 
They are also used in development work for comparison and for predicting 
the field behavior of preservatives when supplemented by artificial weather- 
ing cycles. The Signal Corps test is used as an acceptance test of fungi- 
cidal coatings which are sprayed on electical equipment. Since the criterion 
is the inhibition of fungus growth at some distance from a paper impreg- 

* Nutrients here include the sugars and compounds used by the fungi for food purposes, 
and nutrilites will be referred to in this paper as those compounds necessary to fungus 
nutrition, such as vitamins, growth substances and minerals, Williams, R. J., 1928. (See 
Bibliography at end of this paper.) 


nated with the fungicidal coating it is fundamentally a quantitative measure 
of the amount of fungicide which diffuses into the agar from the impregnated 
paper specimen. 

A third group of test procedures employs soil or soil suspension in con- 
junction with the preservative materials. Here the soil furnishes an active 
microbial culture and supplementary nutrients and nutrilites. The soil 
suspension method has been described by Furry, and Zametkin 7 , 1943, and 
the soil burial method by the American Society for Testing Materials. 

The techniques included in the first group are time saving, permit of 
replication, and are readily, duplicated by other investigators. However, 
the results in the agar-fungicide system do not apply to a cellulose-fungicide 
system and are therefore a source of confusion resulting from their mis- 
interpretation when so used. Agar-fungicide systems as originally de- 
scribed by Richards are quantitative tests and have been used principally 
for comparative toxicity studies. From such comparative studies attempts 
to predict the behavior of a preservative in subsequent field tests have 
been generally unsuccessful. Examples of the discrepancies between the 
results from field and petri dish tests will be discussed later in this paper. 

In general, the second group of methods takes a longer time, and replica- 
tion leaves much to be desired. Since the preserved material is the same 
for laboratory and field tests, better agreement between field and laboratory 
results should be obtained with the kolle flask-wood block method and 
the A.S.T.M. fabric methods. However, the Signal Corps method for 
testing fungicidal coatings used on electrical equipment is not a a true test 
of the coating material per se. 

The third group of methods introduces a large number of variables through 
the use of soil. Previously, replication of results and concomitant duplica- 
tion by other investigators had been lacking, due to microbial activity, 
physical properties, nutrient properties, and moisture variations of the soil. 
However, during experimental work with termites, the author 8 made certain 
observations on the various factors involved in the decay process. These 
led to an intensive study of the problem resulting in the development of a 
test method for wood preservatives which overcomes many of the limitations 
of earlier methods. The soil burial method is a severe test of fungicide 
treated material, and, with the modification to be discussed in this paper, 
it is anticipated that the variables which cause non-uniformity of results 
can be eliminated. The method is also evaluated by comparison between 
the results obtained in the laboratory and those obtained from parallel 

field tests. 

Rapid decay of wood in contact with soil was observed during an attempt 
to establish experimental termite colonies in the laboratory (Leutritz 8 , 
1939). Instead of becoming infested by the termites, nearly all the blocks 


decayed more rapidly and more completely than in any previous hboratory 
test. Preliminary experiments were devised to ascertain the factors re- 
sponsible for the accelerated decay and to establish optimum conditions for 
growth of fungi in laboratory tests of wood preservatives. As a result of 
this exploratory work a laboratoiy technique was devised which permitted 
study of these factors and which offered a convenient means of evaluating 
toxicity and preservative properties of chemical compounds. Further in- 
vestigation was made on the effect of nutrients and nutrilites in the soil, 
temperature, and the moisture content of the wood. Parallel with this 
laboratory investigation, a study was made of the fungus attack on wood 
under climatic conditions very favorable for decay at Gulfport, Mississippi. 

Initial Experiments and Results 

As a preliminary step, the moisture content of the soil from the termite 
colonies was determined by oven-drying 100-gram samples. This was found 
to average 22% of the oven-dry weight of the soil. Tests with several soils 
showed that approximately the same moisture content could be obtained by 
merely adding to dry soil just enough water to make the mixture cohere when 
squeezed in the hand. 

A one-hundred-gram sample of moist soil was placed in each of 24 large- 
mouthed, eight-ounce, screw-capped bottles (12 cm. high and 6 cm. in 
diameter). A weighed oven-dry block of southern pine sapwood, 2x2x2 
cm., was pushed to a depth of about 2 cm. into the soil in each bottle. The 
caps were put 'on, and the preparations were sterilized for 30 minutes at 15 
pounds' pressure in an autoclave. After cooling, the block in each of twelve 
of the bottles was inoculated with a pure culture of one of seven common 
wood-destorying fungi— Lentinus lepideus, Fomes roseus, Porta microspore,,* 
Polyporus vaporarius, Coniophora cerebella, Porta incrassata, and Lenzites 
Irabea. Twelve bottles, not inoculated, were used for moisture determina- 

The bottles were then placed in an incubator maintained at 26°-28°C. 
At the end of each month three of the bottles inoculated with each fungus 
were taken from the incubator. Each block was removed from the soil and 
weighed immediately; it was then allowed to dry in an oven at 105°-110°C. 
to a constant weight. The average percentage loss in dry weight due to 
decay was calculated. The results, recorded in Fig. 1, show that the very 
rapid decay of wooden blocks in contact with the soil is not the result of any 
one particularly active fungus. Each of the seven species produced ex- 
ceedingly rapid decay under the conditions of the soil assay. 

* This fungus was designated BTL U-10 until recently identified as Porta microspora 
by Miss Mildred K. Nobles, Dept. of Agriculture, Ottawa, Canada, 1943. (See Bibliog- 
graphy at end of paper.) 


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uj y 

> UJ 


O uj 

Gi 3 

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>- < 

Z t- 

o z 


I 8 







3/ X. 


^ \ 

1 NJ 

2 ^^"C — 









2 I^- 3 **^ 

2 3 


Fig. 1— Wood soil contact technique 



For comparison, a similar test was made according to the method de- 
scribed by Waterman, Leutritz, and Hill 8 , 1938, in which the test blocks are 
placed on inoculated sapwood slabs supported over water in capped wide- 
mouthed bottles. Comparison of the average percent weight loss due to 
decay for all organisms by both methods, Fig. 2, shows that the water-wood 
method is far less effective in producing decay than the soil method. 

















a. < 

O 03 




























Fig. 2 — Comparison of average weight loss and final moisture content by wood-water 

and wood-soil techniques 

An additional experiment was conducted with several strains of two of 
the fungi previously used, Coniophora cerebella and Lentinus lepideus. The 
Coniophora cerebella strains were as follows: 

Baarn, from Dr. Johanna Westerdyjk, Holland 

Liese, from Dr. Liese, Germany 

Idaweiche, from Dr. Idaweiche, Germany 

Madison, from Forest Products Laboratory, Madison, Wisconsin, iso- 
lated from oak, November 13, 1919 

BTL, also from Forest Products Laboratory, Madison, Wisconsin, 1930 
The Lentinus lepideus strains were from the following sources: 

No. 534, from Forest Products Laboratory, Madison (No. 534) 

BTL U-l, U-13, U-14, and U-32, from creosoted pine telephone poles 
which had failed in service 


Gulfport, from a test post in Gulfport, Mississippi, used for our assay 

The results of the assay with these strains of Coniophora cerebella and Lenti- 
nus lepideus showed the average weight loss in percent due to decay to be 
32.0 and 27.3 percent respectively which was as great as that in the previous 
soil tests with the single representative of these species. A greater amount 
of decay was obtained with one strain of Coniophora cerebella due to a slight 
change in technique, i.e., the fungus was first established on small slabs of 
southern pine sapwood, and then sterile oven-dry blocks were dropped on 
the vigorously growing fungus. The large amount of decay (60%) which 
resulted led to the adoption of this modification in all subsequent tests. 

The foregoing tests may be regarded as supporting the use of the criteria 
previously employed in the selection of fungi for laboratory tests— namely, 
their occurrence as saprophytes of wood, their isolation from service ma- 
terials for example, pine telephone poles or tests posts, and their demon- 
strated ability to bring about decay of wood in the laboratory. 

As a result of these preliminary experiments, the use of soil as the medium 
in testing procedure was adopted. 

Soil Contact Technique 

On the basis of the foregoing experiments and in view of the rapidity of 
the decay occurring on test blocks in the soil-contact test, the following 
method is described as a means of evaluating the effectiveness of preserva- 
tives or toxic materials which are recommended for the protection of wood 
or other cellulosic materials. The method may also be used to study en- 
vironmental factors which affect decay or it may be adapted to the study of 
fungi other than the wood-destroying fungi of the Basidiomycetes. 

Ordinary top soil, such as a florist would use for potted plants, is satis- 
factory for the test. While experience has shown that top soil from a 
number of different sources may be used without materially affecting the 
results, standardization would be desirable. Therefore the term "soil" 
will be defined as a sandy loam type which contains 4-6 percent of organic 
matter and a pH originally between 5-7. The soil is passed through a 
coarse-mesh screen to remove rubble, stones and other debris; this is most 
easily accomplished when the soil is dry. The screened soil is moistened 
with just enough water to effect cohesion into a soft ball when squeezed in the 
hand, and a check may then be made by determining the moisture content 
of the soil. When prepared in this manner the moisture content of the soil 
should be 20-25% on an oven-dry weight basis. In an alternative procedure, 
the moisture content of the dry soil is ascertained and then sufficient water is 
added to give a moisture content of 20-25%. 

Bottles, 12 cm. high and 6 cm. in diameter, are half filled (60-100 grams) 



with the moistened soil. Two pieces of southern pine sapwood "feeder 
strips" (3.5 x 2.0 x 0.3 cm.) are placed on the soil in each bottle, Fig. 3. 
The bottles are closed tightly with screw caps and then autoclaved for 30 
minutes at 15 pounds' pressure. When the bottles have cooled, a small 
inoculum (a few millimeters square) cut from a pure culture of a suitable 
wood-destroying fungus is placed on the sapwood substrate. Each bottle 
contains a single dominant fungus culture. It is best to use at least four to 
eight selected species of fungi for an assay. The bottles are again capped 
and placed in an incubator, or a controlled temperature room, held at 26°- 
28° C, for at least one month. Any contaminated or weak cultures are 




Fig. 3 — Schematic diagram of wood soil contact method 

discarded. This completes the preparation of the pure fungus cultures, 
Fig. 4, and they are now ready to receive the test blocks. 

To each bottle containing a culture established on sapwood substrate 
are added an untreated control block and a block treated with a preserva- 
tive according to the following method: 

The required number of \" cubes of sapwood blocks are placed in a humid- 
ity chamber at 30°C. and 76% relative humidity until the blocks have 
reached a constant weight. Then the necessary number of weighed blocks, 
weighted to ensure immersion, are placed in a container of convenient size 
under a bell jar fitted with a separatory funnel. After evacuation of the 
bell jar to a pressure not greater than 2 cm. as measured by a mercury man- 
ometer, the vacuum is held for 5 minutes. The stopcock in the pump line 
is then closed, and sufficient solution is admitted from the separatory funnel 



Fig. 4 — Pure fungus culture of Poria incrassata 


to submerge the blocks completely when the air is admitted. After re- 
maining in the solution for 5 minutes, the blocks are wiped superficially and 
and weighed. This treated weight is used for calculation of the theoretical 
retention according to the following formula: 

100 V 

in which R* = pounds of preservative per cubic foot of wood, G = gain in 
weight in grams, C= grams of the preservative in 100 grams of solution, and 
V = volume of the test piece in cubic centimeters. When the solvent has 
evaporated from the blocks, they are placed on racks, returned to the 
humidity chamber and brought to constant weight. The difference be- 
tween the humidity weights before and after treatment serves as the basis 
for calculating the actual retention, and the final equilibrium weight is used 
also as the initial weight of the treated block before exposure to the fungus. 

The cross-section side of the blocks is placed in contact with the vigor- 
ously growing mycelia of the sapwood culture which in turn is in contact 
with the soil. For each concentration of preservative, three treated blocks 
and three untreated control blocks are exposed to each species of fungus. 
The bottles are recapped and place in an incubator or constant temperature 
room at 26°-28° C. with a relative humidity of 85-95%. 

Exposure of the blocks to the fungus for from twelve to twenty-four weeks 
gives satisfactory results. If a sufficient number of treated specimens 
is exposed to the same organism, one or two specimens may be removed 
at the end of twelve weeks; and if considerable decay has taken place, 
the test may be concluded. At the end of the exposure period the blocks 
are brushed free of mycelia and immediately weighed to determine their 
moisture content. The blocks are then allowed to stand in the room until 
dry, after which they are again transferred to the humid chamber (tem- 
perature 30°C, relative humidity 76%) for two-three days until a con- 
stant weight is attained. 

In the toxicity work reported in this paper an untreated reference block 
was added to each test bottle. If a large number of assays are contemplated, 
the number of weighings may be reduced by eliminating the untreated 
blocks except for occasional reference purposes. The reduction in the num- 
ber of reference blocks may be accomplished by establishing a decay norm 
for each test organism. This norm would be based on data similar to that 
used in Fig. 1 except that the procedure would be the same as that described 
for treated blocks. Comparison of the percentage weight loss due to decay 

* To express the retention metrically R X 16.018 = Kll °g rams . 

Cubic meter 


of the norm with the percentage weight loss due to decay of the test block 
may be used as a measure of the effectiveness of the preservative or toxic 
material. An index of the value of a preservative treatment may be ob- 
tained from the following computation: 

% loss of norm - % loss of treated block lQQ 
% loss of norm 

Values of the index would range from 100, representing complete protection 
against decay, to 0, representing no protection whatever. 

In most cases, especially when no volatile preservative is present, the 
untreated reference block disintegrates completely within twelve to six- 
teen weeks. An inspection rating based on strength may be used to supple- 
ment weight loss due to decay. The rating is made on the basis of appear- 
ance and strength: 10 denotes a sound condition, 9 superficial decay, 8 
superficial decay in spots or streaks, 7 general surface decay, 6 considerable 
decay but not enough to allow specimen to be broken easily, 5 advanced 
decay, and 4, 3, 2, and 1 different stages of advanced decay, determined pri- 
marily by the ease with which the specimen is broken; denotes complete 
disintegration. Ratings of 5 and below are considered failures. Similar 
ratings have been used for sticks in field work. This method of rating was 
used in the field trials of preservatives which appear later in this paper. 
Although the system is an arbitrary one, considerable correlation has been 
shown between these dissection ratings and the weight loss in percentage. 
With a series of field sticks or blocks treated with the same low amount of a 
preservative, ratings based on strength for the series are found to be very 
closely correlated with weight losses, even when the ratings are made by dif- 
ferent workers. 

The Influence of Moisture on Decay 

The first experimental factor studied was the moisture content of the wood 
preceding and during the time that decay took place. Statements in the 
literature concerning the optimum moisture content for the decay of wood 
have placed the figure variously from fiber saturation, 27-30% to 60% 
(Schorger, 1926) 10 and 150% (Benton & Ehrlich, 1940) 11 of the wood sub- 
stance based on the oven-dry weight. 

Figure 1 gives data on the moisture content of blocks exposed to the seven 
species of fungi for periods of one, two, three, and four months. Un- 
inoculated control blocks, removed at the end of each of these periods, were 
found to be at fiber saturation, indicating 100% relative humidity in the 
bottles and little or no migration of liquid water. 

During the progress of decay there is a rapid decrease in the weight of 
the wood substance. But the amount of water present in each block does 


not decrease and at all stages of decay corresponds to about 35% of the 
original dry weight of the block. Since the amount of water does not de- 
crease as the amount of wood substance decreases, there is an increase in the 
percentage of water expressed in terms of dry weight as shown by curves 
labeled 3 in Fig. 1 . Such an increase in the amount of water relative to the 
remaining wood substance would tend to limit decay if the optimum moisture 
content for initiating decay is considered to be at or near fiber saturation. 

If the absolute amount of water in the blocks does not change during the 
progress of decay, then the final amount of moisture divided by the initial 
weight of the blocks should give a percentage figure fairly close to the initial 
fiber saturation of approximately 30%. The curves labeled 2 in Fig. 1 show- 
ing the moisture content based on the initial weight indicate that this is the 
case. For example, Fig. 1 shows that the average for all organisms is 4% 
greater than 30% after one month, 7% greater after two, 6.3 after three, and 
only 2.1 greater after four months. Between the third and fourth months 
the soil showed signs of drying out and examination of all of the moisture 
curves, Fig. 1, indicates a loss of water through the bottle caps, which 
accounts partially for the discrepancy. The 5-10% increase in water over 
the original fiber saturation may be due to slight condensation on the 
blocks or to the respiratory activity of the fungi in breaking down carbo- 
hydrates into COo and water. 

The average amount of decay (curve 1) and the final moisture content 
referred to the initial weight (curve 2) for all the organisms obtained by 
the wood-water (A) and wood-soil (B) assays are compared in Fig. 2. The 
water content of the blocks in the water test varied from 55-165% based 
on the oven-dry weight of wood and the decay was much less in amount 
and uniformity than that obtained by the wood-soil technique with the 
same organisms. From the results for individual blocks, the limiting water 
content at which no decay took place was determined as 78% for Porta 
incrassata, 84% for Coniophora cercbclla, and 66% for Polyporus vapor- 
arius. In a few instances, despite the full cell saturated conditions, decay did 
take place. Examination of these blocks indicated that most of the decay 
was confined to the surfaces of the blocks. This indicates that lack of 
oxygen was the limiting factor. 

When wood was supported on glass rods over agar (kolle flash technique) 
full cell saturation of the blocks often occurs due to capillarity of the glass, 
condensation of water, accidental contact between wood and agar, and 
conduction by the fungus filaments. That the water content of the wood 
in the kolle flask technique is also too high is indicated by the "optimum" 
moisture content of 150% and the relatively small weight losses due to 
decay, less than 10%, cited in the experiments of Benton and Ehrlich. 11 
The amount of decay was again shown to be affected by the water content 


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 Nutrilites 

Nitrogen in the form of asparagine has been shown by Schmitz and 
Kaufert 12 (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 


top soil than of that on the mixtures of sand and fuller's earth indicates 
that the nutrients present in the soil were not all removed by the water 
extraction. Further information was obtained by adding soil extract and 
other nutrient solutions to the sand-fuller's earth mixtures. The following 
materials were used: 

4500 grams of washed beach sand 
900 grams of fuller's earth 
300 ml. of soil extract 
60 ml. of malt extract (2% water solution) 
50 leached blocks 

vitamins Bi, Be and biotin (free acid) 
stock mineral solution of the following composition: 
1.5 grams per liter of KHjPO« 
1.0 gram per liter of MgS0 4 7HiO 
plus the following elements in parts per million: 

0.02 Cu, 0.01 Mn, 0.005 B, 0.10 Fe, 0.01 Mo, 0.09 Zn 
pure cultures of the fungus Porta incrassala 
42 eight-ounce bottles, screw capped (12 cm. high by 6 cm. diameter) 


The sand and fuller's earth in the proportions mentioned above were 
mixed in a porcelain jar on a ball mill for six hours. The blocks were 
leached for two years with weekly changes of distilled water, using 50 ml. 
of distilled water for each block. Before the test, the blocks were oven- 
dried to constant weight at 105°C. and a volume measurement was made 
by mercury displacement method. The volume at fiber saturation was 
calculated from the oven-dry volume according to the formula: Volume 
at fiber saturation = Oven-dry volume -f- 0.25 X oven-dry weight. 

One hundred grams of the soil moistened with 20 ml. of the appropriate 
nutrient solution (Table 1) was placed in an eight-ounce bottle for each 
block. The weighed block was pushed into the soil with a cross-section of 
the block facing- upward until the top of the block was level with the soil. 

The bottles were capped and autoclaved for 20 minutes at 20 pounds' 
pressure. After sterilization and cooling, an inoculum from a pure culture 
of the fungus Porta incrassala was placed on the top of each block. The 
bottles were then placed in a controlled temperature room (26°-28°C, 
relative humidity 90-95%) for 16 weeks. 

At the end of this time the blocks, brushed free of soil and mycelia, were 
weighed immediately, and the volume was measured. Finally, the blocks 
were again oven-dried to constant weight and the volume was measured 

The average volume at fiber saturation of the 42 blocks calculated from 
the initial volume when oven-dry was found to be 7.24 cc, and the final 
average volume when removed from the test was 7.26 cc. No shrinkage 
took place until the blocks were oven-dried, and then the distortion became 


permanent. This constancy of the volume during the decay period indi- 
cates the mechanism by which the water is held practically constant during 
the decay period. Any loss of water would result in a shrinkage from 
which there would be no recovery. 

Table 1 gives a list of the solutions used to moisten the artificial soil and 
the average weight loss for 3 blocks in percentage for each variation. Several 

Table 1 

Effect on the Decay of Wood in Contact With Sand and Fuller's Earth 

Mixtures Moistened with Various Nutrients and Nutrilites 

Organism Porta Incrassata. Time 12 Weeks 

Solution Used to Moisten Artificial Soil 

Top Soil (Control) •.■•••• 

M.S.* + 0.2% Ammonium Nitrate + 1% Maltose + Vitamins B,, 

B 8 and Biotint 

2% Malt Extract 

M.S. 4- 2% Ammonium Nitrate + Vitamins 

M.S. + 2% Ammonium Nitrate ; 

M.S. 4- Various Combinations of Vitamins! 

Distilled Water 

M.S. + Vitamins + 1% Maltose 

M.S. + 1% Maltose 

Average Weight Loss in 
Per Cent Due to Decay 



* M.S. = mineral solution containing the following minerals: 

Potassium Dihydrogen Phosphate 1.5 grams per liter 

Magnesium Sulfate 1.0 gram per liter 

Copper 0-02 parts per million 

Manganese. 0.01 parts per million 

Boron 0.005 parts per million 

I ron ._ 0.10 parts per million 

Molybdenum 0. 10 parts per million 

Zj nc 0.09 parts per million 

t The vitamins used and the concentrations per liter were as follows: 

d 0.1 milligrams per liter 

B„. ... . 0.1 milligrams per liter 

Biotin 0.02 milligrams per liter 

X The vitamins were added singly and in the following combinations: 

B! + B a + Biotin— Concentration of each vitamin as listed above. 

B, + B, 

B, + Biotin 

Bo + Biotin 

conclusions may be drawn. It is evident that the soil greatly accelerates 
decay, and that the soil extract contains a large portion of the nutrients 
and nutrilites which accelerate decay. Malt extract, which contains 
proteins and sugars, and the mineral solution fortified with ammonium 
nitrate also stimulate decay. The effect of nitrogen in increasing decay 
confirms the experiments made by Schmitz and Kaufert, 12 1936. When 
nitrogen is lacking and a simple sugar is present, the fungus consumes the 
simpler sugar instead of the more complex carbohydrate cellulose. This 
preference is not evident if sufficient nitrogen is present since both carbohy- 



drates are destroyed. There is a slight indication that the vitamin mixtures 
promote decay but the effect as measured by the weight loss is not very 
pronounced. The basic mineral solution which was used in this experiment 
promoted only slightly more decay than the distilled water. While the 
results are not included in the table, it may be stated that leaching of the 
blocks had no apparent effect on decay when compared with unleached 

Influence of Temperature on Decay 

Most of the early work in the Bell Telephone Laboratories was conducted 
by the petri dish method at temperatures in the range 26°-28°C, following 
the recommendations of Richards, 1923. But certain fungi, including 
Merulius lachrymans, failed to grow at this temperature. When several 
inocula of Merulius lachrymans that had failed to grow at 26°-28°C. were 
transplanted to sterile blocks, according to the earlier sapwood-water tech- 
nique, the loss in weight due to decay after six months averaged 34% at 
21°C. and only 7% at 26°-28°C. 

An experiment was planned to test the influence of a wide range of tem- 
peratures on the decay of wood in the soil contact assay method. Four 
kinds of fungi were established under sterile conditions on untreated wood 
slabs laid on moist garden soil. An abundant growth of the fungi was 
secured within one to two months. Cubes of sapwood were placed on the 
vigorously growing mycelia, both of which had been conditioned by exposure 
overnight to the various temperatures. Sterile soil, also conditioned to 
the temperatures, was used to cover the blocks. After 15 weeks' exposure, 
the results were as follows: 

Average Weight Loss in Per Cent 

o # c. 












The results indicate that the standard temperature, 26°-28°C, was optimum 
for the four fungi tested. No decay was produced by any of the fungi at 
0°C. A temperature of 35°C. was too high for active decay; in the case of 
Poria microspora, for instance, only a single block was attacked. The 
series at 35°C. was repeated because the soil in some of the bottles seemed 
to have become rather dry, although the blocks contained 30% moisture. 
In the new series the humidity was maintained at 76% around the bottles 
to reduce loss of water, and three more organisms were used. The results 



of the previous temperature tests were confirmed and the weight losses 
due to decay by the three additional fungi, which are known to tolerate 
higher temperatures, were as follows: 


Lentinus lepideus 

Lenzites sepiaria 

Lenzites trabea (BTL U-40) 

Percentage Weight Loss 
Due to Decay 


These results indicate that certain fungi are able to bring about decay 
of wood over a wider temperature range than others. It is clear that a 
complete statement cannot be made until the effects of various tempera- 
tures between 0°C. and 21°C. have been ascertained. In the light of results 
with Lentinus lepideus, Lenzites sepiaria, and Lenzites trabea showing con- 
siderable decay at 35°C, the upper limits of temperature should be deter- 
mined for these fungi. 

Humphrey and Siggers, 13 1933, studied the effects of different temperatures 
on the growth of sixty-four fungi. Two different nutrient substrates were 
used, but the optimum temperature with these rarely differed by more 
than 2°C. The following summary shows a comparison of their results 
with those obtained in the above tests: 

Optimum, "C. 

Upper Limit, °C. 

















* Bottle method used; no test has been made yet with soil. 

Two of the fungi, Merulius lachrymans and Lentinus lepideus, brought 
about decay at limits higher than those reported for cessation of growth 
by Humphrey and Siggers. Poria incrassata had the same limiting tem- 
perature in both tests. The temperatures for maximum growth and maxi- 
mum decay check rather well in both tests. 

Field Studies 

The rapid decay obtained in the foregoing laboratory experiments based 
on the soil technique was further evaluated by investigating the rapidity 
of decay in the field. 



Selection of Wood: 

For both laboratory and field assays care is exercised in selecting the 
wood. Boards of southern pine sapwood of the shortleaf type, which 
includes Pimis echinata, and Pinus taeda, are obtained from local lumber 
dealers and are cut into sticks f x f x 32 inches. Since the square sticks 
facilitate calculation of volume and retention of toxics or preservatives, 
they have superseded the round saplings cited by Waterman and Williams, 14 
1934. The sticks are selected on the basis of uniformity of growth, density 
and ratio of springwood to summerwood. The presence of any heartwood, 





(±2-0) v 








Fig. 5 — Field test of untreated sapwood squares exposed at Gulfport, Mississippi, 1941-42 

sap stain or other indication of incipient invasion by fungi is cause for re- 
jection. After classification into piles according to arbitrarily chosen 
weight increments, twenty to twenty-five 32" sticks, for each concentration 
of preservative used in field studies are selected by taking the appropriate 
number of specimens from each pile to give a representative distribution 
based on density. Since the specimens are subsequently cut in half each 
individual treatment is represented by 40-50 specimens. Sticks in the 
median range of density are generally used for laboratory studies after 
they have been cut into \" cubes (8 cc. volume). 

An experiment with untreated sticks was carried out at Gulfport, Missis- 
sippi, where the climatic conditions are very favorable for decay and also 


for termite attack. Six hundred 8-inch lengths were dried in the oven in 
the laboratory at 105°-110°C. and then weighed. The specimens were 
then shipped to Gulfport, Mississippi, and distributed throughout the test 
plot in April. Each eight-inch specimen was buried in the soil until the 
end of the specimen was even with the level of the soil. At the end of 
each two-month period subsequent to exposure about 80 specimens were 
removed, brushed free of dirt and mycelia, oven-dried, and reweighed. 
In order to study decay during the winter months, 150 additional pieces 
were planted in October; seventy-five of these were removed after four 
months and the rest after six months exposure. 

From Fig. 5 it is evident that in the field test the loss in weight due to 
decay was far less than that obtained in the soil test in the laboratory. 
The maximum amount of decay in the field after two months was as high 
as 10% in only two out of the 81 samples exposed; after four months it was 
19%, after six months 30%, after eight months 20%, after 10 months 30%, 
and after 12 months 50%. While the maximum percentage loss applies 
only to one specimen in each case, in general the average percentage losses 
noted in the figure were far below these figures. Therefore, decay in the 
field does not approach in uniformity and rapidity that occurring under 
controlled conditions in the laboratory. 

Experiment at Chester, New Jersey, Using Various Nutrients: 

Another field experiment was devised in which an attempt was made to 
increase the rate of decay by using nutrient materials and salts which would 
change the pH of the soil. Sixteen-inch untreated sticks were selected for 
uniformity within a very narrow density range and exposed in each of six 
specially prepared plots in northern New Jersey. The ground was first 
plowed, then harrowed and raked free of stones so that the soil in all plots 
was nearly uniform before treatment. Then fifty sticks were buried to a 
depth of 7 inches in each plot in rows of five, with two feet between each 
row and one foot between the sticks in each row. The plots were treated 
as follows: 

Plot No. 



Barnyard manure 

5 pounds lime 

5 pounds commercial fertilizer (5-10-5) 

5 pounds aluminum sulfate 

Nutrient solution* 

* Containing the following minerals dissolved in ten gallons of water, then sprinkled 
over the entire plot: 



A) Ca(N0 3 ) 2 -4HjO 297.0 

MgS0 4 -7H 2 118.8 

KH*P0 4 83.6 

B) ZnS0 4 0. 176 

MnSCWHoO 0.572 

Boric Acid . 704 

A1*(S0 4 ) 3 0. 176 

C) FeS0 4 5.0 

Note: The salts in A, B, and C were dissolved separately and then the three parts 

The sticks were removed and examined at intervals of three, twelve, and 
fifteen months. The percentage failure, as determined by the ease with 
which the specimen could be broken, is shown in the following: 

Percentage Failure 

3 mo. 

12 mo. 

15 mo. 















Aluminum sulfate 



At the end of 12 months the greatest number of failures due to decay 
was observed in the plot treated with the acid salt (aluminum sulfate). 
Colorimetric determinations of the pH of the soil showed it to be between 
5.8 to 6.0 for the soil treated with the acid salt and about 6.6 to 6.8 for the 
control plot. This experiment needs to be repeated for confirmation of 
results, but the present indications are that the acid soil was much more 
favorable for decay than the soil in the control plot. Limed and fertilized 
soils gave results comparable to those of the control plot, with an indication 
that the fertilizer increased the decay. The complete disintegration of 
the sticks in the acid treated soil was particularly noticeable, whereas the 
sticks in the soil treated with manure were intact, though easily broken. 
The plot treated with the nutrient solution showed the same rate of decay 
as the manure plot at the end of the twelve-month period. Ten pounds 
of aluminum sulfate were then added to the nutrient plot, and three months 
later all the sticks were completely disintegrated. The rapid disintegration 
of the sticks in the plot treated with the acid salt points to the importance 
of further work on the effect of the pH on the rate of decay. 

Comparison of Soil Technique With Other Toxicity Assays 

In the selection of a method for testing relative toxicity of chemicals to 
microorganisms, the rapidity of test, the standardization of the medium, 


the choice of test organisms, the ease of manipulation, the replication of 
results, and the duplication by other investigators have been the paramount 

A hypothetical test which would meet these requirements could be 
performed with distilled water to which could be added increasing concen- 
trations of the chemical to be tested. A known amount of fungus mycelium 
or spores could be shaken with the toxic solution and left for several different 
time intervals. The fungus filaments or spores could then be removed to a 
nutrient agar, and the viability of the fungal filaments or percentage of 
spores germinating could be readily determined. Comparative toxicity of 
a large number of compounds could be quickly and easily ascertained. 
The results, however, would be applicable only to a distilled water-poison 
system, and the concentration of most toxic materials necessary to inhibit 
growth would be very low. 

The addition of nutrients would necessitate larger amounts of the toxic 
materials (Van den Berge, 16 1935). Therefore the mineral solutions- 
nutrient agar, soil extract agar, soil or wood substrate would in general 
necessitate an increase in toxic material, the amount of increase depending 
upon which substrate best meets the nutritional requirements of any partic- 
ular fungus. Some toxicity values would also be affected by chemical or 
certain physical changes resulting from interaction between the toxic 
material and the substrate. 

In the petri dish method, the fungi selected for studies of wood destruction 
grow well on the nutrient substrate containing 1.5% malt extract and 2% 
agar. Although such a mixture has been recommended as a standard 
substrate, 2 it should be pointed out that the malt syrup is somewhat variable 
in composition and constituents and that even the agar varies in the amounts 
of various growth substances present, Robbins and Ma, 16 1941. The inter- 
pretation of results obtained by the assay of a fungicide when dispersed 
in an agar system should be restricted to that specific system and not 
applied to a wood-fungicide system. 

When wood preservation studies are carried out, reliance cannot be placed 
on the results of petri dish tests. The use of wood permits the testing of a 
large variety of the more common preservatives and fungus-proofing agents, 
many of which may react with the wood or are precipitated in the wood 
upon loss of solvent. Organic preservatives which are relatively insoluble 
in water are not readily tested by petri dish assay. 

Comparison of the wood-soil contact method with the wood-water method 
when untreated wood blocks are used is shown in Fig. 2. The greater 
uniformity in the amount and rapidity of decay and the better control of 
moisture showed the soil technique to be superior. It is obvious that if 
the amount of decay is variable and adversely affected by other factors, 


the effect of the preservative or fungicide will be obscured. When the 
sapwood-water method 3 was published, comparison between it and the 
kolle flask method showed that the wood-water method had certain 

Comparing the method of soil contact with that of soil burial, the principal 
point of difference is that a pure culture is used in the soil contact method 
and a mixed culture is used in the soil burial method. Common to both 
are the moisture-regulating and nutrient properties of the soil. Since the 
microbial activity of unsterile soil is diverse, depending on the type and 
source of soil, uniform results from soil burial could not be expected. When 
wood specimens were exposed individually in bottles of non-sterile soil in 
the laboratory, the amount of decay after 12 weeks' exposure was less than 
10% for all specimens. The results were similar to those obtained by the 
exposure of untreated wood out-of-doors at Gulfport, Mississippi, for the 
two-month period (Fig. 5). Since decay-producing organisms were shown 
to be present, the other organisms in the soil must have interfered with the 
growth of the wood-destroying fungi. The antagonism between the wood- 
destroying fungus Lentinus lepideus and a contamination is shown in Fig. 6. 

The soil-contact technique instead of the soil burial method has been 
used extensively to test cotton fabric, thread, paper, jute, fibers, and a 
variety of other materials. The organisms have been varied according to 
their occurrence on the particular substrate in nature. The fungi Chae- 
lomium globosum, Aspergillus niger, Stachybotrys atra, Stysamis media, and 
Metarrhizium have been established with excellent results on a substrate 
of cloth when testing fabric. The loss in tensile strength of an unprotected 
cotton thread which had an initial absolute pull of 30 pounds was 90-100% 
after two weeks' exposure to Chaetomium globosum. Treated threads or 
other cellulosic materials may be tested as satisfactorily as treated wood. 

Examination of numerous reports from soil burial studies of treated textiles 
indicates that organisms which tolerate certain types of chemicals become 
dominant in the test beds. As a result, a preservative which shows great 
promise initially may suddently fail when the test is repeated. If a pure 
culture technique were used, a better evaluation of the preservative would be 

Similarly the controversies which have arisen over the ability of certain 
fungi to destroy cellulose could be resolved by using the suitable cellulose 
soil technique. At least there is very good evidence that many of the en- 
vironmental variations affecting decay are at or near the optimum. 

Toxicity Tests 

In the toxicity tests which follow, petri dish results are given for several 
compounds, soil contact test results are given for compounds not readily 


assayed by the petri dish method, and the field test results are included for 
comparison with the results of the soil contact test method. 

Fig. 6 — Antagonism which has persisted for over one year between the wood-destroying 
fungus, Lentinus lcpideus (outer portion) and contaminating fungus (inner portion) in a 
soil culture. 

During the initial stages of this research on the evaluation of toxic prop- 
erties of various compounds for wood preservation, the petri dish method 
was used for assay studies in the laboratory and the modified sapling method 



of Waterman and Williams 14 (1934) was used for the field tests at Gulf- 
port, Mississippi. Table 2 shows the results obtained with the petri dish 
method on the toxicity of four common inorganic salts and a creosote to 
several of the usual test fungi. 

Table 2 

Toxicity Expressed in Per Cent Toxic Agent Present in Nutrient Agar as 

Determined by Petri Dish Assay 





Arsenic Tricoride 

Madison #517 



Porta incrassata 



Lentinus lepideus 




Fomes roseus 



Porta micros pora 



Poly poms vaporarius 



Zinc Chloride 

Poria incrassata 



Madison #517 



Lentinus lepideus 



Fomes roseus 



Poria micros pora 



Polyporus vaporarius 



Mercuric Chloride 

Madison #517 



Lentinus lepideus 



Polyporus vaporarius 



Poria incrassata 



Poria microspora 



Fomes roseus 



Copper Sulfate 

Lentinus lepideus 



Madison #517 



Lenzites sepiaria 



Fomes roseus 



Poria incrassata 



Polyporus vaporarius 



Poria microspora 




Poria incrassata 



Polyporus vaporarius 



Lenzites sepiaria 



Poria microspora 



Lentinus lepideus 



Resistance of the fungi to the four salts is variable, but it will be noted 
that Lentimis lepideus, which is most sensitive to copper sulfate, tolerates 
the highest concentration of creosote. Poria incrassata, which is fairly 
tolerant of copper salts by petri dish test, is the most sensitive to zinc chloride 
and creosote. Poria microspora tolerates relatively greater concentrations 
of all the compounds than any of the fungi tested. 

The four salts assayed can be easily dissolved in an agar medium in con- 
centrations high enough to be toxic, but uniform dispersal of insoluble salts 



in the agar is not so easily accomplished. Many salts may be made soluble 
by dissolving them in dilute ammonia or acetic acid solutions. For example, 
copper arsenate or zinc meta-arsenite are soluble in ammonia or acetic acid, 
and by evaporation of the volatile portions of the solvent the salts are pre- 
cipitated. When precipitation of the salts from ammoniacal or acetic acid 
solution is carried out in treatments of wood, subsequent evaporation of 
ammonia or acetic acid from the wood is rather rapid. In agar solutions, 
uniform precipitation of the salts through evaporation of the ammonia and 
acetic acid is not easily attained. 

Table 3 

Twenty Four Week Soil Assay of Wood Preservative Compounds Not Readily 

assayable by petri dlsii methods 

Mixture #1 

Poria incrassata. 
Poly poms vapor aritis. 
B.T.L. U-ll 

Mixture *2 

Poria incrassata 

Polyporus vaporarius . 
B.T.L. U-ll 

Mixture S3 

Poria incrassata. 
Polyporus vaporarius. 
B.T.L. U-ll 

Mixture *4 

Poria incrassata. 
Polyporus vaporarius. 
B.T.L. U-ll 

Average Weight Loss in Per Cent 

1.60 Ibs/cu.ft. 


2.79 lbs/cu.ft. 


0.35 lbs/cu.ft. 


0.72 lbs/cu.ft. 



0.80 lbs/cu.ft. 


1.40 lbs/cu.ft. 


0.15 lbs/cu.ft. 


0.36 lbs/cu.ft. 


0.41 lbs/cu.ft. 


0.68 lbs/cu.ft. 








* Average per cent weight loss of untreated blocks in the same bottles with the treated 

Agar cannot readily be used for assays of two other types of compounds 
used as wood preservatives. The first type depends on chemical reactions 
with and also within the wood. Specific examples of this type are the series 
of compounds fixed in the wood by the reduction of chromium salts which was 
first studied by Kamesam, 17 1934. It is now the generally accepted view 
that the reduction of the chromium is brought about by various sugars in 
the wood. Subsequent research led to the use of Ascu (Kamesam) or Green- 
salt K and to the later development of Greensalt "O" by the Bell Telephone 
Laboratories in the United States and the Bolidens' salts in Sweden. These 
inorganic salt mixtures were developed in the search for preservatives which 



would be fixed in the wood and thus resist leaching when exposed to the action 
of ground waters. 

The second type is comprised of organic compounds or mixtures of organic 
compounds, such as creosote, which has had an excellent service record as a 
preservative. Also included in this type of compounds (which are insoluble 
in water and have a relatively low vapor pressure) are certain chlorinated 
phenols and cresols. Because of the low water solubility or immiscibility 
with agar solutions, the uniform dispersal of the toxic agents in the agar 
system, which is essential to reproducibility of results, is almost impossible. 
Uniform injection of these materials into wood, however, presents no par- 
ticular problem. 

Assays of four representative mixtures not readily assayable in the petri 
dish are included in the results of Table 3. The composition of the treating 
solutions of the mixtures 1, 2, 3, and 4 is given below: 

Mixture 1 

Mixture 2 
Mixture 3 

Mixture ■( 

Zinc oxide 
Chromic acid 
Arsenic acid 

Copper phenolate 
Zinc phenolates 

Sodium fluoride 
Disodium hydrogen arsenate 
Sodium chromate 

Zinc oxide 
Arsenic oxide 
Sodium carbonate 
Acetic acid 

49.6% dissolved in a 10% ammonia 







In the assays shown in Table 3 the maximum retention of the mixture by 
the wood is that recommended for the treatment of wood that is not to be 
used in contact with the ground. To make the test as severe as possible, 
organisms were selected which were known from petri dish, kolle flask, and 
wood-water assays to have a high tolerance for various inorganic salts. 

Examination of the data in Table 3 shows that the compounds produced 
in the wood by mixture 4 afford almost complete protection to the wood 
which was treated with 0.72 pound of the salt per cubic foot. The vigorous 
attack on the untreated blocks is evidence of the severity of the test. Sim- 
ilarly, the comparable treatment of the wood with 0.8 pound of mixture 1 per 
cubic foot was very effective in protecting the wood against fungus attack. 
If the amount of mixture 1 in the wood is doubled, almost perfect protection 
against decay may be obtained. Wood treated with 0.35 pound of mixture 
3 per cubic foot is protected against Polyporus vaporarius and BTL U-ll but 



not against Porta incrassata. The latter fungus completely disintegrates 
both the treated and the untreated wood. Despite the fact that the wood 
treated with mixture 2 represented the highest concentration of any pre- 
servative used (2.8 pounds per cubic foot), complete disintegration of the 
wood results from the action of the three fungi just mentioned. 

Compounds of the Greensalt type were also assayed by means of the soil- 
contact method. The solution commonly used for treatment of wood with 
Greensalt K contains three chemicals in the following proportions: 

Potassium dichromate 
Copper sulfate 
Arsenic acid 


CuSCv5H 2 

As»Ob-2H 2 


After treatment of the wood with this solution, reduction of the chromium 
by the sugars in the wood together with evaporation of water precipitates 
in the wood fibers several complex insoluble salts, among which presumably 




S> ^,v 









2. \ 



*RIUS ''-^ 




1.36 0.45 0.91 136 


Fig. 7— Ccmparison by wood-soil assay of components with the whole salt of Greensalt 
K. Organisms Poria incrassata and Polyporous vaporarius. 24 weeks' time. 

are copper arsenate and copper dichromate. Results of soil-contact assay 
of wood treated separately with solutions of these two components and with 
the whole Greensalt K complex are given in Fig. 7. Before exposure to the 
fungi, the wood specimens were leached by a diffusion method described by 
Waterman, Leutritz and Hill 3 , 1938. No untreated control blocks were 
included in the bottles with this test, which was conducted for 24 weeks. 
The copper arsenate component of the Greensalt K complex is shown to be 
much more effective as a preservative than the copper dichromate component 
but not as effective as the whole K salt complex. Figure 8 shows the setup 
for the wood-soil assay of 0.75 lbs/cu. ft. of Greensalt K from a recent series 
of cooperative experiments conducted by the Forest Products Laboratory, 
Madison, Wisconsin. Figure 9 is a comparison of the Greensalt K treated 
and untreated reference blocks in the same bottles after exposure to the 
following fungi: 


Blocks Fungi 

A Lenziles Irabea #617 F.P.L. 

B Poria incrassata #563 F.P.L. 

D Lenlinus lepideus #534 F.P.L. 

E Porta microspore # 106 F.P.L. 

F Poria luleofibrata (Baxter) 

The distortion and shrinkage of the blocks can be used as a visual confirma- 
tion of the weight loss due to decay. 

The more recently developed Greensalt is similar to the K salt. The 
treating solution of this salt mixture is composed of copper oxide, hydroxide 
or carbonate, chromic acid anhydride, and arsenic acid in percentages based 
on the chemical equivalents of the copper, chromate and arsenic salts in the 
K salt solution. The toxicity from the wood-soil assay of Greensalt O is 
given in Table 4 for fourteen fungi and for three concentrations of the pre- 
servative. The effectiveness of the Greensalt treatment of wood is ap- 
parent from examination of the toxicity index. The weight losses due to 
decay of the untreated reference blocks indicated in general that conditions 
for decay were again very severe, but the reference blocks exposed to the 
fungus Lenlinus lepideus were protected by their proximity to the treated 
specimens. As previously pointed out, the fungus Lenlinus lepideus does 
not tolerate even slight concentrations of copper, which is a major component 
of the Greensalt complex. 

The fungus Poria incrassata was again shown to be the least affected by 
the toxicity of the preservative. The toxicity index for the blocks treated 
with the highest concentration of the preservative was 94% after 16 weeks' 
exposure to this organism. In view of the excellent field record for the 
equivalent K salt preservatives ratings 90 to 100% by the toxicity index 
would be satisfactory. Additional data will undoubtedly determine the 
limits of the toxicity index. 

At the end of the 24-week period, the fungi BTL U-ll, Lenziles Irabea, 
Trametes serialis, Polyporus vaporarius, and one strain of Coniopliora cere- 
bella caused slight decay of one or more blocks treated with the maximum 
concentration of preservative, the fungi Lenziles Irabea, Coniopliora cerebella, 
and Trametes serialis were still capable of causing only slight losses. Ex- 
posure of the blocks treated with the low contentration of preservative to 
Polyporus vaporarius and BTL U-ll resulted in an increase in the amount 
of decay. 

Since the resultant salts of the Greensalt O reaction should be similar 
to those produced by Greensalt K, field trials of these materials would be 
expected to give comparable results. Extended field tests of wood treated 
with one pound of Greensalt K per cubic foot have been in progress for ten 
years without a single failure having occurred in more than 40 specimens. 
Specimens treated with Greensalt O have been tested in the field for only 










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'I' ,. 
o 5 o o 





















j Sj ^ &•' 

a; r- fa b -C 

Lt. — 5C ^o « 
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is . K 5 s 3 

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the relatively short period of three years, during which time all the specimens 
have remained sound. 

Results of other field trials for a three-year period with the same mixtures 
1, 2, 3, and 4, listed previously, copper arsenate, Greensalt O, and a creosote 
are given in Fig. 10. Twenty-five untreated controls showed 84% failure, 
4% sound, and 12% badly infected in one year. All had failed at the end of 
the second year. Twenty specimens were used for each retention of the 
individual preservatives, with the exception of the creosote. The reason for 
fewer creosote specimens within the correct retention is that the empty- 
cell treatments of wood with creosote give a wider range of retention than the 
full-cell treatments of the wood with water solutions of the salts. 

Table 4 
Twenty Four Week— Wood Soil Assay of Greensalt 

Wood Destroying Fungi 

'0" Using 13 Species of 

Poria incrassata (16 wks) 

B.T.L. U-ll 

Poly porus vaporarius .... 

Lenzites trabea 

Coniophora cerebella 

Trameies serial is 

B.T.L. U-4 

Poly porus anceps 

B.T.L. U-53 

B.T.L. U-24... 

Lenzites sepiaria 

Poria micros pora 

Pomes roseus 

Lenlinus lepideus 

Toxicity Index* 

1.17 lbs/cu.ft. 


0.96 lbs/cu.ft. 


0.476 lbs/cu.ft. 


Toxicity Index = 

% loss of norm - % loss of treated block 
% loss of norm 

X 100. 

In the three-year period of exposure only the treatments of wood with 1.3 
pounds of copper arsenate and 7 pounds of creosote per cubic foot showed a 
perfect record. Copper arsenate had previously been shown to be a very 
effective component of the Greensalt complexes when tested by the soil- 
contact method. Mixture 2 was found to be the poorest by soil-contact 
assay and also in the field trials. Mixtures 4, 1, and 3 were rated in that 
order of decreasing effectiveness in the field test. In the soil-contact assays 
at comparable retention, 0.72 and 0.80 pound of salt per cubic foot of wood, 
respectively, mixture 4 was better than 1; and at 0.41 and 0.35 pound of 
salt per cubic foot of wood, respectively, compound 1 was slightly better 
than 3, especially against Poria incrassata. 

Results from field and laboratory tests show good agreement in the evalu- 



ation of the compounds. The advantage of the laboratory method in the 
matter of time is a decided one, but the field trial is valuable for testing the 
permanence of the preservative. For example, mixture 4 was found in the 
laboratory test to be a very effective preservative, but the initial preserva- 
tive properties were dissipated by exposure to the weather, since 50% of the 



7 0.34 


Fig. 10 — Results of field exposures on the four mixtures of Table 3 and other compounds 
creosote, Greensalt K, copper arsenate (retent in pounds per cubic foot). 

specimens had to be removed because of failure within three years. When 
a compound is a poor preservative, as in the case of mixture 2, both labora- 
tory and field trials serve to eliminate it from further consideration. By the 
soil-contact method every specimen treated with mixture 2 was badly 
attacked by all the organisms used, whereas at a comparable retention only 
35% of the specimens in the field trials had failed after three years' exposure. 


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. 


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- 

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- 

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 possibility 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 


permanence of. the preservative. However, heating, leaching and other 
simulated weathering cycles may be used in conjunction with the soil- 
contact method to determine the stability of a preservative to evaporation, 
to the solvent action of ground water, and to chemical deterioration by ultra- 
violet light. Futher comparisons between soil-contact assay and field test 
of a wood preservative such as Greensalt confirms the fact that conditions 
for decay are more nearly optimum in the former and result in an unusually 
severe test of the preservatives. 

The soil-contact method is an excellent laboratory tool easily adapted to 
fundamental studies of the decay process and to evaluation of preservatives. 
The method has shown considerable promise in evaluating preservatives for 
a wide variety of materials, including leather, cotton, felt, paper and jute. 
Since the factors influencing decay are very near optimum in the soil-contact 
method, any preservative that prevents decay in this laboratory test and is 
permanently retained will be effective under any climatic conditions. 

I wish to express my appreciation to Dr. S. F. Trelease of Columbia 
University for his help and guidance in the preparation of the manuscript 
and to Dr. R. H. Colley and other members of the Bell Telephone Laborato- 
ries Staff for their advice and counsel throughout the work. 


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