As we look for “natural” solutions for pest and disease problems, we sometimes get more than we may have bargained for. In a recent publication of the Journal of Agronomy for Sustainable Development, researchers (Montes-Molina et al) found that the antibacterial properties of neem (Azadirachta indica) leaf extract significantly lower the soil population of the bacterium Rhizobium spp. in plantings of beans (Phaseolus vulgaris). Neem is a tree we have written about in the past, with natural pesticidal, antiseptic and anti-microbial properties. After reading a summary of the research, we wondered whether or not a warning about using neem on legumes was warranted. I located the original article and took a closer look.
In the experiment mentioned above, neem leaf extract was compared to extracts of leaves of the tree Gliricidia sepium (common names include mata-ratón, madre de cacao or simply gliricidia) and a synthetic pyrethroid, lambda cyalothrin (Warrior® or Karate®), all three of which are commonly used insecticides. A commonly grown bean cultivar was planted in individual 7-gallon plastic bags containing soil from local fields where beans had been previously cultivated. One-half of the bags were amended with composted cow manure. The neem and gliricidia extracts were taken from fresh, chopped leaves soaked for 72 hours in water. The plants were tended and measurements were made after 1, 2 and 3 months.
Normally pesticides such as neem, gliricidia and lambda-cyalothrin are applied to the foliage, but in this experiment the chemicals were intentionally applied to the soil surface and not to the foliage to test the effects of the active ingredients on the soil environment, and on plant growth and development. Also noteworthy is that the number of nodules on neem-treated plants in both types of soil did not differ statistically with nodule numbers in non-treated soil.
(As an interesting aside, plants treated with gliricidia produced significantly more pods than the neem and control, in both soil treatments. Perhaps there is a growth-stimulating hormonal effect from the gliricidia.)
So what can we conclude? Though neem is known to have antimicrobial properties, I do not think the information to date suggests that we should stop using neem on legumes. First of all, neem is normally applied to the foliage and not to the soil. In this experiment, the neem was applied to the soil and watered in so it would be in the root zone before having a chance to be broken down. Potential for movement in the soil of the active ingredients in neem is very low, so once sprayed on the plant, any that lands on the soil will soon (within 4 days) be broken down by sunlight and ambient moisture. Thus it will have little potential for affecting changes in the root zone (Martineau, 1994). Secondly, the final result of the experiment showed superior bean production in the neem-treated plants regardless of the number of nodules.
Azadirachtin, Pesticide Information Profile. EXTOXNET [Extension Toxicology Network]:
Martineau, J. 1994. MSDS for Azatin-EC Biological Insecticide. AgriDyne Technologies, Inc. January 26, 1994.
Montes-Molina, J.A. et al. 2008. Effect of pest-controlling neem and mata-raton on bean growth, soil N and soil CO2 emissions. Journal of Agronomy for Sustainable Development 28:187-194.