Understanding WA Rangeland Soils

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Detailed understanding of the chemical, physical and biological properties of mine site soils is essential for rehabilitation of waste landforms and other disturbed areas for site closure.  Unlike the majority of soils on the earth, which are generally considered very young by geological time, WA rangeland soils are extremely old and infertile as a result of hundreds of thousands of years of weathering.  Most of the earth’s soils have formed in the last 10,000 years following thawing of ice sheets that covered much of continental northern Europe, Asia and America and are regularly rejuvenated by deposition of minerals and nutrients by large rivers, glaciers and volcanic activity.  On the other hand, Western Australia has not been covered by ice sheets or experienced significant volcanic activity for millions of years.  As a result, “soil” from upland areas are regularly depleted by wind erosion, sheetwash erosion and leaching of nutrients and minerals by chemical weathering.  This has also resulted in accumulation of silts, clays and salts in broad, almost indistinct “paleochannel valleys” that have not drained to the ocean for hundreds of thousands of years.

In many ways, assessing the “health” of soil is similar to human health care in that it requires regular assessment by skilled professionals supplemented by periodic testing of blood and other biological specimens.  Despite what many health professionals will tell you, interpretation of a blood test laboratory report is relatively straightforward by referring to tables that indicate whether the test result is with the “normal” or “abnormal” ranges.  Soil health interpretation is far more complicated, both in selection of suitable test methods and interpretation of the results.  Take for example measurement of “soil pH”, which is considered one of the most important soil properties.  Being a solution property, pH of a dry soil is in effect meaningless.  To generate numbers reflecting “soil pH” conditions, laboratory analysts revert to equilibration of a soil sample with an extracting solution at a standard solid to solution ratio.  Several tests for “soil pH” are offered by Australian laboratories including extraction with deionised water (1:2 and 1:5 ratios), 1 M KCl and 0.01 M CaCl2.  The results for “soil pH” can vary by more than 1 pH unit, depending on which test is used and this can result in significant differences in interpretation of the result.

WA miners are required the rehabilitate waste landforms using the most infertile soils on Earth, either highly weathered lateritic soils and saline and sodic silts and clays
WA miners are required to rehabilitate mine waste landforms using some of the most infertile soils on Earth, either highly weathered lateritic soils or saline and sodic silts and clays

Once the issue of which laboratory test method to use has been resolved, the next challenge is interpretation of the result.  For a sensible interpretation to be made, it is essential to refer to a source of data that relates the soil test result to specific soil health indicator or management outcome.  This information is known as a “decision support system”.  It is essential that data used to populate the decision support system are relevant to local soil types, environmental conditions and land uses.  For example, a WA cereal farmer would be most unwise to base his fertiliser requirements on a decision support system developed for corn growers on prairie soils in Illinois, America.

Much of the data used to develop decision support systems for Australian soils have focused on agricultural production, for which the area of potentially productive soils in WA represents less than 10% of all soils.  In response to the paucity of data for WA rangeland soils, on which most of WA’s mines are located, soil professionals usually have to resort to published information sources to assess soil test results.  Invariably, most of the readily available information is focused on either Eastern States or Northern Hemisphere soils, or WA agricultural soils.

MBS Environmental has established a decision support system for assessment of WA rangeland soils, drawing upon more than 20 years of field and laboratory experience. Principal Environmental Geochemist, Dr David Allen, has supervised and assessed laboratory analysis of thousands of WA soil samples from surveys conducted by the Department of Agriculture and Food and the Department of Parks and Wildlife, developing a decision support system directly applicable to the mining industry.  When preparing soil assessment reports MBS now include a detailed technical paper within the appendices that describes the methodology adopted for assessment of soil chemical and physical properties and ratings tables for interpretation of results.  If you require a soil assessment, please contact Dr David Allen to discuss how we can assist you on 9226 3166 or info(at)mbsenvironmental.com.au