January
2005, SERAIEG6*5
Plant
available phosphorus (P) and potassium (K) in soil are estimated by a soil test
method that involves extracting a portion of total P and K from soil. The extracting solution contains a mixture of
various chemicals that react with soil and release some soil P and K into
solution. Because of the variable nature
of soils, many different extracting solutions have evolved. Soil test extractants
used in
State 
extracting
reagent for P 
extracting
reagent for K 

Mehlich1 
Mehlich1 

Mehlich3 
Mehlich3 

Mehlich1 ABDTPA
(soils with pH>7.4) Acetic
acid (organic soils) 
Mehlich1 

Mehlich1 
Mehlich1 

Mehlich3 
Mehlich3 

Bray
P2 
Ammonium
acetate 




Mehlich3 
Mehlich3 

Mehlich3 
Mehlich3 

Bray
P1 Olsen
(calcareous soils) Bray
P2 
Ammonium
acetate 

Mehlich1 
Mehlich1 

Mehlich1 
Mehlich1 

Morgan
(modified) 
Morgan
(modified) 

Mehlich1 
Mehlich1 
Phosphate
and potash fertilizer recommendations from a soil test were developed from
field studies in which crop yield response to P and K fertilizer additions were
calibrated against the amount of P and K extracted with a soil test. Since each extractant
will measure a different amount of P and K, each calibration is different
depending on the extractant used. Therefore, if an individual or consultant is
providing their own recommendation from a soil test, it is extremely important
to note the extract used to obtain the soil test value since the
recommendations should be for that particular extractant.
The
availability of multiple soil test extractants from
various laboratories sometimes makes it necessary to convert a soil test value
to its equivalent value for another extractant that
is well calibrated and accepted for fertilizer recommendations in the
area. Converting one soil test value to
another may be done if many comparisons are made between soil test extractants and if conversion equations are developed from
the comparisons. The purpose of this
publication is to provide conversion equations based upon published and
continuing research. Converting one soil
test value to another should be done with extreme caution. There are some cautionary words that should
be heeded when converting soil test values.
These cautions are also outlined.
Currently, this fact sheet has conversion equations for converting
between Mehlich3 and Mehlich1 for phosphorus and potassium.
M1P
= Mehlich1 Phosphorus, M1K = Mehlich1 Potassium, M3P = Mehlich3 Phosphorus,
M3K = Mehlich3 Potassium, NAPT = North American Proficiency Testing
Cautionary
words on using conversion equations
When
converting from one soil test value to another, there are two errors associated
with the converted number. The first
error arises from obtaining the original result. The second error arises from doing the
conversion since the equations are not perfect.
The r^{2} value is a measure of these errors. If there is no error at all, the r^{2}
is 1.0. If the error or uncertainty in
the original result is 8% and there is no conversion error, r^{2} will
be around 0.97. An error of 8% means
that for a soil test P value of 30 mg/kg, the actual value may be as low as
27.6 mg/kg or as high as 32.4 mg/kg.
With r^{2} of 0.95 and 0.90, total relative errors are 10% and
15%, respectively. With r^{2} of
0.70, total relative error rises to 30%.
The higher the r^{2 }value, the better the conversion because
errors in the converted number are lower.
Using conversion equations with r^{2} below 0.80 is discouraged
because total relative error in the converted number is greater than 22%.
The
conversion equations should be used only when the soil test result to be
converted is within the range of data used to create the equation. There is some level of certainty, defined by
the r^{2} value, that the conversion equations have predictive
abilities within the data range used to create the equation. There is no level of certainty about the
equations predictive abilities outside of the data range.
Just
as a soil test value to be converted needs to lie within the range of data used
to create the conversion equation, the soil tested needs to be similar to the soils
used to create the conversion equations.
Conversion equations are presented for soils from various parts of the
When
making conversions, it is important to know the unit of the soil test value
being converted. There is no standard
unit for presenting soil test values.
The most common units used are ppm and
lbs/acre. The ppm
stands for parts per million and is equivalent to milligrams of nutrient per kg
soil (mg/kg). A unit used in scientific
literature and advocated by Mehlich (1972) is
milligrams of nutrient per cubic decimeter (mg/dm^{3}). All units have the weight of nutrient measured
in the numerator. This weight can be in
the metric unit of milligrams (mg) or English unit of pounds (lbs). Difference in the units mentioned arises from
what is in the denominators. The
denominator contains either a measure of soil metric
weight (kilogram, kg), land area (acre), or volume of processed soil (cubic
decimeter, dm^{3}). There are no
assumptions made when presenting the weight of nutrient measured in the
numerator (mg or lbs) or when presenting the volume of tested soil in the
denominator (dm^{3}).
Assumptions are necessary when presenting the weight of processed soil
(kg) or area of soil (acre) in the denominator if soil was measured with a
scoop. Most laboratories measure the
amount of soil tested on a volume basis using a scoop after drying and grinding
the soil. The volume of soil used can
range from 1.7 to 5 cm^{3} and depends on the test performed. To present test results on a weight basis of
soil an assumption is made on the density of the processed soil which usually ranges
from 1 to 1.2 g/cm^{3}. To
present test results on an acre basis of soil, an additional assumption is
required on the quantity of soil in an acrefurrow slice which is commonly
assumed to be 2 million pounds. Another
method that is used by some laboratories is to present a soil test result as an
index related to nutrient recommendation.
This value has no units.
The
conversion equations in this fact sheet are in units of mg/kg. When a soil test value is in a unit other
than mg/kg, you first have to convert the value to mg/kg. The laboratory providing the test result
should be able to provide information on how to convert the value to
mg/kg. In general, lbs/acre divided by
two equals mg/kg if the laboratory assumed 2 million pounds of soil in an acrefurrow
slice. After converting the value to the
desired soil test extractant, you may then still need
to convert the result to another unit used in a fertilizer calibration
table. The laboratory that developed the
calibration table should be contacted on how to make this conversion.
The
conversion equations are intended for occasional soil test results that need
converting to make proper recommendations using recommendation data derived
from another soil test extractant. They can also be used for wholefarm
conversion of soil test records if there is a need or desire to switch to
another soil test method. The conversion
equations presented in this paper are not intended for routine analysis of
soils with one extractant with the values converted
so fertilizer recommendations could be made based on another extractant. The
equations should not be used in this manner because there is an error
associated with the conversion.
Even
when using the same soil test extractant, there are
differences that can occur in test results from various labs due to minor
differences in protocols on performing the test. These protocol differences include method and
speed of shaking the sample, weighing versus scooping a sample, soil:solution ratio, and
analytical method of determining the nutrient in the extract. Because these minor differences exist from
lab to lab, the best approach is to have soil tested at the same laboratory
that provides the lime and fertilizer recommendation from the test result. Good soil test laboratories have evaluated
and calibrated lime and fertilizer recommendations against soil testing
protocols they perform. An appendix is
available in this report with the methodology for the soil test extractions used
to derive conversion equations in the Southern Region. A section on miscellaneous conversion
equations from the literature from a wider region of soils is presented in this
report without detailed methodology. The
methodology for this data can be found in the literature cited for these equations.
Care
must be taken in obtaining a converted soil test value to be used in nutrient
management costshare programs or for regulatory purposes. The governing authority that makes decisions
based on soil test values may or may not accept a converted value. The governing authority should be contacted
to determine if converted soil test values are acceptable.
MEHLICH1 AND
MEHLICH3
Mehlich1,
Mehlich2, and Mehlich3 soil test extractants were
developed by Adolf Mehlich
in
Soils
across
The
There was considerable error
and uncertainty in the comparison between M3P and M1P when considering all 519
soils as indicated by a low r^{2} value of 0.64 (Table 2). When just the lower values where considered
with M3P less than 445 mg/kg and M1P less than 240 mg/kg, there was some
improvement in the comparison with an r^{2} value of 0.72. The conversion equation at the lower MehlichP levels indicates the M3P values are approximately
1.5 times greater than M1P values.
The comparison between M3K
and M1K was much better with an r^{2} of 0.97 (Table 2). The conversion equation between M3K and M1K
indicates that these values are nearly the same.
Equation 
M3 range* 
M1 range* 
N** 
r^{2} 

M3P=0.725(M1P)+52.7 
0 to 900 
0 to1300 
519 
0.64 

M3P=1.43(M1P)+18.6 (for lower range of data) 
0 to 445 
0 to 240 
490 
0.72 

M3K=0.957(M1K)+2.54 
0 to 530 
0 to 588 
519 
0.97 

*^{ }Values are in units of mg/kg.^{}
** N is the number of
samples
Phosphorus
and potassium fertilizer recommendations from the
Soils
were collected from five of the six physiographic regions in
To
obtain M3P from M1P test results, the soil pH has to be known. Mehlich1 P is then multiplied by the
appropriate conversion factor according to soil pH. As found in Table 1, the conversion factors
are 2.13 with soil pH < 6.0 and 1.44 with soil pH >= 6.0.
Soil pH 
Equation 
M3P range* 
M1P range* 
N** 
r^{2} 
<6.0 
M3P=2.13(M1P) 
3 to 326 
2 to 136 
143 
0.92 
>= 6.0 
M3P=1.44(M1P) 
4 to 320 
1 to 200 
366 
0.86 
*^{ }Values are in units of mg/kg.
** N is the number of
samples
Equation 
M3K range* 
M1K range* 
N** 
r^{2} 
M3K=1.27(M1K)+12.0 
34 to 598 
24 to 479 
578 
0.89 
*^{ }Values are in units of mg/kg.^{}
** N is the number of
samples
A survey of
The conversion equations for
P and K tested by Mehlich3 and Mehlich1 are presented in Table 5. The r^{2} of the relationship was
0.90 between M3P and M1P and 0.97 between M3K and M1K. Mehlich3 P and M3K were both greater than
the Mehlich1 values as indicated by slope values of 1.45 and 1.23 for P and K,
respectively.
Equation 
M3 range* 
M1 range* 
N** 
r^{2} 
M3P=1.45(M1P)+10.8 
0 to 350 
0 to 220 
89 
0.90 
M3K=1.23(M1K)+7.34 
0 to 330 
4 to 263 
100 
0.97 
*^{ }Values are in units of mg/kg.
** N is the number of
samples
The
Soil Science Society of America sponsors a program for soil test laboratories
to check on the quality of the data they produce. This program is entitled the North American
Proficiency Testing (NAPT) program (Soil and Plant Analysis Council,
2000a). Five soil samples are sent to
participating laboratories every 3 months.
Soil samples are tested at each laboratory using their normal extractants. The
test results are compared with the results obtained from other laboratories
performing the same test. The program
allows laboratories to check their level of accuracy on various analytical
tests they perform. The program had 167
laboratories enrolled in the fourth quarter of 2002.
Equations
for the conversion of Mehlich1 P and K to Mehlich3 P and K were developed
from 116 soil samples from the NAPT program and are shown in Table 6. Samples were collected from across
Soil
pH was not a factor affecting the relationship between M3P and M1P or M3K and
M1K. Therefore, one equation for M3P
versus M1P and one equation for M3K versus M1K for all 116 samples of the NAPT
program are shown in Table 6. Mehlich3
P was about 20 mg/kg greater than Mehlich1 P values. There was a very poor relationship between
M3P and M1P as indicated with a low r^{2} of 0.46.
Mehlich3 K was about double the M1K values.
Equation 
M3 range* 
M1 range* 
N** 
r^{2} 
M3P=0.976(M1P) + 18.9 
3 to 611 
2 to 434 
116 
0.46 
M3K=1.95(M1K) 
21 to 1710 
3 to 652 
116 
0.88 
*^{ }Values are in units of mg/kg.^{}
** N is the number of
samples
^{†
}unpublished data, J.
KotubyAmacher, Utah State University and R. Miller,
Colorado State University.
Relationships
between M3P and M1P and M3K and M1K from various sources are shown in Table
7. Mehlich2 P (M2P) is shown for a study
conducted in
State

Soil 
Equation* 
N**

r^{2}








M2P=(1.61)
M1P – 8.2 
60 
0.81 
(Shuman
et al., 1988) 
Pacolet
scl 
M2P=(1.39)
M1P + 2.4 
60 
0.87 


M2P=(1.43)
M1P + 0.3 
60 
0.94 





19
states in the southeast, north central, and northeastern regions 
noncalcareous 
M3P=(1.85)
M1P – 4.3 
91 
0.75 
(Wolf
and Baker, 1985) 










Coastal
plain (all) 
M3P=(1.61)
M1P + 21.8 
400 
0.91 
(Sims,
1989) 

M3K=(1.12)
M1K + 5.9 
400 
0.90 






Coastal
plain 
M3P=(2.04)
M1P + 0.80 

0.80 

(less
than high) 
M3K=(1.21)
M1K + 0.7 

0.77 







M3P=(2.04)
M1P + 14.8 
300 
0.91 
(Gartley et al., 2002) 

M3K=(1.37)
M1K – 7.7 
300 
0.97 
* Values in units of mg/kg except for data
from
** N is the number of
samples
CONCLUSIONS
Conversion
equations for soil test methods should be used with considerable caution. Errors are introduced with the use of
conversion equations because of the variable nature of soils and how they react
with different soil test extractants. Conversion equations are not recommended for
routine use. The best approach is to
obtain fertilizer and lime recommendations from the laboratory conducting the
test. Occasionally, recommendations are desired
from a soil test that was not used to calibrate the recommendations. When conversion equations are used on an
occasional basis, the soil that was tested should be similar to the soils used
to derive the conversion equations.
Several equations were presented for conversion between Mehlich1 and
Mehlich3 with emphasis on soils in the Southern Region.
The
chemical composition and original method for using Mehlich1 and Mehlich3 are
presented in Table 8. General methodology
for soil tests using Mehlich1 and Mehlich3 are described in “Soil Analysis
Handbook of Reference Methods” (Soil and Plant Analysis Council, 2000b), Mehlich (1953), and Mehlich
(1984). For Mehlich3, 2.5 cm^{3}
of soil is mixed with 25 ml of the extract (Mehlich,
1984). For Mehlich1, 4 cm^{3}
or 5 g of soil is mixed with 20 ml of Mehlich1 extract (Mehlich,
1953). Soil and extractant
are shaken for 5 minutes and filtered.
Specific details on methodology used in developing conversion equations
from the Southern Region and the NAPT program are presented below.
Soil Test 
pH 
Total acidity (eq/L) 
Chemical composition 
Soil:extractant ratio 
Shaking time 
Mehlich1 
1.25 
0.075 
0.05
N HCl, 0.025 N H_{2}SO_{4} 
4
cm^{3} (or 5 g) : 20 ml 
5
minutes 
Mehlich3 
2.50 
0.22 
0.2 N acetic acid, 0.015 N NH_{4}F, 0.25
N NH_{4}NO_{3}, 0.013 N HNO_{3}, and 0.001 M EDTA 
2.5 cm^{3} : 25 ml 
5 minutes 
The
study conducted to obtain the conversion equations is reported in Mylavarapu et al. (2002).
A total of 519 acid mineral soils routinely submitted to the
For
the Mehlich1 extraction, 5 g soil was shaken with 20 ml of extractant. For the Mehlich3 extraction, 3.1 g soil was
shaken with 25 ml of extractant. Soil was shaken with the extractant
for 5 minutes. The soil slurry mixture
was filtered and P and K in the filtrate were determined via inductively
coupled plasma spectrophotometry (ICP). Results are presented in units of mg/kg. No assumptions are made in reporting the
value as mg/kg since soil was weighed for the extraction.
Regression
analysis was performed on the data to establish the linear dependence of the
two Mehlich procedures.
Soils
were selected from 5 physiographic regions in
Five
cm^{3} of soil was shaken with 20 ml of Mehlich1. Two cm^{3} of soil was shaken with 20
ml of Mehlich3. Shaking occurred on an
endtoend shaker at 240 cycles per minute for 5 minutes. The soil slurry was filtered through Whatman no. 2 filter paper for 10 minutes. The filtrate was analyzed for P and K via
inductively coupled plasma spectrophotometry
(ICP). Results are presented in units of
mg/kg assuming a processed soil density of 1 g/cm^{3}.
Comparison
of data sets was conducted with regression analysis using linear models. If the 95% confidence interval for the y
intercept overlapped 0, the regression analysis was conducted again forcing the
y intercept to be 0. Soils were grouped
according to counties of origin or soil pH level. Two soil pH groups were defined as <6 or
>=6. If the 95% confidence intervals
for the slopes of the linear models overlapped one another, the slopes were not
considered to be different.
Soils
were collected from 50 sites in
For
the Mehlich1 extraction, 10 g soil was shaken with 40 ml of extractant. For the
Mehlich3 extraction, 4.5 g soil was shaken with 45 ml of extractant. Soil was shaken with the extractant
for 5 minutes. The soil slurry mixture
was filtered and P and K in the filtrate were determined via inductively
coupled plasma spectrophotometry (ICP). Results are presented in units of mg/kg. No assumptions are made in reporting the
value as mg/kg since soil was weighed for the extraction.
Regression
analysis was performed on the data to establish the linear relationship between
the two Mehlich procedures. Outliers were excluded from the P analysis.
Samples
from the third quarter of 1998 through the first quarter of 2004 in the North
American Proficiency Testing (NAPT) program was used to assess relationships
amongst Mehlich1 and Mehlich3 extractable P and K. Median values were used in the analysis. In 2001, NAPT began reporting two median
values for M3P. One was for
determination via ICP and the other was via colorimetry. An average M3P value was determined for each
sample according to
M3P
= [ (N_{ICP }x M3P_{ICP}) + (N_{color} x M3P_{color}) ] / (N_{ICP}
+ N_{color})
where N is the number of samples reported for the ICP or colorimetic
determination of M3P. M3P_{ICP}
and M3P_{color} are the median values reported for ICP and colorimetric
analyses of M3P, respectively. One
sample was omitted from the group because of extremely high levels of extractable
P and K. The sample ID is 99104 and had
median M3P and M3K values of 1385 and 2737 mg/kg, respectively.
There
are no strict stipulations on method protocol for analysis of NAPT
samples. Samples should be analyzed
according to accepted standard methodologies as listed in one of the five soil
workgroup regional publications. In
general, the Mehlich1 extraction involves 5 g of soil and 20 ml of Mehlich1
and the Mehlich3 extraction involves 2.5 cm^{3} soil and 25 ml of
Mehlich3 (Table 8). Values are reported
in units of mg/kg in the NAPT program.
Variations can exist across different labs because of differences in
scoop volumes, scooping techniques, and calculations to arrive at soil test
concentrations on a soil mass basis.
Comparison
of data sets was conducted with regression analysis using linear models. If the 95% confidence interval for the y
intercept overlapped 0, the regression analysis was conducted again forcing the
y intercept to be 0. Soil pH was
considered as an independent variable in a multiple regression of M3P versus
M1P and soil pH. Likewise, soil pH was
considered in a multiple regression of M3K versus M1K and soil pH. Soil pH was not a statistically significant
variable in the regression models.
Benbi, D.K., R.J. Gilkes, and M.D.A. Bolland. 1988.
An assessment of soil tests for phosphate for the prediction of cereal
yields on a sandy soil in
Franklin, R. and L.D. Simmons.
2005. Correlations of Mehlich 1 and 3 extractable plant nutrients in
Gartley, K.L.,
J.T. Sims, C.T. Olsen, and P. Chu. 2002.
Comparison of soil test extractants used in
midAtlantic
Karathanasis, A.D. 1991. Phosphate mineralogy and equilibria
in two
Hanlon,
E.A. 2001. Procedures used by state soil testing
laboratories in the southern region of the
McFarlan, A.C. 1950. Geology of
Mehlich, A. 1953. Determination of P, K, Ca,
Mg, and NH_{4}. Soil Test
Div. Mimeo, N.C. Dept. of Agri., Raleigh.
www.ncagr.com/agronomi/pdffiles/mehlich53.pdf, accessed
Mehlich, A. 1972.
Uniformity of expressing soil test results. A case for calculating results on a volume
basis. Commun. Soil Sci. Plant Anal. 3(5): 417424.
Mehlich, A. 1974.
Uniformity of soil test results as influenced by extractants
and soil properties. Proc.
7^{th} Int. Col.,
Mehlich, A. 1978.
Influence of fluoride, sulfate, and acidity on extractable phosphorus,
calcium, magnesium and potassium. Commun. in Soil Sci. Plant Anal. 9(6):
455476.
Mehlich, A. 1978.
New extractant for soil test evaluation of
phosphorus, potassium, magnesium, calcium, sodium, manganese, and zinc. Commun. in Soil Sci.
Plant Anal. 9(6):477492.
Mehlich, A. 1984. Mehlich 3 soil test extractant: A
modification of Mehlich2 extractant. Commun. Soil Sci. Plant Anal. 15(12):
14091416.
Mylavarapu, R.S.,
J.F. Sanchez, J.H. Nguyen, and J.M. Bartos. 2002.
Evaluation of Mehlich1 and Mehlich3 extraction procedures for plant
nutrients in acid mineral soils of
Shuman, L.M., P.L. Raymer, J.L. Day, and M.J.
Cordonnnier.
1988. Comparison of four
phosphorus extraction methods on three acid southeastern soils. Commun. in Soil Sci.
Plant Anal. 19(5): 579595.
Sims,
J.T. 1989. Comparison
of Mehlich1 and Mehlich3 extractants for P, K, Ca,
Mg, Mn, Cu, and Zn in Atlantic Coastal Plain Soils. Commun. in Soil Sci.
Plant Anal. 20 (17&18), 17071726.
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methods. CRC Press,
Tucker,
M.R. 1992. Determination of Phosphorus by Mehlich 3 Extraction.
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Commun. in Soil Sci.
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F.J. Sikora^{1}, R.S. Mylavarapu^{2}, D.H. Hardy^{3},
M.R. Tucker^{3}, and R.E. Franklin^{4}.
^{1} University of
^{2} University of
^{3}
^{4} Clemson University