Companion Plant for Soil Health

Driven by a need for natural methods of pest control as well as sustainable soil practices, organic gardeners are renewing the traditions of companion planting. They have found the benefits to include improved soil health.


When we think of companion planting generally pest control and attracting predator insects comes to mind. Improving the health of our soil is a tremendous benefit of certain companion planting that is not discussed often. We have some awareness that plants can change chemistry in our gardens but we tend to think primarily of cover crops in that regard. But when we understand how specific plants change what is happening in our soil, we can pair those plants with plants that can benefit from the change.

The biology of the changes plants make is termed allelopathy. The phenomenon of course can get complicated but we can boil it down to which plants, and how, can influence nutrient availability and uptake, seed germination, plant growth and photosynthesis. Other plants with corresponding needs then are easily paired up. And just as important, we need to avoid certain pairings will have a negative affect. And to make the most use of nutrients in the soil, alternating companions with different root levels will allow greater nutrient uptake for all the plants.

It is helpful to not just have a list of “best companions”, but have some knowledge of how specific plants benefit the soil and therefore plants that you might choose as a companion.

  • Legumes such as peas and beans, as well as clover and alfalfa, fix nitrogen in the soil from the air. Allow the plants to die back to take advantage.

  • Heavy feeders planted together can certainly negatively affect each other, quickly depleting soil nutrients. Broccoli is a notorious heavy feeder, especially of calcium.

  • Comfrey also has long been an all around garden companion. It has an amazing tap root that goes several feet deep to pull up nitrogen and minerals into its’ leaves. Comfrey in your compost adds rich minerals including calcium, magnesium, potassium, iron and phosphorus, as well as vitamin A, C and B-12. Comfrey is an excellent compost accelerator.

  • Caraway loosens soil.

  • Beets add minerals to soil. The leaves are loaded with magnesium for your compost.

  • Clover helps aerate soil and adds nitrogen.

  • Dandelions pull deep minerals up to the surface to improve topsoil and they attract earthworms. Ethylene gas is produced by dandelions.

  • Soybean not only enriches the soil as a nitrogen fixer, but will choke out weeds.

  • Mustard disinfects the soil as well as being a great green manure cover crop.

  • Fennel seed is coated with a growth inhibitor. Generally may not be a good companion to anything.

  • Buckwheat does well in poor soil and will mine and store phosphate and calcium. Leave to wither back for a great green manure.

All that information can be used to make the best pairing of plants.

  • Borage is an all around great companion especially for edible gardens. It will deter cabbage worms and tomato hornworms. Borage adds trace minerals to the soil. End of season plants should go in the compost heap as the leaves are loaded with calcium, potassium, vitamin C and mineral salts. All benefiting the soil when you dig in your compost.

  • Generally heavy feeders such as broccoli, asparagus, cantaloupe, pumpkin, sweet corn and watermelon can be paired with light feeders such as nasturtiums and beets.

  • Pair up and alternate plants that are deep rooted and shallow rooted. They can make use of nutrients at different levels in the soil without competing.

  • Clover is great in a lawn or with pasture crops to keep soil aerated and nourished with nitrogen.

  • Dandelions are also great in the lawn to improve topsoil and deliver minerals to the lawn.

  • Ethylene gas from dandelions will aid plants to ripen early.

  • Mustard’s disinfecting properties can benefit plants susceptible to nematode problems, cutworms and snails.

  • Corn and grain are heavy nitrogen users and will benefit from legumes replacing the nitrogen that they consume from the soil.

  • Soybeans are great companions to corn and potatoes, choking out the weeds and feeding the soil with nitrogen.

  • Caraway can loosen soil to aid carrots and other root vegetables. Sprinkled through your garden, loosening the soil can be an all around benefit to plants.

  • Buckwheat will benefit corn by mining and replacing nutrients when used as green manure.

When planning a garden then, decide first which edibles you want to grow. Then select companions to interplant that will benefit and support their needs. There are many lists of companion plants available, and many of those companions benefit each other by making sometimes subtle changes to the soil that benefit each other. Sometimes the change is using something out of the soil or adding something to the soil. The changes can affect growth, flavor, yield, or repel soil insects or inhibit disease.

Sharon DwyerComment