San Antonio Express News
GARDENING, Etc.
Sunday, November 6, 2005
By Dr. Jerry Parsons
To read some of the material “out there” and hear some of the weekend organo-maniac radio programs, one would think that agriculture has been waging “all out” chemical warfare on soil microbes and that we have all but annihilated them in our soils. This misinformation is being touted by people who owe their very existence and expanding waist lines to modern agriculture. To try to bring some clarity to the subject, I would like to make the public aware of factual, scientific information from an article entitled “Microbes in soil and sand-based root zones: A few of the basics” written by David Zuberer, a real soil scientist in the Department of Soil and Crop Sciences at Texas A&M University. The entire article is available at: http://www.plantanswers.com/garden_columns.htm as the first column under the November heading and titled: “The Truth About Microbes.”
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 recommend using mulching mowers and returning grass clippings and the nutrients in them back to the soil where they belong.
There are countless microbes in soils and literally tons of microbial biomass in normal, healthy turfgrass systems. Grasslands have long been known to support large populations of soil microbes. 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! The scientific data dispels 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. 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.
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.
What do you need to do for soil microbes? This is probably the question that generates the most confusion among the mislead public as this is an area where there exists a lot of information not based on the science of what true scientists 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!
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 plant culture, fertilization, aerification, drainage, etc., will insure that the microbial populations are not being adversely affected!
So the next time you hear a radical, uninformed person make a statement such as, “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.” …Start looking for a reliable source of information.
Dr. Jerry Parsons is a Professor for Texas A&M University and a Texas Cooperative Extension Horticulturist for over 30 years in South Central Texas. For more information on this or other horticulture topics, go to www.plantanswers.com and our County Extension website at http://bexar-tx.tamu.edu.
