SOIL MICROBE BIOLOGY FOR THE SPORTS TURF MANAGER

In sports field turfgrass management, we tend to focus most of our soil vision on the physical characteristics of the soil. Will it drain? What are the particle and bulk densities? In sands, we include things like particle size distribution curves, coefficient of uniformities and perched water tables. We also tend to look relatively closely at nutrient levels, salts, pH etc. when we have a lab chemically analyze the soil through soil testing reports. As we know, a large number of turfgrass problems, or most any plant maladies for that matter, are likely soil related in some way. So it makes sense to analyze our sports field soils in these two ways, chemically and physically, and while there is plenty of differences of opinions when it comes to soil testing and nutrient analysis, there is a fairly decent body of science to support our approach.

May I suggest we look a little deeper at the biology of the soil also? In particular, I’m talking about the microorganisms that live in your soil. This soil “microbe-biology” is not very well researched and understood, but that does not belittle the importance of this part of soil management, the backbone of agronomy. Good science on anything related to turfgrass microbes is very tedious, very expensive and often yields inconsistent results. It seems to me that any work done in this area would require a lot of expensive DNA assays and such. Even so, there has been work done, just not nearly the body of work we can rely on in soil fertility management and soil physics. But new technologies in genetic engineering may change all this. A glance at many turfgrass science textbooks usually reveals a short section or very little dedicated to soil microbes, usually they are mentioned in the sections corresponding to the unique functions they carry out. Still, for the turfgrass manager I believe we need to start getting our arms around this subject and begin incorporating some of the more anecdotally proven practices after careful scrutiny and careful implementation, finding some metrics to evaluate for ourselves.

On a more general perspective we really should be looking at all the soil organisms as interconnected in the soil ecosystem, large and small, to give the topic a solid treatment. This would include larger invertebrates like nematodes, mites, ants, earthworms and things like that and even vertebrates like moles, mice and other burrowing animals. But for today I’m focusing mostly on the tiny guys, the microorganisms, maybe the least understood of the soil creatures. These are largely the soil bacteria, but also large populations of soil fungi, actinomycetes, protozoa and even algae. And I’m probably missing a few on that list.

Think of your soil as an interconnected ecosystem of tiny organisms we can’t see with the naked eye. It has been estimated that soils can contain as many as 9 billion microorganisms (or microbes) per gram of soil, that’s about 1 teaspoon. There are more microbes in this gram of soil than people on earth! And it is an astonishingly diverse population with some estimates of 10,000-50,000 species in that same gram of soil! This congested population is very dynamic, it changes with time, conditions and food sources, just like critters in any ecosystem do. There are many more microbes near the surface of the soil, as this is where most of the best food is for many, organic matter. There is generally more activity and larger populations in the warmer soil months than colder. And these 9 billion microbes in a small dirt clod, much like people on earth, don’t all get along so well. There are predators and prey, just like on the savannahs of Africa. There is competition for food. There are specialty creatures that fill a niche. The same natural selection factors you see in most any above-ground ecosystem also apply to the soil ecosystem. But it’s more of a war zone than a peace rally in my book and this makes any real and significant change to the soil microbial system population dynamics very difficult in my opinion. But I do believe that until the science catches up, there are things the field manager can do to improve our production and effectiveness in managing the soil microbes, as best we can.

Microbes carry out many important functions in the soil ecosystem that support healthy turfgrass growth and development, the cornerstones of a high-quality natural grass playing surface, in my opinion. Without getting into the weeds about nitrification and mineralization, I’m here to tell you that your turfgrass wouldn’t get anywhere near enough nitrogen or many other nutrients into the plant without the aid of soil microbes. Many of the bacteria are the grazers, so to speak, eating and breaking down the accumulation of organic matter in the soil that would otherwise build-up and pretty much choke off the system to healthy turfgrass growth. Others, often fungi, can actually eat the tough, high lignin content of thatch, which can overtake a turfgrass stand also, reducing surface quality. Other microbes are masters at breaking down and rendering harmless some of the nastier man-made chemicals like pesticides and such. Some like mycorrhizae form a symbiotic relationship with the roots of your turfgrass and aid in the uptake of nitrogen by the plant. We are only recently beginning to understand the important role microbes play in the carbon cycle with potential effects on carbon sequestration. Still other microbes poop out sticky glue-like substances that bind smaller soil particles together and build soil structure which aids in drainage and soil aeration.

But it’s not all peace, love and happiness down there! There are pathogenic microbes in the soil that want to infect and harm your turfgrass plants. These are largely soil-borne fungi, but even some bacterial and viral turfgrass pathogens can live in your soil. Luckily, there are microbes paid to hunt out and destroy these pathogenic freeloaders! Some microbes even attack, infect and kill larger invertebrate pests like insect grubs for example. There are things going on down in our soils, battles, alliances, friendships, famines, climate change and wars we don’t yet know about, believe me.

We are probably a ways off from understanding much of this and as a turfgrass manager discharging limited resources in preparing and maintaining natural grass playing surfaces, a cautious approach is always advised. But neglect of the issue is not smart either, not by a long shot. Healthy turfgrass stands would be impossible without soil microbes, or at least very difficult. Our cloudy scientific position notwithstanding, doesn’t it stand to reason that we manage this system as best we can, as we do others?

Turf Tips 101: Soil Microbe Management.

With a basic understanding of the microbial population dynamics, my approach is that we can best and most prudently manage our soil microbe health with a “feed them and let them fight it out” approach. Microbial nation-building rarely works, just as it often fails with humans in my opinion, but others more knowledgeable than me would disagree.

Using an example of a new sand rootzone that is seeded to turfgrass can help understand how microbe dynamics cycle. Usually, we add in a small amount of high-quality organic matter to our sand rootzones like peat moss. This is usually done for water-holding improvement but I’ve also felt that it may be a good way to get the microbial race off and running. Bacteria are usually the first to populate and often ride in on the organic matter itself in a relatively sterile sand rootzone. But have you ever tried to break down and eat peat moss? It’s a tough job to be sure and even these specialized bacteria look for help initially by consuming water-soluble sources of energy that washes down from mower clippings and other sources. These water soluble starter energy sources are laden with certain sugars and other jump starters that get the hard work going, so to speak. But these bacterial pioneers can’t live forever off this initial sugar high and the populations may decline as they get into the harder work of eating the tough lignin and “leftovers” of the organic matter buffet. If only this tough organic matter is there to eat (high carbon:nitrogen ratio organic matter), the microbes can actually compete with your turfgrass for nitrogen. This is where the fungi are good at coming in to finish up the tough work and poop-out digestible organic compounds for the bacteria. Fungi are better at breaking down the tougher organic compounds and are also best at breaking down thatch. Even earthworms and their poop do this kind of food prep for the bacteria.

So really, managing your soil microbes to optimal performance for your turfgrass, however murky and anecdotal, comes down to two basic approaches. Many soil-microbe related turfgrass products are microbe inoculants or microbe feeders, or both in my view. Inoculant products, as I understand them, add in beneficial microbes with the intent of gaining the upper hand in the populations and also jump-starting new beneficial pioneers. I could be wrong, but I believe there are plenty of eager microbe pioneers of vast diversity just waiting to go to work in even depleted, tired soils. I say, feed and support the microbial homesteaders that are already in large numbers rather than just add more to the doles that will probably not last the first winter anyway, so to speak. But I certainly don’t know and could be wrong I readily admit.

To me that looks more reasonable and maybe even safer than introducing microbial populations into the soil. Compost teas have gained popularity in recent years. Again, the formal research is sparse. The idea is for the turf manager to “brew” their own “tea” of microbes and corresponding broken down organic compounds (microbe food) on site. This is done in a kettle of some type over a period of time for populations to grow and organic compounds to be broken down somewhat. The resulting tea is then applied to the turfgrass as a spray “biostimulant”. While some turfgrass managers do things themselves and even use things like on-site raked leaves for a brew-starter, there are now more standardized commercially available brew kits which include inoculants and food.

I admit I have never tried a compost tea, but I guess I’m not very comfortable brewing bacterial tea in my shop, maybe I’ll brew up some bacteria that is nasty to humans? I imagine it is pretty hard to ship inoculant products and this may make them more expensive than simple, proprietary soil microbe food products. Smarter people than me would certainly disagree, but I figure why not do my own brew and tea in my soil, where there is microbial law and order and less risk of problems in my book.

I look to products that simply feed your soil microbes and let the population dynamics pretty much fight themselves out in a safe cage of soil. The question would then become “What do I feed my soil microbes”? It seems soil microbial populations wax and wane based on access to easier to digest organic materials and compounds and may be jump-started with sugars and enzymes, which is where I look for microbe “food”. There are many good available products out there that feed soil microbes, but for the reasons described above, peer-reviewed science that can be replicated is expensive, time consuming and is lacking. Most products have to keep their recipes proprietary for business reasons, and this can make fertile ground for charlatans, even if they are by far the exceptions in my experiences.

Us turf managers are a crafty lot with a lot on the line. We can usually get a pretty good idea of what works and what doesn’t with some simple control plots and scaling up use as is justified by what we observe. Some of the microbe feeder products will combine their microbe food with support products like surfactants to aid in getting the products down through thatch and soil surfaces, or maybe naturally occurring enzymes that aid the microbe digestion and chemically “pre-chew” the tougher organic matter for them, so to speak. Some may add naturally occurring plant hormones and other plant stress reducing compounds and chemistries like processed tidal kelp and such. This may cloud the turf manager’s evaluation of effectiveness. If we get a positive response from use compared to a simple control plot (maybe a small piece of plywood we lay in the same spot every app), is it the microbe food or maybe one of the other useful components? I suggest not overthinking this too much beyond getting a basic understanding and awareness of this underground biosphere so critical to turfgrass health, and so, critical to playing surface quality. Do some of your own homework and try some different products on a small scale in non-critical parts of the turfgrass stand. Design some basic experiments in your shop. Build on your successes. You may be surprised!

Resources of the Month

The Ohio State University has a great online factsheet about soil microbes and their role in nutrient recycling.

Interesting read in The Atlantic magazine about soil microbes and their potentials in some fields including human health.

Another interesting article on potential human health benefits by injecting a harmless soil microbe Mycobacterium vaccae.

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“The nation that destroys its soils destroys itself.” Franklin D. Roosevelt