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Molybdenum (Mo) is an essential element for
proper growth and development of the major-
ity of living organisms, but it is required in very
small amounts and has a narrow range between
deficiency and toxicity. In plants, it plays a part in
nitrogen metabolism as a component of enzymes
such as nitrate reductase and nitrogenase. At the
same time, Mo participates in the metabolism of
sulphur, biosynthesis of plant hormones and ca-
tabolism of purine compounds (Kaiser et al. 2005).
Overall content of molybdenum in agricultural
soils ranges from 0.2 to 5.0 mg/kg (Scheffer and
Schachtschabel 2002). The plants take up Mo in
the form of the molybdate anions (MoO4
2– and
HMoO4
–) which are the predominant species in
soil solution. A release of molybdenum from solid
mineral forms to soil solution is determined by
different soil properties, such as soil pH as well as
soil content of Fe, Mn, Al oxides, clay minerals and
organic carbon. Among these factors, soil pH has
the strongest effect on the processes of adsorbing
and releasing MnO4
2– ions into the soil solution.
The maximum adsorption of molybdenum onto
positively charged metal oxides occurs between
pH 4 and 5 (Riley et al. 1987, Xie et al. 1993, Gupta
1978, Xu et al. 2013). In acidic soils, molybdate
Prediction of molybdenum availability to plants
in differentiated soil conditions
B RUTKOWSKA1, W SZULC1,*, E SPYCHAJFABISIAK2, N PIOR3
1Agricultural Chemistry Department, Faculty of Agriculture and Biology,
Warsaw University of Life Sciences-SGGW, Warsaw, Poland
2Department of Agricultural Chemistry, Faculty of Agriculture and Biotechnology,
UTP University of Science and Technology, Bydgoszcz, Poland
3Institute of Intercultural Studies, Jagiellonian University in Krakow, Krakow, Poland
*Corresponding author: wieslaw_szulc@sggw.pl
ABSTRACT
Rutkowska B., Szulc W., Spychaj-Fabisiak E., Pior N. (2017): Prediction of molybdenum availability to plants
in differentiated soil conditions. Plant Soil Environ., 63: 491–497.
e aim of the study was to assess of plant available molybdenum (Mo) resources in the solutions of soils as well
as to evaluate the effects of selected soil properties on changes of the Mo concentration in the soil solution. Sixty-
two soil samples were investigated. e soil solutions were obtained by modified vacuum displacement method.
e results showed that Mo concentrations in the soil solutions were much differentiated, ranging from 0.002 to
approximately 0.100 µmol/L. Positive correlations were found between soil solution Mo concentration and soil pH
as well as the contents of available phosphorous and organic carbon in soil. At the same time, Mo concentration
was higher in the soil solutions obtained from soils with larger amounts of soil particles with diameter lesser than
0.02 mm. Among the analysed soil parameters in this study, soil pH is the most important factor that influences the
Mo concentration in soil solution. Studies have shown that in acid sandy soils the amount of molybdenum found
in the soil solution is too small to cover the nutritional requirements of the plants. is indicates the need of fertil-
ization with this element. Regular liming of soils and fertilization with phosphorus can improve the availability of
molybdenum to plants.
Keywords: micronutrient; mobility; leaching; solubility; acidic soils
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Plant Soil Environ. Vol. 63, 2017, No. 11: 491–497
doi: 10.17221/616/2017-PSE
anions are adsorbed onto positively charged Fe,
Mn and Al oxides as well as clay minerals and
organic colloids. Availability of molybdenum to
plants increases together with increasing soil pH.
For each unit of pH rise above 3, MoO4
2– solubility
increases about 100-fold, mainly through decreased
adsorption of metal oxides (Smith et al. 1997,
Jiang et al. 2015). The poorly drained wet soils,
rich in organic matter tend to accumulate MoO4
to high levels and from well-drained sandy soils
molybdenum is readily leached away (Riley et al.
1987). The availability of Mo to plants primarily
depends on the supply of soil available Mo and
is also related to the species of plant (McGrath
et al. 2010b).
The present study was undertaken with the aim
to (i) assess the amount of easily available re-
sources of molybdenum in the soil solutions of
agriculturally used soils; (ii) evaluate the effects
of some soil properties on changes of concentra-
tion of this element in the soil solution and (iii)
determine the degree of supply of selected crop
plants in Mo by the amount of this element that
is in the soil solution.
MATERIAL AND METHODS
Sixty-two soil samples were collected by the
Regional Agro-Chemical Laboratories from con-
trol points included in the system of the State
Environmental Monitoring. The control points
are located on arable land, characteristic for the
soil cover of the country. One control point cov-
ers an area of 650 km2. Depending on the area of
agricultural land, 2 to 6 soil samples were col-
lected from each voivodship. Soil samples were
taken as follows: GPS-determined point was the
central point of the square of 100 m2, from which
each individual sample was taken with a steel soil
probe from a depth of 0–30 cm. The combination
of individual samples was a collective sample rep-
resentative of the control point. Soil samples were
taken from the most common Polish soils: Haplic
Luvisols, Haplic Cambisols, Haplic Arenosols with
granulometric composition from loose sands to
heavy loams.
The soils were air-dried, and ground in an agate
mortar to pass through a 2.0 mm sieve for analysis.
Soil samples were characterized for: pH – by
the potentiometric method after extraction with
1 mol/L KCl (10 g of soil was suspended in 25 mL
of KCl and equilibrated for 24 h) using a pH meter
(apparatus: Schott, Mainz, Germany); available
P – by the Egner-Riehm (DL) method (Egner and
Riehm 1958); available Mo – after extraction in
1 mol/L HCl (10 g of soil was shaken with 100 mL
HCl on a rotary shaker for 2 h at 120 rounds per
min) by the inductively coupled plasma-atomic
emission spectrometry (ICP-AES, IRYS Advantage
ThermoElementar, Cambridge, UK); total Mo
by the aqua regia digestion, determined by the
ICP-AES; total organic carbon content – by dry
combustion at high temperatures in a furnace
with the collection and detection of evolved CO2
(Tiessen and Moir 1993); content of soil parti-
cles < 0.02 mm – by the laser diffraction method
(Ryżak et al. 2007). In Poland, the content of soil
particles < 0.02 mm determines the agricultural
usefulness of soil. On this basis, four categories
of soils are identified: very light (< 10% particles
< 0.02 mm), light (10–20%), medium (20–35%)
and heavy (> 35%).
The samples differed in terms of their physi-
co-chemical properties, such as: content of soil
particles < 0.02 mm, soil reaction, organic car-
bon content, available forms of molybdenum and
phosphorus in soil (Table 1).
The soil solutions of all the observed soils were
obtained by the modified vacuum displacement
Table 1. Properties of the investigated soils
Content of soil particles < 0.02 mm (%) < 10 (17) 10–20 (17) 20–35 (17) > 35
pHKCl < 4.5 (12) 4.6–5.5 (17) 5.6–6.5 (12) > 6.6 (21)
Available phosphorus content in soil (mg/kg) < 22 (7) 22–44 (15) 44–66 (12) > 66 (28)
Soil organic carbon content (g/kg) < 5 (3) 5–10 (30) 10–15 (25) > 15 (4)
Available molybdenum content in soil (mg/kg)* low (0.008–0.059) (42) medium (0.026–0.070) (20)
Total molybdenum content in soil (mg/kg) < 0.5 (21) 0.5–1.0 (32) 1.0–1.5 (7) > 1.5 (2)
*depending on soil pH and soil available phosphorus content in soil. Number of soils are in brackets
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doi: 10.17221/616/2017-PSE
method of Wolt and Graveel (1986). Air-dried soil
samples (100 g each) were wetted with redistilled
water to 100% field water capacity and then in-
cubated at room temperature for 72 h. Once the
balance between solid and liquid soil phases was
established, the soil solution was obtained with the
use of a vacuum pump (Dynavac OP4, Melbourne,
Australia) under pressure 0.08–0.09 MPa. The
obtained soil solutions were filtered through a
filter paper. The total concentration of Mo in
soil solutions was determined by the inductively
coupled plasma- atomic emission spectrometry
(ICP-AES).
The ICP-AES apparatus calibration was made
based on patterns prepared from the Single Element
Standards for ICP Solution by Ultra Scientific
company. To check the calibration curve, the solu-
tions were used to check the instrument and the
calibration (QC) at concentrations of 0.1 ppm
and 1 ppm – before the samples were studied, and
at every 20 samples according to the Combined
Quality Control Standard from Ultra Scientific
company.
Relationships between the concentration of Mo
in the soil solution and selected soil properties
were analysed with simple regression and corre-
lation at a significance level P = 0.05. Statistical
analyses were performed using the Statgraphics
Plus Professional software (StatPoint Technologies,
Inc., The Plains, Virginia, USA).
Relevant data on plant yields in Poland in the
year 2015 (Concise Statistical Yearbook of Poland
2016) and average molybdenum contents in plants
(Jadczyszyn 2000) were used in evaluating whether
molybdenum quantity in the soil solution was
sufficient for plant nutritional needs.
RESULTS AND DISCUSSION
The concentration of molybdenum in the
soil solutions analysed ranged from 0.002 to
0.100 mol/L and was differentiated depending
on soil properties. Soil solution Mo concentration
with range 0.005–0.035 mol/L was found in 66%
of all the analysed soils (Figure 1).
A similar range of Mo concentrations in the soil
solutions of Poland’s agricultural soils was earlier
observed by Rutkowska (1999). Wolt (1994) re-
ported that natural Mo concentration in the soil
solution observed in the USA and Great Britain
was 0.02 mol/L. On the other hand, Balík et al.
(2006) stated that Mo concentration in the soil
solutions of agriculturally used soils ranged from
0.006 to 0.06 mol/L.
Molybdenum concentration in the soil solutions
analysed was determined by the properties of the
observed soils. For strongly acidic (pH < 4.5) and
0
5
10
15
20
25
30
35
< 0.005 0.005–
0.020
0.020–
0.035
0.035–
0.050
0.050–
0.065
> 0.065
Number of soils
Mo in soil solution (mmol/L)
Figure 1. Range of soil solution molybdenum (Mo)
concentration in agricultural soils in Poland
Minimum Maximum Mean Standard
deviation
Variation
coefficient
0.0021 0.091 0.027 0.022 81.48
Table 2. Average soil solution molybdenum (Mo) con-
centration according as soil physico-chemical proper-
ties (mol/L)
Content of soil particles < 0.02 mm (%)
< 10 10–20 20–35 > 35
Mo 0.018 0.039 0.041 0.047
pHKCl
< 4.5 4.6–5.5 5.6–6.5 > 6.6
Mo 0.009 0.014 0.037 0.072
available phosphorus content in soil (mg/kg)
< 22 22–44 44–66 > 66
Mo 0.011 0.015 0.017 0.048
soil organic carbon content (g/kg)
< 5 5–10 10–15 > 15
Mo 0.007 0.021 0.041 0.056
soil available Mo
low medium
Mo 0.023 0.045
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acidic soils (pH 4.6–5.5), Mo concentration in the
soil solution was lower than 0.02 mol/L. It was
significantly increased at rising soil pH values
and reached on average 0.072 mol/L in soils with
neutral and alkaline soil pH (pH > 6.6) (Table 2,
Figure 2a). In the soil solution of soils with pH
value higher than 5.0 MoO4
2– ions dominate. Above
pH 4.2, MoO4
– is the common anion followed in
decreasing order by MoO4
– > HMO4
– > H2MO4
0 >
MoO2(OH)2
+ > MoO2
2+ (Lindsay 1979). In acidic
soils with pH 4–5, molybdate anions are strongly
adsorbed by positively charged oxides of Fe, Mn
and Al, and this holds back availability of Mo for
plants (Smith et al. 1997). With increasing soil pH
the concentration of MoO4
2– ions in the soil solu-
tion increases. As Lindsay (1979) and McGrath et
al. (2010a) reported, for each unit of the pH value
above 3.0, the concentration of molybdate ions can
increase even one hundred times. Enhancement
of Mo mobility in soil at high pH values is caused
by an increase of free negative charges on soil col-
loids, stronger competition between molybdates
and hydroxyl anions for adsorption sites, as well as
by lower activity of Al and Fe oxides, which is the
cause of reducing the amount of free positive sites
able to adsorb molybdenum (Jarrell and Dawson
1978, Jiang et al. 2015).
The content of soil particles also determined Mo
concentration in the soil solution. In heavy soils
(> 35% of soil fractions < 0.02 mm), soil solution
Mo concentration was more than two times higher
when compared with that in very light soils (< 10%
of soil fractions < 0.02 mm) (Table 2). Low Mo
concentration in the soil solution of very light
soils was connected with their strongly acidic soil
reaction. On the other hand, soils with more than
35% content of fractions with particle diameter
< 0.02 mm were characteristic of pH higher than
5.6. Studies of Jones and Belling (1967) showed
enhanced molybdenum leaching in well-aired sandy
soils. This process depends on soil pH. As indicated
by Riley et al. (1987), molybdenum leaching from
sandy soils with acidic soil reaction is limited be-
cause solubility of this element is restricted. The
concentration of Mo in soil solution increased
with the content of available molybdenum in soil
(Figure 2b). The results of this study showed that
with an increasing content of available phospho-
rous in soil the concentration of molybdenum in
the soil solution increased, which was proven by
y = 0.0002e0.7809x
r = 0.91**
0
0.02
0.04
0.06
0.08
0.1
3.5 5.5 7.5
Mo (mmol/L)
pH
y = 0.7301x + 0.0028
r = 0.68**
0
0.02
0.04
0.06
0.08
0.1
0 0.05
Mo (mg/kg)
y = 0.0004x + 0.0008
r = 0.85**
0
0.02
0.04
0.06
0.08
0.1
0 50 100150 200
Mo (mmol/L)
P (mg/kg)
y = 0.0042x – 0.011
r = 0.62**
0
0.02
0.04
0.06
0.08
0.1
0 5 10 15 20
Corg (g/kg)
Figure 2. Relationship between soil solution molybdenum (Mo) concentration and (a) soil pH; (b) available Mo;
(c) available phosphorus (P), and (d) organic carbon (Corg) content in soil. **P < 0.01
(a) (b)
(c) (d)
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doi: 10.17221/616/2017-PSE
correlation coefficient r = 0.85 (Figure 2c). Average
molybdenum concentration in the soil solution
depended on the content of available phospho-
rous in soil. In soils exceedingly rich in available
phosphorous (> 66 mg/kg) soil solution Mo con-
centration was four times higher when compared
with soils with low available phosphorous contents
(< 22 mg/kg). In soil, phosphates compete with
molybdates for adsorption sites on the surface of
the solid soil layer, and with increasing available
phosphorous amounts numerous adsorption sites
with high affinity for molybdenum can be blocked
by phosphates (Xie and MacKenzie 1991, Vistoso
et al. 2009). As a result of this process, desorp-
tion of MoO4
2– into the soil solution is enhanced.
The process does not depend on soil reaction and
is started in acidic soils. The soils tested were
of low content of organic carbon. Nevertheless,
significant relationships between soil solution Mo
concentration and soil organic carbon contents
were shown (Figure 2d).
In soils with organic carbon content > 15 g/kg,
soil solution Mo concentration was eight times
higher when compared with soils < 5 g/kg of or-
ganic carbon (Table 2). Studies on the effects
of organic matter on molybdenum mobility are
scarce and their results are not consistent. Kasimov
et al. (2011) showed that the humus content in
soil strongly affects molybdenum mobility. The
strength of Mo absorption increased with higher
humus in soil. Karimian and Cox (1978) and Xu
et al. (2013) indicated that the soil solution Mo
concentration decreased with increasing contents
of organic carbon in soil, most probably as a result
of formation of complexes with humic acids. On
the other hand, Jenne (1977) as well as Reddy et
al. (1997) showed that in the soil solution MoO4
2–
anions could form complex ions with metal cations
(K, Na, Ca, Mg) and also with humic and fulvic
acids. This influences indirectly the increase of
molybdenum availability for plants through re-
straint adsorption of ions MoO4
2– on Fe, Mn and
Al oxides, especially in acidic soils.
Based on the multiple regression analysis, the
relationship between the concentration of molyb-
denum in soil solution and the content of available
molybdenum, available phosphorus and organic
carbon in soil was significantly determined by
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0 0.02 0.04 0.06 0.08
Mo (µmol/L)
Mo (mg/kg)
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
0 50 100 150 200
P (mg/kg)
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
0.1
0 5 10 15 20
Mo (µmol/L)
Corg (g/kg)
0
0,01
0,02
0,03
0,04
0,05
0,06
0,07
0,08
0 0,02 0,04 0,06 0,08
Mo (µmol/L)
Mo (mg/kg)
pH < 4.5 pH = 4.6–5.5
pH = 5.6–6.5 pH > 6.6
pH < 4.5 pH = 4.6–5.5
pH = 5.6–6.5 pH > 6.6
0
0,01
0,02
0,03
0,04
0,05
0,06
0,07
0,08
0,09
0 50 100 150 200
Mo (µmol/L)
P (mg/kg)
0
0,01
0,02
0,03
0,04
0,05
0,06
0,07
0,08
0,09
0,1
0 5 10 15 20
Mo (µmol/L)
Corg (g/kg)
0
0,01
0,02
0,03
0,04
0,05
0,06
0,07
0,08
0 0,02 0,04 0,06 0,08
Mo (µmol/L)
Mo (mg/kg)
pH < 4.5 pH = 4.6–5.5 pH = 5.6–6.5 pH > 6.6
pH < 4.5 pH = 4.6–5.5
pH = 5.6–6.5 pH > 6.6
0
0,01
0,02
0,03
0,04
0,05
0,06
0,07
0,08
0,09
0 50 100 150 200
Mo (µmol/L)
P (mg/kg)
0
0,01
0,02
0,03
0,04
0,05
0,06
0,07
0,08
0,09
0,1
0 5 10 15 20
Mo (µmol/L)
Corg (g/kg)
Figure 3. Effect of soil pH on the relationship between
molybdenum (Mo) concentration in soil solution and
(a) available Mo; (b) available phosphorus (P) and (c)
organic carbon (Corg) content in soil
(a) (b)
(c)
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soil pH (Figure 3, Table 3). The concentration of
molybdenum in the soil solution of strongly acidic
soils was the smallest and significantly increased
with the increase in the amount of molybdenum
available in the soil. However, in soils with pH >
6.6, the concentration of molybdenum in the soil
solution was significantly higher. Under such condi-
tions, the increase in the amount of molybdenum
available in the soil caused a much higher increase
in the concentration of molybdenum in the soil
solution than in strongly acidic soils (Figure 3a).
Similar trends were found in the relationship be-
tween molybdenum concentration in soil solution
and phosphorus content in soil (Figure 3b) and
organic carbon content in soil (Figure 3c).
Average uptake of molybdenum by plants in
Polish agriculture ranged from 1.68 to about 10 g
per hectare (Table 4). The average content of ac-
tive molybdenum in the arable layer of soil is suf-
ficient to cover the nutritional needs only in the
case of rye. This corresponds to a concentration of
molybdenum in the soil solution at 0.027 µmol/L.
57% of the analysed soils are characterized by a
concentration less than sufficient to cover the
nutritional needs of plants (Figure 1).
The results of the study carried out indicate that
molybdenum concentrations in the soil solutions of
agriculturally used soils in Poland are very variable
and depend on physico-chemical soil properties.
A significant positive correlation was found be-
tween soil solution Mo concentration and soil pH,
available phosphorous contents in soil as well as
soil organic carbon contents. In the soil solution
of soils with pH > 6.6, the concentration of Mo
was eight times higher when compared with soils
with pH < 4.5. Also an eight-fold increase of soil
solution Mo concentration was observed under
the influence of increased soil organic carbon
contents from below 5 to more than 15 g/kg. At
the same time, the concentration of molybdenum
in the soil solutions increased in the observed soils
with increased both the content of soil particles
with the diameter lesser than 0.02 mm and the
quantity of molybdenum forms. Plants cultivated
Table 3. Regression equation between molybdenum (Mo) concentration in soil solution and selected soil properties
Dependent variable Independent variable Equation P R2 (%)
Mo soil solution (Moss)
Mo available (Moav) soil pH Moss = –0.06 + 0.51 Moav + 0.11 pH < 0.01 75.06
P available (Pav) soil pH Moss = –0.06 + 0.001 Pav + 0.14 pH < 0.01 56.23
Corg soil pH Moss = –0.06 + 0.002 Corg + 0.01 pH < 0.01 50.70
Table 4. Yield (t/ha) and molybdenum (Mo) uptake (g/ha) by selected plants and content of Mo in soil
Plant Yield Molybdenum uptake
minimum maximum average minimum maximum average
Winter wheat (Triticum aestivum)3.20 7.20 4.13 2.24 5.04 2.89
Rye (Secale cereale)2.30 5.80 2.40 1.61 4.06 1.68
Green forage corn (Zea mays)30.00 58.00 48.00 4.50 8.70 7.20
Winter rape (Brassica napus)2.20 4.00 2.24 2.20 4.00 2.66
Potato (Solanum tuberosum)12.00 45.00 23.20 1.32 4.95 2.55
Sugar beet (Beta vulgaris)15.00 69.00 57.40 2.55 11.73 9.76
Mo quantity in the solution of soil arable layer
Minimum maximum average minimum maximum average
(µmol/L) (g/ha)
0.0021 0.091 0.027 0.19 8.25 2.45
*according to the Concise Statistical Yearbook of Poland (2016) and Jadczyszyn (2000)
496
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doi: 10.17221/616/2017-PSE
in sandy, acidic soils may show deficiencies of Mo
due to the low molybdenum concentration in soil
solution as well as soil characteristics that limit its
availability to the plants. Regular liming of soils
and fertilization with phosphorus can improve the
availability of molybdenum to plants.
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Received on September 27, 2017
Accepted on November 6, 2017
Published online on November 16, 2017
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Plant Soil Environ. Vol. 63, 2017, No. 11: 491–497
doi: 10.17221/616/2017-PSE