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| Magnesium cycle |
Magnesium (Mg) is an essential plant nutrient that is too
frequently overlooked. Although weathering of primary and secondary minerals
may provide adequate Mg in some soils, there are some soils that benefit from
Mg additions. There are various soluble and slowly soluble Mg sources available
to meet crop demands.
Magnesium is a common
constituent in many minerals, comprising 2% of the Earth’s crust. It is also a common component in
seawater (1,300 ppm). Magnesium is present in the divalent Mg2+ form in nature,
but can be processed into a pure metal. Since metal Mg is one-third lighter
than aluminum (Al), it is commonly used in lightweight alloys for aircraft and
automobiles. In the powder or ribbon form, metallic Mg burns when exposed to
air. China is the largest producer of Mg metal, although the USA and the former
Soviet Union (FSU) also produce significant amounts.
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| Magnesium deficiency symptom |
The
importance of Mg for human and plant nutrition has been well established. This
article will review the behavior of Mg in rocks and soils, and describe some of
the common Mg sources used for plant nutrition.
Magnesium
in Primary and Secondary Minerals
Several
ferromagnesian minerals (such as olivine, pyroxene, amphibole, and mica) are
major Mg sources in basic igneous rocks. Secondary minerals, including
carbonates... for example, dolomite [MgCO3 .CaCO3], magnesite [MgCO3], talc [Mg3Si4O10(OH)2],
and the serpentine group [Mg3Si2O5(OH)4] ...are derived from these primary
minerals.
When
serpentine is present in large amounts, it gives rise to the term “serpentine
soil.” In these ultramafic serpentine soils, high Mg concentrations lead to
poor plant growth and poor soil physical conditions. Undesirably high
concentrations of nickel may also occur in these soils.
These
primary and secondary minerals are important sources of Mg for plant nutrition,
especially in unfertilized soil. But plant-available Mg concentrations cannot
be accurately predicted based only on the parent material composition due to
differences in mineral weathering rates and leaching. In some cases, the
contribution of minerals to meeting the entire crop demand for Mg during a
single growing season is insufficient to prevent plant and animal deficiencies.
Non-Exchangeable
and Exchangeable Magnesium
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| Magnesium as part of soil exchange capacity |
Magnesium
is located both in clay minerals and associated with cation exchange sites on
clay surfaces. Clays such as chlorite, vermiculite, and montmorillonite have
undergone intermediate weathering and still contain some Mg as part of their
internal crystal structure. The Mg release rate from these clays is generally
slow. Illite clays may also contain Mg, but their release rate is even slower.
The details of clay weathering and mineralogy are available elsewhere.
The
gradual release of non-exchangeable Mg has been demonstrated in a variety of
conditions, but the amount of Mg dissolved from these minerals is often small
compared with the amounts required to sustain high crop yields for multiple
years. This non-exchangeable Mg may be coming from the octahedral clay layers
as well as the interlayer material. In low-productivity agriculture, this slow
release of Mg may be sufficient to replenish the soil solution and meet plant
nutritional demands.
In
alkaline to slightly acidic soils, Mg is usually second in abundance to Ca on
cation exchange sites. Magnesium ions generally resemble Ca in their behavior
in ion exchange reactions. These cation exchange reactions are generally
reversible, where even strongly adsorbed cations can be typically replaced by
manipulation of the soil solution.
To
become soluble, Mg adsorbed on a clay particle must be replaced by a cation
present in the soil solution. Cation exchange reactions are stoichiometric,
meaning that the charge balance must be maintained. For example, two K+ ions
are required to replace a single Mg2+ ion. The exchange reactions are very
rapid, but the limiting step is usually the diffusion of the cation to or from
the colloid exchange site.
Certain
clays, such as vermiculite, have a special affinity for soluble Mg. The
hydrated Mg ion fits well between the partially expanded sheets of vermiculite,
making this clay an excellent Mg scavenger.
An
excessively large proportion of Mg on the cation exchange sites can lead to
degradation of the soil physical condition. Since Mg cations have a larger
hydrated radius than Ca, the attractive forces that tend to aggregate soil
colloids in typical conditions are diminished with an over-abundance of Mg. A high proportion of Mg on soil exchange
sites results in dispersion of clay particles, leading to decreased porosity
and reduced infiltration rates typically found in serpentine soils.
Pathways
of Magnesium Loss
When
removal of Mg from the soil is greater than the release rate of Mg from mineral
sources and fertilizer additions, Mg concentrations in solution and on the
exchange sites will decline. This low-Mg situation is most frequently observed
on sandy soil with low exchangeable Mg, soils receiving repeated applications
of calcitic limestone, and due to a competition with other cations, such as K.
Long-term sustainability requires balancing the Mg supply with removal from
crop harvest, leaching, and runoff.
Crop
removal
A
wide range of Mg crop removal data exists in published literature, depending on
the soil Mg supply, growing conditions, the specific plant species, and yield levels.
For example, a high-yielding crop of sugarbeets may take up as much as 80 lb
Mg/A, and high-yielding forages and corn silage may remove 50 lb Mg/A. In
general, cereal crops remove smaller amounts of Mg at harvest compared with
root crops and many fruit crops. Of all the pathways of loss, removal of
abundant crops at harvest is the desired outcome.
Leaching
Losses
Loss
of soil cations through leaching can result in significant decline in nutrient
availability over time. The extent of Mg loss from the rootzone to lower soil horizons
will vary greatly depending on the soil properties, the amount of water passing
through the soil, and local conditions. In some circumstances, leaching losses
as low as a few pounds of Mg/A/yr are reported. However in other conditions,
losses exceeding 100 lb Mg/A/yr are not unusual.
Fertilization
with other cations, such as K+ and Ca2+, frequently leads to enhanced Mg
solubility in the soil as they exchange on the clay sites and ultimately make
Mg more susceptible to leaching. Decreases in exchangeable Mg are often
correlated with the amount of salts added as fertilizer
or
soil amendments. In soils where Ca and Mg are leached following repeated K
fertilization, an undesirable enrichment of K on the cation exchange sites can
result. Leaching losses
of
nitrate accelerate Mg loss, especially under urine and dung spots in pasture.
Erosional
Loss
The soil
surface is the zone that generally contains the most organic matter and
essential plant nutrients. Runoff water leaving the field may carry with it
valuable organic matter and nutrients associated with the eroding clay. Minimizing
water runoff from fields by use of conservation techniques such as vegetative
buffers or irrigation tailwater return will help reduce losses of Mg, as well
as protect adjacent surface water.
Interactions
Magnesium
deficiencies are not uncommon in low pH, sandy soils where Al dominates the
soil cation exchange sites. Magnesium assimilation is also depressed in the presence
of Al3+, which has a detrimental effect on root growth as well as through a
competitive cation effect for root uptake.
High
exchangeable K concentrations can have an adverse effect on Mg availability for
plants. The competition between these two cations for root uptake appears to be
the primary cause, although high K may also impair Mg translocation within the
plant. Low forage Mg concentrations following K fertilization have been linked
with low Mg in the blood of grazing animals (called grass tetany) where it is
essential for certain enzyme and metabolic reactions.
Magnesium
Sources
There
are many excellent sources of Mg that can meet crop demands. Surface placement
of the soluble Mg sources is usually satisfactory, but incorporation of the
less-soluble Mg materials into the soil is recommended. Since there are no
serious environmental issues associated with agricultural uses of Mg, no
special precautions are needed. Contributions of Mg in rainfall are generally
less than one lb/A/yr.
Common
Mg fertilizers are typically divided into two classes: soluble sources and
semi-soluble sources. The particle size of semi-soluble Mg sources in large
part determines the rate of dissolution, while this factor is not significant
for the soluble sources.
Soluble
Mg Sources (with approximate solubility at 25°C)
Kieserite
– MgSO4 .H2O;
17% Mg – Kieserite is the monohydrate of magnesium sulfate, produced primarily
from mines located in Germany. As a carrier of both Mg and S, kieserite finds
multiple applications in agriculture and industry (360 g/L)
Kainite
– MgSO4 .KCl.3H2O;
9% Mg – Kainite is the mixed salt of magnesium sulfate and potassium chloride.
It is most commonly used as a K source, but is useful where both
Mg
and K are required (variable solubility).
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| Langbeinite (mixture of potassium, magnesium, and sulfur) |
Langbeinite
– 2MgSO4 .K2SO4;
11% Mg – A widely used source of Mg, as well as K and S, this mineral is an
excellent multi-nutrient source. While totally soluble, langbeinite is slower
to dissolve than some Mg sources and not typically delivered through irrigation
systems (240 g/L).
Magnesium
Chloride – MgCl2;
25% Mg – Generally sold as a liquid due to its high solubility, this material
is frequently used as a component in fluid fertilizers (560 g/L).
Magnesium
Nitrate – Mg(NO3)2
.6H2O; 9% Mg – Widely used in the horticultural industry to supply Mg in a form
that also provides a soluble N source (1,250 g/L).
Magnesium
Sulfate (Epsom salt) – MgSO4 .7H2O, 9% Mg – Epsom salt derives its name from naturally
occurring geologic deposits in Epsom, England. It is a common mineral and a
byproduct from various brines that makes an excellent Mg source. It is similar
to Kieserite, except it contains seven water molecules associated with the MgSO4
(357 g/L).
Schoenite
– K2SO4 ·MgSO4 ·6H2O;
6% Mg – Although more commonly used as a K source, it is also a useful soluble
Mg fertilizer material (330 g/L).
Animal
Wastes and Composts The concentration of Mg in these organic materials is low compared
with mineral sources. However, high application rates can supply significant quantities
of Mg to the soil. Magnesium in these materials is generally considered to be
totally plant available within a growing season.
Foliar
Sprays These may
contain one or more of the soluble Mg materials discussed above. Specialty
materials containing EDTA, lignosulfonate, and other complexing agents may be
used with soluble Mg sources to improve foliar uptake. Leaf sprays are
effective at correcting Mg deficiency, but they generally must be repeated to
maintain maximum plant growth and are usually considered a temporary resolution
before the soil can be modified.
Semi-Soluble
Mg Sources
Dolomite
– MgCO3 .CaCO3;
6 to 20% Mg – Depending on the geologic source, the concentration of Mg will
vary considerably. Pure dolomite contains 40 to 45% MgCO3 and 54 to 58% CaCO3.
However a concentration of 15 to 20% MgCO3 (4 to 6% Mg) is common for material
called “dolomitic limestone”. Dolomite is often the least expensive common
source of Mg, but may be slow to dissolve, especially where soil acidity is
lacking.
Hydrated
dolomite –
MgO.CaO/MgO.Ca(OH)2;18 to 20% Mg – This product is made by heating dolomitic
lime (calcined) to form MgO and CaO. It is then hydrated to form dolomitic hydrated
lime, which may contain only hydrated calcium oxide or it may also contain hydrated
magnesium oxide. These compounds dissolve faster than untreated dolomite.
Magnesium
oxide – MgO; 56%
Mg – Composed of only magnesium and oxygen, it is formed by heating MgCO3 to drive
off carbon dioxide. It contains the highest concentration of Mg of common
fertilizers, but is rather insoluble. Applying in advance of plant demand and
using a fine particle size will help make this nutrient source useful for plant
growth.
Struvite
– MgNH4PO4 ·6H2O;
10% Mg – Struvite is produced primarily during the recovery of P in wastewater
from animal manure and municipal treatment plants. While slow to dissolve,
struvite also provides a valuable supply of N and P, nutrients not found in
other Mg-containing fertilizers.
Crop
fertilization practices continue to intensify with the demand for high yields.
Magnesium is an essential plant nutrient that is frequently overlooked and may
be limiting plant growth. Soil testing should be used to identify potential
deficiencies, and there are many excellent Mg sources available for farmers
when needed.
This article is available as a pdf from Better Crops here BC
