Roadside Revegetation

IMPLEMENTATION GUIDES
10.1.1 FERTILIZERS
Introduction
Fertilizers are used to bring soil nutrients up to levels essential for establishing and maintaining
a desired plant community. When applied within a soil fertility strategy, fertilizer can be a
great tool for revegetation. In recent years, however, the use of fertilizers on roadsides has
come under greater public scrutiny and more restrictive water quality laws. Many roads are
adjacent to streams, lakes, or residential areas which can be affected by runoff or leaching of
inappropriately applied fertilizers. It is important for the revegetation specialist to learn how
to develop fertilizer prescriptions that integrate short- and long-term site fertility goals with
water quality objectives.
Use of commercial fertilizer is only one of many options to increase nutrient levels. A soil
fertility strategy should also consider the application of topsoil, mulch, compost, wood
waste, biosolids, and/or the planting of nitrogen-fixing species. This section will guide the
revegetation specialist through the steps necessary to develop a site
specific fertilizer prescription. The process for developing a fertilizer
3,000
prescription follows these steps:
◾◾
Determine nutrient thresholds and deficits
◾◾
Delineate areas to be fertilized
◾◾
Select fertilizer analysis
◾◾
Select fertilizer release rates
◾◾
Determine application rates
◾◾
Determine timing and frequency
◾◾
Select application method
The fertilizer prescription is the basic instruction for ordering and
applying fertilizers.
Develop Nutrient Thresholds and Determine Deficits
All sites have a minimum, or threshold, level of nutrients that must be met for each plant
community to become functioning and self-sustaining (see Section 5.5, Nutrient Cycling).
Threshold values can be determined by comparing soil tests from several disturbed and
undisturbed reference sites (see Chapter 4). Disturbed reference sites should range from poor
success to good. Based on nutrient values from good and poor revegetation sites, a target
can be estimated between these values. Figure 10-1 gives an example of how a nitrogen
threshold value was obtained by evaluating the total soil nitrogen levels from two disturbed
reference sites, one considered “fair” revegetation and one considered “poor.” The threshold
was set between these two nitrogen levels. Threshold levels represent the minimum level
of nutrients needed for a site. However, higher nutrient levels are more desirable. In fact,
the target nitrogen levels in this example for establishing and maintaining the original plant
community would be closer to the undisturbed reference site.
To determine whether any nutrient is deficient, post-construction soils must be collected and
tested. The values obtained from these tests are compared against target values to determine
if a deficiency exists. By comparing post-construction nutrient values against threshold
values, the nutrient deficit can be estimated for each nutrient. Figure 10-2 shows an example
of how nitrogen deficits are calculated based on post-construction soil tests and established
threshold levels. In this example, total soil nitrogen is determined from soil tests. Since soil
testing facilities report nutrients in a variety of rates, it is important to convert the rates to
percentages. This is done by dividing values that come as gr/l, ppm, mg/kg, and ug/g by
Total N (lbs/ac)
2,500
2,000
1,500
1,000
500
0
D
Undisturbed
RS - Topsoil
B
Disturbed RS
(“fair”
revegetation)
A
Disturbed RS
(“poor”
revegetation)
Soil Condition
C
Post
Construction
Soils
Target Nitrogen
Levels
Threshold
Deficit
Figure 10-1 | Threshold
values of nitrogen
Threshold values are determined from
reference sites. In this example, the
threshold was established at 1,100 lb/
ac, which was between the total N of
a disturbed reference site with “poor”
revegetation (A) and one with “fair”
revegetation (B). Total N in post- construction
soils was 650 lb/ac (C), making
these soils deficient by 450 lb/ac. The
undisturbed topsoil of reference sites
showed a total N of 2,430 lb/ac (D),
which sets the target levels of nitrogen
between 1,100 and 2,430 lb/ac.
Roadside Revegetation: An Integrated Approach to Establishing Native Plants and Pollinator Habitat
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