Download Revisiting the role of Calcium and Gypsum for Sodic

PRODUCT INFORMATION BULLETIN
Revisiting the Role of Calcium and Gypsum for Sodic Soils
There is an abundance of literature citing soluble Calcium and/or gypsum as recommended treatments for sodic soils. The
general approach offered is to provide the soil solution with sufficient amounts of multivalent Calcium (Ca++) ions to reduce
the Exchangeable Sodium Percentage (ESP) via replacement of monovalent Sodium ions at soil exchange sites – increasing
the potential for sodium leaching from the soil profile..
However, restoring soil quality and improving soil stability of sodic soils is much more complex. Multi-component
biogeochemical systems (pH, cation exchange capacity, mineral and organic composition), their reactions and interactions,
need to be considered. All too often, the existence of these complex chemical reactions is ignored favoring more simplified
solutions that rely on electrostatic exchange reactions involving calcium- or gypsum-based products.
As a result, there is a lot of confusion and many misconceptions as to the benefits of using just soluble calcium and/or gypsum
products to treat sodic soils. This product bulletin revisits calcium’s role in the reclamation of sodic soils and introduces silicate
chemistries for improved sodic soil reclamation.
Surface Chemistry of Soils and Organic Matter
Adsorption of Cations and Anions
Clay colloidal particles and soil organic matter (SOM) form
the structural basis of most chemical, physical and biological
properties of soils.
Adsorption is a general term referring to the accumulation of
a substance between a solid surface and the solution. Forces
involved in adsorption include:
The surfaces of clay colloids and organic matter
characteristically carry negative and/or positive charges called
surface functional groups (SFGs). These surface charges can be
constant or variable.
Physical forces: Forces dependent on distance and valence.
• Van der Waals (weak electrostatic forces between
nonpolar molecules)
• Electrostatic complexes (ion exchange)
Constant surface charges are found on soil particles and
are the result of isomorphic substitution of one element for
another in ionic crystals. These are structural in nature and are
not affected by pH.
Chemical forces: Forces dealing with electron reconfiguration
(breaking/making bonds)
Variable surface charges are found on soil particle surfaces
and organic matter. They are termed “pH -dependent charges”
because these charges depend on the pH of the soil solution.
These charges are primarily associated with hydroxyl (–OH)
functional groups.
Outer-sphere complexes are created when at least one water
molecule is present between the ion or molecule and the SFG.
These complexes have the following characteristics:
As the pH rises, Hydrogen (H+) ions are removed from surface
OH groups on the colloids/SOM, leaving behind a negatively
charged (–O-) exchange site. These negative sites attract cations.
In highly acid soils surface groups may have a positive
charge that results from the attachment of H+ ions to surface
–OH functional groups. The resulting positively charged
functional groups (–OH2+) attract anions.
• Ligand exchange, covalent/ionic bonding (chemisorption
or specific sorption)
• Form weak bonds
• Electrostatic interaction – the surface must be charged
• Are reversible (exchangeable)
• Are affected by the ionic strength of the solution
CrossOver – from soil to plant
It is electrostatically held ions that make up the cation
exchange or anion exchange capacity. Electrostatically bound
ions can be displaced by other ions or displaced simply due
to a diffusion gradient. Exchangeable ions are essential for
maintaining plant nutrient levels. Calcium and sodium are
examples of outer-sphere complexes.
H
O
H
H
O
H
H
H
H
H
O
+1
Na
O
O
H
H
H
O
H
H
+2
Ca
O
H
H
O
H
O
H
O
H
H
O
O
H
H
O
H
O
O
H
O
H
Na
O
O
H
O
H
H
O
• Moisture-holding capacity is reduced
H
H
• Reduction in Plasticity (soils that can be transformed from
a solid to a putty-like or fluid-like state by adding water)
H
O
H
O
H
O
H
H
O
H
H
H
Inner Sphere
H
O
H
H
H
O
+2
H
H
Outer Sphere
+1
H
O
H
H
H
H
H
H
• Swell reduction
H
Ca
O
H
H
H
O
Soil Modification. It has long been established that the use
of calcium or gypsum will produce short-term modification
of soils. Reactions that result from calcium replacing sodium
in the outer-sphere complex of clay soils will cause improved
flocculation and agglomeration of dispersed clay particles.
Clay surface mineralogy is altered, with the following effects:
H
O
H
O
H
H
H
O
O
H
O
H
H
H
H
H
H
H
H
+1
Na
O
H
H
H
O
O
H
O
O
H
H
H
H
O
H
H
H
O
H
H
H
H
H
O
H
O
H
H
Calcium Short-Term Soil Modification Solutions
H
O
H
H
+2
Ca
O
H
O
H
H
O
H
O
H
H
Illustration of weak outer-sphere complexes formed by calcium and sodium near negative exchange sites on surface of soil particle. Note that neither calcium nor sodium
ions are attached to soil surface. They are merely held by electrostatic attraction to
the negative exchange sites. Also note the presence of water molecules between the
cations and the soil surface.
Inner-sphere complexes are formed with no water molecule
between the SFG and the ion or molecule. These complexes
have the following characteristics:
• Form strong associations via chemical reaction
• Exchangeable
• Can be irreversible depending upon environmental
conditions
• Weakly affected by solution ionic strength
• Charged surface is not required for complexation
Phosphorus and Silicates are a good examples of molecules
that form inner-sphere complexes with clay particle surfaces.
Soil Surface
Inner-Sphere
O
Fe
O
P
O
Phosphate
H
O
O
Soil Surface
Inner-Sphere
H
O
O
Fe
O
Si
O H
Monosilicic acid
O
H
Illustration of strong inner-sphere complexes formed by phosphate and monosilicic
acid at exchange sites on surface of soil particle. Note that both phosphorus and
monosilicic acid have formed adsorption complexes with no water molecules between
the molecules and the soil surface.
Addition of multi-valent calcium ions increase mutual attracton due to their ability to
form bridges between negative charges on platelets.
Long-term reactions or reactions that strengthen the soil
structure and create soil stability are not feasible from the
use of calcium alone because calcium complexes form weak
outer-sphere electrostatic bond associations and are subject to
reversibility (exchange with other ions including sodium).
In order to establish long-term improvements in sodic soils, it is
essential that soluble silicates (and to some degree aluminates)
are present to form complexes comprised of calcium-silicatehydrates (CSH) and/or calcium aluminate hydrates. CSH
molecules are inner-sphere complexes and form “binders” that
produce long-lasting strength gains and improved flocculation
and agglomeration.
Clays contain high amounts of silica and alumina but both
may exist as non-available crystalline structures. Pozzolanic
reactions require that silica and alumina be in amorphous and
soluble form. Without sufficient quantities of these important
constituents, the use of calcium or gypsum amendments alone
may not provide long-term bond strength and could become
susceptible to reversal should sodic conditions return.
The pH stability, and increased flocculation strength
of silicate polymers suggest that silicate polymers can
significantly improve flocculant performance -- when used
separately or in combination with calcium complexes.
CrossOverTM is a highly refined, calcium and magnesium
silicate amendment in pelletized form. Silicate anions and
calcium cations released into the soil profile following
CrossOver applications are particularly effective in treating
sodic soils – establishing soil surface complexes that
improve flocculation and agglomeration of dispersed clay
particles and formation of unique soil binders that lead
to increased particle aggregation, structural strength and
long term stability.
Adsorption Processes
Long-Term Strength and Stability. Long term soil
stabilization occurs in alkaline sodic soils when calcium
ions chemically react with soluble silica (monosilicic acid)
and alumina to form calcium-silicate-hydrates (CSH) and
calcium-alumino-hydrate (CAH) polymer gels that function
Soil Surface
Soil Surface
H
Soil Surface
Soil Surface
O
H
O
O
O
Fe
O
H
Si
O
Al
Fe
O
Si
O
Al
O
O
Al
O
O
O
O
O
Fe
O
Si
O
H
Application of CrossOver creates an increased concentration of
silicate species and calcium ions in the soil solution, initiating
formation of silicate, calcium, calcium-silicate-hydrate (alkaline
soils) and hydroxy-alumino-silicate (acid soils) adsorption
complexes that contribute to successful reclamation of sodic soils.
Silicate Adsorption Processes. Application of calcium silicate
initiates the release of monosilicic acid in the soil solution. As
concentrations of monosilicic acid increase, polymerization
reactions result in unique Silicon-rich inner-sphere complexes
being adsorbed at soil surfaces. Adsorption complexes can be
either outer-sphere or inner-sphere interactions.
Soil Modification. Silicic polymers, when added to a
suspension of colloidal particles, adsorb onto them in such
a manner that an individual chain can become attached to
two or more particles thus “bridging” them together.
Soil Surface
O
Fe
H
H
H
O
O
O
Si
O H
O
Fe
O
Si
O
Si
O
O
O
H
H
H
Monosilicic Acid
Soil Surface
H
H
O
O
O
O
Fe
Fe
O
Si
O
Si
O
O
H
H
O
Al
O
Illustration of strong inner-sphere silicate ligand complexes at exchange sites on surface of soil particle. Soil modification by flocculation and aggregation is significantly
improved over calcium bonds and are less susceptible to desorption.
as “binders” between soil particle surfaces.
These reactions are called “pozzolanic reactions.” Of the two, calcium
silicate hydrates produce the strongest mechanical strength.
Ca(OH)2 + H4SiO4 → Ca2+ + H2SiO42- + 2 H2O → CaH2SiO4 • 2 H2O
Soil Surface
O
Soil Surface
O H
Polymerization
Soil Surface
Polysilicic Acid
H
O
O
Fe
O
Si
O H
O
O
Fe
O
Si
O H
O
H
Siloxane Linkage
On average, bridging flocculation results in aggregates
that are much stronger than those produced by calcium
complexes alone. Charged surfaces are not required. Bonds
can be formed via direct ionic or covalent interactions.
Silicate-based bonds are less susceptible to reversibility.
Depiction of calcium-silicate-hydrate. C-S-H gels can be approximately viewed as
layered structures, in which calcium oxide sheets are ribbed on either side with silicate
chains, and free calcium ions and water molecules are present in the interlayer space.
In acid soils the silicate binders are formed in a reaction
of silicates with aluminum resulting in the production of
hydroxy-alumino-silicates (HAS).
Pozzolanic reactions needed for long-term stability are
generally enhanced by incorporating an additional amount of
soluble, amorphous silica over and above the amount of native
silica present in the clay soils. This is particularly true in highly
weathered and/or acid soils where levels of soluble silicon are
low due to long periods of leaching of silicon from soil profiles.
The addition of CrossOver that can release soluble silicon, can
significantly improve the reactive potential of clay soils while
also providing calcium for traditional sodic soil remediation
processes.
Beyond Sodicity
It is well known that the properties of clays affected by
sodicity can be improved with the addition of amendments
containing calcium – particularly in the area of soil modification
(flocculation and agglomeration).
However, with the addition of silicate chemistries, not only
is soil modification improved to levels beyond what can be
attained by calcium or gypsum alone, but a more consistent
stabilization of such soils is realized – soils that demonstrate
longer term improvement in shear strength, aggregate
formation, porosity, cation exchange and reduced swelling.
The uniqueness of silicon’s inner-sphere adsorption
characteristics also contribute benefits to soil profiles beyond
the reclamation of sodic soils.
Improved phosphorus availability and immobilization of
toxic metal contaminants in the soil are two very important
processes associated with the activity of silicate ligand
exchange adsorption processes.
These are addressed in separate CrossOver Information
Bulletins.
Graphic of improved agglomeration of soil particles by calcium silicate hydrate binders.
Silicon (Si) is now designated as a “plant beneficial substance”
by the Association of American Plant Food Control Officials
(AAPFCO). Silicon formulations that contain measurable
soluble silicon rather than silica can now be listed on fertilizer
labels with the new designation backed by an established
protocol for product quality, production, and accurate
labeling for commercialization of silicon fertilizers.
Graphic of fully agglomerated soil particle stabilized by calcium silicate hydrate.
Purchase Information for CrossOver
is available at:
numerator
TECHNOLOGIES, INC.
©2013 Harsco Corporation. World Rights Reserved. CrossOver is a trademark of Harsco Technologies, LLC
P.O. Box 868
Sar asota, Fl or i da 3 4230
941.807.5333
www.nume r ator t e ch.com
CrossOver – from soil to plant
For technical advice, contact:
Harsco
359 North Pike Road
Sarver, PA 16055
Phone: 1-800-850-0527
Email: [email protected]
Web: www.crossover-soil.com