Research indicates that soil treated with compost tends to produce plants that have fewer insect and disease problems. There are two general reasons for this phenomenon. In the first place, compost added to the soil encourages a more active and numerous population of beneficial micro-organisms which, in turn, keep harmful ones in control. In the second place, as we have seen, compost added to the soil produces healthier plants, which are much better able to resist insects and disease through chemical changes and by virtue of basic good vigor. Insects and diseases always strike the weakest plants first.
Plants sometimes sicken even though soil and growing conditions are excellent. They may be suffering from root knot nematodes, a problem in many parts of the South. These wormlike pests live deep in the soil, and their feeding on plant roots causes them to develop nodules, or knots. They interfere with the plant's ability to take up nutrients, so the plants weaken. In this case, compost is an indirect pest control. Research by Safer Agro-Chem Ltd. of Victoria, British Columbia has shown that one of the byproducts of microbial activity in compost is fatty acids similar to those which are the basis for the commercial insecticidal soap sprays. These fatty acids control pests such as nematodes without harming beneficial insects. Since no chemical nematocide is registered for use in home gardens, compost or leaf mold is one of the few control methods for nematodes at present.