The Truth About
Microbes-- Microbes in soil and sand-based root zones:
A few of the basics.
David A. Zuberer
College Station, TX 77843-2474
Portions of this article
Dept. of Soil and Crop Sciences appear in the July, 2005,
Texas A&M University issue of SPORTTURF
Throughout my career as a university researcher
and teacher, I have studied microbes in soils from a number
of viewpoints. Mostly my interests have been focused on the
roles of so-called beneficial microbes in soil-plant systems
ranging from agricultural fields to reclaimed surface-mine sites.
More recently, perhaps because of an interest in golf, but perhaps
more so because of the need for basic information for turfgrass
managers (my job brings me in contact with many students aspiring
to work in the sports turf arena), our studies have focused
on some of the microbiological aspects of sports turfs on native
soils as well as those established on sand-based systems. As
I read various publications, from trade magazines to information
on the World Wide Web, I find that there is a wealth of misunderstanding
and misconceptions among the general public regarding not only
the real functions of soil microbes but also of what it takes
to maintain or manipulate them. To read some of the material
'out there" one would think that agriculture, including
turf management, has been waging "all out" chemical
warfare on soil microbes and that we have all but annihilated
them in our soils. To try to bring some clarity to the subject,
I would like to discuss some of the issues surrounding soil
microbes in turf with special reference to sand-based root zones,
as that seems to be an area of some confusion.
In the space remaining, I would like to address
the following questions:
1. Are native soils and sand-based root zones different?
2. What microbes are in sports-field soils?
3. How many are there; how much biomass?
4. What do they do?
5. What do you need to do for them?
I will try to provide some relevant information from the standpoint
of what we know about soil microbes and their activities and I
will try to indicate areas where the science is still uncertain.
1. Are native soils and sand-based root zones different?
To some, this is obvious; there are differences in the two systems!
But let's take a look from a microbiological perspective. Some
of the major ways in which soils and sand-based root zones might
differ are listed in Table 1. It is likely that native soils will
have a greater content of silt and clay than a sand-based root
zone as that is one of the principal reasons for developing the
sand base; it cuts down on the finer particles and leads to coarser
(more sandy) textures with the presumed advantage of better aeration
and drainage. It is also likely that native soils have better
aggregate-forming potentials than the coarser sands. Thus, if
managed properly, native soils can exhibit good drainage and will
likely have a more variable range of pore sizes. Pore size is
critical for air and water retention and finer textured soils,
if not well aggregated, tend to drain poorly making the soil environment
less suitable for microbes and roots. One might envision that
the chemical properties of soils would be more complex than those
of the sands although many different ingredients, e.g., zeolites
or diatomaceous earth, have been added to sand mixes to improve
their chemical properties (cation exchange capacity, etc.). One
might also expect that native topsoil would have more organic
matter than a sand mix and that it might be more complex in its
chemical composition. Sand-based systems are generally constructed
with peat or some other organic material (for example rice hulls)
making up as much as 20% of the mix. One should realize that sand-based
systems can also accumulate new organic carbon fairly quickly
in the surface few inches as roots and microbes grow, die off
and decompose. Thus, grasses in sand-based systems become sources
of microbial substrates relatively quickly in their early development.
Table 1. Some major
characteristics which differ between native soils and
and-based root zones.
||Varies but likely to have silt
|Sand of specified particle?size
||Probably aggregated with a mixture of pore sizes Mechanical
properties may deteriorate when wet
||Probably lacks aggregation; pore size less variable. Better
mechanical properties than soil when wet
||Variable, mix of pore sizes. Better water retention.Drainage
can be poor depending on texture and structure.
||Rapid drainage, pore space mostly due to packing of the
of the larger sand particles.Water movement retarded due to
perched water table.
||More complete, greater cation exchange capacity due to clays.
||Less complex; lower cation exchange capacity. Chemical properties
derived mainly from the organic matter content
||Probably greater and perhaps older; more humified
||Can be quite high in surface few inches. Different composition
during early stages
2. What microbes are in sports-field soils?
Sand-based root zones contain abundant populations of bacteria
and fungi as well the other major microbial groups; actinomycetes
(a specialized group of mostly filamentous bacteria and well known
for their ability to produce many of our modern, medically useful
antibiotics), algae (and cyanobacteria -formerly known as the
"blue-green algae") and protozoans. Bacteria and fungi
generally dominate the soil microbial population and this is probably
true of sand-based root zones as well (see Table 2 and Figure
1). It probably is not inaccurate to say that we know less about
the microbial ecology of sand-based root zones than we do about
"normal"' soils. But that is changing as more research
efforts are focused on these highly managed systems. However,
what we do know is that they tend to function like regular soils
once vegetation is established and regularly maintained as a healthy
turf. Numerous studies document the abundance of microbes in sand-based
turfgrass systems and they indicate that microbial numbers equal
or exceed those of turf growing on various soils. Thus, one might
expect that microbes in sand-based systems would behave like their
counterparts in soil-grown turfs.
The major role of the bacteria and fungi is to decompose organic
materials in the root-zone mix (or soil), including the cells
of their recently dead microbial colleagues. It is precisely this
turnover of root tissues and microbial cells that releases organically
bound N and P as plant-available, inorganic ('mineral") forms.
This so-called mineralization process is the essence of what soil
microbial activity is all about. Yes, they do bring about other
important processes, some beneficial and some detrimental, but
their primary benefit is to decompose organic materials, make
more microbial cells and synthesize some soil organic matter (humus)
along the way. This is why we can use mulching mowers and return
grass clippings and the nutrients in them back to the soil where
Algae and cyanobacteria occur in very small numbers unless a soil
is kept overly moist. They can be a problem on closely mown turf,
like putting greens, where they may form slick spots if they are
not shaded out by the grass canopy. More often than not, they
are only problematic in very wet soils. On the other hand, in
arid soils they represent a source of new organic matter albeit
a relatively small one.
Protozoa probably deserve more research attention in turf systems.
Grass roots generally support abundant bacteria and that is where
you'll find the protozoa. Soil protozoa are effective "grazers"
of soil bacteria and other microbes. In fact, this may be their
most important role. By eating bacteria, they not only keep a
check on the size of the population but they speed up the rate
at which nutrients locked up (immobilized) in those microbial
cells are recycled (mineralized) for uptake by plants and other
3. How many are there; how much biomass?
There are countless microbes in soils and literally tons of microbial
biomass in normal, healthy turfgrass systems, including sand-based
systems. Grasslands have long been known to support large populations
of soil microbes. Some figures for numerical abundance and microbial
biomass of various microbial groups are listed in Table 2. For
perspective, one gram of soil is about the size a kidney bean
in the palm of your hand.
Table 2. Numbers
and biomass values for the major groups of microbes found
(Wet weight. in pounds per acre)
Number per Gram of Soil
10 Million — 1 Billion
10 Million — 1 Billion
10 Million — 100 Million
Algae and Cyanobacteria
100 - 1 Million (Bloom)
1000 - 100,000
10 Billion - 100 Billion
*A bloom is a visible overgrowth
of algae on the soil surface.
Alexander, 1977, Sylvia et al., 2005
But what about numbers of microbes in intensively managed, sand-based,
sports fields? Are the populations somehow compromised? Research
suggests that the answer to this question is, No! Results from
multi-year monitoring of microbial populations in sports fields
at Texas A&M University show that bacteria consistently number
in the tens of millions (Log10 7.0 = 10 million; Log10 8.0 = 100
million) per gram of sand (Kyle Field, Soccer Field) or soil (Intramural
Field) and fungi number in the tens to hundreds of thousands per
gram. The soccer field was first sampled just two weeks after
washed bermudagrass sod had been laid on an 11-inch base of pure
sand with no organic amendments (peat). The sand used in construction
of the field contained only 100,000 (Log10 5.0) bacteria per gram.
Thus, the microbial numbers increased rapidly (10- to 100-fold)
as the grass "grew in" and new roots of the washed sod
were the primary source of microbes and the carbon sources to
sustain them. Fluctuations did occur during the seasons and they
appeared to be most associated with the moisture status of the
fields when collecting samples. However, populations remained
high throughout the year and they were similar in the sand-based
and soil root zones.
Similar populations have been found in common bermudagrass
with and without compost additions (15 or 90 tons per acre), sand-based
putting greens under dwarf bermudagrass varieties and even under
common bermudagrass treated with molasses at 16 times the suggested
rate of the vendor. These data dispel the notion that sports turf
is "lacking soil microbes" and that microbial preparations
(microbial inoculants, small amounts of carbon sources like molasses
or sugar, etc.) are needed to restore them.
While the numbers of microbes in soil are no doubt impressive,
it is the biomass (weight) of the microbes that truly indicates
their abundance. Though not all soil microbes are actively growing
at any given point in time, a large biomass indicates great potential
for the many biochemical activities of the microbes under appropriate
conditions for their growth! A healthy stand of grass can literally
contain tons of soil microbes! Thus, we know that soils with large
active populations do in fact mediate lots of beneficial processes
in the soil.
We are only at the beginning of our understanding of the microbial
biodiversity in soils and sand-based systems. Molecular biology
research from the past two decades suggests there may be as many
as 4000 - 13,000 species of bacteria in a single gram of soil.
Moreover, we have managed to culture only a very small percentage
of these in the lab. The challenges of understanding and harnessing
this diversity are many but they must be understood in order to
determine if we can actually manipulate soil microbial populations
to our benefit under "real world" conditions!
4. What do they do?
What do soil microbes really do? The fact is that they do all
sorts of things in the soil when active, but mostly, they just
"hang around" waiting for something to eat! Contrary
to what some might think, soils are not seas of organic soup.
Rather, they tend to be limiting in supplies of organic carbon
to feed microbes and the competition for that carbon is fierce.
This is one reason why the rhizosphere, the zone of soil immediately
around a plant root, is such a "hot spot" for microbial
growth. Roots, as it turns out, give off organic carbon in a variety
of forms (sloughed cells, exudates, etc.) that are exploited by
the nearby microbes. So, one of the things that microbes do in
soil is to reprocess these materials into available forms (i.e.,
mineralization) and into microbial cells and humus (recalcitrant,
stable organic matter). They are also involved in many other processes
too numerous to describe here in detail. For example, many soil
bacteria can fix atmospheric nitrogen (N2) in order to grow in
areas where available soil N is scarce. Note, that I said where
N is scarce! They're "smart enough" not to rely on N2
fixation when soil N is sufficient because the process of biological
N2 fixation is energetically very "expensive" for them.
A common misconception is that one can apply small numbers of
nitrogen-fixing bacteria to turfgrass and they will supply nitrogen
for the plants. While some N2 fixation might occur, it is unlikely
that one could achieve a healthy stand of turfgrass on such minuscule
amounts of nitrogen. Perhaps more likely than N2 fixation in turfgrasses
is the process of denitrification, the microbial conversion of
plant-available nitrate to gases such as nitrous oxide (N20) and
dinitrogen (N2). This process occurs when soils become saturated
oxygen is depleted within the soil/sand matrix. Then, denitrifying
bacteria convert the nitrate to gases which escape from the soil
taking with them one of the most expensive turf management inputs,
namely, fertilizer nitrogen.
These are just a few of the processes brought about by microbes
in soil. Others are listed in Table 4. What is most important
to realize, is that most of these benefits can be derived simply
by maintaining a healthy stand of grass!
Table 4. Beneficial processes often attributed
to soil microbes:
Soil microbes may play a role in the processes below under suitable
conditions for their growth:
- Recycle nutrients in, or added to, soil
- Contribute to aggregate stabilization
- Help to solubilize some nutrients, for example Phosphorus.
- Fix N2 under appropriate conditions
- Mycorrhizal fungi help plants explore soil for nutrients
- Afford some natural protection from disease, a "rhizosphere
- Degrade contaminants, e.g., pesticides, spilled hydrocarbons,
The discussion above about the abundance and functions of soil
microbes leads us to the final question:
5. What do you need to do for soil microbes?
This is probably the question that generates the most confusion
among turfgrass managers as this is an area where I see a lot
of information not based on the science of what we know about
soil microbes. It is in answers to this question that we find
much misinformation! A common misconception about soil microbes
is that using synthetic fertilizers and other management inputs
(pesticides, etc.) somehow kills the soil microbial population
leading to "dead" or "sterile" soils. The
Internet abounds with information (in some cases posted by well-meaning
individuals and, in others, by persons selling miracle cures)
that is just patently false!
Take the following statement gleaned from the internet
"Chemical fertilizers will eventually destroy even the best
soils by killing the beneficial organisms that plants rely on
to gather nutrients and moisture. Growers are then forced to pour
on larger and larger amounts of expensive petroleum-based fertilizers
to maintain yields, but the overdoses create unbalanced "dead
A recent search of the World Wide Web for the term "dead
soil" returned 96,000 hits.
While it is true that fertilizers may inflict some harm on microbes
directly exposed to granules or to anhydrous ammonia, the overall
effect of fertilizer applications is to markedly increase microbial
numbers and activity in soil through increased plant growth. We
have known this for decades! As I mentioned earlier, the majority
of soil microbes require organic carbon to grow and produce new
cells. In grass systems, the vast majority of organic matter is
produced from decomposing roots and leaves. Fertilization increases
the amount of organic substrates available to soil microbes by
increasing its source, the grass plants themselves. Thus, rather
than producing "dead soil", judicious use of fertilizers
invigorates soil microbes by allowing plants to produce more resources
for them! Remember though, all management inputs must be used
carefully and correctly. Too much of a good thing can produce
negative consequences. Excessive fertilizer applications will
likely lead to enhanced runoff and leaching and the undesirable
environmental consequences that go with those processes!
So, do you need to add "beneficial microbes" to the
soil to make it function properly? That's highly unlikely! Many
studies of turfgrasses, whether in sports fields, golf courses
or home lawns, have shown that soil microbial populations are
not compromised by normal management practices. The best thing
that you can do to "manage" the soil microbes under
your care is to grow a healthy stand of turf and pay close attention
to the condition of the soil or root zone supporting it. Paying
attention to the agronomics of grass culture, fertilization, aerification,
drainage, etc., will insure that the microbial populations are
not being adversely affected!
Alexander, M. 1977. Introduction to Soil Microbiology, 2nd ed.
John Wiley and Sons, Inc. New York.
Elliott, M. L. and E. A. Des Jardin. 2001. Fumigation
effects on bacterial populations in new golf course bermudagrass
putting greens. Soil Biology and Biochemistry. 33:1841-1849.
Elliott, M. L., and E. A. Des Jardin. 1999. Effect of organic
nitrogen fertilizers on microbial populations associated with
bermudagrass puffing greens. Biology and Fertility of Soils 28:43
Kenna, Mike.2001. Nature Will Find a Way: Common myths about soil
microbiology. U.S.G.A. Green Section Record. May-June 2001.
Kerek, M. R.A. Drijber, W.L. Powers, R. C. Shearman, R.E. Gaussoin,
and A.M. Streich 2002. Accumulation of Microbial Biomass within
Particulate Organic Matter of Aging Golf Greens. Agron. J. 2002
Nelson, Eric B., Microbiology of turfgrass soils. Grounds Maintenance.
Sylvia, D.M., J. J. Fuhrmann, P.G. Hartel and D.A. Zuberer. 2005.
Principles and applications of soil microbiology, 2nd ed. Pearson-Prentice
Hall, Upper Saddle River, New Jersey.
Waltz, C., H. Skipper and B. McCarty, The living
earth. Grounds Maintenance. May 1, 2001.