Soil Test Note – No. ST002
Dr. Stanley L. Chapman, Extension Soils Specialist
THE NEW MEHLICH 3 SOIL EXTRACTANT
The University of Arkansas
soil testing lab at Marianna began using a new and improved extractant for
routine soil samples on September 1, 1986.
It is called Mehlich 3 and is the nearest thing to a universal extractant
ever developed for soil testing. This
one extractant takes the place of three different ones formerly used for different
elements at the Marianna laboratory. It
does a good job of extracting phosphorus, potassium, calcium, magnesium, sodium,
iron, manganese, zinc, copper, and sulfur with reproducible results.
It does not do a good job of extracting boron.
The Mehlich 3 extractant
is compatible with our lab’s inductively coupled argon plasma (ICAP) unit
that determines several elements simultaneously.
Also, the actual readings will be automatically transferred from it
to our computer without the data having to be reentered by a data entry person.
This lessens the chance of human error.
The Mehlich 3 soil extractant is made
up as follows:
a.
0.20 M Acetic Acid
d. 0.013
M Nitric Acid
b.
0.25 M Ammonium Nitrate e. 0.002
M EDTA (chelate for
c.
0.015 M Ammonium Fluoride
micronutrients)
A
soil:extracting solution ratio of 1:10 was used in 1987. This resulted in extraction levels that were
5 to 20% higher than those obtained by the “old” Bray 1 and ammonium acetate
extraction methods. Conversion factors
were provided so that past readings by the “old methods” could be compared
to those obtained by the “new methods.”
The
Mehlich 3 procedure was further modified as of January 12, 1988, to lower
the soil:extractant ratio to 1:7. The
result is slightly lower extraction levels of P, K, and other elements. Average levels of P are about 10 lbs/A higher
than measured by the Bray 1 method used by the lab prior to 1987 or about
15-20 lbs/A lower than those measured by the 1:10 Mehlich 3 procedure used
in 1987. Average values of K are about
25 lbs/A higher than those by the ammonium acetate extraction method used
prior to 1987 or about 75-100 lbs/A lower than those obtained by the 1987
procedures. Present values of sodium
and magnesium are about the same as those obtained by the “old” ammonium acetate
extraction procedure.
Calcium
readings are not greatly affected by dilution changes and thus the following
formulas can be used to make conversions from one extraction procedure to
the other:
1.
Ammonium acetate Ca = Mehlich
3 Ca – 82
1.14
2. Mehlich
3 Ca = (Ammonium acetate Ca x 1.14) + 82
Methods
of determining organic mater, pH, EC, and Nitrate-N have not changed. The calculated
CEC and
percent saturation values for total bases, K, Mg, and Na generally average
only slightly higher with the Mehlich 3 method for sands, sandy loams, and
silt loams. However, calculated CEC
values for clays and clay loams are considerably higher than was measured
by the ammonium acetate method. This
should have little or no affect on the lime and fertilizer recommendations
you receive.
Sulfate-sulfur,
iron, manganese, copper, and zinc readings are not used to make recommendations.
Other parameters are used to make recommendations for specific elements
for specific crops. However, values for certain micronutrients
extracted by Mehlich 3 may be useful in confirming that the recommendations
are in order.
Soil
test levels in lbs/A are considered satisfactory as long as iron is above
50, manganese is above 25, copper is above 1.0, zinc is above 1.0, and sulfate-S
is above 10. However, recommendations
may be given for sensitive crops when soil test levels are above these breaking
points.
May,
1995
The Arkansas
cooperative Extension Service offers its programs to all eligible persons
regardless of race, color, national origin, religion, gender, age, disability,
marital or veteran status, or any other legally protected status, and is an
Equal Opportunity Employer.