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Soil
 
                   
 
 
‘Symbiotic relationships’ between plants
and organisms
Soil organisms and organic matter decomposition

A closer look at who lives underground

Fabulous fungi
Arthropods
protozoa

nematodes

Providing an inviting
home for
microorganisms
in the garden
Organic matter decomposition
 
 
 

An introduction to soil bugs


The world underground is an amazing frontier of bizarre organisms and complex interactive networks. Though some people may find it hard to believe, our lives depend on the minuscule creatures living in the soil beneath our feet. Without fungi, for example, we would be smothered by thousands of years of fallen trees, branches and other dead plants.

There are many important functions in the soil that are dependent on the organisms that have made it their home. Nutrient cycling, pesticide decomposition, soil structure improvement and pest control are but a few. More often than not, it is actually not individual species that are contributing to overall soil health but the `interactions’ between the many species that are important.

‘Symbiotic relationships’ between plants
and organisms

The dictionary describes symbiosis as ‘a close union of two unlike organisms that live together in a mutually beneficial intimate association’. In 1885, the scientist A B Frank discovered that certain fungi form massive webs of hyphae called mycorrhizae and live in a symbiotic relationship around the roots of certain plants. Via these mycorrhizae the roots are able to scavenge nutrients and water from a lot further afield than just the immediate vicinity of the roots. The mycorrhizae can therefore be thought of as an extension of the roots. In return, the plant roots will share carbon and energy captured from the air and sun with the fungi.

Other important ‘symbiotic relationships’ exist between plants and organisms such as those of the nitrogen-fixing bacteria. Certain plants or trees (a special type known as legumes) share the carbon and energy they have obtained through photosynthesis in return for the much needed nitrogen that only certain specialised bacteria are capable of obtaining from the atmosphere. White clover is an example of a legume that is host to such bacteria. In New Zealand, clovers are a very important pasture plant species (heavily relied upon by pastoral farmers), as not only do grazing animals find them very tasty, but they also add significant amounts of nitrogen to the soil free of charge. In the garden, it is important to know that incorporating legumes into bed rotations is a good idea, but one must remember that the bacteria will only ‘fix’ (or scavenge nitrogen from the atmosphere) when they do not already have a supply of nitrogen at their ‘fingertips’. Indeed if the soil has plenty of nitrogen already (e.g. after a recent nitrogen fertiliser application) then the bacteria will not need to bother fixing atmospheric nitrogen, but instead will just use up what is already around them in the soil.

The rhizosphere is the area of the soil immediately adjacent to the plant roots and it is where large amounts of organisms congregate. Just as we may linger around the kitchen if we are hungry and know a good meal is about to appear, the rhizosphere is where the bacteria can find large amounts of dead root cells and root excretions (produced by plants) to feed upon. The bacteria, in turn, release substances that can improve plant growth or protect the plant from pests. Indeed some plants release special substances hoping to attract specific bacteria.

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Soil organisms and organic matter decomposition

Organic matter decomposition is the main process that recycles nutrients back into the soil as well as releasing water, heat and CO2 back into the atmosphere. Soil bugs are the decomposers that break down dead plants and animals, releasing nutrients back into the soil so that plants can make use of them again to grow. They can therefore be thought of as the drivers of the carbon cycle.

Decomposition of dead organic matter begins with large soil organisms like earthworms, arthropods (ants, beetles, and termites), and gastropods (slugs and snails). These organisms breakdown the organic matter into smaller pieces that can be decomposed by smaller organisms like fungi and bacteria. There is no doubt that the effects of soil microorganisms ‘soil bugs’ on the soil system far outweigh their size. Pick up a pinch of topsoil and you may be holding a billion microscopic decomposers!

Though soil organisms are important for improving a soil’s structure (they release certain glues that bind soil aggregates together), under certain conditions (for example after tillage when lots of oxygen is mixed with the soil) they can actually decompose more organic matter than is desirable. The sudden release of nutrients as a result of their activities will be beneficial for a short while, but may become a problem further down the track if the organic matter ends up being broken down at a faster rate than it is being added. This process explains why newly broken down sod was initially so productive for early settlers in many parts of the world and why the ‘slash and burn’ agriculturalists have to move on to ‘fresh’ areas once the soil becomes ’spent’. It is now widely known that reduced tillage is a quick way to build up soil organic matter due to the fact that organisms cannot break the organic matter down so quickly.

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A closer look at who lives underground

 

The mighty bacteria  

Bacteria

By permission Saskatchewan Interactive
http://interactive.usask.ca

Actinomycetes

 

 

Bacteria are one of the simplest forms of life – they are single celled with a length of only a micrometre or so. Soil bacteria outnumber all other soil organisms by far. In a healthy soil their numbers can exceed 109 or one thousand billion per gram of soil with over 20 000 different species present. The weight of all the bacteria in one acre of soil can equal the weight of a cow or two!

These microscopic organisms decompose all kinds of organic matter, releasing nutrients that plants can use (a process called mineralisation). Sniff a handful of soil and the familiar, fresh aroma comes from a substance that scientists have shown is released by actinomycetes, a particular group of soil bacteria. Actinomycetes are important in helping to breakdown some of the more resistant compounds in plant material such as cellulose and chitin. A single gram of soil can contain hundreds of millions of soil bacteria!

Specialised groups of bacteria are extremely important in soil since they promote vital biochemical reactions. Some bacteria help protect environmental quality by degrading compounds that would otherwise become pollutants. They do this by using the complex pollutive compounds to support their growth, thereby degrading the pollutants and improving environmental quality. For example, ammonia can be turned into nitrate through a process known as nitrification and this process is brought about by bacteria. Similarly bacteria are responsible for turning elemental sulfur into the plant available form of sulfur, namely sulfate.

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Fabulous fungi

There are over 50 000 species of fungi. Most are very, very small and play an essential role in the recycling of nutrients from dead plant matter. They are capable of decomposing (breaking down) the parts of dead leaves and wood that generally cannot be broken down by many of the other soil decomposers, like bacteria. They do this by releasing specialised chemicals (enzymes) that help them absorb the nutrients from the organisms they are decomposing! You may have noticed a lot of mushrooms growing on a lawn where a tree once stood. This is because many fungi, including those that make mushrooms, thrive on decomposing wood.

There are many fungi that are helpful, e.g. penicillin and other antibiotics are made from fungi. Fungi like mushrooms, mildew, mold, and toadstools are not plants. They don't have chlorophyll so they can't make their own food. The mushroom is actually just the ‘tip of the iceberg’ as it is the fruiting body of large underground networks of fungal hyphae that might be compared to a mass of white shoelaces. In 28 g of fertile soil there may be as many as 2.3 km of these thread-like mycelia.

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Arthropods

Beetles, spiders, mites, millipedes, and other arthropods have jointed legs, segmented body parts and no backbones (though they do have an exoskeleton). Many of them are in the front line of the army of organisms that decompose organic matter as they shred the larger plant debris, mixing it with microbes and soil so it is ready for the next stages of decomposition. Arthropods also have other uses in the garden as many prey on disease-causing pests and influence soil structure through their activities in the soil.

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Protozoa

Protozoa are a group of tiny one-celled creatures. Many of them like to eat bacteria so they are common near plant roots where bacteria like to congregate. Protozoa numbers will go up and down depending on what bacterial numbers are doing. For example, after tillage or rainfall bacterial numbers generally increase and therefore so too do protozoa numbers. Plants can benefit from the protozoa grazing on bacteria simply because the protozoa have lower nitrogen requirements than do most bacteria. So when they consume bacteria the protozoa ooze out the excess nitrogen that they do not require – and it happens to be in the form of inorganic nitrogen – exactly the form that plants like best for uptake.

Nematodes

Nematodes tend to get labelled with a ‘bad name’ because some of them attack roots and cause plant diseases (e.g. potato cyst nematode). Nematodes are, however, an incredibly diverse group and most species are actually beneficial. Nematodes are grazers just like protozoa and they similarly help to release plant available nutrients into the soil and therefore provide a useful service to plants by releasing the nitrogen previously taken up by other micro-organisms.

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Providing an inviting home for
microorganisms in the garden

Organisms do not merely live in the soil, but are also a component of soil and can significantly influence both its physical and chemical make-up. Just how well the organisms perform all these important functions is affected by how well we manage the land. The more we mistreat a soil, the more ‘sterile’ it will become and all that beneficial work that the organisms do will have to be done by you (or not get done at all)! Compaction of the soil, for example, has been shown to reduce the populations of beneficial nematodes and organisms.

It is important to provide an inviting home for the diverse array of organisms that will be beneficial in your garden. Luckily, to some extent, this will happen by default as long as you follow the basic guidelines in the garden to create or maintain healthy soil (i.e. by making sure there are adequate inputs of organic matter, and that water inputs and drainage are managed efficiently).

Organic matter decomposition

 

A simplified nutrient cycle


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