What's in Your Garden Soil? - Nematodes

Nematode loa loa removed from eye.
Source: dated.org

I missed Halloween for this post; but given that it seems  horror movies are released year round, mentioning parasitic nematodes along with the beneficial ones seems okay to do. Most of us probably  have heard about diseases stemming from parasitic nematodes,  but hopefully have not suffered from ascariasis, trichinosis, hookworm or worse. we'll try  just to focus on what's in our garden's soil and not our guts...

Nematodes, named after their phylum, Nematoda, ["nema" means  "thread-like"] are unsegmented round worms that live in all the environments that other earthly lifeforms do.  They actually constitute the most numerous of animal lifeforms. In fact, for pratical purposes, nematodes are inseparable from other lifeforms... If you know what nematodes exist in a particular environment, you can predict what  other lifeforms will be also be found there.

According to the Wikipedia entry, about 25,000 species so far have been identified with probably tens of thousands more awaiting discovery.   They are typically described as being  just a "tube within a tube" with  a front and back end.  Their front ends are adopted for their feeding preferences. 'Herbaceous' types,  like  root feeders, insert long microstylets  into plant cells to syphon nourishment; fungal  feeders use shorter stylets to puncture cell walls; and  bacterial eaters have more recognizable mouth-like openings for collecting their lunches as they wiggle through  soil.  Most nematodes reproduce sexually, but a few are harmaphroditic; and many  are able to enter a dormant state to survive extreme environmental surprises like drought or extreme cold.

Cross section of typical  nematode.
Source: Biology Stack Exchange

Because nematodes closely co-exist with other lifeforms in their environments, their minescule wiggles are sufficient to meet their daily needs. Their greater travels depend upon outside factors such as wind, water or physical transport by some other agent -- animal, insect, garden tool or tractor tire for dispersal.

Soil nematodes are very small ranging in size between  1 mm and 50 ml; but some, like the parasitic ones that attack vertebrates,  can be large. One parasitic nematode that abides in the placenta of sperm whales   reaches 30+ feet!  

Minor carrot damage from root piecing nematodes.

Parasitic  nematodes can affect plants in several ways.  Some appear in foliar tissue (leaves); others burrow into roots or stems; while others just attach themselves externally to roots. Gardeners have probably encountered the effects of these latter pesty varieties on their cabbages and carrots. Small infestations of these "herbaceous" feeders cause carrots to respond with those little bumps, called 'root galls'.  Bigger infestations of this type of nematode (e.g. the NRK nematode - our local favorite) show up as "root knots" on cruciferous veggies like cabbage and broccoli, and they also can  cause serious crop damage to  legumes  like soy beans. Various root knot nematodes affect more than 2,000 agricultural  crops and ornamentals.

N.B. - Root knot nematodes also affect many common weeds such as lambsquarter, purslane, 

and dandelions -- Better keep weeding!

Severe root knot.

If you think that you have a root knot problem like the one pictured to the right, take a look at this visual assessment technique from Cornell Extension. You can probably use it as an initial guide for sizing up your  situation. Cornell's Plant Disease Diagnostic Clinic for a small fee will analyze soil samples for nematodes and other pathogens. There are also  commecial services that will analyze soil samples and identify what you've got.  Beneficial nematodes can be purchased to improve your soil if it turns out that you have a nematode deficiency or infestation.  Suppliers offer both  tailor made and  standard  mixes for particular crops and soils.

What Good Nematodes Do

Because nematodes occur naturally in the environment, their uses in pest control and sustaining soil health make them  important components in  organic farming practices.

Grasshopper demised by its parasetic nematode.
Photo: U. of Fl.

Nematodes in soil are particularly important  for protecting healthy young plants by controlling a lot of plant pests and pathogens. They also   increase soil fertility   by  making more nutrients available for plant use through their digestive processes. Among these is the ammonium cation (NH4+). Because ammonium is very soluble in water, it is easily taken up by plants - although too much of it, like all good things, is toxic. NH4+ is critical for plants. They convert it  into organic compounds like amino acids.  Animals, like us, and other creatures  depend on plants to supply us with these compounds.  Both plants and  us omnivores should be grateful for these beneficial nematodes. So, continue to tread lightly on your garden soil.

Although there are lots of potential nematode bad guys even in healthy soil, these are generally outnumbered by beneficial nematodes. Among the ranks of beneficial nematodes are the predatory   fellas that devour bacteria, fungi, protozoa along with other nematodes. Unfortunately, getting rid of an infestation of the bad dudes  using chemicals also  eleminates the good guys too. A balance of beneficial versus parasitic nematodes in healthy soil actually stimulates plant growth. Healthy plants simply  outgrow any nutrient challenge posed by low levels of parasite activity.  Finally, by their wiggling through  soil nematodes also promote soil fertility by redistributing both  good and bad bacteria and fungi.

What's a Normal Nematode Load in Soil? 

The US Dept. of Agriculture reports that a healthy census is around 100 nematodes in a teaspoon of dry garden soil. Given their microscopic size, this is a surprizingly low number.  But nematode populations can rapidly expand, e.g.  a rotten apple on the ground might contain 90,000+ nematodes while it still  vaguely resembles an apple.  Even this remains a low number compared with the  abounding nematode populations found in  sea beds that can reach 1,000,000 per square foot!

Good Gardening Practices 

Backyard gardeners can employ a number of techniques to minimize harmful nematodes. Among these are

Clean equipment and boots.
Source: Northville Dairy

• Using certified seeds and choosing nematode-resistant or tolerant varieties;

• Cleaning tools and equipment;

• Weeding as well as removal of infected stock;

• Rotating crops and allowing some areas to remain fallow during  growing seasons;

• Using cover crops and green manure along with compost both to keep up soil nutrient levels. 

Now stop worrying about what you might be walking on,  and remember to:

"...trust dirt. I don't trust diamonds and gold." - Eartha Kit