Magnesium mobility in soils as a challenge for soil and plant analysis, magnesium fertilization and root uptake under adverse growth conditions

Abstract

Background
Due to its unique chemistry magnesium (Mg) is subject to various cycling processes in agricultural ecosystems. This high mobility of Mg needs to be considered for crop nutrition in sustainable agricultural systems. The Mg mobility in soils and plants and its consequences for crop nutrition are understood, but recent findings in crop Mg uptake, translocation and physiology particularly under adverse growth conditions give new insights into the importance of Mg in crop production.

Scope
The aim of this review is to combine the knowledge on the origin and mobility of Mg in soils with the role of Mg in plant stress physiology and recent evidence on the principles of crop Mg uptake. The question is addressed whether the progress made in Mg research, particularly on the role of Mg in stress physiology, makes a revision of the development of Mg fertilization recommendations necessary.

Conclusions
New insights into Mg uptake and utilization but particularly into the role of Mg in increasing crop tolerance to various stresses indicate changes in the crop Mg demand under adverse growth conditions. Future work should incorporate these findings in optimization of site-specific balanced fertilization programs particularly under stress conditions.

Full text

REGULAR ARTICLE
Magnesium mobility in soils as a challenge for soil and plant
analysis, magnesium fertilization and root uptake
under adverse growth conditions
A. Gransee & H. Führs
Received: 14 June 2012 /Accepted: 10 December 2012 /Published online: 29 December 2012
#
The Author(s) 2012. This article is published with open access at Springerlink.com
Abstract
Background Due to its unique chemistry magnesium
(Mg) is subject to various cycling processes in agri-
cultural ecosystems. This high mobility of Mg needs
to be considered for crop nutrition in sustainable agri-
cultural systems . The Mg mobility in soils and plants
and its consequences for crop nutrition are understood,
but recent findings in crop Mg uptake, translocation
and physiology particularly under adverse growth con-
ditions give new insights into the importance of Mg in
crop production.
Scope The aim of this review is to combine the knowl-
edge on the origin and mobility of Mg in soils with the
role of Mg in plant stress physiology and recent evi-
dence on the principles of crop Mg uptake. The question
is addressed whether the progress made in Mg research,
particularly on the role of Mg in stress physiology,
makes a revision of the development of Mg fertilization
recommendations necessary.
Conclusions New insights into Mg uptake and utiliza-
tion but particularly into the role of Mg in increasing
crop tolerance to various stresses indicate changes in
the crop Mg demand under adverse growth conditions.
Future work should incorporate these findings in
optimization of site-specific balanced fertilization pro-
grams particularly under stress conditions.
Keywords Antagonism
.
Carbohydrate formation and
partitioning
.
Fertilization
.
Leaching
.
Stress
physiology
.
Soil and plant analysis
Introduction
Magnesium (Mg) is an essential element for plant growth
and development. The availability of Mg to plants
depends on various factors: the distribution and chemical
properties of the source rock material and its grade of
weathering, site specific climatic and anthropogenic fac-
tors and, in agricultural systems, to a high degree on the
agronomic management practices established at the spe-
cific production site including the cultivated crop species
and crop rotation, cropping intensity and organic and
mineral fertilization practice (Mikkelsen 2010; Scheffer
and Schachtschabel 2002).
The importance of Mg in crop production was under-
estimated in the last decades (Cakmak and Yazici 2010).
Indeed, compared to other nutrients little attention has
been paid on this mineral nutrient by agronomists and
scientists in the last decades. Therefore, the term ‘the
forgotten element’ was introduced and used (Cakmak
and Yazici 2010). One decisive reason for this gap in
research may be that Mg deficiency is often not recog-
nized in agriculture, so that there is no concrete stimulus
for enhanced (research) activity in this area. Indeed,
Plant Soil (2013) 368:5–21
DOI 10.1007/s11104-012-1567-y
Responsible Editor: Ismail Cakmak.
A. Gransee
:
H. Führs (*)
K+S KALI GmbH,
Bertha-von-Suttner-Str. 7,
34131 Kassel, Germany
e-mail: hendrik.fuehrs@kali-gmbh.com

acute Mg deficiency is typically correlated with visible
interveinal chlorosis and growth reduction, whereas the
more frequent latent deficiency is often not visible and
hardly to diagnose but negatively affects yield of crops
(Cakmak and Yazici 2010).
Latent and acute Mg deficiencies are common phe-
nomenons in crop production (Römheld and Kirkby
2007). A typical Mg deficiency symptom is leaf inter-
veinal chlorosis. Development of chlorosis requires pre-
ceding degradation of chlorophyll, since Mg acts as
central atom in the chlorophyll molecule. As Mg is
strongly bound to this molecule chlorosis appears to be
a late response to Mg deficiency. In plants well supplied
with Mg only about 20 % of the total Mg is bound to
chlorophyll, whereas the remaining about 80 % are
present in more mobile forms (Marschner 2012).
Magnesium is phloem mobile and readily translocated
within the plant to actively growing plant parts acting as
sink (White and Broadley 2008). Consequently, due to
the high mobility under Mg starvation Mg deficiency
symptoms typically appear on older leaves of the plant
(Bergmann 1992). As chlorosis is a late visible response
to Mg deficiency considerable decreases in yield forma-
tion can be expected. Therefore, the diagnosis of chlo-
rosis is not a suitable tool for diagnosis of Mg deficiency
as basis for fertilization recomme ndations. There is
some evidence that Mg plays specific roles in dry matter
formation and carbon partitioning to sink organs, as
under Mg deficiency carbohydrates accumulate in
source leaves (Cakmak et al. 1994a, b;Dingetal.
2006). Therefore, an earlier response of plants to Mg
deficiency is carbohydrate accumulation in source
leaves and reduced root growth due to restricted supply
of the roots with carbohydrates (Cakmak et al. 1994a,
b), even though some contrasting results exist for some
plant species, for example sugar beet (Hermans et al.
2004, 2005). Hence, disturbed carbohydrate partitioning
may be regarded as latent deficiency which per defini-
tion also impacts yield formation.
It is assumed that t he disturbed metabolite (e.g.
carbohydrates and amino acids) partitioning under
Mg deficiency is a consequence of impaired phloem
loading of metabolites. When the root system as an
important and heavy sink for metabolites suffers from
limited carbohydrate supply the reduced root growth
further enhances the risk of other nutrient deficiencies
and environmental stresses (e.g. drought stress) due to
less explored soil volume and, therefore, less access to
soil resources (Cakmak and Kirkby 2008). This may
be underlined by results of Damm et al. (2011), who
recently published results on the effect of a long-term
combined Mg and K fertilization on soil p hysical
properties and root penetration of several plant species
with different rooting systems. In this study the com-
bined application of K and Mg increased the soil water
content within the whole range of water retention up to
the permanent wilting point (PWP). This soil physical
effect was mainly attributed to K. However, in parallel
to the K-induced soil physical and water-storage
changes also the rooting depth was increased, partic-
ularly for sugar beet. In view of the decisive role of
Mg in carbohydrate partitioning the increase in rooting
depth through combined Mg and K fertilization may
be at least partly attributed to the Mg supply.
In principle there are two reasons for Mg deficiency to
occur, absolute deficiency and cation competition.
Absolute deficiency can be a consequence of (1) low
Mg contents in the source rocks (Papenfuß and
Schlichting 1979), (2) of Mg losses from the soil e.g. by
mobilization and subsequent leaching (Schachtschabel
1954;Grzebisz2011) and (3) of long-term unbalanced
crop fertilization practice neglecting Mg depletion of soils
through crop Mg removal (van der Pol and Traore 1993).
Cation competition is a consequence of nutrient imbal-
ances in soils. It is commonly known that the uptake of
Mg is strongly influenced by the availability of other
cations like NH
4
, Ca and K (Jacoby 1961;Diemand
Godbold 1993; Fageria 2001; Römheld and Kirkby
2007). Nutrient imbalances in the soil can have soil-
born and/or fertilization practice-associated reasons.
Both deficiencies are closely interrelated and should be
considered together .
In order to get information on the nutritional status of
a crop and production site soil and plant analyses are
carried out. In practice the challenge is to relate results of
soil analysis to crop growth and phytoavailability of the
respective nutrient and to correctly interpret the infor-
mation for the development of recommendations.
According to Kopittke and Menzies (2007) there are at
least two principle methods of interpreting the cation (K,
Mg, Ca) soil concentrations, the sufficiency level of
available nutrients (SLAN) and the basic cation satura-
tion ratio (BCSR). The SLAN concept focuses on the
availability of individual nutrients to plants, whereas the
BCSR concept determines a site-specific optimal base
cation saturation ratio of the CEC for all cations at the
same time. Any deviation from this ‘ideal’ ratio would,
according to the concept, lead to production losses. In
6 Plant Soil (2013) 368:5–21

their review Kopittke and Menzies (2007) revise the
BCSR concept and conclude that ‘within ranges com-
monly found in soils, the chemical, physical, and bio-
logical fertility of a soil is generally not influenced by
the ratios of Ca, Mg, and K’. Indeed, recent molecular
insights into the capability of plants to cope with highly
variable nutrient concentrations not only externally (up-
take from the substrate/soil) but also internally (ion
homeostasis) (see secion ‘Mechanisms of Mg uptake
by crops’) underlines that crops do not implicitly need
a fixed base cation saturation ratio in the soil to reach a
theoretic yield potential. In fact highly sophisticated and
interconnected ion transport systems were evolved en-
abling the plant to quickly (sometimes within minutes in
nutrient solution experiments) adapt to changing con-
ditions of nutrient availability. However, despite these
fundamental qualms about the suitability of the BCSR
concept for recommendation development considerable
amounts of soil tests are currently interpreted according
to the BCSR concept in private laboratories (Kopittke
and Menzies 2007). In contrast, in university laborato-
ries soil analysis is done almost exclusively according to
the SLAN concept. Nevertheless, even though base
cation saturation ratios do not appear to reflect plant
nutrient availability in a proper way, the individual base
saturation of the CEC giving the exchange sites (accord-
ing to CEC) occupied by base cations (Ca, Mg, Na, K)
may be in principle an important measure for agricul-
tural practice as it gives important information on the
potential of a production site to provide cations.
This review aims at (i) s ummarizi ng the current
knowledge on soil and plant factors affecting Mg nutri-
tion of plants, (ii) relating newest insights into Mg
uptake and its role in plant (stress) physiology to the
current methodologies of soil and plant analysis (iii)
highlighting future research areas e.g. with respect to
combining agronomic practice with plant physiology.
Magnesium in the soil and rhizosphere
Origin of magnesium in soils
Magnesium is the 8th most abundant mineral element
on earth (Maguire and Cowan 2002). Magnesium in
soils originates from source rock material containing
various types of silicates. The Mg content of different
silicate types varies considerably (muscovite > biotite >
hornblende > augite > olivine). The reason for variation
in the Mg content is that within the silicates the Al
3+
is
substituted for Mg
2+
. This well described phenomenon
causes typical permanent charge surpluses of silicates
decisive for the physico-chemical properties of soils.
Among silicates, carbonates (e.g. magnesite [MgCO
3
]
and dolomite [MgCO
3
×CaCO
3
], but also in calcite
[CaCO
3
] in concentrations of about 1–3 %) are substan-
tial sources of Mg as well (Scheffer and Schachtschabel
2002). Due to high variation in Mg content of the source
material the total content of Mg in soils varies consid-
erably between 0.05 and 0.5 % (Grimme 1991;Maguire
and Cowan 2002). Differences in silicate contents of
soils also explain higher Mg contents typically found in
clay and silty soils compared to sandy soils. In extreme
cases, in soils high in Mg-depleted silicates, mainly
chlorites, Mg fixation can o ccur (Papenfuß and
Schlichting 1979; Scheffer and Schachtschabel 2002).
Magnesium bound in the interlayers of silicates is not
mobile and is only r eleased into mobile fractions
through weathering processes, which is regarded as a
long-termed, slow process (see also later sections). Plant
available Mg concentrations in the soil solutions have
been reported to vary between 125 μM and 8.5 mM
(Barber 1995).
Factors determining the mobility of Mg in soils
In typical agronomically used, deep-grounded soils a
considerable amount of the total soil Mg is bound in
an exchangeable form. This fraction is characterized by
reversible binding of the Mg to permanent and/or vari-
able charge in soils. Whereas the permanent charge
surplus in soils is a consequence of Al su bstitution
(see previous section), the variable charge is provided
by organo-mineral soil components. The variable charge
is stro ngly dependent on the pH of soils. Both the
permanent and variable charge of soils form the cation
exchange capacity (CEC). Two different CECs are dif-
ferentiated: (i) the potential cation exchange capacity
(CEC
pot
), which is determined at a soil pH around 7–
7.5 and (ii) the effective cation exchange capacity
(CEC
eff
) measured under the actual soil pH underlining
the high dependence of the CEC on pH. In fact, the CEC
can be regarded as the (potential or actual) capacity of a
soil to reversibly bind or buffer cations (Mehlich 1948;
Scheffer and Schachtschabel 2002).
In contrast to other cations like K, Ca, and NH
4
+
Mg is comparatively mobile in soils. The properties or
the ‘behaviour’ of Mg in soils can be ascribed to its
Plant Soil (2013) 368:5–21 7

unique chemical properties. Whereas the ionic radius
of Mg is smaller than that of Ca, K or Na, its hydrated
radius is substantially larger (Shaul 2002; Gardner
2003; Maguire and Cowan 2002). One consequence
is that Mg is less strong bound to soil charges (CEC)
leading to compared to other cations higher Mg con-
centrations in the soil solution. This has consequences
for the mobility of Mg in the soil and implications for
plant Mg nutrition.
Consequences of the mobility of Mg in soils
Two points need to be mentioned, which appear to be
the most important consequences of the high Mg mo-
bility. First the high concentration of Mg in the soil
solution explains the extraordinary high contribution
of mass flow to plant Mg nutrition (Barber et al.
1963). The contribution of mass flow to Mg nutrition
of crops can be calculated from the Mg concentration in
the soil solution and the amount of water transpired by
the crop. However, even though the contribution of
mass flow to Mg nutrition compared to K is principally
sufficient under optimum conditions (soil humidity, Mg
concentration in soil solution) in agriculturally used
soils as a consequence of the high Mg concentrations
in soil solution (Fig. 1a, Karley and White 2009; Barber
1995), it also depends on the species-specific demand
and root system of a crop species (Fig. 1b, Strebel and
Duynisveld 1989; Marschner 2012). However, under
adverse conditions like drought, this system of delivery
to the roots can be disadvantageous as the transport of
Mg to the roots can be impaired.
Second, Mg is subject to leaching in considerable
amounts. Koc and Szymczyk (2003) and Grzebisz
(2011) gave an overview over seasonal Mg leaching
from typical soil types present in Poland. The major part
is leached in autumn and winter months due to the high
positive water balance. Independent of the soil type Mg
leaching can reach up to 25 kgMg ha
−1
. Boysen (1977)
investigated nutrient leaching in different soils under
different cropping systems and found that under forest
ca. 9.5 kgha
−1
was leached, whereas under arable land
ca. 20 kgha
−1
was lost by leaching. Another study
reports Mg leaching from a soil of limited fertility in
the magnitude of 45–70 kgha
−1
depending on crop type
and developmental stage, N-fertilization, precipitation
quantity and intensity and consequently the drainage
volume (Mesić et al. 2007). This high variation in Mg
leaching can be explained by the numerous factors that
influence the magnitude of Mg leaching which include
the amount of leaching water, soil acidity (the presence
of H
+
ions), the Ca concentration (liming), the bicarbon-
ate (HCO
3
−
) concentration and the cation exchange
capacity as affected by the clay and o rganic matter
content. In view of NPK-dominated fertilization practice
often neglecting Mg these Mg losses together with
considerable Mg offtake with harvested plant products
soil mining of Mg may occur frequently (Römheld and
Kirkby 2007, see also later sections).
Mechanisms of Mg uptake by crops
To understand the Mg nutrition of crops knowledge on
the processes involved in Mg uptake and uptake of
other cations is necessary. Therefore, this section fo-
cuses on the physiology cation uptake with special
emphasis on the question of the physiological back-
ground of K-Mg antago nism.
An interesting phenomenon is the competition of
cations for uptake (Jacoby 1961; Diem and Godbold
1993; Fageria 2001; Marschner 2012;ChenandMa
2012) which is often experienced in agriculture under
unbalanced soil nutrient composition. In nutrient solu-
tion experiments it has been shown that high availability
of the cations Ca, K and Mn can lead to strong decreases
in Mg uptake (Marschner 2012). Fageria (1973) reported
on the effect of increasing Ca concentration in a nutrient
solution on the uptake of Mg and K by rice plants
(Table 1).TheauthorshowedthatincreasingtheCa
concentration to a certain level increases the uptake rates
of Mg (and also K, not displayed here). It is assumed that
at very low availability of Ca (Ca deficiency) the plasma
membranes of root cells are adversely affected leading to
ion leakage and unspecific uptake. Slightly increasing
the Ca concentration in the nutrient solution then rapidly
restores the membrane functionality, so that the uptake of
other cations is enhanced and leakage reduced. Further
increasing the Ca concentrations in the nutrient solution
then turns the positive synergistic effect of the nutrients
into an antagonistic cation competition for uptake. This
is reflected by a reduction in uptake of Mg (and K) when
the Ca concentration in the nutrient solution is further
increased (Table 1, Fageria 1973, 2009; Marschner
2012). The picture gets even more complex when non-
essential cations are included. For example de Wit et al.
(2010) recently provided evidence for a specific impact
of toxic Al ions (as released by acid depositions) on the
8 Plant Soil (2013) 368:5–21

uptake of Mg but not Ca in Norway spruce. The reason
for this specific antagonistic effect on Mg uptake is not
known. However, reduced forest vitality due to Al
excess-inhibited Mg uptake has implications for risk
evaluations of forest health under acid deposition
highlighting the importance for future research not only
for agriculture.
Which are the mechanisms leading to such antagonis-
tic effects? In the following sections some ideas are
presented based on the current knowledge on cation
uptake by plants and very recent results particularly on
transporters involved in K and Mg uptake. Two facts
may already imply the need for specific and distinct
uptake systems for K and Mg in crops. First, there are
differences in the hydrated ionic radius between both
ions (Maguire and Cowan 2002). Even though the ionic
radius of Mg is smaller than that for K the hydrated
radius of Mg is bigger indicating the need for ion-
specific transport proteins. In addition, as a consequence
of the different delivery mechanisms for K and Mg to the
roots (mass flow versus diffusion), the concentrations of
both nutrients in the soil solution typically differ that
much particularly in the rhizosphe re (Zhang and
George 2002; Marschner 2012), that specific uptake
systems are required to meet the actual crop demands.
From investigations on the course of K uptake as a
function of the K concentration in the substrate a so-
called ‘High-Affinity Transport System’ (HATS) was
proposed, which could describe the uptake for a specific
K concentration range (Epstein et al. 1963; Britto and
Kronzucker 2008). It has been shown that the K status of
the plant regulates V
max
of the HATS. Low plant K status
thereby increases the V
max
by HATS and high plant K
decreases V
max
. However, in their early studies Epstein
et al. (1963) found that at higher external K concentra-
tions the uptake pattern was different, as it did hardly
show any saturation level. This means that K supply and
K uptake showed an almost linear relationship over wide
concentration ranges. For this mechanism the term
‘Low-Affinity Transport System’ (LATS) was estab-
lished. The efficiency of the LATS is thereby modulated
by the accompanying anion (Britto and Kronzucker
2008 and literature cited therein). Chloride as accompa-
nying anion strongly increases the uptake rates as com-
pared to sulfate and phosphate, which is most probably a
consequence of a (temporarily) increased electrochemi-
cal gradient established by Cl
−
.IncontrastSO
4
2−
and
H
2
PO
4
−
are metabolized or accumulated in lower
amounts in the plant tissue that the electrochemical gra-
dient remains almost unaf fected.
How does the plant realize HATS and LATS? In
principle three main transport systems can be distin-
guished, which modulate the HATS and LATS in a
Fig. 1 Contribution of root
interception, mass flow and
diffusion to the K and Mg
demand of maize (a)and
contribution of mass flow to
measured uptake of K and
Mg to spring wheat and sugar
beet (b). Data from a Barber
(1995)andb Strebel and
Duynisveld (1989)
Table 1 Effect of increa sing Ca concentrations in nutrient
solution on uptake rates of Mg and K in rice (adapted from
Fageria 1973, 2009)
Ca concentration (μM) Mg uptake rate
[μg (g root dry weight * h)
−1
]
6.23 3.80
12.47 5.06
49.90 25.70
74.79 52.59
124.75 57.41
249.50 62.49
499.00 46.46
748.00 30.76
R
2
0.66*
Plant Soil (2013) 368:5–21 9

coordinated interplay: passive transport, secondary ac-
tive transport and primary active transport (Marschner
2012). For passive and secondary active transport of
ions across plasma membranes the work of H
+
-
ATPases is vital. These proteins form electrochemical
gradients across the plasma membrane providing en-
ergy for cation transport. Passive transporters use the
electrical force, whereas secondary active transporters
or so-called ‘coupled transporters’ use the energy of
the electrochemical gradient in order to co-transport
(either in the same (symport) or the opposite (antiport)
direction) a H
+
ion and the nutrient ion. In contrast to
the first two transport systems the primary active
transporters directly use energy from energy rich com-
pounds like ATP and couple this to the uptake of
nutrients. The electrochemical gradient has no impact
on this transport system. For the HATS mecha nism of
K the secondary active transport (symport) was sug-
gested, whereas for the LATS mechanism the passive
transport via channels wasproposed(Brittoand
Kronzucker 2008). In principle the concept of HATS
and LATS including the respective transport mech-
anisms was described for K but was also suggested
to be present in plants for Mg (Shabala and Hariadi
2005).
As described there are numerous reports showing
that cations can re duce Mg uptake, wher eas to our
knowledge there are very few studies available showing
that Mg hampers the uptake of K or other cations to the
same level. For example Scharrer and Jung (1955)
showed that Mg fertilization of perennial ryegrass re-
duced the Ca and Na contents of the plants but did not
affect the K uptake. But why does K affect Mg uptake
and not or not so drastically vice versa? This phenom-
enon is not yet fully understood. However, from the
available information it may be concluded that the ob-
served competition for uptake is a consequence of the
lack of specificity of the individual uptake systems for
each cation (Gardner 2003; Shaul 2002; Deng et al.
2006; Marschner 2012). Some Mg transporters may also
transport other cations like K. As a consequence a high
plant available K concentration in the soil/rhizosphere
blocks these unspecific Mg transporters for Mg uptake.
On the other hand, high Mg concentrations in the soil
solution do not inhibit K uptake as K uptake even at low
external concentrations in the soil solution is facilitated
by very specific K transporters of the HATS which are
not blocked by Mg. For the sake of completeness it has
to be mentioned that the antagonistic effect can occur
not only in the roots, where the nutrients are taken up but
also within the plant. It has been frequently observed
that Mg concentrations in roots are not affected by K-
Mg antagonism, but the translocation from the root to
the shoot is impaired by high K. It can be suggested that
similar mechanisms operate here as well.
As proof of concept very recently Horie et al. (201 1)
could show for two members of the class II of HKT
transporters in rice, that they are principally (among K)
also capable of transporting Mg and Ca but that under K
supply the uptake of these cations is hampered giving a
first molecular hint for the mechanisms underlying cation
competition. In addition, the data provided by Horie et al.
(201 1) show that the considerable differences in K and
Mg chemistry, e.g. the differences in the radius of the
hydrated ions, may be not sufficient to reason specific
uptake systems (see discussion above). Also, Shabala and
Hariadi (2005) provided evidence that Mg uptake in leaf
mesophyll cells of broad bean is facilitated by two sys-
tems, a non-selective ion channel and at lower Mg con-
centrations by a H
+
/Mg
2+
exchanger. The non-selective
channel system was also capable of K and Ca transport.
Provided that such transport systems also appear in root
plasma membranes this is another hint that K and Ca can
reduce Mg uptake by these non-selective ion channels.
Future work is necessary to further clarify the underlying
mechanisms. The interested reader on the current knowl-
edge on molecular and physiological aspects of Mg
nutrition and homeostasis in plants is referred to other
reviews on this topic (Shaul 2002; Gardner 2003;White
and Broadley 2008; Chen and Ma 2012). Other important
areas of research for the future should also focus on the
question on how plants react to Mg deficiency. Are there
shoot/root-derived signals leading to physiological or
morphological changes with the aim to improve Mg
acquisition? Does the uptake kinetic for Mg change
under Mg deficiency in a similar way as described for
K? New insights in such processes could provide impor-
tant information which can be incorporated in sophisti-
cated breeding programs for Mg-efficient crops.
Methods of determining the plant available Mg in soils
In view of the various functions of Mg in plants, the
chemistry and mobility of Mg in soils and the mech-
anisms plants evolved to acquire Mg this section
describes how plant and soil Mg level s can be deter-
mined. The challenge is to relate the data obtained
10 Plant Soil (2013) 368:5–21

from soil analysis to the phytoavailabil ity and plant
growth (Kopittke and Menzies 2007). Particularly
questions related to determination of pla nt and soil
thresholds for Mg deficiency and how these can be
translated into appropriate agronomic strategies is dis-
cussed. A tool to diagnose the Mg status of a given
production site is the evaluation of the Mg status of the
soil and the crop at a pre-defined location and time
(soil and plant analysis). For correct interpretation of
the results obtained from such analyses calibration
experiments are required for each crop and soil deter-
mining critical individual plant and soil Mg levels
leading to deficiency. Also dose–response curves are
needed in order to find plant and soil Mg level s lead-
ing to a yield threshold ( concentr ation of a given
nutrient in the plant dry matter leading to maximum
yield). A good example for the combination of soil
and plant analysis and calibration experiments is sum-
marized in the review by Edmeades ( 2004). He
reviewed the research conducted in New Zealand in
the last 40 years on the development of diagnosis criteria
to predict and strategies to manage soil, plant (pasture),
and animal Mg nutrition. Soil, plant and animal Mg
analysis were taken into account as well as further
meta-data like rainfall. From this meta-analysis he con-
cluded that on the one hand there are considerable
amounts of Mg available in the topsoil but that these
are slowly declining as a consequence of reduced Mg-
fertilizer input. Moreover, he could show that extreme
deficiencies can be avoided in New Zealand by appli-
cation of about 25 kgMg ha
−1
year−
1
. A balanced state
of soil Mg was achieved by applying 5–20 kgMg ha
−1
year −
1
. Interestingly, the author went further and could
show that avoiding Mg deficiency in pasture was not
sufficient to reduce the risk of hypomagnesaemia devel-
opment in ruminants. The required higher concentra-
tions in the pasture could only be achieved by high
soil Mg levels or, when the Mg levels were low, large
inputs of Mg to the soil (> 100 kgMg ha
−1
year
−1
).
Soil analysis
Relating the Mg content of a soil to plant growth and
phytoavailability is a challenging topic (see previous
section). Several different soil analysis methods are used
to predict phytoavailability of nutrients making a direct
comparison of the results sometimes rather difficult.
However, some of the most important questions, which
need to be answered and standardized, are: What are the
decisive soil Mg fractions (water-soluble, exchangeable,
fixed) for plant Mg nutrition? How can they be deter-
mined (which extraction methods)? What is an appro-
priate sampling type to cover the plants’ ability to take
up Mg from the soil (topsoil and subsoil)?
Ortas et al. (1999) state, that there is often a poor
relationship between the plant growth response and
extractable nutrients in the soil. What is the reason
for this phenomenon? One reason is that such a soil
analysis just gives a picture of the current situation at a
given site, which is difficult to extrapolate to the
future. It is not capable of perfectly simulating the
above mentioned plant characteristics on Mg uptake
(see section ‘Mechanisms of Mg uptake by crops’).
Also, other agronomic and environmental (drought
stress etc.) factors are not included. Consequently,
and this does not only apply for Mg, soil analysis only
gives information on the potential of a soil to provide
the respective nutrient. An idea of the complexity of
soil analysis is given by a recent study of Staugaitis
and Rutkauskienė (2010) who compared six different
magnesium extraction and determination methods.
From the results shown in Table 2 it can be clearly
seen that the extraction procedure strongly impacts the
outcome. Nevertheless, all mild extraction procedures
including CaCl
2
, KCl, NH
4
OAc and the Mehlich 3
method showed quite high correlations over all three
sampling depths investigated indicating similar extrac-
tion characteristics for Mg from soils. In contrast
particularly the strong A-L method and the H
2
O extrac-
tion method showed only very little correlation with
almost all other extraction procedures over all sampling
depths. These results show the importance of chosing an
appropriate extraction method for correct evaluation of
the Mg availability to a crop.
Concerning the evaluation of the contribution of
different soil Mg pools to plan t Mg nutrition early
results indi cate that the water-soluble and exchange-
able soil Mg fractions do not always reflect the crops’
capacity to mine the soil. In fact, no close relationship
between the water-soluble and exchangeable soil Mg
status in the topsoil and the Mg concentration in the
plants was observed (Fig. 2, Grimme 1978, exchange-
able fraction extracted with CaCl
2
). From Fig. 2 it can
also be concluded that depending on the crop type the
subsoil considerably contributes to the Mg nutrition of
crops. In another early study Papenfuß and Schlichting
(1979) employed two different extraction metho ds
simulating the ext raction of the water-soluble and
Plant Soil (2013) 368:5–21 11

exchangeable pool on the one hand (CaCl
2
) and the
fixedpoolontheotherhand(HCl)andcorrelated
these findings with M g uptake by plants. From
Fig. 3 representing the data published by Pa penfuß
and Schlichting (1979) i t c an b e concluded, that
among the water-soluble and exchangeable soil Mg
pool also the fixed Mg pool of the soil can substan-
tially contribute to plant Mg nutrition. However, in
literature inconsistent results are reported on the
contribution of the fixed soil Mg pool to plant Mg
nutrition: in some intensive cropping systems a negli-
gible contribution of the fixed soil Mg pools to crop
nutrition has been found (Salmon and Arnold 1963;
Christenson and Doll 1973; Kidson et al. 1975),
whereas other authors report more variation in the
magnitude of Mg delivery depending on the soil type
(Schroeder et al. 1962; Rice and Kamprath 1968;
Lombin and Fayemi 1975). However, there are only
very few reports available on the potential of the fixed
Mg fraction to contribute to plant nutrition.
Another potential source of Mg in soils is the soil
organic matter. There is only rare information available
in literature on the contribution of soil organic matter to
the Mg level of soils and its contribution to plant mineral
nutrition (Mayland and Wilkinson 1989). With respect
to plant Mg nutrition soil organic matter is widely
treated as provider of variable charge in soils increasing
the CEC of soils. However, the organically bound Mg
can be an important source of Mg in some specific soils
(Mayland and Wilkinson 1989). Nevertheless, the lack
of knowledge on the contribution of Mg mineralization
to Mg nutrition of crops again points to the importance
of future research efforts to be put on this topic.
Plant analysis
Development of appropriate agronomic strategies to
prevent Mg deficiency requires careful diagnosis of
Mg deficiency in plants. Indeed, already the diagnosis
of Mg deficiency in crops represents a challenge on its
Table 2 Correlation coefficients of different Mg extraction pro-
cedures. Soil samples from 21 different Lithuana sites (differing in
topology, pH and texture) were sampled over three layers
(0–30 cm, 30–60 cm, 60–90 cm). Six different extraction
procedures were compared. D ata modified after Staugaitis
and Rutkauskien ė (2010)
Methods
a
A-L CaCl
2
KCl NH
4
OAc Me-3
0–30 cm layer
CaCl
2
0.204
KCl 0.217 0.982
NH
4
OAc 0.250 0.974 0.996
Me-3 0.302 0.970 0.991 0.995
H
2
O 0.248 0.533 0.544 0.570 0.551
30–60 cm layer
CaCl
2
0.418
KCl 0.394 0.972
NH
4
OAc 0.456 0.981 0.984
Me-3 0.520 0.967 0.975 0.993
H
2
O 0.026 0.298 0.270 0.263 0.247
60–90 cm layer
CaCl
2
0.546
KCl 0.509 0.956
NH
4
OAc 0.595 0.969 0.985
Me-3 0.626 0.956 0.979 0.997
H
2
O 0.087 0.395 0.327 0.331 0.284
a
Methods used were:
A-L – 1 M lactic acid, 3 M acetic acid, 1 M ammonium acetate,
pH 3.7, soil:solvent ratio 1:20, extraction duration 4 h
Me-3 – 0.2 M acetic acid, 0.015 M ammonium f luoride,
0.013 M nitric acid, 0.25 M ammonium nitrate, 0.001 M ethyl-
enediaminetetraacetic acid (EDTA), pH 2.5, soil:solvent ratio
1:10, extraction duration 5 min.
CaCl
2
– 0.0125 Calcium chloride, soil:solvent ration 1:20,
extraction duration 1 h
KCl – 1 M Potassium chloride, soil:solvent ratio 1:10, extraction
duration 1 h
NH
4
OAc – 1 M ammonium acetate, pH 7.0, soil:solvent ratio
1:10, extraction duration 1 h
H
2
O – water, soil:solvent ratio 1:5, extraction duration 1 h
Mg content in topsoil [mg Mg 100g
-1
soil]
051015
Mg concentration in the plant [% d.m.]
0
20
40
60
Wheat
Sugar beet
Fig. 2 Relationship between Mg content in the topsoil and Mg
concentration in wheat and sugar beet. (Graph redrawn after
Grimme 1978)
12 Plant Soil (2013) 368:5–21

own. For correct identification of Mg deficiency the
relationship between Mg concentration in the plant
matter and occurrence of deficiency symptoms needs
to be determined. The following questions have to be
addressed and standardized by the use of calibration
experiments: What is the deficiency threshold for a
crop(Mgconcentrationintheplantdrymatterat
which deficiency symptoms occur)? What on the other
hand is the yield threshold for a given crop (concentration
of Mg in the plant dry matter to obtain (economically)
maximum yield)? Which plant organ needs to be ana-
lyzed (old leaves, young leaves)? Are there specific
growth stages which need to be particularly addressed?
To answer these questions basic knowledge on the func-
tions of Mg in plants is necessary in order to correctly
sample the plants.
Magnesium deficiency symptoms are typically asso-
ciated with interveinal chlorosis particularly on older
leaves as Mg is readily phloem-mobile and, therefore,
retranslocated (White and Broadley 2008). Hence, defi-
ciency symptoms of Mg should appear on older leaves,
making these leaves the most important ones for deter-
mining deficiency thresholds. A standardized sampling
time is of particular importance due to the high variation
in the Mg concentrations during the growth period
(Čmelik 2000; Dow and Roberts 1981). However , chlo-
rosis is a quite late expression symptom of Mg deficien-
cy. It can be assumed that (particularly in phases of high
crop growth rates) latent Mg deficiency frequently
occurs. Latent deficiency , however, is already yield rele-
vant. As mentioned in the introduction part a typical early
response of plants to Mg deficiency is reduced root
growth (Cakmak et al. 1994a, b), even though this effect
obviously does not appear in all crops (for example sugar
beet, see Hermans et al. 2004; Hermans and Verbruggen
2005). Future work should clarify the reason for the
discrepancies in the sensitivity of root growth to Mg
deficiency observed in these studies. However, it can be
stated that root growth might be a more reliable indicator
for Mg deficiency than chlorosis. Reduced root growth is
most likely a consequence of impaired phloem transport
of carbohydrates (and other metabolites) from the shoot
to the root. Nevertheless, a cost ef fective and reliable
diagnosis of root growth under field conditions is almost
impossible to realize and methods are not available.
Another more promising and practicable link for recog-
nizing Mg deficiency at early stages could be the deter-
mination of the sugar concentration in leaves upon Mg
deficiency as this was shown to be an early deficiency
response and thought to be the ‘precursor’ of reduced
root growth (Cakmak et al. 1994a, b; Hermans et al.
2004; Hermans and Verbruggen 2005). It would be in-
teresting to correlate the Mg content with sugar accumu-
lation and root growth in order to develop a reliable field
test for latent Mg deficiency. Also it would be interesting
whether such a method would be more precise in dis-
playing latent Mg deficiency than measuring the Mg
concentration in the plant dry matter .
Strategies to bring Mg availability and demand
in accordance
In this section environmental, soil- and plant-related
factors leading to Mg deficiency are highlighted and
strategies to cope with these challenges, e.g. those based
on soil and plant analysis, are presented. Where appro-
priate, also the question is addressed whether new plant
physiological insights need to be applied to fertilization
recommendations.
Conditions of absolute deficiency and imbalanced
nutrient availability
There are at least two main sometimes interrelated rea-
sons for absolute Mg deficiency: (i) The Mg content of
the source material forming the soil is low (see section
‘Origin of magnesium in soils’). (ii) Long-term Mg
depletion of soils. The depletion is a consequence of
Mg leaching or soil mining due to imbalanced crop
Mg supply from different soil fractions [mg]
0 10203040506070
Mg uptake [mg]
0
10
20
30
40
50
60
70
CaCl
2
2 N HCl
Fig. 3 Relationship between Mg uptake by plants and Mg
supply from different soil fractions. (Graph redrawn after
Papenfuß and Schlichting 1979)
Plant Soil (2013) 368:5–21 13

offtake / resuppl y (fertilization) ratios (see section
‘Consequences of the mobility of Mg in soils’). In such
soils it is indispensable to supply the soils with Mg in
order to maintain or even improve soil fertility.
However, of course this not only applies to Mg but also
to other base cations. It is obvious that the fertilization
practice modulates the soil fertility, e.g. through direct
modification of the plant availability of the nutrients in
the soil. Thereby, fertilization not only changes the
availability of the nutrients supplied with fertilization
but also affects the availability of nutrients present in the
soil. With respect to Mg nutrition K and Ca modulate
plant Mg availability due to two reasons: (i) increased
Mg exchange from soil exchange sites and, therefore,
increased Mg concentrations in the soil solution with
increased risk of Mg losses by leaching and (ii) shifts in
the quantitative and qualitative cation composition in
the soil leading to cation competition at the plant root for
uptake (see section ‘Mechanisms of Mg uptake by
crops’). Table 3 shows how K fertilization increases
the concentration not only of K but also of Mg in the
soil solution (Seggewiss and Jungk 1988). These inter-
relationships particularly between the base cations high-
light the importance of a balanced fertilization.
Cation competition due to imbalanced nutrient avail-
ability to plants is due to imbalanced nutrient supply
either through agronomic practices (liming (see above),
fertilization strategy) or delivery from the soil. The
phenomenon of cation competition was already de-
scribed previously (see section ‘Mechanisms of Mg
uptake by crops’). Cation competition is a complex
problem and points to the need of (in certain limits)
balanced nutrient supply to crops for efficient crop
production. This is particularly important for soils in-
herently not capable of providing balanced nutrient
supply. Soils rich in chlorites are principally capable of
fixing Mg (see section ‘Origin of magnesium in soils’).
In such soils the risk of Mg deficiency is not only due to
the absolute deficiency but also due to enhanced risk of
induced Mg deficiency resulting from cation antago-
nism as a consequence of increased availability of other
cations like K and Ca. In such soils improvement and
maintenance of a balanced nutrient supply to crops is
extraordinary important. The fixing sites need to be
filled up with Mg through a fertilization strategy which
provides Mg in amounts much higher than extracted
with the crop. Few reports are available on Mg fixation
in soils, e.g. in response to fertilization practice. In their
report on the effect of liming on yield parameters
Sumner et al. (1978) suggested that often observed
negative yield responses after liming could be a conse-
quence of Mg fixation. The data set the authors provided
was not sufficient to completely clarify the mechanism
of Mg fixation after liming but provided some ideas: (i)
formation of Mg-silicates, (ii) immobilization of Mg in
the interlayer of aluminous chlorites, (iii) formation of
insoluble mixed hydroxides with active Al under alka-
line conditions, (iv) absorption of Mg on Al-hydroxides.
Mokwunye and Melsted (1973) evaluated the extent of
immobilization of Mg added to soils of temperate and
tropic origin and concluded that it was not possible to
improve the ‘long-term Mg-supplying power of soils’
which indicates ‘the importance of repeated application
of Mg fertilizers, especially in highly weathered soils’
(Mokwunye and Melsted 1973).
Magnesium deficiencies related to adverse growth
conditions
Light, drought, and heat stress
High light stress, often associated with drought and heat
stress, represents a major challenge in crop production
(Mittler 2006). According to the forecasts the frequency
of extreme weather conditions will increase due to pro-
cesses involved in climatic change (Easterling et al.
2000). The roles of Mg in plant metabolism particularly
under stress conditions are well known (Cakmak and
Kirkby 2008). Magnesium is involved in carbohydrate
formation and translocation. However, under Mg defi-
cient conditions these processes of the primary metabo-
lism are severely disturbed. Cakmak and Kirkby (2008)
concluded from experiments investigating the effect of
light treatments on Mg-deficient leaves that the Mg
Table 3 Effect of an increased K fertilization on the concentration
of exchangeable K in the soil and the concentrations of K and Mg
in soil solution (adapted from Seggewiss and Jungk 1988)
K fertilization Exchangeable K Concentration
in soil solution
(μmolK 100 g
−1
soil) (μmolKl
−1
)(μmolMgl
−1
)
0 153 58 1180
200 220 108 1620
400 280 142 2000
800 390 278 2630
1600 760 1600 3400
14 Plant Soil (2013) 368:5–21

requirement is increased under high-light conditions.
The higher requirement of Mg under high light might
be reduced to the fact that under suboptimal Mg supply
and high light processes are induced which finally lead
to accumulation of reactive oxygen species (ROS) and
thus plant damage. Much higher activities of antioxida-
tive enzymes such as superoxide dismutase and ascor-
bate peroxidase in Mg -deficient leaves compared to
Mg-adequate leaves indicate that Mg deficiency stress,
indeed, induce generation of reactive oxygen species as
a consequence of impairments in photosynthetic elec-
tron transport and utilization of p hotoassimilates
(Cakmak and Marschner 1992). Hence, the described
higher Mg demand is simply due to the essential roles of
Mg in primary metabolism, which cannot be optimally
fulfilled under Mg deficient conditions. Consequently,
in plants well supplied with Mg differences in light
stress susceptibility are not observed, so that under
optimal Mg supply no further demand for Mg can be
suggested. However, as initially mentioned high light,
drought, and heat stress typically occur at the same time.
Therefore, in agricultural practice there is an increased
risk of temporarily occurring Mg deficiency due to
reduced delivery of Mg by mass flow. With respect to
the crop Mg demand plant biomass production typically
follows a sigmoidal curve with low biomass accumula-
tion after emergence, followed by an exponential vege-
tative growth and finally a plateau during generative
growth and yield formation. Particularly in periods
where an increased risk of low Mg availability (e.g.
drought) is synchronized with high crop growth rates
and therefore high Mg demand, there is an increased risk
that the soil Mg pool and soil applicated sparingly
water-soluble Mg fertilizers/limes cannot meet the actu-
al crop demand for Mg. This is particularly true for
important crops exhibiting extraordinary high growth
rates and, therefore, nutrient demands (C4 plants like
maize and sugarcane). Here leaf and/or soil application
of readily available Mg sources can be advantageous
(Römheld and Kirkby 2007). This view was underlined
by Härdter et al. (2004), who investigated the effect of
different Mg fertilizer sources on Mg availability in
soils. The authors recommended that for maximization
of crop uptake while minimizing Mg losses Mg sources
exhibiting a gradual but strong release matching the
plants requirements should be applied. The source of
Mg applied to meet actual high demands should among
water-solubility also consider the accompanying ion,
which is introduced into the system and, particularly
for leaf fertilization, the capacity of a specific Mg salt
to induce leaf burning symptoms. Figure 4 shows the
solubility of some different Mg salts. Römheld and
Kirkby (2007) state that foliar Mg fertilization would
be effective particularly in combination with N quality
fertilization during reproductive stages as the assimilate
transport into harvest organs (tubers, grains, fruits etc.)
is enhanced and thereby the competition between the
root as second strong sink is reduced. However, in terms
of yield security maintained root growth further offers
the advantage of continuous water and nutrient supply
under drought stress.
Acid soils—low pH and metal toxicities
Magnesium deficiency is problem inherent in acid soils
due to the high saturation of the soil CEC with H
+
ions
and consequent Mg leaching for long time periods and
impaired Mg uptake (see sections above). Indeed, al-
ready 60 to 70 years ago first studies reported that a low
pH of soils exhibit higher leaching rates for Mg
(Schachtschabel 1954, and literature cited therein).
Vo n U ex ku ll a nd M ut er t (1995) estimated that about
30 % of the total ice-free land area in the world is acidic
(defined as pH<5.5 in the top soil layers) representing
about 4 billion ha. In view of arable land expansion
Haug (1983) even estimated that about 70 % of the
potentially arable land is acidic. Soil acidity is not only
a soil immanent problem but is additionally further
enhanced by agricultural fertilization practice. Indeed,
Fig. 4 Solub ility of different Mg fertilizer salts in water at
20 °C. Black bars show the solubility of the Mg salts and white
bars show the corresponding amounts of solubilized Mg
Plant Soil (2013) 368:5–21 15

Guo et al. (2010) could measure a significant effect of
excessive N and base cation fertilization on soil acidifi-
cation in China by comparing soil pH and fertilization
practice of different Chinese soils in the 1980s and
2000s. Among the quantitative cation composition of
soils the soil acidity also impacts Mg uptake by plants as
low pH impairs the uptake of base cations. The pH
dependency of the plant availability of three base cations
is schematically shown in Fig. 5 for a pH range realistic
for arable land. The decrease of plant availability partic-
ularly of the two base cations Ca and Mg at acidic pH
(< pH 6) is a consequence of the increasing inability to
build up and maintain a sufficient pH and hence elec-
trochemical gradient across the plasma membrane of
root cells (Schubert et al. 1990). Among an impaired
availability and uptake of base cations like Ca and K soil
acidity leads to several additional adverse effects. At soil
pH<4–4.5 the root growth is directly inhibited by H
+
toxicity with severe consequences for crop production
(Islam et al. 1980;Koyamaetal.1995;Rangeletal.
2005). However, acid soils bear also the risk of element
toxicities, particularly Mn and Al. Manganese is an
essential plant micronutrient, whereas Al is not required
by plants, even though some plant species accumulate
Al in their tissue (Klug and Horst 2010;Maetal.2001
[Fagopyrum esculentum], Matsumoto et al. 1976
[Camellia sinensis], Ma et al. 1997 [Hydrangea macro-
phylla]) This section is included due the substantial
progress made in the last years in clarifying the role of
Mg in metal toxicity stress alleviation.
Starting with Mn there is a considerable inter- and
intraspecific (between plant species and between culti-
vars within a plant species) variability in tolerance to
excess Mn (Foy et al. 1978). However, several studies
report a beneficial effect of Mg supply on Mn tolerance
of crops (Le Bot et al. 1990;GossandCarvalho1992).
For wheat Goss and Carvalho (1992)reportedthatMg
“increased the tolerance of plants to high concentrations
of manganese in shoot tissue and also increased the
ability of the plant to discriminate against manganese
ions in translocation of nutrients from roots to shoots.”
Le Bot et al. (1990) found that increasing the Mg/Mn
ratio in the plant tissue alleviates Mn toxicity in tomato
and wheat. It is noteworthy here that obviously Mg
reduced Mn toxicity not only by reducing Mn uptake
(cation antagonism) but also by increasing the plant
tissue tolerance. Malcová et al. (2002) found that the
toxic effect of excess Mn can be alleviated by adding
Mg. This was not only true for in-vitro experiments but
also for the symbiotic association between the arbuscu-
lar mycorrhizal fungi and maize as host plant. To our
knowledge the mechanisms underlying the increasing
effect of Mg on Mn tissue tolerance are not yet
understood.
The alleviative effect of Mg on Al toxicity is recog-
nized for a long time in several plant species (Edmeades
et al. 1991 [Triticum aestivum], Keltjens and Tan 1993
[Helinathus annuus, Glycine max, Vigna unguiculata,
Arachis hypogaea, Cucumis sativus, Lycopersicum
esculentum, Brassica capitata ssp., Oryza sativa, Zea
mays, Secale cereale, Sorghum bicolor, Triticum aesti-
vum, Hor deum vulgar e, Avena sativa]; Tan and Keltjens
1995
[Sorghum bicolor]; Silva et al. 2001 [Glycine
max]; Hecht-Buchholz and Schuster 1987 [Hordeum
vulgare]), but up to date the exact mechanisms underly-
ing this phenomenon are not yet fully understood as
well. Bose et al. (2011) summarized the potential posi-
tive effects of Mg in the plants’ physiology to increase
the resistance/tolerance of crop plants to toxic Al
3+
.It
appears that there is not a single reason responsible for
the alleviative effect of Mg on Al toxicity expression but
a coordinated interplay between several factors includ-
ing better carbohydrate partitioning and organic acid
synthesis and secretion (see also Ma et al. 2001),
Fig. 5 Scheme of the
plant availability of mineral
nutrients in dependence
of the soil pH
16 Plant Soil (2013) 368:5–21

enhanced phosphatase activity, better H
+
-ATPase activ-
ity and cytoplasmic pH regulation, protection from Al-
induced increase in cytosolic Ca concentrations and
reactive oxygen species. Interestingly, Deng et al.
(2006) provided evidence that overexpression of a mag-
nesium transport gene from Arabidopsis thaliana
(AtMGT1) conferred tolerance to Al in Nicotiana ben-
thamiana indicating that increased internal Mg concen-
trations are necessary for this effect. The association of
increased Mg uptake and Al tolerance was also very
recently verified for rice, an extraordinary Al tolerant
plant species (Chen et al. 2012;ChenandMa2012).
The expression of a rice Mg transporter (OsMGT1)was
regulated by Al availability, and knockout of this gene
enhanced Al sensitivity of rice. The upregulation of a
Mg transporter under Al stress was accompanied by
increased Mg uptake by the rice plant through enhanced
V
max
of the protein. Whether the increased tolerance of
rice due to increased Mg uptake is a consequence of a
specific Mg function or of the more efficient metabolic
regulation of Mg-associated pathways in the plant as
described by Bose et al. (2011), remains to be elucidat-
ed. Future work should clarify the exact underlying
mechanisms as well. Moreover, it would be interesting
to investigate whether Al stress induces a higher Mg
demand for optimal plant growth also in the field, and
when yes, to which extent (Is an adaption of the recom-
mendation for Mg supply necessary?).
Even though Mg is obviously capable of alleviating
the adverse effects occurring on acid soils, on the
long-term the most important agronom ic tool to cope
with soil acidity is liming. Among reducing soil acid-
ity and associated adverse growth conditions liming is
supposed to also improve and sustain soil structure
and fertility (Haynes and Naidu 1998). Commonly
used lime types are dolomitic lime (CaMg(CO
3
)
2
),
calcitic lime (CaCO
3
) and dolomitic lime (MgCO
3
)
(Fageria 2009). However, depending on the lime type
considerable amounts of Mg and Ca reach the soil
which may then interfere with the uptake of other
cations through antagonistic effects (see also section
‘Mechanisms of Mg uptake by crops’). Stevens et al.
(2005) could not find negative impacts of short- and
long-term liming with calcite or dolomite on cotton
growth in field experiments with well drained soils,
whereas Sumner et al. (1978) reported from their
comprehensive data ev aluation of soils from North
America and Africa consistent reductions in Mg up-
take by plants when soils were limed to neutrality
which could be reduced to pure Ca containing lime
application (see also previous section dealing with Mg
fixation). These two contrasting studies may indicate
the complexity of this topic.
Use of appropriate nitrogen form
For wheat it has been show n that the N form supplied
impacts not only the Mg uptake but also the translo-
cation from the root to the shoot. Increasing the NO
3
−
supply increased the uptake of Mg but decreased the
translocation (Huang and Grunes 1992). Particularly
under low Mg conditions there was a higher risk of
Mg deficiency symptom development. In contrast, in
tomato it was shown that increasing the NO
3
−
nutri-
tion increased the cation concentration in the shoots
(Kirkby and Knight 1977). It was suggested that cat-
ions act as counter-ions for NO
3
−
in root to shoot
translocation (for more information on the effect of
mineral nutrition on partitioning of mi nerals and
metabolites see Marschner et al. 1996). These con-
trasting results show that future research is necessary
to understand these and additional differences in nu-
tritional physiology in order to improve crop-specific
fertilization recommendations.
Use of Mg efficient cultivars
A long standing research area in crop sciences and
particularly in the field of plant nutrition is the investi-
gation of nutrient use efficiency (NUE). Several meas-
ures of NUE were developed over the last decades (see
for example Gourley et al. 1994). However, NUE is
typically regarded as a crops capacity to produce high
yield under limited nutrient supply. Hence, choice of Mg
efficient crops/crop cultivars could be another important
agronomic factor to deal with low plant Mg availability.
This appears to be particularly true for low input agri-
cultural systems. However, so far breeding programs
more or less neglected the plant nutrient Mg, so that
there is no or only rarely respective germplasm available
(Clark 1983; Beets et al. 2004 [Pinus radiata]). Future
research programs should, therefore, also include breed-
ing programs producing Mg efficient crops. Alternatively
growers can draw back on existing crops/cultivars with
low demand for Mg. However, this mostly excludes most
important crops typically grown in subsistence farming
like maize and pulses, further narrowing food availability
and diversity.
Plant Soil (2013) 368:5–21 17

Soil organic matter and organic fert ilization as sources
of Mg and modulators of Mg availability
As mentioned above the contribution of soil organic
matter (SOM) on plant Mg nutrition is mainly reduced
to provision of charge to the CEC, whereas the Mg
content of SOM is thought to be rather limited
(Mayland and Wilkinson 1989). Unfortunately there
is only very little information available on the contri-
bution of SOM to plant Mg nutrition. With respect to
organic fertilization the situation is different. Among
modification of the CEC of soils organic fertilization
can also have an effect on crop nutrition with respect
to Mg supply. As the mean Mg concentrations of
different organic material sour ce s us ed for organic
fertilization vary consi derably the applied amounts of
Mg and also other relevant cations need to be taken
into account for sustaina ble agricultural practice.
Eghball et al. (2002) stated that more than 55 % of
the organically applied Mg is plant available. Allison
et al. (1997) reported that in sugar beet grown on soils
where straw was incorporated the uptake of Mg but
also of phosphorus (P) and Ca was not significantly
affected, whereas the K uptake was increased by about
40 kgha
−1
. However, it has been shown, that organic
fertilization increases the Mg content of soils and pre-
vents higher amounts of Mg to be leached into soil layers
not or sparingly reached by plant roots (Piechota et al.
2000;Grzebisz201 1).
Conclusion
The mobility of Mg in soils and plants is principally
understood and transferred to soil and plant analysis
methods aiming at developing appropriate recommenda-
tions for crop nutrition. However, inherent to soil analy-
sis methods is that they can only predict the potential of a
soil to deliver Mg to the plant roots. Accordingly plant
analysis methods reflect only the current nutritional sta-
tus of plants but are not capable of predicting the Mg
availability. Combination of soil and plant analysis and
the use of calibration experiments on production sites
admittedly allow developing recommendations for Mg
nutrition of crops to a certain degree. However , the often
observed contrasting results on the effect of Mg fertiliza-
tion on crop yield and other parameters show that other,
additional factors are involved in crop Mg nutrition,
which are not covered by the current analysis methods.
Indeed it is not new, that the actual Mg availability over a
growing season heavily depends on (i) various environ-
mental factors (rainfall and timing etc.), (ii) site-specific
conditions (soil type, availability of other nutrients etc.),
and (iii) the crop species making a precise prediction
almost impossible. Environmental factors can hardly be
influenced and comple tely forecasted, whereas site-
specific soil conditions can be determined by soil and
to a certain degree by plant analysis. Consequently, an
improvement of the development of recommendations
on Mg nutrition is most promising when the crop species
is regarded in more detail. It is, therefore, concluded that
the newest findings on the physiology of Mg uptake by
plants (e.g. antagonistic effects as affected by transport
systems) and the role of Mg in stress physiology
(drought, heat, high radiation, low pH, metal toxicity)
should be increasingly recognized and incorporated into
site-specific recommendations.
Acknowledgments The authors would like to thank the
reviewers for their helpful suggestions.
Open Access This article is distributed under the terms of the
Creative Commons Attribution License which permits any use,
distribution, and reproduction i n any medium, provided the
original author(s) and the source are credited.
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