Not all soil nematodes are bad!

June 15, 2009
By: Mark Kesby, Projects Officer

Whenever you mention nematodes, farmers immediately think of the ones that attack their crops. Root Lesion Nematode attacks the roots of susceptible crops (mainly wheat) and is a significant pest especially in southern Australia. However, overall nematodes are an important part of soil biology and should be encouraged!

Let’s go back to basics and understand what nematodes are and what they do. Nematodes are small, non-segmented worms and are less than 1mm long.  Nematodes live mainly in the soil where they feed on fungi, bacteria and other soil organisms and, in some cases, plant roots. They live in the thin film of water surrounding soil particles and require water to move. They have a resistant cuticle (skin) which enables them to adapt well to environmental change, in fact, they are so good at adapting that they are the most abundant multi-cell organism on the planet!

There are three main functional groups of nematodes:

• Decomposers which feed on organic matter in the soil, breaking it down to release nutrients for plants use. They also improve soil structure, water holding capacity and drainage.
• Predatory nematodes that feed on other nematodes as well as bacteria, fungi and small single celled organisms (protozoa). Some have become specialised predators of insects. The digested pests help build the soil organic matter reserve
• Parasitic nematodes feed on plant roots and slow plant growth, often causing problems in agricultural production.

As indicated above, nematodes play an important role in the soil. When they eat soil organisms such as bacteria and fungi, nutrients such as ammonia have been stored in the organisms are released. These nutrients are then available for plant use.

Bacteria and fungi cannot move through the soil themselves. However, their dispersal is critical for ongoing soil health. Nematodes are parasitised by some fungi which helps their spread.

Beneficial nematodes attack and kill a range of pests such as borers, grubs, thrips and beetles. Some find their prey by waiting for the prey to move past them and ambush them – usually near the soil surface. Others actively search out slow moving prey at depth in the soil.

When the nematode catches its prey, it penetrates through the outer cuticle. Once inside the body, the nematode releases bacteria from its gut. Within a couple of days the bacteria kill the prey. The nematode continues to feed on the multiplying bacteria while maturing and producing a new generation of nematodes.

Nematodes are generally found in well structured soils with large pore spaces. For nematodes to flourish there needs to be enough organic matter (food), suitable hosts, water and minimal disturbance of soil. Some agricultural chemicals adversely affect nematodes.

The diversity and complexity of nematode communities can be used as an indicator of soil health. The different ratios of bacterial and fungal feeders and other types indicate the type of soil functions that are occurring. Varying ratios can indicate if the food web is disturbed, maturing, structured or degraded.

Further information can be sourced from two excellent websites:

• www.dpi.nsw.gov.au/agriculture/reources/soils/biology
• www.soilhealthknowledge.com.au/