HELPFUL ALLIES IN THE ORCHARD

NITROGEN FIXATION

The importance of nitrogen in the soil cannot be stressed enough. It is essential for all life forms, as it is needed to make important molecules for life, including RNA, DNA, and proteins. It is among the top three most vital nutrients required by plants, alongside potassium and phosphorus. Nitrogen assists with chlorophyll production, photosynthesis, and the formation of amino acids (proteins that are vital to plant health and growth).

So why all this talk about nitrogen? Well, it is unreachable by life forms, including plants. That is because, despite the fact that 78% of the atmosphere’s is made up of nitrogen, it isn’t usable by plants. It is an inert form of nitrogen, called atmospheric nitrogen (N2), therefore, it cannot perform any reactions to form the important molecules for life. Additionally, the soil is easily depleted of nitrogen since it is used in such high quantities, and it is easily leached away by the sun and water.

This is where nitrogen fixers jump into the picture. What nitrogen fixers do is transform inert atmospheric nitrogen into ammonia (NH3) and ammonium (NH4+), a usable form of nitrogen, via the nitrogen fixation process. The ammonia can further break down into nitrates and nitrites, which are also usable by life forms. The primary nitrogen fixers are the rhizobia bacteria (soil bacteria that fix nitrogen). They live in the soil and on plant roots of legume plants (peas, beans, etc.) within nodules growing on the roots. The fix nitrogen for the plant and nearby plants in exchange for the carbohydrates the roots provide for the bacteria. Once this process occurs, it can be passed down to other life forms through the food web.

NITROGEN FIXING PLANTS

Nitrogen fixing plants are our allies in the garden. They make more nitrogen available for themselves and other plants. But, as we know, they don’t do it on their own. These plants have a symbiotic relationship with the rhizobia bacteria mentioned above. These bacteria form root nodules on the plant roots, within which bacteria can convert nitrogen into ammonia. This symbiosis mostly occurs with plants in the legume family, but not exclusively. These leguminous plants come in all forms, ranging from groundcovers to trees. Some examples are: peas, beans, baptisia, black locust trees, and the list goes on.

Root nodules on a fava bean plant.
Source: Wikimedia Commons

NITROGEN FIXING PLANTS IN THE ORCHARD

There is a wide variety of nitrogen fixing plants to choose from in an orchard setting. Criteria for the preferred type of nitrogen fixing plants in a holistic orchard setting are plants that not only improve soil, but may offer some human or ecological benefit, whether that be food, medicinal products, nectar, etc. It is also important to be aware that some nitrogen fixing plants are much more efficient at fixing nitrogen than others- check out Eric Toensmeier’s article comparing nitrogen-fixing capacity!

Incorporating nitrogen-fixers within an orchard setting can be done a few ways. There are also preferred species to consider.

  • Growing an annual climbing bean plant under a tree and letting it climb the trunk.
  • Cover-cropping in the fall with a leguminous cover crop, such as crimson clover, vetch, alfalfa, etc.
  • Planting a nitrogen fixing shrub for every 2-3 trees.

Shrubs are often ideal in an orchard setting because they fix more nitrogen than herbaceous plants and don’t need to be replanted every year. It is best practice to place one nitrogen fixing shrub per 2-3 trees, especially trees that have high nutrient needs and/or are more susceptible to pest and disease pressures.

Please view our previous blog post for more information on POP’s favorite nitrogen fixing shrubs, including goumi, bayberry, peashrub, new jersey tea, and more!

Nitrogen Fixing Perennial Flowers

Below is a list of nitrogen-fixing perennial herbaceous plants commonly used at POP orchards, including the POP Learning Orchard at The Woodlands:

  • Baptisia (Baptisia australis)
    • Baptisia, also called Wild Indigo, is a native shrubby perennial with blue-green small leaves and purple or yellow pea-like flowers, which turn into dark pods. This plant was used to produce a blue dye by both Native Americans and settlers before the discovery of the higher quality true indigo (Indigofera tinctoria). The many different species of Baptisia are native to eastern or midwestern North America. They grow very easily in a wide range of conditions and are drought tolerant once established but prefer full sun and well-drained soil. It is in the bean family, making it a nitrogen fixer.
Source: Wikimedia Commons
  • Thermopsis (Thermopsis villosa)
    • Thermopsis, also called Carolina Bushpea, is an upright, clump forming perennial that forms long, yellow flower spikes. These flowers turn into hairy pods. It closely resembles a lupine. It is native to forest openings in the Appalachians, but is adaptable to areas throughout the eastern United States. It likes humus-rich, well drained moist soil and tolerates partial shade, but prefers full sun. It needs water during droughty summers. It is in the bean family, making it a nitrogen fixer.
Source: Wikimedia Commons
  • Purple Prairie Clover (Dalea purpurea)
    • Purple Prairie Clover is a prairie plant from the Midwest. It is easily grown in any sunny location with average to dry soils. The deep taproot allows it to withstand heat and drought. It has pink-purple cone-like flowers and lacy foliage. It is in the bean family, so has nitrogen-fixing abilities in the garden.
Source: Wikimedia Commons

Nitrogen Fixing Cover Crops

There are a variety of herbaceous legumes commonly used at cover crops because they fix nitrogen and improve soil fertility and soil texture. In the wrong context, these plants may sometimes be considered “weeds”, but we are generally happy to see them pop up in an orchard or food forest setting and sow them intentionally when cover crops are needed:

  • Crimson clover
    • Nitrogen fixer
    • Annual in our zone
    • 1-3 feet tall
    • Shade-tolerant
    • Winter and summer cover crop
    • Used for livestock grazing before it flowers
    • Used for erosion control
    • Great for pollinators
Crimson clover
Source: Wikimedia Commons
  • White clover (Trifolium repens)
    • Nitrogen fixer
    • Perennial in our zone
    • 6-12 inches
    • Low growing groundcover
    • Partial-shade tolerant
    • Attracts pollinators
    • Winter and summer cover crop
    • Used for livestock grazing
    • Used for erosion control
    • Incorporated into lawn
White clover
Source: Wikimedia Commons
  • Red clover (Trifolium pratense)
    • Nitrogen fixer
    • Winter cover crop
    • Short-lived perennial in our zone
    • 1-2 feet tall
    • Shade tolerant
    • Medicinal blooms that can be used in tea
    • Used for livestock grazing
    • Used for erosion control
    • Incorporated into lawn
Red clover
Source: Wikimedia Commons
  • Hairy vetch (Securigera varia)
    • Nitrogen fixer
    • Annual in our zone
    • Low growing but vining habit
    • 1-3 feet tall
    • Shade-tolerant
    • Winter cover crop
    • Used for erosion control
    • Great for pollinators
    • Habitat for wildlife
Hairy vetch
Source: Wikimedia Commons

SOURCES

All Nitrogen Fixers are Not Created Equal– Eric Toensmeier

Nitrogen Fixation– Nature.com

Nitrogen Fixing Plants– Gardenia.net

Siberian Peashrub-Pfaf.org

Weeds aren’t actually that bad– Permaculturenews.org

Cover Crops-Sustainable Agriculture Research Education

DISCLAIMER

The information presented on this website is for informational, reference, and educational purposes only and should not be interpreted as a substitute for diagnosis and treatment by a health care professional. Always consult a healthcare professional or medical doctor when suffering from any health ailment, disease, illness, or injury, or before attempting any traditional or folk remedies. Keep all plants away from children. As with any natural product, they can be toxic if misused. 

This POP Blog was written by Orchard Assistant Simone Shemshedini with help from Co-Executive Director Phil Forsyth.

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