My idea of a healthy plant has changed over the past 40 years. I once thought that the biggest, greenest, most vigorous, nitrogen-filled plants would give me the best quality and highest yield. (Thus my dedication in times past to using manure-based compost.)

A healthy plant is one in which the main nutrients (such as nitrogen, phosphorus, and potassium) and the micronutrients are all balanced in relation to one another. No one nutrient should be out of a healthy range.

For example, I still strive to grow vigorous plants, but now I understand that high levels of nitrogen in plant tissues don’t necessarily make plants healthier.

In fact, high nitrogen levels have been shown to make some crops more susceptible to insect pests such as aphids, thrips, and mites.

Lower concentrations of soluble nitrogen (specifically nitrate-nitrogen) in plant tissue seem to make crops less susceptible to feeding damage by most types of insects. In several studies, applying fertilizers that rapidly release nitrate-nitrogen led to increases in sap-feeding insects.

And in one review that spanned 50 years of insect and nitrogen research, 135 studies showed more plant damage and/or greater numbers of leaf-chewing insects and mites in nitrogen-fertilized crops. Only 50 studies reported less pest damage with nitrogen fertilization.¹

Just as important, fast-release nitrogen fertilizers often cause plants to reduce investments in their “immune system”— their natural chemical defenses designed to fight off insect attacks and disease.

High nitrogen in the soil around plant roots can diminish the ecological efficacy of important interactions among plant roots and microbes that help to suppress insects and disease.

And as I’ve already discussed, it is becoming clear that higher-carbon, lower-nitrogen fertilizers increase soil microbial biomass and diversity as well as root-microbe interactions.

These beneficial belowground soil microorganisms help plants better tolerate stress and streamline nutrient uptake, leading to optimal plant nutrient balance. All in all, healthy plants are those that are connected to and interacting with a healthy, microbially active soil agroecosystem.