United States

Department of

Agriculture

Forest Service

Forest

Products

Laboratory

Research

Paper

FPL–RP–571

Soil-Contact Decay

Tests Using Small

Blocks

A Procedural Analysis

Rodney C. De Groot

James W. Evans

Paul G. Forsyth

Camille M. Freitag

Jeffrey J. Morrell

In cooperation

with the

United States

Department of

Transportation

Federal

Highway

Administration

AS

RP

AS

RP

HM

HM

LP

LP

DF

DF

SG

RO

YP RM WH

RO

YP

RM

WH

SG GF GF WO

WO EHEH
Abstract

Much discussion has been held regarding the merits of

laboratory decay tests compared with field tests to evaluate

wood preservatives. In this study, procedural aspects of soiljar decay tests with 1 cm3

 blocks were critically examined.

Differences among individual bottles were a major source of

variation in this method. The reproducibility and sensitivity

of the soil-jar method using small blocks must be further

characterized before it can be accepted as a standard protocol

for evaluating preservative-treated wood.

Keywords: Decay, preservatives, methods

March 1998

De Groot, Rodney C.; Evans, James W.; Forsyth, Paul G.; Freitag, Camille

M.; Morrell, Jeffrey J. 1998. Soil-contact decay tests using small blocks—a

procedural analysis. Res. Pap. FPL–RP–571. Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory. 7 p.

A limited number of free copies of this publication are available to the

public from the Forest Products Laboratory, One Gifford Pinchot Drive,

Madison, WI 53705–2398. Laboratory publications are sent to hundreds

of libraries in the United States and elsewhere.

The Forest Products Laboratory is maintained in cooperation with the

University of Wisconsin.

The use of trade or firm names is for information only and does not imply

endorsement by the U.S. Department of Agriculture of any product or

service.

The United States Department of Agriculture (USDA) prohibits discrimination in its programs on the basis of race, color, national origin, sex,

religion, age, disability, political beliefs, and marital or familial status. (Not

all prohibited bases apply to all programs.) Persons with disabilities who

require alternative means for communication of program information

(braille, large print, audiotape, etc.) should contact the USDA’s TARGET

Center at (202) 720–2600 (voice and TDD). To file a complaint, write the

Secretary of Agriculture, U.S. Department of Agriculture, Washington,

DC 20250, or call 1–800–245–6340 (voice), or (202) 720–1127 (TDD).

USDA is an equal employment opportunity employer.

Acknowledgments

This research was partially supported by the Federal

Highway Administration. We appreciate the data processing

assistance of Victoria Herian, Forest Products Laboratory.
Soil-Contact Decay Tests

Using Small Blocks

A Procedural Analysis

Rodney C. De Groot, Research Plant Pathologist

James W. Evans, Research Statistician

Forest Products Laboratory, Madison, Wisconsin

Paul G. Forsyth, Former Post Doctorate Research Associate

Camille M. Freitag, Senior Research Assistant

Jeffrey J. Morrell, Associate Professor

Oregon State University, Corvallis, Oregon

Introduction

Although considerable debate exists concerning the merits of

using laboratory decay tests compared with field tests to

evaluate wood preservatives, the use of small-volume wood

blocks in laboratory soil-jar decay studies holds promise as a

rapid method to evaluate new preservatives in a variety of

wood species (Bravery 1979; Scheffer and others 1987,

1988). The study herein was initiated to (1) characterize the

capacity of white- and brown-rot fungi to decay different

wood species when using this soil-jar method and

(2) examine potential sources of variability within the

procedure.

Procedurally, this soil-jar technique parallels that of the

standard ASTM soil-block test (ASTM 1995) in that weight

loss is used as a measure of fungal attack in blocks that are

incubated under conditions of controlled temperature and

humidity for defined periods. The major differences are that

the wood blocks are 1 cm3 rather than 6.9 cm3 (19- by 19- by

19-mm) in the ASTM technique and the incubation vessels

are smaller than those used in the ASTM procedure.

Developmental research (Duncan 1958) that led to the ultimate standardization of a soil-block procedure showed that

species of decay fungi respond differently to microenvironmental conditions in the standard 60 ml (8-oz) jars. Most

fungi were unaffected by a range of soil moisture levels, but

fungi with a relatively high tolerance to a particular preservative were sensitive to variations in soil moisture content.

The threshold values for preservatives were judged to be

similar for soils having a water-holding capacity within a

range of about 20% to 40% if the initial moisture content

was about 130% of that capacity.

In a preliminary study, we used the soil-jar method to investigate the effects of both soil moisture content and incubation

temperature on the capability of several white-rot fungi to

decay wood. Unfortunately, the variability in our results

diminished our ability to interpret the potential effects of

these two experimental parameters. Subsequently, we conducted a second study at a constant, presumed optimal temperature and with soil at a uniform initial moisture content.

Methods

Both softwood and hardwood species were used in this study

(Table 1). Seven decay fungi were used (Table 2), but not in

a completely balanced design. P. merismoides was used only

with softwoods; X. frustulatus was used only with hardwoods.

Untreated 1-cm3 wood blocks were cut from defect-free sapwood of respective tree species. Blocks were ovendried at

54oC, weighed before being placed in vessels containing

deionzed water, subjected to a 30-min vacuum at −92 kPa,

followed by a 1-h pressure of 0.86 MPa (125 lb/in2). This

process was used to minimize the time normally required for

moisture uptake by the block from the soil and feederstrip.

Blocks were at 80% to 100% moisture content at the start of

the test. The blocks were blotted dry to remove excess moisture, weighed to determine uptake of water, then sterilized by

subjecting them to 25μGy (2.5 mrads) ionizing radiation

from a cobalt 60 source.

Small glass bottles (60 ml) were filled with 25 g of forest

soil that had a minimum moisture holding capacity of 60%.

Moisture content of the soil was adjusted to 100% (wt/wt).

A small feederstrip of sapwood (3 by 15 by 15 mm long)

was placed on the surface of the soil. Feeder strips of western

hemlock (T. heterophylla) were used with brown-rot fungi.
2

Feeder strips of sweetgum (L. styraciflua) were used with

white-rot fungi. Then, the jars were loosely capped and

sterilized (45 min at 121°C). After cooling, the feeder strip

was inoculated with a small agar disc cut from the edge of an

actively growing test fungus. The jars were incubated at

room temperature until the fungus nearly covered the wood

surface. Then, sterile test blocks were added transverse face

down. After the sterile blocks were inserted, bottles were

incubated at 32°C for 12 weeks. The incubators did not

allow for humidity control. Humidity was maintained by

placing trays of water on the bottom of the incubators and

minimizing opening and closing of the incubator doors.

Two blocks of each wood species were incubated in each of

two bottles per fungus. The identity of the bottles was followed in this study. This enabled comparison of results

between paired bottles per wood species and results for

blocks within bottles. The design used for this analysis was

a random design, with subsampling and a two-way factorial

treatment structure.

Possible changes in wood moisture content during the course

of the decay trial were also examined as a source of variation

within the experiment. Moisture content of the wood was

determined when blocks were removed from the bottles at the

conclusion of the incubation.

Results

Data were initially analyzed using a general linear model

(SAS 1989). The difference between bottles in rates of decay

in blocks of the same wood species was a significant source

of variation. Comparisons between paired bottles for each

wood species revealed significant differences between bottles,

compared to within bottle variation, in percentage weight

loss for all species except eastern hemlock, white oak, and

Douglas-fir. Results for Douglas-fir were only marginally

nonsignificant at the 95% level of probability. Western

hemlock and Douglas-fir usually had less weight loss than

did most other wood species, regardless of fungus. The

susceptibility of white oak to decay varied from midrange to

the lower third probability level, depending upon fungus.

When the difference between pairs of bottles and within

bottle variation was examined for individual fungi across all

wood species, the between bottle difference was significant for

G. trabeum, P. subserialis, and T. versicolor. These fungi

were the three most virulent against hardwoods (Tables 3–6)

Table 1—Wood speciesa used in the decay test

Softwoods Hardwoods

Common name Scientific name Common name Scientific name

Grand fir Abies grandis (Dougl.) Lindl. Red maple Acer rubrum L.

Lodgepole pine Pinus contorta Dougl. Hard maple Acer saccharum Marsh.

Red pine Pinus resinosa Ait. Sweetgum Liquidambar styraciflua L.

Douglas-fir Pseudotsuga menziesii (Mirb.) Franco Yellow poplar Liriodendron tuliplifera L.

Western hemlock Tsuga heterophylla (Raf.) Sarg. Aspen Populus tremuloides Michx.

Eastern hemlock Tsuga canadensis (L.) Carr. White oak Quercus alba L.

Red oak Quercus rubra L.

a

Sapwood was used with all wood species except aspen, which was used without determining heartwood or sapwood.

Table 2—Fungi used in study of the decay capacity of fungi in different wood species

White-rot fungi Brown-rot fungi

Trametes versicolor (L.:Fr.) Pilát R105 Postia placenta (Fr.) M. Larson et Lombard MAD698

Phlebia merismoides (Fr.:Fr.) Fr 514A Gloeophyllum trabeum (Pers.:Fr) Murr. MAD617

Phlebia subserialis (Bourd & Galzn) Donk RLG10693-SP Wolfoporia cocos (FAWolf) Ryv. and Gilbt. FP104264-SP

Xylobolus frustulatus (Pers.:Fr.) P.Karst 106073-R
3

and were nearly as aggressive in softwoods (Tables 3–5, 7).

This finding indicates that the number of replicates per species in this experiment was two (bottles) rather than four

(blocks). Therefore, subsequent discussion of results are

based upon a statistical analysis of an experiment with two

replicates. The mean percentage weight loss of the two

blocks in each bottle was used as an individual resultant

variable.

When evaluated for all wood species and fungi, final wood

moisture content was significantly different between the two

bottles per wood species by fungus combination.

Only a few fungi caused a relatively large weight loss. One

white-rot fungus, P. subserialis, and two brown-rot fungi,

P. placenta and G. trabeum, caused the most decay in nearly

all wood species. Exceptions were the maple species, which

were most severely degraded by T. versicolor, a white-rot

fungus. In softwoods, the maximum weight loss was caused

by G. trabeum or P. placenta. G. trabeum caused more

decay in red oak than did X. frustulatus.

The complete array of wood species was challenged with

three brown-rot fungi (G. trabeum, P. placenta, and

W. cocos) and two white-rot fungi (P. subserialis and

T. versicolor). Statistically significant differences were not

detected between many wood species in susceptibility to

those fungi, even though actual differences between means

were of some magnitude (Tables 4,5). Parametric and nonparametric analyses of variance (ANOVA), followed by a

Tukey multiple comparison test, produced similar results.

The absence of a significant difference between species attacked by G. trabeum was particularly striking (Fig. 1), even

with substantial differences among species in average percentage weight loss. This result was due to the heterogeneity of

variance in the experiment. A large variation between bottles

was observed with some species but not with others

(Fig. 1).

The weight losses caused by white-rot fungi (Table 5) in

hardwoods and softwoods were not significantly different,

even though a considerable range in weight loss occurred

among the wood species. Hardwoods and softwoods were

intermixed in their susceptibility to brown-rot fungi.

Hardwoods are considered to be more susceptible to whiterot fungi, but this distinction can disappear when wood is

in soil contact.

Table 3—Rank of various decay fungi by percentage weight loss caused within each respective wood

species

Ranka

 of decay in softwoods

Fungus

DouglasfirEastern

hemlock

Western

hemlock

Grand

fir

Lodgepole

pine

Red

pine

G. trabeum 1 2x 1 1

P. merismoides 33 xb 2 3

P. subserialis 2 33 x 3

P. placenta 2 1 11 2 2

T. versicolor xx x

W. cocos x x

Rank of decay in hardwoods

Fungus Aspen

SweetgumHard

maple

Red

maple

Yellow

poplar

Red

oak

White

oak

G. trabeum 1 3 2 x 1x

T. versicolor x 11 2 3

P. merismoides x x

P. subserialis 2 1 3 2 1 x2

P. placenta 3 2 x 3 3 21

W. cocos x x 3x

X. frustulatus x x

a

Ranked in decreasing order; 1 indicates most decay.

b

Within each wood species (column), percentage weight loss caused by fungus (indicated by an "x") was less

 than but not significantly different from maximum weight loss (ranked 1) observed in that wood species.
4

As shown in Tables 4 and 5, aspen, hard maple, red maple,

sweetgum, and yellow-poplar were the species most susceptible to decay. No significant difference was detected among

fungi in their ability to decay sweetgum and yellow-poplar.

Even though significant differences were detected among

fungi in their capacity to decay aspen and the two maple

species, both white- and brown-rot fungi were included

within the group that produced the most decay within each

respective wood species (Table 6).

Douglas-fir, eastern hemlock, grand fir, red oak, and white

oak were the least susceptible to decay. Mean percentage

weight loss caused by fungal species was not significantly

different for grand fir. G. trabeum caused the most decay in

red oak and the pines. With hardwoods (Table 6) and softwoods (Table 7), the relative difference between mean percentage weight loss caused by the most virulent or the two

most virulent fungi and the remaining fungi was often quite

large.

The initial uptake of water was uniform within individual

wood species, but varied among species. At the conclusion

of the test, the mean wood moisture content was not limiting

for decay.

Table 4—Weight loss of wood species as a

result of decay by brown-rot fungi

Fungi Species

Mean

weight

loss

(%)

Significancea

G. trabeum

Aspen 73.9 A

Red pine 60.3 A

Hard pine 53.6 A

Lodgepole pine 50.7 A

Douglas-fir 50.7 A

Sweetgum 42.5 A

Red oak 42.1 A

Yellow poplar 38.5 A

Red maple 37.2 A

Western hemlock 34.9 A

Grand fir 20.1 A

White oak 17.7 A

Eastern hemlock 15.9 A

P. placenta

Yellow poplar 55.1 A

Red pine 51.7 A B

Douglas-fir 50.1 A B

Red maple 45.9 A B C

Sweetgum 43.8 A B C

Lodgepole pine 43.6 A B C

Eastern hemlock 43.3 A B C

Hard maple 42.1 A B C

White oak 41.5 B C

Aspen 41.1 B C

Western hemlock 40.5 B C

Red oak 33.7 C D

Grand fir 26.4 D

W. cocos

Yellow poplar 40.7 A

Aspen 35.9 A B

Sweetgum 22.2 A B

Hard maple 16.7 A B

Red maple 16.2 A B

Grand fir 14.9 A B

Eastern hemlock 13.4 A B

Lodgepole pine 10.2 A B

Douglas-fir 8.4 A B

White oak 7.7 B

Red pine 6.1 B

Western hemlock 3.5 B

Red oak 3.5 B

a

Means with same letter are not significantly

 different (α = 0.05).

Table 5—Weight loss of wood species as a

result of decay by white-rot fungi

Fungi Species

Mean

weight

loss

(%)

Significancea

P. merismoides

Lodgepole pine 29.1 A

Grand fir 23.3 A B

Eastern hemlock 18.3 A B

Douglas-fir 14.8 A B C

Red pine 7.5 B C

Western hemlock -1.6 C

P. subserialis

Aspen 71.4 A

Yellow poplar 67.7 A B

Sweetgum 57.1 A B C

Red maple 54.1 A B C

Hard maple 51.2 A B C

White oak 29.5 A B C

Red pine 28.8 A B C

Western hemlock 25.2 A B C

Grand fir 22.9 A B C

Lodgepole pine 21.5 A B C

Eastern hemlock 20.2 B C

Red oak 18.9 B C

Douglas-fir 13.6 C

T. versicolor

Red maple 82.8 A

Hard maple 71.8 A

Yellow poplar 59.5 A B

Aspen 40.3 A B C

Sweetgum 38.1 A B C

White oak 18.0 B C

Red pine 15.9 B C

Red oak 13.6 B C

Lodgepole pine 12.4 B C

Douglas-fir 9.8 C

Grand fir 6.2 C

Eastern hemlock 4.6 C

Western hemlock 1.1 C

a

Means with same letter are not significantly

 different (α = 0.05).
5

There was a significant correlation between percentage weight

loss in individual wood species and final moisture content of

wood blocks at time of removal. The most pronounced

correlation between these two parameters within an individual wood species was demonstrated by red oak (Fig. 2).

A similar, but less well-defined, correlation occurred for red

maple. The comparison of moisture content and weight loss

data for aspen fell into two clusters. One cluster of data indicates 30% to 40% weight loss was associated with final

moisture content of about 50% to 90%. Two other observations of weight losses of about 70% had wood moisture

content levels greater than 200%. The relationship between

percentage weight loss and final wood moisture content was

less apparent for the other wood species.

There was no overall correlation between percentage weight

loss as a result of individual fungi and final wood moisture

content (Fig. 3). Only with G. trabeum was there a suggestion of a correlation between final wood moisture content

and percentage weight loss, particularly at weight loses

greater than 30%. Final wood moisture content levels were

grouped rather tightly across all wood species for W. cocos,

but not for the other fungi.

Discussion

Although the practice of incubating two experimental wood

blocks in one vessel is not unique to this study (AWPA

1994), the fundamental consequence of potential loss of

independent observations must be recognized. Furthermore,

it would seem that verification of independence of replicates

should be a criterion for acceptance of any comparative data

that utilizes this procedure. Definition of the number of

replicates needed to add precision to this type of test also

seems in order. In this study, we often observed substantial

differences of magnitude between mean values for various

experimental combinations, without detecting a statistically

significant difference.

Table 6—Weight loss caused by six decay fungi

in three hardwoods

Hardwoods Fungi

Mean

weight

loss

(%)

Significancea

Aspen

G. trabeum 73.9 A

P. subserials 71.4 A

P. placenta 41.1 B

T. versicolor 40.3 B

W. cocos 35.9 B

X. frustulatus 31.9 B

Hard maple

T. versicolor 71.8 A

G. trabeum 53.6 A

P. subserials 51.2 A

P. subserials 42.1 A B

W. cocos 16.7 B C

X. frustulatus 10.0 C

Red maple

T. versicolor 82.8 A

P. subserials 54.1 A B

P. placenta 45.9 A B

G. trabeum 37.2 B

W. cocos 16.2 B

X. frustulatus 11.9 B

a

Means with the same letter are not significantly

different (α = 0.05).

Table 7—Weight loss caused by six decay fungi

in three softwood species

Softwoods Fungi

Mean

weight

loss

(%)

Significancea

Douglas fir

G. trabeum 50.5 A

P. placenta 50.1 A

P. merismoides 14.8 B

P. subserials 13.6 B

T. versicolor 9.8 B

W. cocos 8.4 B

Eastern hemlock

P. placenta 43.3 A

P. subserials 20.2 B

P. merismoides 18.3 B C

G. trabeum 15.9 B C

W. cocos 13.4 B C

T.versicolor 9.8 C

Lodgepole pine

G. trabeum 50.7 A

P. placenta 43.6 A B

P. merismoides 29.1 A B

P. subserials 21.5 A B

T. versicolor 12.4 B

W. cocos 10.2 B

a

Means with the same letter are not significantly

different (α = 0.05).
6

Because of the magnitude of differences in apparent capacity

of fungi to decay either specific wood species or selected

groups of woods, it seems that efficiency with this type of

procedure could be gained by minimizing the number of

assay fungi and maximizing the number of independent

replications. This would require careful selection of fungi on

the basis of presumed tolerance to the preservatives being

assayed and the capacity to decay the wood species that are

included in the test.

The final wood moisture content did not appear to be a

limiting factor for the weight loss of the wood. Some correlations between high levels of weight loss and final moisture

content were expected, recognizing that as decay progresses,

the reference base for ovendry weight determination decreases.

Conclusions

The soil-jar procedure using small wood blocks requires

only small amounts of wood material and soil. In this study,

the procedure was prone to a large variability within results.

One important source of variation was the difference among

bottles. Before this procedure can be accepted as a standard

protocol for evaluating preservative-treated woods, the source

of procedural variability, reproducibility, and sensitivity

must be defined.

Aspen

Douglas-fir

Eastern hemlock

Grand fir

Hard maple

Lodgepole pine

Red maple

Red oak

Red pine

Sweetgum

Western hemlock

White oak

Yellow poplar

AS

DF

EH

GF

HM

LP

RM

RO

RP

SG

WH

WO

YP

AS

RP

AS

RP

HM

HM

LP

LP

DF

DF

SG

RO

YP RM WH

RO

YP

RM

WH

SG GF GF WO

WO EHEH

80

70

60

50

40

30

20

10

0

Average weight loss (%)

Figure 1—Variation between paired bottles in average

percentage weight loss (two blocks of one wood per

bottle) caused by G. trabeum. Range between paired

bottles is shown as dashed line. Percentage weight loss

between wood species is not significantly different.

90

80

70

60

50

40

30

20

10

0

–10 Average weight loss (%)

Average final moisture content (%)

0 50 100 150 200 250 300

Aspen

Eastern hemlock

Douglas-fir

Sweetgum

Lodgepole pine

Red maple

White oak

Yellow poplar

Red oak

Western hemlock

Grand fir

Hard maple

Red pine

A

E

F

G

L

M

O

P

R

W

X

Y

Z

Figure 2—Relationship, by wood species, between

average final percentage wood moisture content and

average percentage weight loss in wood blocks. Each

entry represents an individual fungus species in wood

(identified by a letter).

90

80

70

60

50

40

30

20

10

0

–10 Average weight loss (%)

Average final moisture content (%)

0 50 100 150 200 250 300

◆◆

◆

◆ ◆

◆

◆

◆

◆◆◆

◆

◆

G. trabeum

P. merismoides

P. placenta

P. subserialis

T. versicolor

W. cocos

X. frustalatus ◆

Figure 3—Relationship, by fungus, between average final

percentage wood moisture content and average percentage

weight loss in blocks. Each entry represents a separate

wood species challenged by fungi (identified by a symbol).
7

References

ASTM. 1995. Standard test method for wood preservatives

by laboratory soil-block cultures. Standard D1413-76. In:

ASTM Annual Book of Standards Vol. 4.10. Philadelphia,

PA: American Society for Testing and Materials: 207–213.

AWPA. 1994. Standard method of testing wood preservatives by laboratory soil-block cultures. Standard E10-91.

In: Book of Standards. Woodstock, MD: American WoodPreservers’ Association: 288–298.

Bravery, A.F. 1979. A miniaturized wood-block test for the

rapid evaluation of wood preservative fungicides. In: Screening techniques for potential wood preservative chemicals.

Proceedings, special seminar. Rep. 136. Peebles. Stockholm, Sweden: Swedish Wood Preservation Institute:

57–65.

SAS Institute Inc. 1989. SAS/STAT User’s Guide,

Version 6. 4th ed. Cary, NC: SAS Institute Inc. Vol. 1&2.

Scheffer, T.C.; Morrell, J.J.; Newbill, M.A. 1987. Shellrot control in western redcedar: potential replacements for

pentachlorophenol spray. Forest Products Journal.

37 (7/8): 51–54.

Scheffer, T.C.; Newbill, M.A.; Morrell, J.J. 1988.

Laboratory evaluation of potential replacements for

pentachlorophenol in sprays to control sapwood decay in

western redcedar poles. Holzforschung. 42: 185–189.

.