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Chapter 3
Soil Microbes in
Plant Health anc! Nutrition
The parts of a plant above the ground can be compared to the tip of an
iceberg, in that the portion under the surface—the root system—is so exten-
sive. The root system is also very active metabolically and provides a continu-
ous source of food for soil microorganisms in the form of secretions of
organic compounds and sloughed-off dead cells and cell debns. Since the zone
where roots and soils meet is a special environment, it has been named the
rhizosphere (root zone). This zone comprises several poorly defined, hetero-
geneous regions in which microorganisms are particularly active (Table 3.13.
Although activity in the rhizosphere is of great importance to the plant, it
affects only a small fraction (about 5 percent) of the root surface. Some
microorganisms are loosely associated with roots, but others develop on the
root surface and many can invade root tissue, with effects that can be bene-
ficial or harmful. Certain soil-inhabiting microorganisms produce diseases of
great significance to agriculture and forestry. Others are beneficial—they part-
ly inhibit the growth of disease organisms or kill them. The vast majority of
the pathogens that infect roots are fungi, and they are exceptionally difficult
to control or eradicate.
In some cases, the invasion of roots by microorganisms is desirable. This is
true for the root-nodule bacteria of the genus Rhizobium that fix nitrogen, as
well as for mycorrhizal fungi, which assist roots in accumulating phosphate
and other essential minerals. Nitrogen fixation is the subject of Chapter 4 in
this report; mycorrhizal fungi are discussed in this chapter. However, it
should be emphasized that rhizosphere microorganisms can affect plant wel-
fare in a number of ways that are not yet well understood or easily con-
trolled. The processes of nutrient cycling, growth stimulation or inhibition,
and diseases are of great significance, but they are very complex population
effects rather than the result of simple interactions between roots and known
microorganisms. The rhizosphere is also influenced by external factors such as
soil moisture and even the intensity of light reaching the plant. No single
microorganism may be essential to the process, but the combined effect of
the rhizosphere population can be profound.
47
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48
MICROBIAL PROCESSES
TABLE 3.1 Comparison of the Numbers of Various Groups of Organisms in the Rhizo-
sphere of Spring Wheat and in Control Soil
Numbers per g Numbers per g Approximate
in Rhizosphere in Control Rhizosphere:
Organisms Soil(x 10-6) Soil(x 10-6) SoilRatio
Bacteria 1,200 53 23 :1
Actinomycetes 46 7 7 :1
Fungi 12 0.1 120 :1
Protozoa 0.0024 0.001 2: ~
Algae 0.005 0.027 0.2 :1
Bacterial Groups
Ammonifiers 500 0.04 12,500 :1
Gas-producing anaerobes 0.39 0.03 13: 1
Anaerobes 12 6 9: 1
Den~trifiers 126 0.1 1,260: 1
Aerobic cellulose decomposers 0.7 0.1 7: 1
Anaerobic cellulose decomposers 0.009 0.003 3: 1
Spore formers 0.930 0.575 2: 1
"Radiobacter" types 17 0.01 1,700: 1
Azotobacter <0.001 <0.001 ?
Adapted from: T. R. G. Gray, and S. T. Williams. 1975. Soil Microorganisms, New York:
Longman, p. 144.
The sum of the various interrelationships of rhizosphere microorganisms
and roots can benefit plant growth by influencing the availability of essential
nutrients, by producing plant growth regulators, and by suppressing root
pathogens.
Mineral Cycling by Soil Microorganisms
By decomposing plant and animal residues, soil microorganisms release
carbon, nitrogen, sulfur, phosphorus, and trace elements from organic mate-
rials in forms that can be absorbed by plants.
This process, known as mineralization, is the primary source of atmo-
spheric carbon dioxide. Without mineralization of organic carbon, the carbon
dioxide content of the air, which is essential for plant photosynthesis, would
be progressively reduced and plant production would ultimately cease. Main-
tenance of the carbon cycle, therefore, is one of the most important biolog-
ical processes on earth.
Microbial activities similar to those responsible for the carbon cycle also
transform soil nitrogen, sulfur, and phosphorus, and to a lesser extent are
instrumental in the conversion of other elements. Although particular atten-
tion has been directed to microbial transformations of nitrogen, plants also
have a nutritional need for sulfur. The microbial transformations of nitrogen
and sulfur are much alike because both elements can be oxidized and re-
duced. Sulfur reduction is necessary for the synthesis of sulfur-containing
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SOIL MICROBES IN PLANT HEALTH AND NUTRITION
49
amino acids. Under anaerobic conditions, sulfur reduction may produce hy-
drogen sulfide, which can be harmful to plants. It accumulates in very wet
soils, like rice paddies, and can cause straighthead disease of rice and other
physiological plant disorders. At low concentrations, however, it can supply
the sulfur requirements of some plants.
A recent report indicates that oxidation of hydrogen sulfide by a bac-
terium in the genus Beggiato a detoxified flooded rice soils. Beggiato a species
may be significant in coastal marshes and estuaries as well as in rice paddies,
and their capacity to influence plant growth favorably deserves further study.
The oxidation of sulfur by bacteria of the genus Thiobacillus is also of poten-
tial significance in agriculture. The product of this transformation is sulfuric
acid, which can dissolve minerals that otherwise would not be available for
plant growth. Farmers who add elemental sulfur to rock phosphate find that
phosphorus is liberated more rapidly and in greater amounts than if the sulfur
is omitted. The explanation for this is that Thiobacillus species oxidize the
sulfur to sulfuric acid, which liberates the phosphorus from the insoluble rock
phosphate.
Microorganisms are also able to promote phosphorus solubilization by the
production of chelators, which form complexes with metal ions and increase
their solubility. Acid and chelate production can be easily demonstrated
under laboratory conditions, but little is known of the phosphate-dissolving
effectiveness of different types of microorganisms under natural soil condi-
tions. Solvent action by microorganisms is not restricted to a few species; it is
characteristic of many members of the rhizosphere population and can be
accomplished in part by plant roots as well. The microbial transformation of
nutrient elements other than those cited above is in some instances similar to,
and in other instances quite different from, the process just discussed. Un-
fortunately, much more is still to be learned about this subject.
Microorganisms require many of the same nutrient elements that are essen-
tial to plants for their growth. When nitrogen, sulfur, or phosphorus is in
short supply, the rhizosphere population will compete with roots for nourish-
ment. Because of their abundance, small size, and relatively large surface area,
and because they surround the absorbing part of the root, microbes will
absorb nutrients at the expense of the plant. Ultimately, plants will display
signs of nutrient deficiency and crop yields may decrease.
Barber and Martin (1976) have recently found that for barley, 10-20
percent of the photosynthate may be released from roots in nonsterile soil.
Less was released in sterile soil. The rhizosphere may exact a price in terms of
energy given up to the soil by the plant.
There has been speculation that in the rhizosphere oxygen consumption
occurs more rapidly than diffusion, so that anaerobic sites may form in places
in the root; such reduced conditions could be important in making ferrous
ions from ferric, for instance, which increases iron solubility. Wheat roots
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so
MICROBIAL PROCESSES
have high populations of denitrifying bacteria, so oxygen-free conditions
must exist in their presence.
Microorganisms are prolific producers of vitamins, amino acids, hormones,
and other growth-regulating substances. Many bacteria and fungi isolated
from soil are able to synthesize compounds that provoke a growth response in
plant tissue. Some produce indoleacetic acid or gibberellins, which are hor-
mones that control plant growth, while others produce vitamins. Many may
also produce unidentified growth factors. Rhizosphere microorganisms are
variously credited with promoting increased rates of seed germination, root
elongation, root-hair development, nutrient uptake, and plant growth.
Inoculation
Root uptake of organic compounds has received more attention in We
U.S.S.R. than elsewhere, and Russian investigators claim rhizosphere micro-
organisms influence the quality as well as quantity of tissue produced by
plants. Although there are no experimental results that convincingly establish
that growth-promoting substances of microbial origin occur in the rhizo-
sphere, speculation persists that such compounds are synthesized in the vicin-
ity of roots and affect crop yields.
Vanous bacterial fertilizers have been marketed at different times, but
commercial preparations known as azotobacterin and phosphobacterin have
received the most attention. Azotobacterin is composed of cells of Azoto-
bacter chroococcum, a bacterium able, under some conditions, to fix atmos-
pheric nitrogen. Phosphobacterin contains the bacterium Bacillus mega-
terium var. phosphaticum, which mineralizes organic phosphorus compounds.
Russian scientists think that growth of these bacteria in soil will supply
plants with nitrogen and phosphorus, but this has not been proved. Treated
plants are favorably affected, but growth is not increased by more than 10
percent. Moreover, the effect is said to be due not to nitrogen fixation or
phosphorus solubilization, but to plant hormones.
Since the benefits are minimal and depend on conditions difficult to con-
trol in the field, and the results are unpredictable, bacterial fertilizers are not
recommended for general use.
Although the potential benefits of inoculating bacteria have not yet been
fully explored, it is questionable how much additional exploration may be
warranted. Present evidence is insufficient to justify the use of inoculants,
other than rhizobia for legumes, to increase crop yields, improve plant quality,
or control disease. A beneficial effect is even less likely when the microorga-
nism used as inoculum is a normal inhabitant of soil. The British soil micro-
biologist S. D. Garrett (1956) has described this problem as follows:
[Such] attempts to boost the population of an antagonistic organism by
inoculation alone have been doomed to failure from their inception, because
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SOIL MICROBES IN PLANT HEALTH AND NUTRITION
51
they are in flagrant contradiction to the ecological axiom that population is a
reflection of the habitat, and that any change due to plant introduction
without change of the habitat must be a transient one.
Mycorrhizal Fungi
Most plants, both wild and cultivated, have roots infected with fungi Mat
increase nutrient and water uptake and may also protect the root from cer-
tain diseases. These infected roots are called mycorrhizae. Although the
mycorrhizal fungi probably increase uptake of all the essential elements, they
are usually most important in improving phosphorus nutrition. Phosphate is
generally present in the soil in low concentrations and it is also highly im-
mobile. Strands of fungal hyphae grow out from mycorrhizae and greatly
increase the volume of soil from which phosphorus is obtained. So mycor-
rhizal plants, in general, can grow and thrive in soils much lower in phosphate
and other essential nutrients than a comparable nonmycorrhizal plant. Many
plants are so dependent on mycorrhizal fungi for nutrient uptake that they
may starve if these fungi are absent. There are a number of types of mycor-
rhizae. The two that occur on the most economically important crops, the
endomycorrhizae and the ectomycorrhizae, are discussed below.
Endomycorrhizae of Crop Plants and Forest Trees
Endomycorrhizae of the vesicular-arbuscular (VA) type occur on nearly
all crop plants (plants in a few families such as the Cn~ciferae [cabbage,
mustard, etc.] and Chenopodiaceae [beets, spinach, etc.] may be nonmycor-
rhizal). They also occur on many trees in temperate regions and on the
majority of tree species native to the subtropics and tropics. VA mycorrhizal
fungi are present in almost all soils and they are not host-specific. Thus, the
same fungus producing VA mycorrhizae on trees will form mycorrhizae on
plants after land is cleared and planted to agricultural crops. The mycorrhizal
condition is normal for most plants, and absence or scarcity of mycorrhizal
fungi can greatly limit plant growth (Figure 3. 14. Introduction of mycorrhizal
fungi to soil environments lacking or with low populations of such organisms,
such as biocide-treated soils, can enhance plant growth. VA mycorrhizae are
particularly important for many legumes in that they stimulate nodulation by
Rhizobium' thereby increasing nitrogen fixation. Improved phosphorus nutri-
tion of the mycorrhizal legume is responsible for increased nodulation.
VA mycorrhizal fungi survive in soil as resting spores. They obtain their
food from the plant roots and they are unable to grow independently in soil.
It is unlikely that they obtain much, if any, organic nutrient from soil.
VA fungi have not been grown in pure culture, which presents an obstacle
to artificial inoculation. However, these fungi produce the largest spores of
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52
MICROBIAL PROCESSES
FIGURE 3.1 Endomycorrhizal (left) and nonmycorrhizal (right) peanut plants (ground-
nuts). (Photograph courtesy of J. W. Gerdemann)
any known fungi, some being 0.5 mm or more in diameter. The spores can be
easily extracted from the soil with sieves and then propagated on the roots of
living plants. The infected roots can also be used for inoculation. Heavily
infected palm roots collected in the wild have been used as a source of
inoculum. If field-collected inoculum is used, it is important that it be free of
dangerous pathogens.
There are situations in which inoculation with VA fungi is highly bene-
ficial. If soil is treated with steam or fumigants to kill pathogens, VA fungi
are also killed, and considerable time is required for them to become reestab-
lished naturally. The nutrient deficiencies and associated stunting that often
result may be prevented by inoculating the soil with VA fungi rather than by
applying excessive rates of fertilizer.
The greatest opportunity for the use of VA fungi is in soils low in available
phosphorus, which includes many untreated soils in tropical regions. There is
evidence as well that many of these soils also contain less than the optimum
number of spores of VA fungi. In such soils inoculation may enable the use of
inexpensive rock phosphate fertilizer instead of the more expensive super and
triple phosphates.
The major obstacle to greater use of VA fungi is the difficulty in obtaining
inoculum. However, there are several commercial companies interested in
producing pure inoculum, and it may become available in the near future. We
are now at the stage where different species of VA fungi should be tested on
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SOIL MICROBES IN PLANT HEALTH AND NUTRITION 53
crop plants and forest trees and their effects on growth compared in the field
in untreated soils and in soils that have been sterilized.
Ectomycorrhizae of Forest Trees
Ectomycorrhiza is the second most common type of mycorrhizae. It
occurs on roots of pine, spruce, fir, larch, hemlock, willow, poplar, hickory,
pecan, oak, birch, beech, and eucalyptus (Figure 3.2). The fungi that form
ectomycorrhizae produce mushrooms and puffballs as their reproductive
structures (fruit bodies). In North America there are more than 2,100 species
of ectomycorrhizal fungi. The fungi are spread in nature by millions of micro-
scopic spores, finer than dust, which are released from fruiting bodies and
moved great distances by winds.
Many forest trees, such as pines, cannot grow beyond the first year with-
out an appreciable number of ectomycorrhizae. Ectomycorrhizae benefit
trees by: increasing nutrient and water absorption from soil; increasing the
tolerance of the tree to drought and extremes of soil conditions (acid levels,
toxins, etc.~; increasing the length of the feeder root system; and protecting
the fine feeder roots from certain harmful soil fungi.
Ectomycorrhizal fungi cannot grow and reproduce unless they are in asso-
ciation with the roots of a tree host. These fungi obtain all their essential
FIGURE 3.2 Examples of pine ectomycorrhizae. Each different ectomycorrhiza is
formed by a different species of fungus. Each main root is approximately 3 cm long.
(Photograph courtesy of J. W. Gerdemann)
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54
MICROBIAL PROCESSES
sugars, vitamins, amino acids, and other foods from their hosts. It is unlikely
that these fungi as a group are directly involved in any significant decomposi-
tion of forest litter.
Certain forest trees, then, must have ectomycorrhizae to survive and grow,
and the ectomycorrhizal fungi need their tree hosts to exist. This means that
the introduction of tree species as exotics into regions where the appropriate
mycorrhizal fungi are absent must be accompanied by the introduction of
their natural ectomycorrhizal fungi. In the past, this introduction has been
accomplished mainly by using soil collected from under healthy trees with
ectomycorrhizae, which is mixed into the upper layer of soil in nurseries. The
seedlings planted in this soil usually form abundant ectomycorrhizae in one
growing season, and they are then transplanted to the field. Unfortunately,
this method is not without risk, since pathogens can be present in the intro-
duced soils and cause serious damage to the trees. The logistics of transport-
ing large volumes of soil great distances is an added problem. By far the most
biologically sound method of correcting an ectomycorrhizal deficiency is by
the use of pure cultures of selected species of ectomycorrhizal fungi.
In recent years, techniques have been devised to inoculate soil with pure
vegetative and spore cultures of Pisolithus tir~ctorius in the United States and
to introduce spore cultures of Rhizopogon luteolus onto pine seed and into
soil in Australia. These two puffball-producing fungi form ectomycorrhizae
on many commercially important forest trees. Currently, research is being
done in the United States on the use of a commercially produced vegetative
inoculum of P. tinctorius, which should be available at economical prices on
the world market in the next few years. Thus far, P. tinctorius appears to
enhance growth more than other ectomycorrhizal fungi, and it can be used to
tailor-make seedlings to improve the performance of trees even in areas where
other ectomycorrhizal fungi are present.
Pines with Pisolithus ectomycorrhizae formed in nurseries and planted in
forest sites in the southern United States have not only survived, but have
grown to twice the heights of comparable pines with naturally occurring
ectomycorrhizae. On sites where it is difficult to establish pines, such as those
created by strip-mining for coal, the only trees capable of growing are often
those that have been inoculated with Pisolithus. The selection and use of
specific ectomycorrhizal fungi may well determine whether a productive
forest becomes established.
The most obvious research need on ectomycorrhizae in developing coun-
tries is to determine whether appropriate ectomycorrhizal fungi are present
prior to the establishment of forests of introduced species. If such forests are
already established or are accessible, then fruit bodies of ectomycorrhizal
fungi can be collected and the spores harvested. The puffball fungi usually
produce an abundance of easily extractable spores. For example, one fruit
body of P. tinctorius may contain 75 grams of spores, and there are more
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SOIL MICROBES IN PLANT HEALTH AND NUTRITION
55
than one billion spores in a gram. Spores of P. tinctor~us when kept dry and
cool have been stored for more than 4 years without losing their ability to
form ectomycorrhizae. The spores can be mixed into nursery soil and the
seed of the desired tree species planted. Moderate levels of fertility and at
least 2-4 percent organic matter should be maintained in the soil. Usually,
the seedlings will have adequate ectomycorrhizae in 6-7 months after seed
germination.
The production of vegetative cultures of ectomycorrhizal fungi requires
aseptic culture technique. This means that the substrate on which the fungi
are grown must be sterilized, usually by autoclaving, and maintained free of
other microbial contamination for at least several weeks, or until the fungus
has produced sufficient growth to overcome contamination. After it has been
leached with water, this inoculum can then be added to soil.
Some species of ectomycorrhizal fungi are more beneficial to tree growth
and development under different soil conditions than others. It is important
to select and test different species to determine which are best suited to
specific locations.
Biological Control of Soil-Borne Pathogens
Microorganisms that cause root diseases are sometimes suppressed by other
microorganisms in the soil. In many instances disease-causing organisms may
be present, but because of naturally occurring biological control, little or no
disease results. The prevalence of pathogens may be reduced by crop rotation
using a non-host crop, which often starves the pathogen and prevents it from
reproducing.
It is also possible to increase the level of organisms that are antagonistic to
soil-borne plant pathogens. This is generally done not by adding antagonistic
microorganisms directly to the soil, but by the use of various organic amend-
ments such as manure or plant residues. These amendments provide a source
of food for soil-borne microorganisms that can inhibit the development of
plant pathogens. Research is needed in order to exploit more fully the use of
various forms of organic matter to enhance this biological control of soil-
borne pathogens.
There have been many attempts to control pathogens in soil by the addi-
tion of specific microorganisms. In general, these attempts have failed to
increase the level of naturally occurring biological control. For example, soil
contains species of fungi that trap and feed on plant parasitic nematodes
(Figure 3.39. However, application of additional nematode-trapping fungi
failed to protect plants under field conditions. It is likely that unless the soil
is altered in some way, it naturally contains the maximum number of nema-
tode-trapping fungi that it can support. There is, however, hope that we may
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56
MICROBIAL PROCESSES
FIGURE 3.3 A nematode-trapping fungus, Dactylella drechsleri, which captures prey
on adhesive knobs. (Photograph courtesy of D. Pramer)
be on the verge of a major advance in controlling soil-borne pathogens by
adding specific microorganisms or by altering the rhizosphere environment.
There are a few examples where disease has been reduced by applying a
hypovirulent strain or a mutant strain of a pathogen that is incapable of
producing disease. Such strains may prevent development of the pathogenic
strains. A nonpathogenic strain of the crown gall bacterium will thus protect
plants from attack by a pathogenic strain.
In soil, most pathogenic fungi must pass through the rhizosphere or must
live within this zone. Their success in colonizing or infecting plant roots
depends upon their ability to compete with other rhizosphere microorgan-
isms. The chemical and microbiological environment in this zone may be
altered slightly, but significantly, to effect changes in the inoculum potential
of pathogens, either by selections of plant genotypes that produce such
changes or by careful regulation of nitrif~cation in soil.
Research on biological control should be highly encouraged, for it could
provide an alternative means of disease control to the use of expensive and
often dangerous pesticides.
References and Suggested Readings
Alexander, M. 1977. Introduction to soil microbiology. 2nd Edition. New York: John
Wiley and Sons.
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SOIL MICROBES IN PLANT HEALTH AND NUTRITION
57
. 1974. Microbial ecology. New York: John Wiley and Sons.
Baker, K. F., and Cook, R. J. 1974. Biological control of plant pathogens. San
Francisco: W. H. Freeman and Company.
Barber, D. A., and Martin, I. K. 1976. The release of organic substances by cereal roots
into soil. The New Phytologist 76:6 9-80.
Barber, D. S. 1968. Microorganisms and the inorganic nutrition of higher plants. Annual
Review of Plan t Physiology 19: 71-8 8.
Barron, G. L. 1977. The nematode-destroying fungi. Topics in mycobiology No. 1.
Guelph, Ontario: Canadian Biological Publications Ltd.
Brown, M. E. 1974. Seed and root bacterization. Annual Review of Phytopathology
1 2:1 81-197.
; Hornby, D.; and Pearson, V. 1973. Microbial populations and nitrogen in soil
growing consecutive cereal crops infected with take-all. Journal of Soil Science
24(3): 296-3 1 0.
Carson, E. W. 1974. The plant root and its environment. Charlottesville: The University
Press of Virginia.
Cook, R. J. 1977. Management of the associated microbiota. In Plant disease: an ad-
vanced treatise in how disease is managed. J. G. Horsfall and E. B. Cowling, eds.,
pp. 145-166. New York: Academic Press.
. 1976. Interaction of soil-borne plant pathogens and other microorganisms: an
introduction. Soil Biology and Biochemistry 8:267.
Doetsch, R. N., and Cook, T. M. 1976. Introduction to bacteria and their ecobiology.
Baltimore, Maryland: University Park Press.
Garrett, S. D. 1956. Biology of root-infecting fungi. p. 11. New York: Cambridge Uni-
versity Press.
1970. Pathogenic root-infecting fungi. Cambridge, England: Cambridge Univer-
sity Press.
Gerdemann, J. W. 1975. Vesicular-arbuscular mycorrhizae. In The development and
function of roots. J. G. Torrey and D. T. Clarkson, eds., pp. 575-591. New York:
Academic Press.
Gray, T. P., and D. Parkinson. 1968. The ecology of soil bacteria. Liverpool: Liverpool
University Press.
, and Williams, S. T. 1975. Soil microorganisms. New York: Longman.
Harley, J. L. 1979. Proceedings of the soil-root interface symposium: London: Academic
Press.
Henis, Y., and Chet, I. 1975. Microbiological control of plant pathogens. Advances in
A pplied Microbiology 19: 85 .
Hornby, D. 1978. Microbial antagonisms in the rhizosphere. Annals of Applied Biology
89 :97-1 00.
Joshi, M. M., and Hollis, J. P. 1977. Interactions of Beggiatoa and rice plants: detoxifica-
tion of hydrogen sulfide in the rice rhizosphere. Science 197:179-180.
Kleinschmidt, G. D., and Gerdemann, J. W. 1972. Stunting of citrus seedlings in fumi-
gated nursery soils related to the absence of endomycorrhizae. Phytopathology
62: 1447-1453.
Krasilnikov, N. A. 1958. Soil microorganisms and higher plants. Moscow: Academy of
Sciences USSR. English translation, by Y. Halperin, 1961. Jerusalem: Israel Program
for Scientific Translations, Ltd.
Marks, G. C., and Kozlowski, T. T. 1973. Ectomycorrhizoe: their ecology and physiol-
ogy. New York: Academic Press.
Marx, D. H. 1977. The role of mycorrhizae in forest production. In Proceedings of the
TAPPI (Technical Association of the Pulp and Paper IndustryJ Annual Meeting,
February 14-16, 1977, held in Atlanta, Georgia, pp. 151-161. Atlanta: TAPPI.
Mosse, B. 1977. Plant growth responses to vesicular-arbuscular mycorrhiza: X. Responses
of stylosanthes and maize to inoculation in unsterile soils. New Phytologist
7 8 :277-288.
1977. The role of mycorrhiza in 1eguMe ~utritiol1 OI1 marginal soils. In Exploiting
the legume-rhizobium symbiosis in tropical agriculture: Proceedings of a workshop,
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MICROBIAL PROCESSES
University of Hawaii, August 1976, College of Tropical Agriculture Miscellaneous
Publication No. 54, pp. 275-292. Honolulu: University of Hawaii.
Rovira, A. D.; Newman, F. I.; Bowen, H. J.; and Campbell, R. 1974. Quantitative assess-
ment of the rhizoplane microflora by direct microscopy. Soil Biology and Biochem-
istry 6:211-216.
Sanders, F. E.; Mosse, B.; and Tinker, P. B., eds. 1974. Endomycorrhizas: Proceedings.
Symposium on Endomycorrhiza. July, 1974, University of Leeds. New York: Aca-
demic Press.
Shipton, P. J. 1977. Monoculture and soilborne pathogens. Annual Review of Phyto-
pathology 15: 387-407.
Smith, A. M. 1976. Availability of plant nutrients in reduced microsites in soil. Annual
Reviewof Phytopathology 14:53-73.
Tansey, M. R. 1977. Microbial facilitation of plant mineral nutrition. In Microorganisms
and minerals, E. D. Weinberg, ea., pp. 343-385. New York: Marcel Dekker, Inc.
United Nations Educational, Scientific, and Cultural Organization. 1969. Soil biology:
review on research. Natural Resources Research, Series No. 9. Paris: United Nations
Educational, Scientific, and Cultural Organization. Distributed in the United States
by UNIPUB, New York.
Research Contacts
Rhizosphere
Richard Bartha, Rhizosphere Group, Cook College, Rutgers University, New Brunswick.
New Jersey 08903, U.S.A.
J. P. Hollis, Department of Plant Pathology, Louisiana State University, Baton Rouge
Louisiana 70803, U.S.A.
A. D. Rovira, Division of Soils, CSIRO, P.O. Box 2, Glen Osmond, Adelaide, South
Australia, 5064.
Endomycorrhizae
J. W. Gerdemann, Department of Plant Pathology, University of Illinois, Urbana, Illinois
61801, U.S.A.
John A. Menge, Department of Plant Pathology, University of California, Riverside,
California 92521, U.S.A.
Barbara Mosse, Rothamsted Experimental Station, Harpenden, Hertshire AL5 2JQ, Eng-
land
T; H. Nicolson, Department of Biological Sciences, University of Dundee, Dundee, DD1
4HN, Scotland
Ectomycorrhizae
Donald H. Marx, Director and Chief Plant Pathologist, Institute for Mycorrhizal Re-
search and Development, Forestry Sciences Laboratory, U.S. Department of Agri-
culture, Carlton Street, Athens, Georgia 30602, U.S.A.
Orson K. Miller, Jr., Department of Biology, Virginia Polytechnic Institute, Blacksburg,
Virginia 24061, U.S.A.
James M. Trappe, Forest Service, Forestry Sciences Laboratory, U.S. Department of
Agriculture, 3200 Jefferson Way, Corvallis, Oregon 97331, U.S.A.
Biological Control of Soil-Borne Pathogens
R. James Cook, Regional Cereal Disease Research Laboratory, U.S. Department of Agri-
culture, Washington State University, Pullman, Washington 99163, U.S.A.
Allen Kerr, Department of Plant Pathology, Waite Agricultural Institute, University of
Adelaide, Glen Osmond, Adelaide, South Australia, 5064.
Representative terms from entire chapter:
mycorrhizal fungi
