Roadside Revegetation

IMPLEMENTATION GUIDES
first growing season. Ammonium nitrate, ammonium sulfate, potassium nitrate, and urea are
several examples of fast-release fertilizers.
“Water soluble” or “available” nutrients do not always remain available or soluble after they
are applied to the soil. Available forms of phosphorus, for instance, react in the soil to form less
soluble compounds; potassium gets bound up in soils with moderate to high proportions of
clay; and many of the micronutrients (e.g., zinc, copper, manganese) become unavailable when
applied to soils with low pH (see Section 5.5.5, pH and Salts). Unless soils
are sandy or rocky, it can be assumed that many of the nutrients stated
as “available,” except for nitrogen and sulfur, will become somewhat
immobile once they are applied. Over time, however, these nutrients
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will become available for plant uptake.
90
The advantages of fast-release fertilizers are they are relatively inexpensive,
easy to handle, immediately available to the plant, and can
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be applied through a range of fertilizer spreading equipment. Disadvantages
are that some nutrients, such as nitrogen, will leach through
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the soil profile if they are not first taken up by plants or captured by soil
microorganisms in the break down of carbon. Nitrates from fast-release
60
fertilizers have been found to leach through sandy soils to depths that
50
are 4 times the rate of rainfall (Dancer 1975). For example, for sites with
annual rainfalls of 12 inches, nitrate could move to a depth of four feet
40
if it was not taken up by plants or soil organisms. At this depth, nitrogen
would be out of range of most establishing root systems.
Since fast-release fertilizers are salts, they have a potential to burn foliage
and roots, especially when fertilizers are applied at high concentrations
or when applied during dry weather (see Section 5.5.5.2, Mitigating for
Low pH Soils). High concentrations of fast-release fertilizers can also
Trifolium
repens
affect germination rates (Figure 10-4) because of the high soluble salt levels (Brooks and Blaser
1964; Carr and Ballard 1979). Salt damage can be reduced by mixing fast-release fertilizers at
lower concentrations or by applying them during rainy weather.
Slow-Release Fertilizers – These fertilizers are designed to release nutrients at a much slower
rate. To be labeled slow-release fertilizer, some states require a specific amount of nitrogen
to be in a slow-release form. Forms of nitrogen shown on the label as “slowly-available” or
“water- insoluble” are good indicators that a fertilizer is in a slow-release form. The advantages of
using slow-release fertilizers are (1) nutrients are supplied at a time when plants are potentially
growing; (2) less frequent applications; (3) less potential for leaching into ground water; and
(4) less potential to cause salt injury. The disadvantages are that many slow-release fertilizers
are bulky, cost more to purchase and apply, and are limited by the type of fertilizer application
equipment that can be used. On the whole, however, slow-release fertilizers have greater
applicability for revegetating disturbed sites than fast-release fertilizers.
Slow-release fertilizers come in either organic or inorganic forms. Organic fertilizers include
animal manures (including chicken, steer, cow), bone meal, fish emulsion, composted sewage
sludge, and yard waste. Unprocessed organic fertilizers are hard to apply to roadside projects
because they are bulky and high in moisture. Commercially available organic fertilizers, such
as Fertil-Fiber and Biosol®, have been processed to remove most moisture, which makes
them easier to apply through most fertilizer spreading equipment.
The agents responsible for release of nutrients from organic fertilizers are decomposing soil
bacteria. When soil bacteria are active, the release of nutrients is high; when dormant, the rate is
low. The release of nutrients is therefore a function of moisture and temperature, which governs
the rate of bacterial growth. Warm temperatures and high moisture, conditions conducive
to plant growth, are also favorable for the break down of organic fertilizers. Because of this,
the release of nutrients from the decomposition of organic fertilizers often coincides with
the period when plants are growing (spring and fall) and the need for nutrients is greatest.
% Germination
30
Without Fertilizer
Festuca
arundinacea
With Fertilizer
Festuca
rubra
Lolium
perenne
Figure 10-4 | Reduced
seed germination after
exposure to fertilizer
Germination of seeds for some species
can be reduced following exposure to
a 10-30-10 fertilizer solution at a rate
of 750 lbs fertilizer per 1,000 gallon hydroseeder
(after Carr and Ballard 1979).
Roadside Revegetation: An Integrated Approach to Establishing Native Plants and Pollinator Habitat
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