Your State University
Laboratory isn't the only place to have a soil sample
tested. Farmers, fertilizer dealers, and consultants
may choose to send samples to a private soil-test lab
or another state lab for analysis. However, they
shouldn't make the mistake of trying to use your
state lab's fertilizer recommendations with the
other labıs results.
In testing a sample, labs use a substance called an
extractant to help identify available plant nutrients
in the soil. After the extractant removes plant
nutrients from a soil sample, laboratory equipment
can determine how much of each nutrient is present.
A lab can then recommend a course of action for
improving the soil with fertilizers.
There are several different extractants
used in the United States. Each was developed for specific soil conditions and they all extract
different amounts of each nutrient. In our region,
the most commonly used extractants are Mehlich-1
and Mehlich-3. In addition, some labs may also
use a Bray P-1 extractant for phosphorus. Results from
any extractant can be used to make good fertilizer
recommendations, but the information from a test
using one extractant canıt be applied to recommendations
which were calibrated for another extractant.
For example, the Clemson Lab soil-test recommendations
for phosphorus, potassium, calcium, magnesium,
zinc, and manganese are calibrated for
extractions done with Mehlich 1. A sample sent to
the state lab in North Carolina will be extracted
with Mehlich 3 and the results will be reported as an index instead of pounds per acre or parts per million as is commonly done by other labs.
Since two different extractant
solutions are used, fertilizer recommendations from
Clemson's lab would not apply to the results received
from the North Carolina lab. Comparing the test
results from these labs would be like comparing
apples to oranges.
Even if labs use the same extractant, they may use
different scales to rate the pounds per acre of
extractable nutrients. Results are usually shown as
high, medium, or low, but the meaning of those
labels can differ from one lab to another. The
amount of a nutrient one lab considers
high might be labeled medium by another lab. The
medium rating would call for adding more of that
particular nutrient to the soil than a high soil-test
rating would recommend.
It is possible, however, to apply your state lab's fertilizer
recommendations to another lab's analytical report
as long as both labs use the same soil extractant.
Consult the information in your state's nutrient management guidelines
to
convert the pounds per acre extractable value for
each nutrient into a rating of high, medium, or low.
Once this interpretation has been made, your state's
recommendations can be used with another lab's
soil-test results.
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