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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.
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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 they belong.

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 microbes.

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.

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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 and
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!

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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 and water
• Afford some natural protection from disease, a "rhizosphere effect"
• Degrade contaminants, e.g., pesticides, spilled hydrocarbons, etc.
  
  
  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 for example:

"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 soil". (Anon.)
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!

Further reading:

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 1-435.

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 94:455-461

Nelson, Eric B., Microbiology of turfgrass soils. Grounds Maintenance. March, 1997.

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.