Interpreting Soil Test Results

What Do All the Numbers Mean?

Pam Hazelton, Brian Murphy

• Publisher: CSIRO PUBLISHING
• Year: 2016
• Print ISBN: 9781486303960
• eBook ISBN: 9781486303984
• Edition: 3
• Topic: Technology & Engineering
• Subtopic: Geology & Earth Sciences
• Index: Technology & Engineering

Interpreting Soil Test Results is a practical reference enabling soil scientists, environmental scientists, environmental engineers, land holders and others involved in land management to better understand a range of soil test methods and interpret the results of these tests. It also contains a comprehensive description of the soil properties relevant to many environmental and natural land resource issues and investigations.

This new edition has an additional chapter on soil organic carbon store estimation and an extension of the chapter on soil contamination. It also includes sampling guidelines for landscape design and a section on trace elements. The book updates and expands sections covering acid sulfate soil, procedures for sampling soils, levels of nutrients present in farm products, soil sodicity, salinity and rainfall erosivity. It includes updated interpretations for phosphorus in soils, soil pH and the cation exchange capacity of soils.

Interpreting Soil Test Results is ideal reading for students of soil science and environmental science and environmental engineering; professional soil scientists, environmental scientists, engineers and consultants; and local government agencies and as a reference by solicitors and barristers for land and environment cases.

1

Soil sampling issues: aspects of design
and implementation of soil investigations

1.1 The fundamental problem of sampling to test soils

A fundamental reality facing soil testing is that the amount of soil in 1 ha of soil to a depth of 10 cm is 1400 tonnes. A 50 ha paddock can have 70 000 tonnes of soil to 10 cm. For a soil test, as little as 0.5 g or 1.0 g of soil may be tested in the laboratory (Price 2006). Therefore it is essential that every effort is made and precaution is taken to ensure that the small amount of soil that is tested is representative of the 70 000 tonnes of soil in the paddock or assessment area. If adequate procedures are undertaken, this can usually be done (Price 2006).

To characterise soils at a site, a suitable sampling design is required. Sampling design depends on the:

• landscape or location from which samples are being taken;

• purpose for which samples are being taken;

• resources available to take and test the samples.

The basic questions to be considered are:

• When is the best time to sample?

• Where should the soil be sampled?

• How many samples are required?

• What spatial pattern should be used to take the samples?

• What depths should be sampled?

For general sampling purposes, the basis for making these decisions is discussed in Petersen and Calvin (1986), Beattie and Gunn (1988), Rayment and Higginson (1992), McBratney (1993), Brown (1999), Price (2006), NEPC (2013a), McKenzie et al. (2008).

Another difficulty in sampling is the problem of temporal variability, where results for samples or measurements taken at one time may be different from results for samples or measurements taken at another time. Some obvious examples are:

• measuring infiltration in a tilled paddock – the infiltration is much higher before rainfall compacts the soil and crusts the surface;

• measuring salinity on a site after a large rainfall event – the soil solution may be diluted, compared with the solution measured after a dry period;

• sampling soils immediately after addition of fertiliser or soil ameliorants such as lime or gypsum;

• sampling soils in paddocks in which sheep and cattle graze, resulting in change in nutrients over time.

These difficulties need to be considered when sampling soils or making measurements on soils and in interpreting the results of any tests carried out. Sampling through time is required, or, alternatively, the conditions when measurements are made or samples are taken should be standardised (or at least recorded).

When interpreting soil test results it is necessary to consider the origin and nature of the samples of soil being taken and the purpose for which interpretations are being made. For example, often a soil sample that is sent for analysis can include a relatively large volume of soil of 1 kg size or larger. Such a sample includes a large range of soil materials on the scale of a plant root. A plant root is often of the order of 1 mm in size or less. There may be microenvironments within the soil that have quite different chemical and physical properties from the bulked soil property measured during analyses. Hence plant root behaviour or plant growth may not always directly reflect the soil properties measured by a large sample. A typical example of this is the comparison of the dispersion behaviour of small aggregates (often 5–10 mm) to the bulk soil properties of exchangeable sodium percentage. At the microscale of soil structural units there may be considerable variation in exchangeable sodium percentage and dispersion behaviour. This microvariation can be masked by the measurement of bulked soil properties. This emphasises the need for adequate sampling of soils and the risks of relying on a single sample to make recommendations.

1.2 Purposes of soil sampling

Soils at a site are sampled and tested for a wide variety of reasons including:

• diagnosis of soil constraints for agricultural production, often in a specific paddock for a specific crop;

• diagnosis of plant nutrition problems and formulation of fertiliser and soil amelioration programs;

• monitoring changes or trends in soil chemical properties including pH, aluminium levels, nutrients, salinity and soil organic carbon concentrations;

• soil testing for engineering and soil stabilisation purposes before the construction of buildings and infrastructure;

• testing of soils for the occurrence of contamination and identification of the type and concentration of contamination;

• estimation of soil carbon stocks for the purpose of determination of potential carbon credits;

• characterisation of soils for mapping and the identificatio...

Table of contents

• Cover
• Title
• Copyright
• Foreword
• Contents
• About the Authors
• Preface
• Acknowledgments
• Introduction
• 1 Soil sampling issues: aspects of design and implementation of soil investigations
• 2 Soil physical properties
• 3 Soil properties and soil behaviour for engineering
• 4 Soil erodibility and erosion hazard
• 5 Soil chemical properties
• 6 Organic matter content of soils
• 7 Application of wastewater and waste materials
• 8 Soil contamination: some considerations
• 9 Units and conversions
• 10 General and technical suggested references
• References
• Appendix 1
• Index