Cover crops like sunn hemp boost beneficial nematodes

Nematodes are microscopic roundworms, and they are abundant and diverse in soil ecosystems. Although plant-parasitic nematodes are considered pests when they feed on our crops, there are also many types of nematodes that are beneficial. They are predators of other nematodes, feed on fungi or bacteria, or are omnivores.

Some of the most common parasitic nematodes are root knot nematodes and reniform nematodes. There are several strategies that help to control these serious pests — the most important is to keep from introducing them to your soil by making sure you do not move soil or infested plants. Unfortunately, much of Hawaii’s agricultural fields and gardens is already infested.

There are organisms that control nematodes, such as nematode-trapping fungi, bacteria and predator nematodes. You can also control the infestation by increasing soil health by using compost, cover crops, manure and keeping the soil covered with mulch.

Sunn hemp (Crotalaria juncea L.) is a leguminous cover crop that grows well in the tropics. It provides the soil with organic matter and nutrients, its residues act as a mulch and it has parasitic nematode-reducing properties when incorporated into the soil.

Sunn hemp has been reported to increase numbers of beneficial organisms, such as ground arthropods and soil-dwelling nematodes. Cover crops have been shown to increase the abundance of bacterivore and fungivore nematodes, and they often have a greater effect on the soil food web than compost additions. Cover-cropping is also used to reduce soil erosion, increase yield and reduce pest damage.

Another effective nematode control method is soil solarization, in which soil is covered with a transparent polyethylene film, which causes the temperature to rise under sunny conditions, killing plant parasitic nematodes and weed seeds. Cover crops have been shown to increase the effectiveness of solarization.

Soil solarization temporarily reduces beneficial nematodes. Sunn hemp cover crop incorporated before solarization reduces this negative effect, while the addition of compost, vermicompost and mulch can increase number of beneficial organisms.