All life on earth depends on a functional microbial community in the soil. By ‘microbial’, I particularly mean bacteria and fungi, but the word ‘community’ can also include actinomycetes, a sort of half-way group between bacteria and fungi. Protozoa and nematodes are sometimes included in the definition too, but are not microbes; they are members of the microflora and fauna. These different groups play varying roles in the soil; many are involved in decomposition, some are highly beneficial to plants because they fix nitrogen or aid nutrient uptake, while others can be harmful, causing disease. The soil is an incredibly rich ecosystem – one gram of healthy soil may contain up to 3 billion bacterial cells of up to 50,000 species. If you add in similar numbers for fungi and the other microbes then there is more life in a teaspoon of soil than there are humans on earth!
I used the words ‘healthy soil’ there – what does that mean? There is no clear definition, but generally it is taken to mean a soil that, through its microbial complement, is able to support good plant growth, and to maintain or enhance air and water quality. Quite what a healthy soil contains compared to an unhealthy one has never been detailed for any crop system, and sports turf is certainly no exception.“
A healthy microbial community can increase root growth and improve the structure of a soil.
I came into the turf industry some years ago, having worked with soil microbes in other industries, and was immediately struck by the lack of appreciation of soil ecology. It’s interesting how things have changed in recent years. On many occasions, I have been asked if I could count the bacteria in a putting green or football pitch. My answer has always been that I could, but I doubt whether the answer would provide any meaningful information. I have seen services advertised in which microbes are quantified, but usually these involve some form of culturing on media plates. Given that about 1% of the microbes in a soil are likely to be culturable, such methods provide data which are so misleading as to be worthless. I could use complex biochemical methods to tell you how much bacterial material there is in the soil beneath your greens, but I would not be able to tell you if they were ‘good’ or ‘bad’ bacteria. I often read that sports turf soils are impoverished when it comes to microbes and that pesticide and fertilisers are to blame. Impoverished they may be, but the critical thing is whether the most useful species, (or those which are beneficial in these circumstances) are still present and that we do not know, as we are yet to characterise the communities. Many ecologists think that a portion of the soil microbial flora is redundant – i.e. there are a number of species that all do the same thing. If that is true, then they argue that losing the odd species or two will have no effect, as those remaining will continue to perform the necessary biological functions.
One aspect of the soil microbial community that is easier to define and measure is the mycorrhizal fungi. These remarkable fungi grow in soils and when they encounter a root, the hypha (vegetative bit of the fungus) enters this root and grows between and within the cells. Inside, they form minute structures called arbuscules, which act as sites of nutrient exchange. The fungus donates minerals, principally phosphorus and nitrogen to the plant, in return for a supply of carbon. There is abundant evidence that plants with a mycorrhiza in their roots grow better and are more resistant to pests, diseases and stresses such as drought. Not all plants respond in the same way though; some exhibit greater benefits than others.
There is good evidence that mycorrhizas are impoverished in fine turf soils, and work in my laboratory found that grass plants with the fungus in the roots were more resistant to Microdochium than those without. In theory, these fungi could be very useful in turf, but we know very little about how we can manipulate their abundance. Experiments have tried to add mycorrhizas to soils (there are plenty of commercial products available), but these don’t seem to produce consistent results.
My view is that for microbes in soils, you will only ever be successful in increasing their numbers if you remove the factor that is limiting their abundance. I believe this is carbon, not pesticides or fertilisers.
While the latter two are not blameless, most soil microbes feed on carbon that is extracted directly from roots or leaks out into the soil (or present in a pesticide molecule!). Because turf is mown continuously, the amount of carbon that a grass plant fixes and translocates to the roots is tiny, compared with a grass plant in pasture or natural grassland. However, it’s not as easy as just adding a bit of carbon to the soil in some easily accessible form. Pathogenic microbes can take advantage of this free food supply too!
So does turf need more microbes? I strongly suspect the answer is ‘yes’, but only of the right type. Currently we do not know if the microbial community present lacks critical elements that might enable turf to resist drought or disease better. Should you use a microbial inoculant? It would probably do no harm and in many instances will be good, though it’s not necessarily grass growth you need to enhance, otherwise you will be mowing it more often!