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Red Clover (L) and Comfrey (R)

Healthy soil is the foundation of a productive and fruitful orchard. When designing an orchard forest garden like many of those POP plants throughout the city, a key consideration in design is choosing plants that will function as soil builders and nutrient accumulators to support the diverse and multi-layered plantings of fruiting trees, shrubs, and perennial herbs and/or vegetables.

Fertility-building plants are critical, especially for new gardens and those producing “harvestables” like fruit because they help to channel and replenish vital soil nutrients that may be limited in the existing landscape or depleted seasonally. Nitrogen is the foremost nutrient needed by plants for healthy growth and one that frequently is in insufficient supply. When nitrogen is deficient, plants may reflect it with a yellowing and dropping of their oldest leaves, stunted growth, or paleness in color. Without it, a plant cannot create its basic building blocks for life–amino acids for proteins and nucleotides for DNA and RNA.

Nitrogen-fixing plants are those that contribute to the conversion of atmospheric nitrogen (N2) to useable ammonia (NH3) that can then be processed and synthesized by plants.  While it’s often expressed that plants–like those of the legume family–are fixing this necessary nitrogen, the truth is that no plant can be credited for this conversion. In actuality, it’s the symbiotic relationship between plant roots and special microbes in the soil–such as Rhizobium–that fix nitrogen. As the bacteria feed on sugary excretions from the plants’ roots, they store nitrogen in root nodules that add dynamic vitality to the soil.

There are several ways to harness the power of nitrogen-fixers within an orchard, as well as several species of plants to consider for this purpose.

  • Surrounding producer plants like a fruit tree with a nitrogen-fixing plant like trellising beans which can then climb the trunk of a tree for support is one way to utilize the structure and function of each! The beans may be grown to fruit, or the leaves may be incorporated as mulch at the tree’s base.

  • Consider growing plants in patches for “green manure.” Comfrey (Symphytum spp.), with its resilient roots and voluminous leaves, regenerates quickly and can be frequently cut back and laid atop beds or used as mulch around producer plants.

  • Cover-cropping in the fall with leguminous alfalfa (Medicago sativa) and red clover (Trifolium pratense) is another way to build soil fertility. Seeded in the fall at season’s end and grown well into spring before grounds are readied, the entire plants and roots are worked into the soil. The extra organic matter builds the soil’s water retaining properties, provides food for worms, and nesting zones for beneficial insects.

  • Add nutrient and mineral rich plants into your orchard plans and composts. A great resource here details the mineral content of many common plants and details what nutrients they add to the soil.

Valuable Soil-Building Plants:

Toby Hemenway recommends some of the following soil-building plants for orchards in Gaia’s Garden: A Guide to Home-Scale Permaculture. Consider what plants work best for your unique growing conditions and needs!

Nitrogen fixers: alder, autumn olive, bayberry, black locust, broom, butterfly pea, cattail, chamomile, chives, collards, common milkweed, false indigo, goumi, licorice, sea buckthorn, wild lilac, wisteria, wild lupine, sweet pea, bladder senna

Annual nitrogen-fixing cover crops: Austrian winter pea, bell bean, crimson clover, Fava bean, Fenugreek, Garbanzo bean, vetch, black eyed peas, cowpeas, lablab, pinto beans, soybeans, Sunn Hemp

Nutrient accumulators: alfalfa, lamb’s quarters, primrose, purslane, stinging nettle, yarrow, sunflower, dogwoods, horsetail

Soil builders: rapeseed, Sudan grass, and crotalaria

Incorporating these suggestions can pave the way to a diverse and thriving orchard!

Resources:

Gaia’s Garden by Toby Hemenway

Landscaping with Fruit by Lee Reich

http://www.eatcology.com/habitat-for-humus

http://www.tropicalpermaculture.com/nitrogen-fixing-bacteria.html

http://www.fedcoseeds.com/trees/companion_plants.htm

http://www.perennialsolutions.org/all-nitrogen-fixers-are-not-created-equal

This edition of POP TIPS was researched and written by 2015 POP Intern, Alyssa Schimmel.

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