Roadside Revegetation

IMPLEMENTATION GUIDES
can be used. Five to ten grams (about 0.2–0.4 ounce) of manufactured inoculant can inoculate
about 500 seedlings, usually exceeding the recommended 100,000 bacteria per seedling. Once
seedlings begin to nodulate, nodules from their roots can serve as the basis for making crude
inoculant as described below. This way, inoculant need only be purchased once for each plant
species grown, and thereafter, crude inoculant can be made from nodules.
Preparing Crude Inoculant – Crude inoculant is made using nodules, the small root structures
that house the bacteria. Each one of the nodules can house millions of bacteria. For Rhizobium,
a brown, pink, or red color inside is usually a good indicator that the millions of bacteria in
the nodule are actively fixing nitrogen. For Frankia, desirable nodules will be white or yellow
inside. Grey or green nodules should be avoided, as they are likely inactive.
To make your own crude inoculant, select healthy, vigorous plants of the same species as the
plants to be inoculated. Expose some of the root system of a nodulating plant in the nursery
or field (Figure 10-41B). If available, choose seedlings that were inoculated with select bacteria.
Young roots often contain the most active nodules. Search for nodules with the proper color
and pick them off cleanly. If possible, collect nodules from several plants. Put nodules in a
plastic bag or container and place them in a cooler for protection from direct sunlight and
heat. As soon as possible after collection (within a few hours), put the nodules in a blender
with clean, chlorine-free water. About 50-100 nodules blended in a liter of water is enough to
inoculate about 500 seedlings. This solution is a homemade liquid inoculant, ready to apply
in the same method as cultured inoculant as described below.
Applying Inoculant – Inoculant must be applied in a timely fashion, when seedlings are just
emerging, usually within 2 weeks of sowing. This helps ensure successful nodulation and
maximizes the benefits of using inoculants. Therefore inoculant must be introduced in the
nursery for nursery-grown plant materials, or introduced at the time of sowing for seeds that
are sown directly at the field site. One liter of liquefied inoculant made from either nodules
or cultured inoculant as per the instructions above is diluted in more chlorine-free water.
For 500 seedlings, about 5 liters of water is used. This solution is then watered into the root
system of each seedling using a watering can. In the field, for direct seeding applications, the
slurry of commercial or crude inoculant can be added to the hydroseeder tank and mixed in
with seed mixes.
Management Considerations for Nitrogen-Fixing Inoculations
Verifying the Nitrogen-Fixing Partnership – Allow two to six weeks for noticeable signs
that the plant has formed a symbiotic partnership with nitrogen-fixing bacteria. Signs include:
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Seedlings begin to grow well and are deep green despite the absence of added
nitrogen fertilizer
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The root systems give off a faint but distinctive ammonia-like scent
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Nodules are usually visible on the root system after about four to six weeks (Figure 10-42),
and nodules are pink, red, or brown (for Rhizobium), or yellow or white (for Frankia)
Post-Planting Care – Several factors are of primary concern when using inoculants for
nitrogen- fixing bacteria:
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Fertilization. The use of nitrogen-fixing bacterial inoculant requires some adjustments
in fertilization. Excessive nitrogen fertilizer will inhibit formation of the partnership.
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Water quality. Excessive chlorine in water is detrimental to Rhizobium and Frankia. The
water source may need to be tested and a chlorine filter used if excessive chlorine is
a problem.
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Micronutrients and soil quality. Some nutrients are necessary to facilitate nodulation,
including calcium, potassium, molybdenum, and iron. Excessively compacted soils,
extremes of pH or temperature also inhibit nodulation.
Photo: Tara Luna
Figure 10-42 | Nitrogen-fixing
bacteria will multiply as
inoculated plants grow
After successful inoculation, nitrogen-fixing
bacteria will multiply on the
root system as plants grow. The arrow
points to a visible Frankia nodule on an
alnus seedling.
Roadside Revegetation: An Integrated Approach to Establishing Native Plants and Pollinator Habitat
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