In some cases the substance has been a waste product (e.g. coffee grounds, newsprint, crab shells, quinoa bran) from a manufacturing process.
In other cases, a crop is grown specifically for this function (e.g. marigolds, vetch, sesame).
In still others, microbial organisms or mixtures of microbials (e.g. Microplex, Microlife, Farewell) have been selected and reproduced in a fermentation process.
The mode of action of most of these natural substances is unclear (as is the mode of action of many chemical nematicides) and because of the variability within agricultural soils, activity could be expected to vary from one situation to another.
Some products claim to be inoculating soils with beneficial microbials originally derived from soils, and multiplied in fermentation media, or with toxins produced by these organisms (e.g. killed microbials). Some products are marketed to reinoculate soils with beneficial organisms after these have been removed by chemical treatments.
Some substances such as those containing chitin (e.g. crab and shrimp shells) are thought to stimulate the growth of nematophagous fungi which utilize chitin for food. Chitin has been shown to be present in the eggshells of nematodes and this is thought to be the target of the nematophagous fungi.
The addition of organic matter to soils is known to improve soil structure, aid in water retention, and provide nutrients. Improving soil conditions reduces plant stress which in turn can make the stress caused by plant parasitic nematodes less severe or apparent.
As some plant based products (e.g. marigolds, brassicas, sesame) decompose in the soil, they release chemicals thought to be nematicidal. Brassicas, for example, have been shown to release a compound similar to the active ingredient in metam-sodium containing (e.g. Vapam, Sectagon) products.
There is abundant evidence in the literature that this group of substances loosely classified as soil amendments, natural products, or organic amendments, can have an affect on nematode populations or stimulate plant growth in spite of the presence of plant parasitic nematodes. There is less evidence that they perform as consistently as traditional chemical nematicides or provide the same degree of nematode control when tested side by side in replicated field trials. In spite of this, there are probably abundant situations in which such products could be utilized in agricultural situations. There are many perennial crops in California, for example, for which no postplant nematicides are registered.
Because of the variability in agricultural soils, and the suspected mode of action of many soil amendments, it is likely that growers and PCA's will need to develop different programs for particular crops and sites. In such instances, it is important to try to leave some replicated areas untreated in order to be able to observe the success of the treatment program.