types of nematodes and one under a microscope

What are soil nematodes?

Soil nematodes are one of many different types of microbes found in soil. There are here are many different morphological groups or types of nematodes in soil.

Soil microbiologists say that nematodes generally are so abundant on this planet that there probably isn’t a place on Earth where we can’t find them. Under a microscope they look like transparent worms. In fact, they are a roundworm. They are microscopic animals.

Most soil microbes need moisture and nematodes are no exception. The rhizosphere is the eco-habitat for most microbes. This is where we also find most types of nematodes. This is the area directly adjacent to a plants roots.

Apparently there are over 15,000 species of nematodes. The vast majority of these are not plant parasitic (root feeding) nematodes. Most types of nematodes, like most microbes are ‘good guys‘. Most soil nematodes are beneficial to plants. They are a key organism in healthy compost and healthy soil.

However, when we use a pesticide designed to target all types of nematodes, we kill all the nematodes. The populations that tend to bounce back first are the herbivores. These are the ones pesticides are designed to kill.


Types of nematodes

Broadly speaking there are free-living nematodes and non-free living types of nematodes.

The non-free living nematodes are herbivores and feed on plant tissues. These plant parasitic nematodes are the target of nematode based pesticides.  This type of nematode is the target of gardeners who use Marigolds and other nematode resistant plants.

A more specific taxonomy is based on their feeding habits. There are five groups or types of nematodes:

  1. Some nematodes eat bacteria and are called Bacterivores. These guys help decompose organic matter. Being bacterivores we can imagine these nematodes also consuming harmful bacteria and pathogens in the soil.       Therefore, we might want to take care of them.
  2. Fungivores are a type of nematode that eat fungi. They have a different mouth part to Bacterivores. These nematodes also aid in the transformation of raw organic matter into compost and are therefore critical in our soils. Also, given so many plants have soil born fungal problems – we can enlist the help of these nematodes to help consume problematic soil born fungi. Where we use a fungicide, and kill off all the soil fungi, we also take away the food source of these nematodes.
  3. Predator nematodes have mouthparts specific to feeding on other nematodes. They’ll eat free-living and plant parasitic nematodes. They are found in most soils but aren’t considered very abundant.
  4. The Omnivores have mouth parts that can feed on plants, bacteria, fungi and other nematodes.
  5. The Herbivores feed on plants. Commonly called the plant parasites or root feeders they are the subject of most funded research. Some live in the soil (Ectoparasites) whilst others (Endoparasites) colonise the plants roots. The well known root-knot nematode is one that lives in the roots of a plant and can cause significant damage to a plant.

Each of these types of nematodes have subclasses. That is well beyond the scope of this blog. However, the image below shows the different mouth parts for these five types of nematodes.

five types of nematodes and their different mouth parts

Image courtesy of Soil Foodweb Institute (Australia)


Gardening with nematodes

When we’re gardening, we’re tending to the plant’s environment. One aspect of the environment is the soil. When we’re feeding the soil, we’re feeding the microbes. Those microbes feed the plant by making nutrients available to the plant. An example of this are nitrogen fixing bacteria.

Healthy soil, healthy microbes, healthy plants… generally. That soil will contain an abundance of food sources for all microbes and all nematodes. That’s what we want – a healthy balance.

When we’re looking at crop rotation systems or inter-planting and companion planting we’re looking at seeking a balance. If we’re looking at introducing a particular plant resistant to a plant parasitic nematode or herbivore insect we’re looking to reduce populations of that nematode or insect.

The relationship between plants and other beings is species specific.

However, nematodes don’t fly… they aren’t particularly mobile over vast distances. Therefore some nematodes aren’t found in some soil. Some plant parasitic nematodes aren’t considered a problem in some countries.

Indeed, some root-knot nematodes (a type of plant parasitic nematode) aren’t found in particular regions. However, all root feeding nematodes tend to have host plants. Those plants are more susceptible to particular root feeding types of nematodes. Others are repelled or poisoned by some resistant plants.

It’s a very complex science. Soil microbiologist tend to suggest increasing the diversity and complexity of our aerobic compost to increase the diversity and populations of microbes generally. This is how nature does it and how we can be more better and mindful gardeners.

Isn’t it interesting how nematodes can be so diverse in their feeding habits?


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