General

The following guidelines should be followed when examining soil landscape report soil test results and their interpretations. The aim is to provide an indication of likely soil conditions.

While every care has been taken and the information presented in the following appendices is of highest practical standard, it is important that the following disclaimers be recognised:

• The soil analyses provided in the tables serve as only a guide to soil conditions.
• The analyses are not definitive except for the points where they were sampled.
• Variations in the field are to be expected.
• Consult soil landscape descriptions for an understanding of soil variability and distribution.

Laboratory test results are based on what are considered to be representative single samples for each soil material that has been observed and described many times by the author during the course of the survey.

Soil Survey Standard Test Methods are used to allow State-wide comparison of different soil types and are outlined in soil landscape report chapter 3 tables.

Similarly, standard information has been used to derive and rank interpretive values. In some cases, the interpreted outcomes may not be appropriate for certain soil types. Where such anomalies occur, notes are usually made within the Soils section of the particular soil landscape. Nevertheless, an understanding of the procedures and the algorithms used is necessary to be able to put soil test results into perspective.

Soil test result rankings have been collected from many references that have derived their data sets from international or regionally collected samples for a variety of purposes. No definitive prescriptive meaning can be provided for ranking names across all test results. Generally, data sets for chemical and exchangeable cation analysis tend to be agronomic; therefore, they are more regional and biased towards topsoil. Data sets for engineering interpretations tend to be derived from subsoil and more universal data sets. Examine the explanatory notes concerning interpretive procedures for a detailed understanding of the rankings.

Regardless of the considerations above, interpretations are usually based on a five-class system and individual ratings can generally be considered meaningful. The degree of cumulative inaccuracy involved with sampling, test selection and laboratory error can usually be expected to be much smaller than interpretive class widths. In other words, the ratings can be considered sufficiently accurate to indicate if a result is, for example, high or very high.

Interpretations in the following tables are based solely on sampled soil material and do not include influences from nearby soils, the site, land use history or landscape considerations. For example, a soil material may be interpreted as capable for small building foundations, but an unsuitable material may underlie the soil or the site may be subject to frequent flooding.

Surface soil materials are composed of at least six and normally 12 to 24 sub-samples collected within a 10 m radius of the point for which they are recorded. Patterson and Wall (1982) found with selected tests that three sub-samplings were sufficient to obtain estimates of many soil properties at the sampling site to within 95% accuracy. Surface vegetation is removed prior to sampling. Standard sampling depth is usually 0 - 10 cm from the mineral surface unless the surface horizon is thinner. Note: Depth range indicates the depth over which the horizon occurs.

Sub-surface samples are collected on the justification that they are morphologically representative of soil materials observed and/or described in the field. Sub-surface samples are usually selected evenly within soil horizons.

Bear in mind that for many broad acre land management purposes, a representative average value is sufficient. Even if laboratory testing were undertaken on 10 times as many samples to gain an indication of variability, there are usually no means available to apply the information on a paddock-by-paddock basis.

Where the depth range of a soil horizon exceeds 30 cm, the sample is taken from the upper 30 cm of the horizon.

It is expected that bulked samples would provide an average value for more than half the variation expected to be encountered for individual soil physical properties within the soil material (Becket & Webster 1971).

Sample Handling

About 2 kg of sample is collected at the time of initial inspection for each major horizon of each profile where a Soil and Land Information System card is completed. Sufficient soil is usually collected to repeat all tests at least once on each sample. Samples are discarded 12 months after publication of the soil landscape map and report.

Samples are stored in calico bags or plastic bags if the soil is saturated at time of collection. Soil drying (at temperatures up to 40⁰C) continues when samples arrive at the laboratory. Several months’ delay between sampling and completion of testing is common. Delays in testing have been well studied and are considered to be mostly negligible on results (Hesse 1971).

Test Interpretations

To best understand test results, it is necessary to understand the rationale of necessary test methods. What Do All the Numbers Mean? (Hazelton & Murphy 1992) includes information on the interpretation of many soil tests.

Soil test interpretations have been applied and tested over two years by the Soil Knowledge Team of the Department on a wide range of soil materials. These have been examined by R.F. Isbell, CSIRO Division of Soils; B. Murphy, Department of Natural Resources; and W. McDonald, Australian Collaborative Land Evaluation Program.

Rock-Adjusted Outputs

Coarse fragments, larger than 2 mm in diameter (gravels), are often removed from samples prior to transport to the laboratory to reduce sample size and transport costs. Soft and highly weathered coarse fragments, such as some siltstones and mudstones, may be easily crushed in transit.

Department of Natural Resources laboratories do not have sample handling facilities for coarse fragments larger than gravels, so their volume is estimated in the field.

Field measurement includes coarse fragment volume size classes 2-6 mm, 6-20 mm, 20-60 mm, 60-200mm, 200-600 mm and >600 mm; abundance 0-2% (very few), 2-10% (few), 10-20% (common), 20-50% (many), 50-90% (abundant) and >90% (very abundant). Coarse fragments do not include charcoal, pumice or shells as these have densities closer to soils than rocks. Total coarse fragment volume midpoints are converted to weight equivalents to provide an estimate for the rock-adjusted factor.

The rock-adjusted factor is multiplied by the fine earth value to gain an estimate for the value of the attribute of interest for the whole soil. As proportions of coarse fragments can vary greatly over short distances and coarse fragment volume classes are wide, rock-adjusted values are intended only as a broad indication.

Estimates are based on the assumptions that, on average, coarse fragments have a density of 2.65 kg/m and that the average bulk density of the soil’s fine earth fraction is 1.4 kg/m³. Actual coarse fragment densities and soil bulk densities are substituted where they are known.

Should field coarse fragment volume estimates be missing, Interp (expert system) defaults to gravel fraction weight percentages derived from the particle size analysis.