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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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