Open Journal of Soil Science, 2012, 2, 256-262
http://dx.doi.org/10.4236/ojss.2012.23030 Published Online September 2012 (http://www.SciRP.org/journal/ojss)
Differences in Modified Morgan Phosphorus Levels
Determined by Colorimetric and Inductively Coupled
Plasma Methods
Zhongqi He1*, Hailin Zhang2, O. Modesto Olanya3, Jonathan M. Frantz4, Robert P. Larkin5
1USDA-ARS, Southern Regional Research Center, New Orleans, USA; 2Department of Plant and Soil Sciences, Oklahoma State
University, Stillwater, USA; 3USDA-ARS, Eastern Regional Research Center, Wyndmoor, USA; 4USDA-ARS, Application Tech-
nology Research Unit, University of Toledo, Toledo, USA;5USDA-ARS, New England Plant, Soil, and Water Laboratory, Orono,
USA.
Received May 2nd, 2012; revised June 5th, 2012; accepted June 18th, 2012
ABSTRACT
Phosphorus (P) fertilization is frequently needed for profitable crop production. Modified Morgan P (MMP) is a soil
test P used to estimate plant available P in soils. The critical values of MMP for P fertilization and maintenance recom-
mendations are based on the P concentrations measured by a common colorimetric molybdenum blue method although
other P quantification methods have also been used for MMP measurements. In this study, we collected 120 surface soil
samples of Caribou Sandy loam under potato cultivation or its rotation crops from Maine, USA, and 72 soil samples of
Cecil sandy loam with cotton/corn crops under conventional tillage and no-till management with chemical and poultry
litter fertilization in Georgia, USA. The MMP levels in all 192 dry samples were greater when they were measured by
an inductively coupled plasma (ICP)-based method, compared to the corresponding data produced from colorimetry.
Our results show the two sets of data were positively and significantly correlated (r = 0.93, P < 0.001). In average, the
ICP-based MMP level of the 192 samples was 23.3 mg·P·kg–1 with standard deviation of 12.9, compared to the average
of colorimetric MMP level of 14.9 mg·P·kg–1 with standard deviation of 8.8. Based on the observations in this work,
both colorimetric and ICP-based methods can be used for P fertilizer recommendation, but a conversion factor should
be applied for ICP data as the current recommendation systems are based on colorimetric M & R data.
Keywords: Phosphorus Measurement; Soil Test Phosphorus; Modified Morgan Phosphorus; ICP; Molybdenum Blue
Method
1. Introduction
Phosphorus (P) is an essential plant nutrient. P fertiliza-
tion is frequently needed for profitable crop production.
Soil test phosphorus (STP) is an estimate of P in soil that
is part of soil P available for plant uptake during a grow-
ing season. Modified Morgan P (MMP) is one of many
STP methods used in the US. This method has been used
to evaluate STP in Maine [1,2] and other northeastern
states [3,4]. Traditionally, P concentration in a soil ex-
tract is measured by a colorimetric method, which mea-
sures the soluble inorganic P [5] or molybdate (Mo)-rea-
ctive P [6]. With the advancement of technology, induc-
tively coupled plasma (ICP)-based methods, which mea-
sure total P in a soil extract, are also used in STP mea-
surement [7]. As colorimetric P determination methods
are initially used to calibrate STP for P fertilizer recom-
mendations, the difference in STP data determined by
colorimetric methods and ICP-based methods need to be
examined and documented in order to appropriately in-
terpret ICP-based STP data [7]. Indeed, the difference in
Mehlich 3 P (M3P), another STP, measured by Mo-re-
active and ICP methods has been rigorously examined by
numerous research groups [7-10]. Based on about 6500
soil samples, Pittman et al. [7] reported that the M3P va-
lues measured by ICP-atomic emission spectrometer (AES)
were higher than those determined colorimetrically.
These researchers proposed two regression equations. for
P> and < 60 mg·kg−1 respectively. The first was to convert
ICP-based data to colorimetric M3P values to make ferti-
lization recommendations if STP < 60 mg·kg–1, and other
conversion was for STP > 60 mg·kg–1 for other uses.
*Corresponding author.
Mention of trade names or commercial products in this publication is
solely for the purpose of providing specific information and does no
t
imply recommendation or endorsement by the US Department o
f
Agriculture. USDA is an equal opportunity provider and employer.

Differences in Modified Morgan Phosphorus Levels Determined by Colorimetric and
Inductively Coupled Plasma Methods
257
The difference in MMP measured by colorimetry and
ICP methods has not been extensively investigated so far.
In the limited literature noted, the ICP method measured
an average of 1.5 mg·kg–1 more MMP than colorimetry
[4]. Some researchers [11] argued that it is a substantial
difference since critical soil MMP levels are in the range
of 4 - 7 mg·kg–1. He et al. [12] found that MMP concen-
trations measured by an ICP method were two times
greater than those measured by a colorimetric method as
the average MMPColorimetric and MMPICP contents of 10
Maine soil samples were 7.24 mg·kg–1 and 14.96 mg·kg–1,
respectively. To further explore the difference in MMP
measured by a different method, we collected 120 sur-
face soil samples from plots cultivated to potato and its
rotation crops in Maine, USA, and determined their
MMP levels using different colorimetric and ICP-based
methods. The study provides useful reference on more
appropriate application of MMP data resulting from dif-
ferent P assay methods.
2. Materials and Methods
2.1. Soil Sample Collection
Soil samples were collected from two sites. The first
sample site was the USDA-ARS research field in Pre-
sque Isle, Maine, USA (Latitude 46˚41'N, Longitude
68v2'W). The soil is classified as Caribou Sandy loam
(fine-loamy, isotic, frigid Typic Haplorthods). Potato or
rotation crops had been planted with five production sys-
tems as 1) Continuous Potato (CP), a non-rotation control;
2) Status Quo (SQ) a typical 2-yr rotation practice in the
area: potato (Yr 1) followed by barley (Yr 2); 3) Disease
Suppressive (DS): mustard green manure/winter rapeseed
(Yr 1)—sorghum sudangrass/winter rye (Yr 2)—potato
(Yr 3); 4) Soil Conserving (SC): barley underseeded with
timothy (Yr 1)—timothy sod (Yr 2)—potato (Yr 3) with
mulch after harvest; and 5) Soil Improving (SI): same as
SC, with compost (20 Mg·ha–1 ) added to each crop [13].
Fertilizer was applied at the annual rate of 2240 kg·ha–1
of commercial 10(N)-10(P2O5)-10(K2O) fertilizer (fertil-
izers forms are ammonium nitrate, diammonium phos-
phate and potassium muriate) in bands approximately 5
cm to the side and 5 cm below the seed [13]. Extra P
input (39 kg·P·ha–1·yr–1 based on five year average) was
added in the SI plots from compost addition. All of the
production systems were managed under both rainfed
and irrigated conditions and there were five replications
of each treatment. Irrigation water (1.25 cm) was applied
to all irrigated treatments when 25% of the tensiometers
placed in irrigated plots register 50 kPa reading. Soil
samples under both irrigated and rainfed managements
were collected in May 2010 after the completion of two
three-year crop rotations.
The second sample site was the water quality facility
at the USDA-ARS, J. Phil Campbell, Sr. Natural Re-
source Conservation Center, Watkinsville, Georgia, USA
(83˚24'W and 33˚54'N) [14,15]. The facility consists of
12 large (10 m × 30 m) tile-drained plots, located on
nearly level (<2% slope) Cecil sandy loam soil (fine,
kaolinitic, thermic Typic Kanhapludults). The facility
was managed over ten years for cotton and corn produc-
tion under combinations of two tillage types, conven-
tional tillage and no-till management, and two fertilizer
sources, poultry litter and inorganic (chemical) fertilizer.
In the fall of 1997, 2000, and 2005, soil samples (0 to 15
cm) were collected with a tractor mounted hydraulic soil
coring device from three locations in each plot. In addi-
tion, in Oct 2006, 0- to 2.5, 2.5- to 5, and 5- to 15-cm
samples were collected from each plot in the same way.
2.2. Modified Morgan Extraction
For modified Morgan P (MMP), soils (5.0 g dry soil or
equivalent) were extracted by 20 mL of 0.62 M NH4OH/
1.25 M CH3COOH (pH 4.8) for 15 mins [12]. MMP was
extracted first from a set of 25 wet soil samples. Later
MMP was extracted from the whole set of 120 dry soil
samples. The extract and soil residues were separated by
centrifuging at 14,000 × g for 30 min at 4˚C. The super-
natant (i.e. the extract) was removed and past through a
0.45 m filter. Those supernatants were saved and kept
at 4˚C prior to P determination. In addition, a set of
Maine soil samples was sent to University of Maine
Analytical Laboratory for MMP measurements for com-
parison.
2.3. Phosphorus Determination
Two molybdium (Mo) blue methods were used in colo-
rimetric MMP determination. The first o was based on
He and Honeycutt (H & H) [5] and the second was the
Murphy and Riley (M & R) method as modified by Wa-
tanabe and Olsen [16]. ICP-AES P was determined using
a Spectra Arco Spectrophotometer (Perkin-Elmer, Nor-
walk, CT) at Oklahoma State University. ICP-AES P
analyzed at University of Maine was conducted using a
Thermo Jarrell Ash IRIS 1000 Dual-view Spectrometer
(Franklin, MA).
3. Results and Discussion
3.1. Difference in P Determined by Two Mo Blue
Methods
Figure 1 shows the MMP concentrations in 25 Maine
wet soil samples collected from five PP and SQ plots.
MMP in most soil samples was greater when measured
by M & R than by H & H. The MMP concentration was

Differences in Modified Morgan Phosphorus Levels Determined by Colorimetric and
Inductively Coupled Plasma Methods
258
0
5
10
15
20
25
1357911 13 15 1719 21 2325
P (mg·kg
-1
of s oi l )
H&H M&R
Figure 1. Modified Morgan P concentrations in 25 Maine
wet soils from Continuous Potato (PP ) and Status Quo (SQ)
plots measured by Mo blue methods based He and
Honeycutt (H & R) [5] or Murphy and Riley (M &R ) [16].
These plots were either rainfed (R) or irrigated (I) with
potato (P) or barley (B) grown.
only slightly lower in M & R measurement than by H &
H in Sample 12, 14 and 25, apparently due to the fact
that the assay errors are greater than the difference be-
tween the two methods. The average of MMP of the 25
samples is 12.13 mg·kg–1 of dry soil with SD of 2.30
measured by H & H, and 12.96 mg·kg–1 of dry soil with
SD of 3.01 measured by M & R. The results were rea-
sonable as H & H method was adapted to measure solu-
ble inorganic orthophosphate only [5,17]. Whereas M &
R measures some organic P that may be hydrolyzed dur-
ing the assay, leading to a greater P measurement than
inorganic orthophosphate [17] alone so the P measured
by M & R has been alternatively termed “Mo-reactive P”
[6,18]. Statistical analysis of data in 1 indicates the two
sets of data are positively and significantly correlated (r2
= 0.944, P < 0.001). The linear regression is MMPM & R =
1.271 MMPH & H = 2.457 in mg·kg–1. Our observations
imply that whereas M & R method is practical for soil P
test measurement, H & H method should be applied for
more sophisticated or highly quantitative research in-
volving accurate measurements of inorganic P and or-
ganic P, such as in hydrolysable organic P studies [19-22].
3.2. Relationship between Colorimetric M & R
MMP and ICP-AES MMP for the Maine
Soil
The MMP concentrations of 120 dry soil samples from
Maine potato fields measured by the two methods are
shown in Figure 2(a). The MMP concentrations of all
120 samples measured by M & R method were less than
by ICP-AES. The average of the colorimetric measure-
ments of all samples was 16.9 mg·kg–1 of soil with SD of
4.5 whereas the average of ICP-AES measurements 27.6
mg·kg–1 with the SD of 5.9. Specifically, the averages
and standard deviations of the colorimetric and ICP-AES
measurements were (15.5 ± 2.3) mg·kg–1 and (24.9 ± 2.3)
mg·kg–1 for 45 soils from rainfed plots without compost
addition, (14.4 ± 2.4) mg·kg–1 and (24.3 ± 3.0) mg·kg–1
for 45 soils from irrigated plots without compost addition,
(23.1 ± 4.8) mg·kg–1 and (34.4 ± 9.2) mg·kg–1 for 15
soils from rainfed plots with compost addition, and (22.2
± 4.3) mg·kg–1 and (32.1 ± 6.1) mg·kg–1 for 15 soils from
irrigated plots with compost addition. Compost addition
contributed an additional 10 mg·kg–1 of MMP in these
soils. However, irrigation did not significantly impact
MMP levels, which is similar to an earlier observation on
labile water soluble P in soils from these plots after the
first 3-year crop rotation in 2007 [23].
Visual examination of Figure 2(a) suggests that the
difference in MMP values between colorimetric and ICP-
AES methods is not affected by the treatments. The P
concentration is a more critical factor as seen in the
changing trend above 27 - 30 mg·P·kg–1 measured by the
colorimetric method. There were 119 soil samples below
the value of 30 mg·P·kg–1. The linear regression of these
data points is MMPICP-AES = 1.1 MMPColoremetric + 8.0 in
mg·kg–1 (r2 = 0.92, P < 0.001). The slope near 1 and in-
tercept near 10 imply that the difference between the two
measurements was mainly due to a stable (about 10
mg·kg–1) organic P component in these soils. Our previ-
ous work after completing the 3-yr crop rotation also
found no significant change of organic P fractions in
these soils attributable to crop rotation or irrigation [23].
Previously, a regression of MMPICP-AES = 0.98 MMPCol-
oremetric + 1.5 (MMPColoremetric < 30 mg·P·kg–1) was re-
ported based on 51 soil samples collected from corn
fields in 10 Northeastern USA states [4]. The lower con-
stant value suggested a lower level of organic P in these
corn fields than that in the potato field we tested. The
eight soils with MMPColorimetric > 27 mg·P·kg–1 all were
from soils where compost was added. Organic P in these
eight soils as determined by the Modified Morgan ex-
traction was greater than for the other soils. These data
points can be mathematically expressed by another re-
gression MMPICP-AES = 6.26 MMPColoremetric − 134 in
mg·kg–1 (r2 = 0.99, P < 0.001).
The relative difference between the two sets of data is
shown in Figure 2(b). The ratio of ICP-AES P/Colori-
metric P is 1.3 to 2.2 with the lowest ratios between 20 -
25 mg MMPColorimetric kg–1 soil. The greater ratio seems to
be due to the indigenous organic P at the low MMP le-
vels. The greater ratio shown by the high MMP levels
could be attributed to external organic P input from com-
post. When all data was combined, the ratio of ICP-AES
P/Colorimetric P could be expressed by a non-linear equ-
ation as shown in Figure 2(b).

Differences in Modified Morgan Phosphorus Levels Determined by Colorimetric and
Inductively Coupled Plasma Methods
259
0
10
20
30
40
50
60
70
51525
ICP-AES P (mg·kg
-1
)
Colorimetric P (mg ·kg
-1
)35
R-Co mpost
I- Com post
R+Compost
I+Com p ost
(a)
y=0.0032x
2
‐0.14x+3.06
R²=0.59,P<0.001
0.5
1
1.5
2
2.5
5 15253
Rati o (I CP-AE S P/C o lo ri metri c P)
Colorimetric P (mg ·kg
-1
)5
R‐Compost
I‐Compost
R+Co mp o st
I+C ompost
(
b
)
Figure 2. Modified Morgan P concentrations in 120 Maine
dry soil samples measured by colorimetric Murphy and
Riley method and ICP-AES. (a) In absolute concentrations;
(b) in ratio of ICP-AES P/Colorimetric P. All samples were
from potato or rotation crop fields under rainfed (R) or
irrigated management with (+) or without (−) additional
compost application.
3.3. Relationship between Colorimetric M & R
MMP and ICP-AES MMP for the Georgia
Soil
The MMP concentrations of 72 dry soil samples from
Georgia cotton/corn fields measured by the two methods
are shown in Figure 3. The colorimetric MMP concen-
trations varied from 0.5 to 41 mg·kg–1 of soil. The ICP-
AES MMP concentrations varied from 2.5 to 102 mg·kg–1
of soil. The greater variation of the Georgia soil data than
the Maine soil data was apparently due to the different
fertilization/soil managements. All Maine soils except
those with compost had received the same NPK 10-10-10
fertilizer at the same rate so that the variation of their
MMP concentration was relatively small. Those Georgia
soils had received different types of fertilizers (chemical
fertilizer and poultry litter) with conventional tillage or
no-till management. Previous data [14] have shown the
no-till effect retained most of P from poultry litter appli-
cation in the surface soil (0 - 2.5 cm). Indeed, the highest
3 MMP concentrations were observed with the 0 - 2.5 cm
soil samples (Figure 3). The low MMP concentrations
0
20
40
60
80
100
120
0 102030405
I CP-AE S P (mg·kg
-1
)
Colorimetric P (mg ·kg
-1
)0
0-15 cm, 1997
0-15 cm, 2000
0-15 cm, 2005
0-2.5 cm, 2006
2.5-5 cm, 2006
5-15 cm, 2006
Figure 3. Modified Morgan P concentrations in 72 Georgia
dry soil samples measured by colorimetric Murphy and
Riley method and ICP-AES. All samples were collected
from cotton/corn plots from the indicated oil depths at the
specific years.
were with subsurface soil samples or with the samples
collected in earlier years as P from poultry litter accu-
mulated in these soils over time [14]. In spite of those
differences, the relationship of the colorimetrically and
ICP-AES-measured MMP concentrations of these Geor-
gia soils was similar to that of Maine soils as the two-
line’s pattern was observed with a changing trend started
at 27 - 30 mg·P·kg–1 measured by the colorimetric me-
thod. The linear regression of the 66 data points with
colorimetric MMP < 30 mg·kg–1 is MMPICP-AES = 1.2
MMPColoremetric + 1.9 in mg·kg–1 (r2 = 0.99, P < 0.001). The
11 data points with colorimetric MMP > 27 mg·kg–1 is
mathematically expressed by the regression MMPICP-AES =
5.57 MMPColoremetric − 124 in mg·kg–1 (r2 = 0.97, P < 0.001).
These two regressions were comparable to those two for
Maine soils, indicating the applicability of the observa-
tions of these regressions to a wider soil types with dif-
ferent cropping/soil managements. Combined all sample
data from both experimental locations, the linear regres-
sions are MMPICP-AES = 1.3 MMPColoremetric + 3.1 in
mg·kg–1 (r2 = 0.94, P < 0.001) with colorimetric MMP <
30 mg·kg–1, and MMPICP-AES = 5.0 MMPColore me tr ic − 102.0
in mg·kg–1 (r2 = 0.91, P < 0.001) with colorimetric MMP >
27 mg·kg–1.
3.4. Effects of Soil Drying and Inter-Laboratory
Assay on MMP Measurements
Figure 4 shows the relationship of colorimetric MMP of
the 25 field moist soil samples in Figure 1 with the cor-
responding dried soils in Figure 2. The fact that almost
all data points are above the diagonal dash line in Figure
4 implies that air drying increased the colorimetric MMP
level. The difference was generally <3 mg·kg–1 since the
average and SD of MMP data were (13.0 ± 3.0) mg·kg–1
and (15.7 ± 2.4) mg·kg–1 for the wet and dry samples,

Differences in Modified Morgan Phosphorus Levels Determined by Colorimetric and
Inductively Coupled Plasma Methods
260
y=0.713x+6.416
R²=0.786,P<0.001
0
5
10
15
20
25
0510 15 20 25
P extracted fro m dry s oil (mg ·k g
-1
)
P extracted fro m we t soil s (m g ·kg
-1
)
Figure 4. The relationship of MMP in wet and air-dried
samples of same Maine soils measured by colorimetric
Murphy and Riley ) method.
respectively. The significant linear regression (Figure 4)
suggested the regression line was different from zero and
that the two sets of data vary. The high value in the dry
soil could be attributed to drying-induced P release
through microbial cell lysis, organic matter destabiliza-
tion, P desorption site exposure [24].
To examine the repeatability of ICP-AES MMP mea-
surement from different laboratories, the 120 dry soils
were analyzed by the University of Maine Analytic La-
boratory. Samples had greater MMP levels, while 109
samples had lower MMP levels when measured at the
University of Maine compared to the measurements
made at Oklahoma State University (Figure 5). The dif-
ference between the two sets of data was around 6
mg·kg–1 and with a SD of (21.1 ± 7.0) mg·kg–1 for the
University of Maine data, and (26.8 ± 5.9) mg·kg–1 for
the Oklahoma State University data. Statistical analysis
indicates that the two sets of data were significantly and
highly correlated (r2 = 0.90, P < 0.001) (Figure 5). Thus,
the MMP measurements seem repeatable and the diffe-
rences between the different labs were within an accept-
able range and were similar to the variance found previ-
ously for an interlaboratory comparison of Mehlich 3 P
[25].
4. Conclusions
Phosphorus (P) fertilization is frequently needed for
profitable crop production. Modified Morgan P (MMP)
is a soil test P used to estimate plant available P in soils.
MMP levels in 120 Caribou Sandy loam soil samples
from potato and rotation crops plots in Maine, USA, and
72 Cecil sandy loam soil samples from cotton/corn plots
under conventional tillage and no-till managements with
chemical or poultry litter fertilization in Georgia, USA,
were evaluated by colorimetric Mo blue and ICP-AES
y = 1.224x -11.676
R² = 0.811, P<0.001
0
10
20
30
40
50
60
70
0 10203040506070
ICP-AES P by UMaine(mg·kg
-1
)
I CP-AE S P by OSU (m g·k g
-1
)
Figure 5. The inter-lab difference in ICP-AES MMP of 120
Maine dry samples measured by Oklahoma state university
and university of Maine.
methods. The MMP levels in Maine soils measured by
different methods were compared. For the 25 field moist
soil samples, the MMP levels measured by M & R method
were greater than those measured by a modified Mo blue
method, in which the modification improved the speci-
ficity on soluble inorganic P measurement. Based on the
observations in this work, we recommend the modified
Mo blue and ICP methods be used for investigating in-
organic P and organic P partition in soil MMP pools.
The data from both Maine and Georgia soil samples
show the same pattern in relationships between colori-
metrically- and ICP-AES-measured MMP concentrations.
That is, there was turning point between 27 - 30 mg·P·kg–1
of soil measured by colorimetry. Combined the two sets
of data (total 192 soil samples tested), 184 samples were
below the value of 30 mg MMPColorimetric kg–1 measured
by Murphy-Riley method. The linear regression of these
data points is MMPICP-AES = 1.3 MMPColorimetric + 3.1 in
mg·kg–1. The 19 data points higher than 27 MMPColoremetric
can be expressed by a different linear regression equa-
tion of MMPICP-AES = 5.0 MMPColoremetric − 102.0 in
mg·kg–1. The highly correlated relationships between the
two sets of data imply that both colorimetric and ICP
methods can be used for MMP measurements. But a con-
version factor should be applied for soil P fertilization
recommendation as the current recommendation system
is based on the MMP vales measured by the colorimetric
method. More field samples should be tested for con-
firming the conversion factor as it could vary with other
soil types and cropping management practices due to the
presence of different pools of P.
5. Ackonwledgements
We thank Peggy Pinette for coordinating sample collec-
tion, treatments, and analysis service at the University of
Maine.

Differences in Modified Morgan Phosphorus Levels Determined by Colorimetric and
Inductively Coupled Plasma Methods
261
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