Sanet Post, Chris Alenson
Plant Resistance to Herbivory
June 25, 1997

Chris Alenson wrote:

Hi to all sanets,
Joel Gruver's posting regarding plant resistance to attack by herbivores is a most interesting one. Many involved in organic farming would suggest that a Healthy soil->Healthy Plant is less prone to attack by pests and diseases. This is fine but what evidence is there to suggest that this might be the case.

There are many scientists that accept that a healthy fertile soil does result in stronger healthier plants. This is not however altogether accepted.

A soil of high fertility is generally accepted as one that will result in a plant being able to maximise both its physical and nutritional capabilities . This means that the soil will be stocked with a wide range of the 13-23 elements required for plant nutrition while having adequate organic matter that will not only support an active microbial population but on decomposition will provide a range of inorganic nutrients and importantly organic substances that will assist in mobilising nutrients through chelation to assist in plant nutrition and protect the plant from many soil borne disease causing pathogens.

The result of this fertility is suggested in the literature as providing a plant with stronger cell walls assisting in pest and disease invasion( Schuphan 1965, Chaboussou, 1986) and a more balanced nutritional make-up of the plant(Chaboussou 1986, Eigenbrode & Pimental 1988). Chabouussou suggests in his theory of trophobiosis (protein synthesis) that imbalanced nutrition leads to free amino acids and other substances circulating in the sap stream of plants that attract pests and disease.

Eigenbrode & Pimental studied collards in the field that were fertilised with organic and chemical fertilisers and found that although the mechanisms remained unclear, resistance of collards to insect attack was greater when fertilised with organic manures.

I would encourage all those interested in this area of research to also search out the paper by Chaboussou as it really does provide some insight into why plants are attacked by pests and diseases and why also importantly that pesticides often increase the likelihood of attack by pests and diseases. Probably the most important paper in my library on plant nutrition and soil fertility.

The references are:

Chaboussou, F. 1986 The Ecologist Vol. 16, No. 1, How Pesticides Increase Pests  Eigenbrode & Pimental 1988 Agriculture & Ecosystems & Environment, Vol. 20 P109-125

Chris Alenson  Organic Advisory Service  Organic Retailers & Growers Association of Australia