This is part two of a four-part series on soil health.
Does the word "nematode" conjure up images of worms that kill your plants? Well, there are those “bad” nematodes but in healthy soil they make up a very small percentage of the soil microorganisms. The number of beneficial nematodes in soil is far greater and keeps the bad guys in check.
What are Nematodes?
Nematodes are roundworms in the Phylum Nematoda. There were 15,000 species and 2,200 genera of nematodes described by the 1980s. They’re microscopic, in 100 cc of soil there are several thousand nematodes. They live in the water film that surrounds soil particles and carry bacteria and fungi on their bodies, as well as in their guts to spread biodiversity in the soil.
Scientists divide them into pathogenic or free-living, based on whether they cause plant damage or are beneficial. How do scientists tell them apart? Usually by their feeding habits. Nematodes are predators in the soil food web. By their activities they improve soil health and keep soil microorganisms in balance.
Most nematodes are quite specific about their diets and their mouthparts are designed for those diets. There are six distinct eating habits:
- Bacterial Feeders - these are nematodes that are free-living and feed only on bacteria. The mouth is a hollow tube that consumes bacteria
- Fungal Feeders - these nematodes feed on fungi. They have a stylet (think of a microscopic spear) that punctures the hyphae of fungi
- Predators - these nematodes eat other (including the root-feeding) nematodes and protozoa. They also eat smaller organisms like bacteria. Equipped with a sharp tooth, they can attach to their prey and eat out the internal organs
- Omnivores - these nematodes eat many different organisms and have different diets at different stages of their life cycle although their diet mainly revolves around bacteria.
- Root Feeders - As their name indicates, they consume roots. They are plant parasites. They are pathogenic and live on and around the roots. These are the ones that come to mind because they affect crop yield.
- Entomopathogenic nematodes (EPN) - beneficial nematodes that work in cooperation with bacteria to control insect pests.
Root-feeding nematodes are always present in soil but the effect they can have on the roots of plants depends on the soil and the nematode diversity available in the soil. Greater the diversity and population of nematodes in (healthy) soil, lower the chances for the root-feeding nematode to thrive and affect your plant/crop. In a healthy soil the different microorganisms are in balance and the good guys are far more prevalent than the bad guys. Disease and insect pests have a hard time getting established in healthy soil. So, if you’re having issues, a soil test for microorganisms, such as a microbial bioassay, will give you a great indication of the microbial life present; from there you can take action.
The Importance of Nematodes in Agricultural Systems
Nematodes are important in nutrient cycling. For example, when a bacteria feeding nematode consumes a bacteria, the nematode excretes excess nitrogen in the form of ammonium, which is a plant available form of nitrogen. Nematodes are also important in the movement of bacteria and fungi throughout the soil. Bacteria and fungi “hitch a ride” on a nematode, or in their digestive system, to an area where more food is available. The rhizosphere of a plant is a particularly rich bacteria, fungi, and nematode space.
You may wonder how nematodes can appear in your soil. If your environment isn’t hospitable for nematodes -not enough food, soil too warm- nematodes will move deeper in the soil looking for food or simply go dormant until conditions are more acceptable.
All microorganisms are readily spread. When soil is eroded by wind or water the microorganisms surrounding those soil particles move also. When you walk in your field you are actively moving microorganisms around on your boots. The natural world of birds, plants, and insects moves microbes around all the time.
This is a positive because the first microbes that appear are usually not the most beneficial for your crops. Plant-parasitic organisms such as bacteria, are harmed the least by soil disturbance.
Tilling and use of synthetics doesn’t have quite the destructive effect on single celled organisms such as bacteria. Those actions have radically detrimental effects on fungal hyphae. Just by natural actions biological control agents – the good nematodes- are deposited in your fields. You can also apply nematodes as an additive for insect pest control.
Nematodes and Soil Fertility
Studies in controlled environments reveal that bacterial-feeders are more involved in N mineralization, whereas fungal-feeders are more involved in P mineralization (Ingham et al., 1985).
Nutrients are cycled through the nematode eating process. They are predators in the soil and most nematode populations are specific about what they eat. Bacteria and fungi break down organic matter into plant soluble forms of nitrogen, phosphorus, and other nutrients. They retain some of those nutrients as their food source. When a nematode eats a bacteria or fungi it excretes excess nutrients, especially nitrogen and phosphorus, that their prey had held in their bodies. Nematodes tend to be concentrated in the rhizosphere (plant root zone) so they make nutrients readily available to plant roots. In a controlled study of nematodes conducted by Dr. Elaine Ingham, bacterial-feeders were responsible for more nitrogen mineralization whereas fungal-feeders mineralized more phosphorus into the soil. According to a study done by Thomas Rosswall and Keith Paustian, “nematodes can account for up to 25% of nitrogen mineralization in the soil.” (1)
In pastures or hay fields, that are under not till management, the bacteria to fungi ratio skews to higher fungi. Hence a greater diversity of nematodes. Fungal feeding, omnivorous, and predatory nematodes are highly sensitive to soil disturbance, tilling not only destroys the nematode population, it also destroys the prey upon which nematodes depend.
In the forest by your fields, or even in the fence line, there is a higher ratio of fungi to bacteria. In those ecosystems fungi eating nematodes are more prevalent. In an undisturbed system, like the woods, there is a greater diversity of all microorganisms.
How Do Predatory Nematodes Improve My Crops?
The root feeders feast on your crops. Predatory nematodes feast on the root feeding nematodes – acting as a biological control in your fields. The question is: do you spray nematicides to destroy the root feeding nematodes? If you do, you’re also killing the nematodes with beneficial roles. This can become a vicious cycle. Investing in predatory nematodes instead of one of those “icidal” products is a wiser long-term investment. As long as predators have prey, they will stay in your soil and if your farming practices allow, they’ll multiply.
What About Those Root-Feeding Nematodes?
Yes, root-feeding nematodes will always be in the soil. No matter how many or how many of these “icidal” products you spray, damaging root-feeding nematodes will always be the first ones to re-establish themselves in your field. But predatory and omnivorous nematodes can take care of those root-feeders if you’ll let them. A study by H. Minoshima et al in 2007 found that "long-term conservation tillage (more than two years) is required before increased abundance of omnivorous or predacious nematodes can be observed." (2) This may mean changing the environmental conditions on your farm to benefit the many different species of microscopic roundworms.
Crop rotations are another way to decrease the number of parasitic nematodes. There are crops that will starve or kill root-feeding nematodes. A cover crop that has allelopathic compounds against nematodes is one option. This is an especially good option for orchards, asparagus, or other perennial farms. These plants produce biochemicals that negatively affect the health of root-feeding nematodes. Some examples are sunn hemp (Crotalaria juncea), French marigolds (Tagetes patula), African marigolds (Tagetes erecta), and rapeseed (Brassica napus).
There are also many natural enemies of root-feeding nematodes including predatory microarthropods (microscopic insects), nematode trapping fungi, and predatory and omnivorous nematodes. Fungi prefer other food but when their preferences are scarce, they weave a net with their hyphae and lure the nematode into it. When it’s in the net water is released into the net, suffocating the nematode and killing it.
As farmers and gardeners, we also have to decide our level of tolerance for parasitic nematodes. They are a natural part of the ecosystem and an essential part of the soil food web. Plant parasitic nematodes are host specific so crop rotation is a powerful weapon to reduce nematode-associated yield losses.
But not all parasitic nematodes are bad. The entomopathogenic nematodes (EPN) work in cooperation with bacteria to control pest insects. If you’re having an insect problem, first identify the insect, then purchase the particular nematode species that targets that pest. Steinernema carpocapsae, for example is effective on the insect larvae of armyworm, weevils, caterpillars, cutworm, and sod webworm. Heterorhabdtis baceriophora is effective for European chafer, Colorado potato beetle, corn root worm, flea beetles, and white grubs including Japanese beetles. Both nematode species have a wide host range which makes them highly beneficial organisms. Beneficial insects seem to move faster and stay out of the way of heterorhabditid nematodes
This video shows how nematodes take care of both underground and above ground pests.
Soil Nematode Communities
Nematodes, like all soil microorganisms, are affected by farm soil management. Tillage, fungicides, and herbicides all decrease the diversity above and below soil. A conventionally operated farm, with tilling and synthetics, will have a far less diverse soil microbiome than a farm operated with minimum tillage and no synthetics.
Fungal feeding, omnivorous, and predatory nematodes are sensitive to soil disturbance and take longer than bacteria to repopulate a field. When you till a field after seedlings have appeared you are no longer disturbing the seed bed, only the furrows. This allows parasitic nematodes, protozoa, and bacteria a chance to repopulate your field without a balance of predatory or omnivorous nematodes.
The soil food web is a community in which each individual is necessary. Bacteria will be too abundant if bacteria feeding nematodes and other bacteria feeding organisms do not maintain the balance. The same is true for all members of the soil food web, from the smallest bacteria to the robin pulling worms from the soil.
Implications for Farm Management
Certain farm practices will hinder crop yields, others will help soil health and farm profit. Nematodes are crucial for soil health. They also facilitate nutrient cycling and act as biological controls.
A farm operating with no-till will have richer nutrient density. It will also have a higher microbial biomass – more diversity – to handle extreme climate conditions. The more fungi you have in your soil the better. For example, in drought situations the fungal hyphae will scout out and return water and nutrients to plants far beyond the plant root zone. If anything gets out of control the nematodes can help create balance.
High organic matter is also critical to fertile soil and a high yielding crop. Organic matter added into tilled the soil will create an increased bacterial population. Bacteria thrive in a moist environment with plenty of food – organic matter. Your fertility from natural sources will be a short-term gain, but a long-term loss.
Bacteria are very efficient at decomposing the organic matter In less than a season your plants will have benefitted from the nutrients released from the organic matter by microbial action. But before harvest the bacteria will run out of food.
Then you will start incurring expenses because of added fertilizers, pest deterrents, and the cost of tractor use. Not to mention soil compaction, loss of soil carbon, and the loss of microorganisms.
Leaving the organic matter on the soil surface is more cost effective and leads to greater microbial diversity. Lower costs, higher profit margins.
Microbial Diversity Leads to Healthy Soil
Synthetic chemicals will decrease all microbial life in your soil, especially fungi and most of the bacteria. When you spray for root-feeder nematodes you also destroy the other beneficial nematodes.
Nematodes serve the function of grazing on their prey, thus keeping a balance of microorganisms in the soil. They’re also large enough to carry bacteria and fungi on their surface and in their guts. They move decomposers to new food sources.
Although we have a tendency to think of nematodes as mainly causing disease, most nematodes are beneficial. They eat root-feeding nematodes as well as pathogenic bacteria and fungi.
Conventional ag practices that include tillage, synthetics, and bare ground leave you with an imbalance of soil microorganisms. This is a vicious cycle that will keep repeating itself.
As Carmen Ugarte has written, “Cultural practices like tillage or cultivation reduce the complexity of the soil food web. Thus, a decrease in the frequency and intensity of tillage may promote the conservation of predatory nematodes and contribute to improved farming system performance.” (3)
Nematodes, bacteria, and fungi are not the only microorganisms in soil. Protozoa are fascinating single celled organisms that come in many shapes and sizes. Stay tuned to find out their roles in your soil.
This article is part of a series of 4, you can explore the others here:
- Part 1: What Is Soil Health?
- Part 2: You are here.
- Part 3: The Role of Beneficial Protozoa
- Part 4: The Importance of Organic Matter