Increased water and nutrients help plants tolerate nematode attack. Soil improvement with organic matter, available as compost or soil improver, helps the soil retain moisture and adds to the available plant nutrients which are released to plants through microbial action. The increased level of microbes in the soil favours the build-up of organisms that feed on other soil microbes, including nematodes. When adding compost, take care that the compost does not include partially decomposed roots containing plant-parasitic nematodes or other soil-borne pathogens.
Green manure crops like legumes, clover, vetch, or rye can also build up organic matter in the soil. These crops are planted in autumn and tilled into the soil in early spring, giving enough time for decomposition before the next crop is planted. Some evidence suggests that the incorporation of a green manure crop produces compounds that are toxic to nematodes. This is especially true for green, leafy brassicas. The soil disturbance occurring with this incorporation can also reduce the nematode levels.
High temperatures can help to control nematodes and some other plant diseases which carry over in the soil. You can treat sections of garden beds by raking out any plant remains, moistening and smoothing the soil surface, then laying and firmly pegging down a sheet of thin clear plastic. Leave this in place for at least a month during mid-summer. The sun’s heat will penetrate deep into the soil, killing many unwanted organisms. To be effective, the soil temperature should reach 37 to 52°C for several months.
Cover crops of marigolds and mustard have been shown to reduce the numbers of root knot nematodes in soil. Sow mustard seeds densely as a green manure crop and dig it into the soil at flowering. Keep the soil moist and as the mustard decomposes it will release chemicals that fumigate the soil and reduce nematode numbers. Plant marigold seeds densely at least two months, preferably three or four months, before sowing vegetable crops. Do not let weeds become established as they can become nematode hosts.
The roots of the living plants produce a chemical that inhibits the hatching of nematode eggs. There is no further benefit by digging the plants into the soil. French marigolds (Tagetes patula) are thought to be effective against a larger range of root knot nematode species than African marigolds (Tagetes erecta).