Technical - Soil Biology
about 50% of this energy goes into top growth, to feed the above ground food chain. In the average diet 50- 80% of the protein and carbohydrate we eat comes from grasses, think cereals, bread, milk, cheese, meats and beer, but nearly half the energy produced by grass goes underground.
So, what happens to the 50% of the protein and carbohydrate that goes back into the soil via the roots? It feeds the underground, mostly invisible, soil food chain that modern management practices overlook.
Since the 1920s, when inorganic fertilisers were introduced to override natural nutrient uptake, sports turf soil has been prone to excessive fungal disease, dry patch, thatch build up and black layer. Turf managers have been taught to use fertilisers and chemicals to treat the symptoms. Excessive fertilisers, fungicides, water and iron very successfully break down the soil food chain, blocking natural growth and disease suppression. Pesticides and wetting agents are introduced to treat more symptoms and the soil becomes less and less fertile. We probably all know of managers following chemical dependent management programmes and losing their jobs when the rootzone becomes too toxic to support decent grass growth.
Out of Sight
Out of sight used to be out of mind, but using molecular analysis and electron microscopes that allow identification of individual bacteria, fungi and other organisms, we can now start to understand how natural processes work on a golf green and how to use them to our advantage.
Food underground comes from exudates from plant roots, in the form of protein sugars and carbohydrates, the stuff you use to make pancakes or biscuits, this is the main food for bacteria.
When grass dies it becomes thatch full of cellulose and lignin. You only have to look at an old dead tree to know this is the favourite food of fungi which convert dead grass, leaves and twigs to humus - the nutrient store and foundation for future plant growth. Bacteria and fungi form the foundation of the whole food chain. The good (saprophytic) microbes only live on dead matter. There are about 20,000 enumerated soil bacteria and 15,000 fungi (Ingham et al), of these most turf managers know the few disease causing (pathogenic) fungi which live on live plants by name.
There are also fungi which live on carbon and sugars inside the grass root. To stop the plant from dying through lack of carbon, these fungi, called mycorrhizal fungi, use their hyphae to suck up phosphate, nitrate, trace elements and water and feed them to the plant, giving the plant a vastly extended root system.
In a gramme of healthy soil, you may find about a billion bacteria weighing about 100ug and a similar weight of fungal biomass, only 10% or so may be active at any time but, for fine grass growth, it is important to have approximately equal bacterial and fungal biomass.
Some bacteria fix nitrogen from the atmosphere, others take it from recycled nutrient in the soil. Nitrogen is not available to the plant when locked up in a bacterial cell or fungal hyphae. The microbes have to be eaten
by the next stage of the food chain comprising protozoa and nematodes. Some nematodes can be seen with a magnifying glass.
Protozoa and nematodes eat the bacteria and fungi, they use the carbon to grow and excrete excess nitrogen as ammonium, which feeds the grass when converted to nitrate by nitrifying bacteria in the presence of oxygen.
There are two other types of nematode, root eating and predatory. Root eating you know about, but they are kept in check by the predatory nematodes which eat all nematodes, These, in turn, are eaten by small worms which create small spaces in the soil, leaving little tunnels full of excrement (ammonium) for roots to grow into and feed on, and so the chain expands up to moles and other burrowing animals.
Working together, all these microbes and organisms ensure a constant supply of nutrient for the plant, avoiding the boom and bust of the fertiliser cycle, they protect the grass from disease because, if the grass dies, the whole food chain dies, and recycle dead grass back into usable nutrient.
For turf management purposes, we try to stop the food chain at non-casting worms but, up to that point, all aspects of the chain are necessary for natural healthy grass growth.
Why are many sports turf rootzones so microbially deficient?
Plants have evolved growth mechanisms in association with soil bacteria and fungi over millions of years. Applying a tonne or more of inorganic fertilisers (which are mineral salts) per hectare, per year, means that the plant is
Degrades thatch Grows perennial grasses Reduces disease Improves drainage Reduces fertiliser and fungicide inputs Provides the best playing surface
Thatch layer Root Feeding
Nematodes Eat roots
Fungi
Degrade cellulose and lignin. Produce humus and humic acid
Pathogenic Fungi Eat living matter. Cause disease
Predatory
Nematodes Eat nematodes and protozoa. Produce ammonium. Aggregate soil
Nitrogen Fixing and
Nitrifying Bacteria Fix nitrogen. Convert ammonium to nitrate
Decomposers Eat root exudates. Aggregate soil. Protect against disease
Mycorrhizal Fungi Make nutrient and water available to the plant. Protect against disease
Flagellates Amoeba Cilliates
Actinomycetes Degrade Cellulose. Produce antibiotics
Actinomycetes Degrade cellulose
Saprophytic Fungi Eat dead matter. Suppress disease. Aggregate soil. Produce humus
Arthropods Shred organic matter. Eat fungi and bacteria. Mineralise organic matter
Fungal Feeding
Nematodes Eat fungi.
Produce ammonium. Aerate soil
Protozoa
Eat bacteria and algae. Excrete ammonium
Earthworms Aggregate soil. Improve percolation. Increase microbial activity.
Bacterial feeding
nematodes Eat bacteria. Produce ammonium. Aerate rootzone
The soil food chain - what is going on below the surface in biologically active soil PC OCTOBER/NOVEMBER 2015 I 123
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