Roadside Revegetation

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PLANNING PHASE TWO: ASSESS SITE
A common assumption is that all labs are of similar quality in their
analytical testing, and that if a group of labs were sent the same soil
sample they would report similar results for most tests. This is not
typically the case, as several university reviews of laboratories have
shown (Neufeld and Davison 2000; Rose 2004). In one comparison,
eight reputable laboratories reported widely differing results for all
soil nutrients when sent identical soil samples (Rose 2004). One reason
for the variation in results is that usually several testing procedures
can be used to quantify a soil parameter. Some methods have greater
accuracy and precision than others. The soil testing industry at this
time has not settled on an agreed upon set of analytical methods
to use. Even when the same tests are performed, labs often report
different levels of accuracy (Rose 2004).
Soil laboratories can voluntarily participate in the North American
Proficiency Testing (NAPT) program that will assess the quality of their
analytical procedures. In this program, NAPT periodically sends all
participating labs identical soil samples. Each lab analyzes the samples
for mineral nutrients using established analytical procedures, then
sends the results back to NAPT. The results from all labs are compiled
and analyzed statistically and each lab is sent a report on how their
results compared to the other participating labs. NAPT suggest that
the accuracy be less than 10 percent of industry values and precision
no greater than 15 percent of industry values (Neufeld and Davison
2000). These reports are not available to the public, but laboratories
might share them if asked. NAPT is not a certification program but is
often a basis for a soil lab quality control program.
The following is a checklist for selecting a high-quality lab (modified
from Neufeld and Davison 2000):
Does the lab have a quality control program? If they do, ask
them to explain it.
Does it participate in a proficiency testing program (such as
NAPT)?
Will they share the results of proficiency testing program?
Does the lab use established analytical methods (the most
appropriate for soils in the geographic area being tested)?
If a “no” is given for the answers to any of these questions, another
soil testing facility should be considered. If the selection is between
a couple of labs, consider sending duplicate soil samples with known
properties (“checks”) to each lab and compare the results using the
NAPT suggested standards for accuracy and precision. Soil “checks”
can be purchased through a proficiency testing program. Once a lab
is selected, continue to ask for quality control reports. If the budget
allows, periodically send duplicate “check” soil samples with regular
soil samples to assess accuracy and precision.
Methods of the Examination of Compost and Composting (TMECC) explained in Section 10.1.3,
Mulches, and Section 10.1.5, Organic Matter Amendments.
Nitrogen Analysis
Soils laboratories report nitrogen in a variety of units, such as gr/l, ppm, mg/kg, ug/g, and
percent. Unless these values are converted to pounds per acre, it is difficult to determine
rates of fertilizer, compost, or topsoil necessary to restore site nitrogen. Use Table 5-2 (Line
E) to convert lab values to total pounds per acre of nitrogen. These calculations account for
Table 5-2 | Calculating the nitrogen deficit of a site – an example
A Total soil nitrogen (N) 0.025 % From soil test of post construction soils - gr/l, ppm, mg/kg, ug/g divide by 10,000 for %
B Thickness of soil layer 0.5 feet The thickness of soil represented in (A).
C Soil bulk density 1.4 gr/cc Unless known, use 1.5 for compacted subsoils, 1.3 for undisturbed soils, 0.9 for
light soils such as pumice
D Fine soil fraction 70 % 100% minus the rock fragment content - from estimates made from sieved soil
prior to sending to lab
E N in soil layer: A * B * C * D * 270 = 331 lbs/ac Calculated amount of total nitrogen in soil layer. To convert to kg/ha: E * 1.12
F Minimum or threshold N levels 1,100 lbs/ac Determined from reference sites or literature
G N deficit: F - E = 769 lbs/ac Minimum amount of N to apply to bring up to threshold
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