Most of us take for granted the quality of the air we breathe and that the oxygen within it is critical to the maintenance of life on this planet. We are all well aware of the increase in atmospheric carbon dioxide levels and almost daily we see news items on the consequences that elevated CO2 will have for global climate. What far fewer people realise is that oxygen and CO2 are also critical to life in the soil.
Scientists have long regarded the soil as a "black box" in which it was best not to peer too closely. However, modern analytical techniques have opened up this world and have enabled us to understand the processes that govern life in the soil.
The predominant life forms in soil are microbes; bacteria and fungi that break down dead plant and animal material and recycle the nutrients therein, making these nutrients available to plants. All of these beneficial microbes also require oxygen to grow and a build up of CO2 is detrimental to them too. Without soil microbes, life above ground would not be able to exist.
However, soil is a physical structure, consisting of aggregates, with air or water-filled spaces between them in which most of the microbes live. Any soil can become compacted by pressure imposed by human feet or machinery, meaning that the spaces between the aggregates become fewer, gaseous exchange with the atmosphere is reduced and oxygen content decreased. If this happens, the microbial population will suffer and, subsequently, the above ground community will too.
In sports turf, compaction is a frequent occurrence and machines such as the Verti-Drain® have been developed to open up (aerate) a compacted soil, to allow oxygen and water to enter it and CO2 to leave. No previous study has ever asked about the consequences of aeration for the microbial community. This unique project examined how aeration can affect soil microbes, using cutting-edge analytical techniques.
The results make interesting reading and are probably the first example of how microbiology can be used to inform engineering practises.
A hidden world beneath the surface
That grey zone where the worlds of turfgrass cultivation and microbiology intersect is an intriguing region about which swirls a confusing amalgamation of facts, half-truths, and downright speculation. It is on this subject, the science behind cultivation practices, that this article will focus, in an attempt to alleviate the ambiguity underlying much of what we "know" about this subject.
For more than 25 years, Redexim has been manufacturing a specialised line of aerators, the Verti-Drain. The products have become such a part of turfgrass cultivation that the phrase Verti-Drain is often mistakenly, but flatteringly, used as a verb by turf professionals the world over. For years, we have observed the beneficial results of Verti-Drain treatments: improved drainage, enhanced root growth, improved turf colour, and better drought resistance. But no one has ever really been able to say how the Verti-Drain worked from a scientific standpoint, or why…until now.
Aeration is certainly one of the most common cultivation practices. In this area of study, the research work of Dr. Donald J. McGregor, conducted at the Royal Holloway campus of the University of London, in Surrey, England, breaks new ground in establishing a greater level of understanding. According to Dr. McGregor, there is much that we simply don't know about microbes and their interactions - directly and indirectly - with plants and other living things in the soil. This complex set of biological processes make up what he calls "the hidden world" beneath the surface.
Just a few years ago, it was impossible for aeration equipment manufacturers - and their customers in the turf maintenance field - to know how effective their products were without understanding the composition of microbial communities in the soil around the turfgrass. This is as true of the Verti-Drain as it is of its various competitors. The new research work at Royal Holloway marks a turning point in that regard.
A little biology refresher lesson…microbes, a conveniently short phrase for microbial organisms, are a rather broad category comprising bacteria, fungi, viruses and protozoa. Most microbes are, in fact, beneficial to higher life forms and form an integral and significant portion of the life processes of any ecosystem.
In a very real sense, microbes are a grounds manager's best friends. "Microbes are the foundation of everything we see around us; without them we would not exist," claims Dr. McGregor. "If anything goes wrong in the environment, the microbes are an early indicator. They will show subtle changes before anything else." And, as we shall see, the health and number of these microbes in the soil has a strong correlation with the health of turfgrass, as it does with other life forms co-existing with the microbes in the soil.
Anyone with an interest in healthy turfgrass is somewhat familiar with the basic processes that make up what we'll call the soil habitat. What biologists are discovering, however, is that this habitat is full of surprises.
(Did you know, for example, that the characteristic "fresh smell" we've all sniffed right after it rains is the scent of chemical secretions from mysterious microbes called actinomycetes, organisms that seem to be halfway between bacteria and fungi? Even today, no one is sure exactly what these microbes do.) One thing we are sure of, when it comes to microbes, is that some are aerobic and others are anaerobic, and that only the aerobic ones show a strong benefit to turfgrass, which is why the potential to link the Verti-Drain to aerobic microbial activity in the soil was an exciting prospect.
But microbes are only part of the picture. They exist in the soil structure right alongside earthworms, organic matter, plant roots and other living and once-living things - the so-called "biota" in the soil. Within this biota we find incredible diversity - aerobic and anaerobic, acidic and basic, wet and dry, and the list goes on. This diversity - and the interdependence of the life forms involved - is the very core of understanding this soil habitat. It is the interactions of these elements that influence all aspects of turfgrass health, and which have a strong relationship with practices like aeration.
When you stop to consider what microbes are capable of, in a biological sense, the effect is mind-boggling:
• Enhancement of plant growth
• Enhancement of drought resistance
• Disease protection
• Reduced need for fertilizers
• Control of Poa annua without using chemicals
And those examples are just biological. Microbes do even more when you examine their roles in the chemical and physical processes of the soil habitat:
• Breakdown of organic compounds
• Breakdown of cellulose and lignin
• Formation of humus
• Soil stabilisation
• Nutrient cycling
• Nitrogen fixation
It's fascinating to contemplate that all this activity is going on beneath the surface. But let's not forget the appearance of the turf plants is also affected. In fact, it is the highly visible condition of the grass, whether it appears healthy or unhealthy, that gives a strong indication of a healthy and productive soil habitat - or the lack of it.
At some point, turf professionals should consider erecting a statue in honour of microbes for their highly effective and productive contributions to a remarkably complex habitat.
The dilemma of conventional wisdom: "We know it works, but ... why?"
Aeration certainly isn't new. For most of this century, mankind has been poking holes in the soil based on the belief that it helped the grass grow. A key point that should be addressed is the notion that "aeration" is all about "pulling cores." Simply put, it's not. Aeration is really the technique, not the machine, and there are a great many ways to aerate, each with its own advantages and disadvantages. What turfcare professionals have known for years is that the Verti-Drain works, but no one could really provide the scientific reason why.
The soil habitat concept introduced earlier by Dr. McGregor proved to be as enigmatic for those who care for turf on a daily basis as it was for those who studied it in a lab. Clearly, aeration was a popular cultivation practice, but how strongly rooted was it in biological processes? When a tine breaks the soil surface, penetrates down to a desired depth, and performs its characteristic "heaving" motion, what is happening from a biological standpoint, and how and why does that affect turf health? Even the manufacturers of the Verti-Drain, could only point to conventional wisdom.
This dilemma recently attracted the attention of the academic community, specifically Dr. Alan Gange and Dr. McGregor at the Royal Holloway, where the Department of Biological Sciences is one of the largest in the college. Some 40 faculty members and about 400 students there routinely explore timely issues in biology, ecology, zoology, biochemistry, and molecular biology - with many research activities yielding practical applications that benefit businesses, consumers and the environment.
While Royal Holloway isn't the first institution to delve into research on microbial activity by any means, it has taken on the task with a rather unique twist. Rather than simply counting the total number of microbes in the soil (the most common practice for studies of microbes), their research involved analysing the composition of the soil's microbial community - exploring in the process whether or not aeration had an effect on that community, in particular, the "good" aerobic microbes with the potential to benefit turf plant growth.
"I've given lots of turf talks in the past, usually on environmentally friendly products which are designed to increase microbial populations," noted Dr. Gange. "What really caught my attention was when people I spoke with told me that many of these were not all that effective. It led me to believe that one could probably get the same effect, increasing the good microbes in the soil, by simply aerating it. So I approached Redexim, to see if they were interested in helping us determine if that statement were true."
According to Dr. Gange, this is the only study done that attempts to link aeration to the numbers and diversity of the microbial community in the soil. "Ecologists couldn't look at the diversity of the microbes before because the technologies were not really available until the last few years," he said. "So, we've had 10 years of just counting the numbers of microbes without anyone profiling the composition of that community in the soil. And no one's made the connection to aeration before."
While Dr. Gange may have started the ball rolling, it was Dr. McGregor who chose to do the research for his thesis publication "The Effect of Cultivation on Microbial Communities in Sports Turf Soils." The research experiment took three years to complete and nearly six months to analyse and write up. The results are extremely useful for anyone responsible for the care of turf grass on sports fields or golf courses.
Anatomy of an experiment: Aeration and Microbes
With some assistance from Dr. Gange, Dr. McGregor conducted two distinct trials: one on two clay-based football fields at the school, and the other at the sand-based turf of a nearby golf course, Liphook Golf Club, in Hampshire.
Feeling a little like an "environmental detective," Dr. McGregor designed the experiment in a way that entailed dividing the football pitches into individual plots, some of which were aerated (using a Verti-Drain 7516 aerator) while others were not. Dr. McGregor: "These were pitches that were used by the college football team on a regular basis, and so were subjected to normal or above-normal real world stresses, like any other sports turf."
He added a degree of complexity by aerating with the Verti-Drain at certain times of the year. "Some areas were aerated in spring only, others in summer only, autumn only, winter only, and still other plots were aerated in all four seasons. Our goal was to mimic the combinations of seasonal aeration typically employed by a groundsman," he adds. A control group of plots on the football pitches were not aerated at all.
Like the football pitches, the test area at the Liphook Golf Club was divided into individual plots, with each one coded for a schedule of Verti-Drain aeration (except the control group, which received no aeration) at different times of the year.
A comprehensive series of measurements were then taken - of pH, water content, gas content, atmosphere, compaction, and make up of the microbial population and community structure - on all the plots at regular intervals before and after treatments with the Verti-Drain, the exact timing of which depended on the technique and the weather conditions. (In some cases, the frozen ground required a slight adjustment to the schedule, for example). Soil samples were taken and returned to the lab at Royal Holloway for analysis. Some tests were done right there in the field, including those measuring the degree of compaction with a penetrometer.
An important note about the measurements: Dr. McGregor was careful to use a variety of measurement techniques ranging from simple soil probes for measuring atmosphere to sophisticated approaches never before available, such as phospholipid fatty acid analysis (PLFA), the technique of choice today for calibrating microbial populations and their structure/make up.
Some of the measurement and analytical tools used by Royal Holloway are fairly routine and are, in fact, decidedly (and appropriately) low-tech in nature. Others represent the most sophisticated methods currently available - short of DNA analysis, which is prohibitively expensive and subject to contamination - for counting and profiling the complex microbial community in the soil.
Static and dynamic changes
According to Dr. Gange, a variety of soil properties were constantly exerting varying degrees of influence on the soil microbial community, and the turfgrass that also called this habitat home. Some of the properties were inherent to the soil (things that changed only over a long period of time) such as: texture (clay, sand, etc.), pH level, and other things relating to the physics of the soil. Other soil properties were dynamic in nature (changing more rapidly) including: gas content, water content, and the most dynamic of all, compaction. ("You get compaction, you aerate to relieve it, and then it's played on and gets compacted again," notes Dr. McGregor.)
Throughout the experiment, the Verti-Drain played a central role by aerating those plots that were not part of the control group. The machine used solid tines to punch holes into the soil down to a maximum depth of 400 mm (16 inches). Unique when compared to most aerators, the Verti-Drain has the added benefit that as the tine goes into the soil, it will kick backwards and heave the soil, lifting the soil profile slightly to relieve compaction.
This allows oxygen and water to get down into the soil, providing a beneficial effect to the turfgrass, possibly due in part to a beneficial effect on the aerobic microbes in the soil. While this idea isn't new, Dr. McGregor's observations would either prove or disprove the assumption.
Key findings ... and what it means for your turf
Among the factors with the greatest influence on microbial activity in the soil are: temperature, water, and oxygen - all of which exerted transient changes on the microbial community.
The importance of water and oxygen in influencing microbial communities bears out the value of compaction relief through aeration. In this experiment, there are a number of benefits that were found to have a direct connection to the use of a Verti-Drain for turfgrass aeration:
Soil Physical Characteristics: The effects on the soil are the most significant result of Verti-Drain treatment, where it reduces compaction in all seasons on both clay and sand-based sports turf, and significantly improves oxygen levels (unless waterlogged conditions exist).
Microbial Structure: Microbial community structure is affected by treatment, but the strength of the response is season dependent and qualitative in nature. The main effect of the Verti-Drain was to change the microbial community structure in favour of more aerobic microbial groups, a factor that is believed to have a profound influence on organic matter dynamics and nutrient supply in sports turf soils.
Aesthetic Characteristics: Changes in the soil and in the microbial structure yield improvements in appearance as well. In this era of televised golf and football games, the visual appeal of the sports turf is often the target of commentary, and viewed as a key indicator of the health and condition of the turf.
In interpreting the results, Dr. Gange stresses that oxygen is a huge consideration, in that unchecked compaction will prevent root growth by causing a shortage of oxygen in the soil and preventing it from being able to filter water down to the root zone. "Once a root zone becomes anaerobic, anaerobic bacteria will flourish, and generally they are toxic to plant life," he says.
Dr. McGregor believes that the key is the prevalence of macropores versus micropores in the soil. "The larger the size of the pores, the more 'connectivity' between pores and the greater the ability of the soil to permit gas exchange and water percolation. This may be the real reason why aeration works in a beneficial way in microbial communities," says Dr. McGregor. "These larger pores also help prevent run-off of water and make irrigation more efficient."
Clays, which have more micropores and fewer macropores, tend to have more physical barriers set up in the soil structure that may prevent bacteria from getting to the organic matter to degrade it. "When you aerate with the Verti-Drain, you're loosening up the soil so you encourage this connectivity of pores," he adds. Also, clay soils require a greater frequency of aeration than sand-based soils.
In the experiment, oxygen concentrations varied from area to area - likely because pore size and connectivity differed from one area to another. And from season to season, oxygen varied because of differences in the biota ("the living things in the soil") including roots, earthworms, bacteria, and other life forms. It is this interconnectedness that reinforces the concept of "soil habitat" to explain the dynamic aspect of such interactions.
Dr. McGregor: "This experiment indicates that encouraging development of microbial communities, particularly aerobic ones, is in the best interest of the average turf plant. Aerobic microbes are more metabolically productive. The chemical reaction that goes on within the respiration system of an aerobic microbe creates more energy, whereas an anaerobic microbe does not use oxygen and cannot achieve the same level of efficiency. The soil habitat is literally a healthier one with such an enriched energy budget."
The greater the number of aerobic microbes in the community, the greater the level of biological processes that are possible: decomposition and nutrient cycling go hand in hand with re-mineralisation. "Enhancement of plant growth is also going on, and it can be either direct or indirect," Dr. McGregor says. "And the natural competition between microbes, aerobic versus anaerobic, can have a useful effect against pathogens that could harm turf plants, since some microbes in the soil produce antibiotics that are antagonistic to the pathogens."
One of the most important findings of Dr. McGregor's work is the revelation of the transient benefits of aeration on microbial communities. "The benefit doesn't last when compaction returns. Sports turf is subject to compaction on an ongoing basis, and so the benefits of aeration are only sustainable when the activity is routinely repeated. Without it, microbes can go into a kind of hibernation if they are deprived of the necessary access to oxygen and water."
The conclusions from this significant research study are important for turf care professionals. A quarter-century of worldwide use of the Verti-Drain convinced us at Redexim, as well as our numerous customers, that the Verti-Drain worked. Now we have a clearer understanding of why this is so. Although science isn't always a neat and tidy process - at times it does seem to creep forward rather slowly - Dr. McGregor's findings are a cause for hope. By understanding that much more about how the Verti-Drain affects soil physical characteristics and microbial community structure, we are more confidently able to explain the science behind a practice that simply works well.
As additional scientific work is conducted, we look forward to sharing it with you to help advance this field of study and the practice of aeration that is such a huge part of our collective work.
Redexim Charterhouse would like to thank the Royal Holloway, University of London, and in particular Dr. Donald J. McGregor for preparing his thesis on this important subject.