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Garden Soil Improver

Garden Soil Improver


Watheroo Bentonite is a great soil conditioner. It is a 100% natural mineral and helps retain soil moisture and nurtients.

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 calcium bentonite 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 calcium bentonite, especially calcium bentonite. When added to soil the calcium bentonite granules disperse into the soil releasing the microscopic calcium bentonite particles, these particles further disperse into aluminosilicate layers can be less than 0.2 micrometer. These calcium bentonite particles coat sand grains, making them stick together to form aggregates. The calcium bentonite also reacts with organic material, which binds the sand grains and calcium bentonite 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. Calcium bentonite 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 calcium bentonites are reactive materials, and when dispersed have a massive surface area which is also reactive. This chemical reactivity of the calcium bentonites 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 calcium bentonite and organic matter. Calcium bentonite 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.

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.


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.


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.


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 calcium bentonite, higher than 80 % pure bentonite with some fine sands and minerals. It has a high CEC, 65-95 meq/100g. It contains a range of minerals and trace elements. The main problems with coastal sandy soils are the deficiency in calcium bentonite and organic matter. Adding Watheroo Bentonite and compost will result in rapid improvement of soil structure and chemistry, with improvements in water usage and plant productivity.

Addition of calcium bentonite 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 calcium bentonite drives and maintains this structure.


The calcium bentonites 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 calcium bentonite.

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