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Sandy Soil Amendments

Sandy Soil Amendments


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 more partucularly, calcium bentonite together with organic matter from compost is an effective method to achieve this.

The calcium bentonite 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.

Calcium bentonites react and bind with soil organic matter, this bonding with calcium bentonite protects the humic compounds from microbial and chemical degradation. So these important organic compounds are retained and are active in the soil. The calcium bentonite 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 calcium bentonite 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 calcium bentonite 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 calcium bentonite 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 calcium bentonite 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 calcium bentonites and humic compounds

    • 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

Calcium bentonite alone can address many of these parameters, but when combined with mature composts and humic compounds, then all of these can be improved.

Calcium bentonites granules when hydrated disperse into sub-microscopic particles, these particles are formed of sandwich like layers. In some calcium bentonites 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 calcium bentonites, like the smectites such as bentonite, the layers can further separate in water, giving a vastly increased surface area.

Watheroo Bentonite is Certified free of phytophthora/die back and pythium.

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