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Watheroo Bentonite as a soil conditioner
(This article was supplied by Watheroo Minerals Pty Ltd)
Coastal areas of the Western Australia have soils derived from ancient weathered
landscapes. The soils are often pure, coarse sand with very low levels of clay and organic
material. This gives them the appearance of beach sand, with the same characteristics: no
soil structure, no aggregates, very rapid drainage, low water holding capacity, low capacity to
retain nutrients and often they develop water repellent properties.
An effective way to improve these soils is to add clay, especially calcium bentonite. When
added to soil the clay granules disperse into the soil releasing the microscopic clay particles,
these particles further disperse into aluminosilicate layers can be less than 0.2 micrometer.
These clay particles coat sand grains, making them stick together to form aggregates. The
clay also reacts with organic material, which binds the sand grains and clay forming
aggregates and giving the soil a cohesive structure, unlike the original sands. These
structured soils have good drainage but also the capacity to hold water and nutrients and
release these back to plant roots. Clay added to sandy soils will react with sand grains and
organic matter to form a structured soil, which is good for soil chemistry and plant growth.
Calcium bentonite added to soil or water, disperses into tiny sub-micron particles. These mix
with sand and other soil components. The clays are reactive materials, and when dispersed
have a massive surface area which is also reactive. This chemical reactivity of the clays
gives soil many of the properties we understand as being typical of good, productive soils.
Soil structure from binding of sand grains into aggregates, ability to hold water, ability to hold
and release nutrients, ability to support diverse and healthy bacterial populations and
produce healthy vigorous plant growth.
A technical term used in soil chemistry is Cation Exchange Capacity or CEC. This is the
ability to hold and release nutrients and minerals, particularly charged ions like potassium,
magnesium, calcium and trace elements. This ability retains these essential plant nutrients in
the soil, but releases them to plant roots as required. When they have been depleted they
can be replenished from added fertilisers or from natural processes like breakdown of
manures and plant materials. The ability of soils to absorb, hold and then release minerals
and nutrients for uptake by plant roots is one of the most important chemical processes in
nature and plant production.
The cation exchange capacity of soil is due primarily to the presence of clay and organic
matter. Clay and organic material are both essential components of soil chemistry and work
together to give the CEC of the soil. Organic material has high CEC and also has an anion
exchange capacity. The organic material can therefore be the most important determinant of
CEC and anionic exchange capacity of soils.
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A number or other mineral based materials can be added to sandy soils to improve their
structure and fertility. These include clay minerals such as sodium bentonite, kaolin, zeolite
and local clay rich soils. Other additives are fly ash and spongolite.
Sodium bentonite can have practical problems because it absorbs water and swells
dramatically, so can cause problems with dispersal, clumping and sealing. Sodium bentonite
is often finely milled so can be difficult to handle. It may also contain a range of additives
required for specific industrial and oil drilling applications.
Zeolite is a hard clay compound with high CEC used in industrial and water treatment
applications. This has problems as a soil additive because it does not disperse well. Kaolin
and heavy clays from some soils, can also have low efficacy because they do not readily
disperse into the soil and combine to form a structured soil.
Spongolite has high surface area, but not much contribution to CEC, so has some benefit to
soils but limited in terms of soil structure and chemistry. Fly ash is a cheap waste product
from coal burning power stations, it has high surface area which contributes to soil structure,
but limited chemical reactivity and contribution to CEC. Fly ash is a waste product and can
have high levels of heavy metals from the coal.
Watheroo Bentonite is a calcium bentonite with no additives, it is a high quality clay, 80 %
pure bentonite with some fine sands and minerals. It has a high CEC, 60-80 meq/100g. It
contains a range of minerals and trace elements. The main problems with coastal sandy soils
are the deficiency in clay and organic matter. Adding pure clay and compost will result in
rapid improvement of soil structure and chemistry, with improvements in water usage and
plant productivity.
Addition of clay and compost will give long term improvement in soil structure and chemistry,
with significant benefits to water efficiency and plant health and productivity. This
improvement of soil structure is as important as the chemical changes in soil, and clay drives
and maintains this structure. The clays are a very stable, persistent component of soils, while
the organic matter fraction is continually being broken down by microbial processes and
needs to be replaced. It will still be necessary to add nitrogen and phosphorus to get good
plant productivity, but minerals and trace elements will be present from compost and the
clay.
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Role of clay in soil improvement and soil amendments for sandy soils
Summary
Sandy soils have poor structure, with large spaces between sand grains. The sand has a low
surface area and little electrical charge on sand grain surfaces.
The result of this is that these soils do not retain water, have rapid leaching of nutrients, and
also have rapid breakdown of organic matter, which then results in development of water
repellency.
Soil amendments should be cost effective and logistically feasible. The use of clay and
organic matter from compost is an effective method to achieve this.
The clay and organic matter help improve soil quality, through development of soil structure
through improving the water retention capacity and increasing the CEC, which improves
retention and bio-availability of nutrients.
Clays react and bind with soil organic matter, this bonding with clay protects the humic
compounds from microbial and chemical degradation. So these important organic
compounds are retained and are active in the soil. The clay and organic compounds can be
seen in soil to have very close interactions on both physical and chemical basis. These
interactions are persistent and vital to soil functioning.
Soil amendments using clay and organic matter rich in humic compounds will have rapid and
long lasting effects in converting unstructured sandy soils into more productive soils, which
are waterwise. This can be done in-situ and is more cost effective and environmentally
appropriate than bringing in top-soils which have been extracted from productive farmland.
Sandy soils of the Perth coastal plain are primarily composed of silica or lime sands, with
very low levels of clay and organic matter.
These soils have poor water retention, poor nutrient retention and limited capacity to hold
and develop organic matter.
The quality of these soils can be improved by amendments that address these issues.
Calcium bentonite granules can be added to sandy soils in combination with composted
organic material. The clay granules disperse into fine particulates, these particles then form
sub-microscopic layered structures that have massive surface area that contains reactive
charged sites. This complex of dispersed clay particulates binds with sand and silt particles
of the soil and with organic matter, especially complex long chain organic polymers, the
humic compounds found in composts and speciality humic additives.
In terms of soil improvement there are a number of criteria that should be achieved:
1. Increased soil water retention; two components to this.
a. Absorption of water by clays and humic compounds
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b. Capillary based water retention due to pores and micropores in soil
aggregates; the sponge effect
2. Increased retention of nutrients, especially calcium, magnesium, potassium and trace
elements
3. Retention of soil organic matter and inorganic nutrients like nitrogenous compounds,
sulphates and phosphates
4. Improved structure of soil through presence of micro and macro-aggregates
Clay alone can address many of these parameters, but when combined with mature
composts and humic compounds, then all of these can be improved.
Clays granules when hydrated disperse into sub-microscopic particles, these particles are
formed of sandwich like layers. In some clays these layers form particles have reactive sites
only along the edges. Because of the small size these still contribute significantly to soil
chemistry. However in other clays, like the smectites including bentonite, the layers can
further separate in water, giving a vastly increased surface area.
Figure 1. Layered structure of clays
Figure 2. Kaolinite clay structure. Note layers stuck together, charged sites that can hold
cations only exist on outer edges of layers and particles.
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Figure 3. Structure of hydrated bentonite clay. Note that when hydrated, the layers separate,
this exposes a large extra area for reactive ion exchange.
In the smectite clay structure, the two layers are separated by water and cations, these ions
are normally sodium, calcium, potassium or magnesium.
When bentonites are differentiated as sodium bentonite, calcium bentonite, magnesium
bentonite (saponite), this classification is based on the predominant ions within this interlayer
zone. When this space hydrates and fills with water the whole clay mass expands, this is the
basis of expanding clays.
So the available surface area of hydrated clay minerals is a function of the clay structure and
the capacity to expose the inner surfaces of the layer structures. There are very large
difference in these surface areas between clays, see Table 1, from White, Principles and
Practice of Soil Science. The surface area is also correlated with the CEC of the clay, so high
surface area clays normally have a higher CEC, although there are other factors that affect
the CEC.
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Table 1. Surface area and CEC of selected minerals and clays
Mineral, class size Surface area m2/g
CEC cmol/kg
Sand 0.01 Fine sand 0.1 Silt 1 Kaolinites 5-40 5-25 Bentonites 300-750 100-120
The negative charged sites on clay layer surfaces can hold and release positive ions, the
cations, for uptake by plant roots. So higher surface area is correlated to ability to hold and
release cations to plants, this is the cation exchange capacity. So the smectite clays have
higher CEC and more reactivity in the soil environment than the kaolin type clays.
Humic compounds in soil derived from compost and plant and animal material are long chain
compounds that also have many negatively charged sites, which have a cation exchange
capacity. The humic compounds have a CEC 4-40 times greater than CEC of clay. So the
humic compounds are vital part of soil CEC and this may be more important than CEC from
clay. However, the clay stabilises the organic material, protecting them from microbial and
chemical degradation and leaching, and therefore enhances their persistence and efficacy in
the soil.
These humic compounds have vital roles in soil quality. The composites of clay and humic
compounds are what create the conditions that result in improved soil quality parameters.
The clay and organic matter combine to bind the inorganic particles to form micro-aggregates
and macro-aggregates. They bind large and small soil particles to form soil aggregates, and
they alter the physical and chemical characteristics of the soil environment. Both of these
processes then facilitate development of soil microorganisms and fauna that further
contributes to, and maintains productive, water efficient soils.
Sandy soils, with low levels of clay and humic materials
Surface of sand grains is hard and inert, so particles stay separate and form loose, unstructured
soil. Sand grains have relatively small surface area, large pores and channels, so water and
nutrients leach through rapidly. The sand grain surfaces are uncharged and therefore do not
retain nutrients or organic compounds, so these leach out and are not kept available to plant
roots.
Sandy soil with added clay and humus
Sand grains become coated with clay and humic complexes. Humic compounds are bound to clay
and are protected from leaching and rapid microbial breakdown.
The coating of clay and humic compounds around the sand and silt material results in micro-‐
aggregates and macro-‐aggregates. This process results in a structured soil. The result of this is
an improved soil structure, better aeration, structural support for plants and greatly increased
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capacity to hold water and nutrients. Good soil structure also contributes to less erosion and
leaching. Another benefit is a better environment for microorganisms and soil fauna, this further
improves and maintains soil quality.
Figure 3. Clay and organic matter composites bind sand and silt particles to form
aggregates. This happens at microscopic scale to form micro-‐aggregates <250 um and at a
larger scale, macro-‐aggregates >250 um.
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