Australian Soil and Land Survey Handbooks Series

George E. Rayment, David J. Lyons

• Publisher: CSIRO PUBLISHING
• Year: 2010
• ISBN:
  9780643102187
• Topic: Technology & Engineering
• Subtopic: Agronomy
• Index: Technology & Engineering

Soil Chemical Methods – Australasia describes over 200 laboratory and field chemical tests relevant to Australasia and beyond. The information and methodology provided across 20 chapters is comprehensive, systematic, uniquely coded, up-to-date and designed to promote chemical measurement quality. There is guidance on the choice and application of analytical methods from soil sampling through to the reporting of results. In many cases, optional analytical 'finishes' are provided, such as flow-injection analysis, electro-chemistry, multiple flame technologies, and alternatives to chemical testing offered by near-range and mid-range infrared diffuse reflectance spectroscopy.

The book supersedes and updates the soil chemical testing section of the 1992 Australian Laboratory Handbook of Soil and Water Chemical Methods of Rayment and Higginson, while retaining method codes and other strengths of that Handbook. Chapters cover soil sampling, sample preparation and moisture content; electrical conductivity and redox potential; soil pH; chloride; carbon; nitrogen; phosphorus; sulphur; gypsum; micronutrients; extractable iron, aluminium and silicon; saturation extracts; ion-exchange properties; lime requirements; total miscellaneous elements; miscellaneous extractable elements; alkaline earth carbonates and acid sulfate soils. In addition, there are informative Appendices, including information on the accuracy and precision of selected methods.

This book targets practising analysts, laboratory managers, students, academics, researchers, consultants and advisors involved in the analysis, use and management of soils for fertility assessments, land use surveys, environmental studies and for natural resource management.

1

Introduction

A plethora of methods for the chemical analysis of soils exists for at least six main reasons: (a) there are many types of soils with widely differing physical and chemical characteristics and problems; (b) the total elemental composition of soils mostly has little consistent association with the ability of soils to provide necessary levels of nutrients for good plant growth; (c) tests that are quick and cheap to perform are often sought for operational and commercial reasons, even when superior procedures exist; (d) technological advances in instrumentation and computing open new analytical opportunities; (e) an ongoing quest by soil scientists and chemists to develop tests superior to those of times past, and (f) broadening demands to deal with emerging natural resource management, soil health and environmental issues. In practice, soil test methodology used by soil testing services varies within and across regions, across states and from one nation to another, making it difficult to exchange meaningful soil chemical information.

Calls from 19th century agricultural chemists failed to ‘settle on a uniform method of soil analysis, so that results obtained by different analysts might be comparable’. A two-part response in harmony with this book provides a way forward. The first, the main focus of this book, is to explain and define the (mostly) empirical methods (see Note 1) of analysis now in use or suitable for use in at least parts of Australasia. The second, also supported by this book, is to periodically assess whether the selected method/s can produce consistent results when used by individual and multiple laboratories. The now superseded Australian Laboratory Handbook of Soil and Water Chemical Methods (Rayment and Higginson 1992) made an important contribution to both components. This book extends that two-part framework for soil chemical methods, with a focus on the Australasian region. For this book, the Australasian region is that accepted by ASPAC, i.e. it comprises Oceania (Australia, New Zealand, Papua New Guinea, other islands in the south Pacific Ocean) plus areas of south-east Asia.

Organisation

This book contains over 200 ready-to-use methods, each accompanied by contemporary background information. There is sufficient detail for the book to be used as an operational laboratory methods manual and as a comprehensive reference to guide educators, researchers and end-users on choice and performance of the various methodological options contained therein. Key references are included to facilitate follow-up by those who seek additional knowledge.

Each method has a unique, three-character or four-character alpha-numeric code number. These codes harmonise with those for soils in Rayment and Higginson (1992). That is, code numbers used for particular methods and analytical finishes in the Rayment and Higginson Handbook are reproduced and extended when necessary. Specifically, the code incorporates two numerals separated by an upper-case alpha-character. The left-hand-side numeral represents methods with related features or applications, such as carbon (C), nitrogen (N), ion-exchange properties and the like. The first alpha-character identifies a variant of the main test, such as a specific extraction for soil phosphorus (P). The next numeral is used to distinguish alternative ways of completing the analytical measurement, or adjusting for variables such as the treatment of soluble salts in ion-exchange determinations. Occasionally, there is a final lower-case, alpha-character to identify a different analytical finish for a specified method that is unlikely to significantly affect the result reported at the three-character code level.

All soil chemical methods, including information on sampling and sample preparation, have been grouped into 20 chapters, mostly the same or similar to those in the Rayment and Higginson Handbook. These chapter numbers are replicated in the left-hand-side numeral of the unique method code. References for each chapter are arranged in alphabetic order. In addition, there is guidance on the preferred way to report results (including codes for preparative details when necessary), and suggestions on the number of significant figures for reporting each relevant test. Appendices cover summary details on the accuracy and precision of key methods. They also provide information on concentrated and dilute acids, year 2007 atomic weights, examples of SI units, etc.

Methodology

Soil chemical methods from the Rayment and Higginson Handbook have been retained to ensure they remain accessible for historical purposes and often for contemporary use. At least some of the technologies specified in that publication, however, have suffered with the passage of time, due to advances in instrumentation and automation. Micro-bore and flow-injection analysis are examples of incremental improvements in continuous-flow technology, while alternatives to conventional chemical testing are offered by NIR and MIR diffuse reflectance spectroscopy these days. In addition, the superseded Handbook did not include quantitative procedures for acid sulfate soils and many other popular tests such as Mehlich No. 3.

For 21st century Australasia, the book needed to embrace both laboratory and field procedures in a comprehensive, informative and consistent style. Soil chemical testing experience regionally and internationally was used to finalise the selections, based broadly on the following criteria:

1 method/s used by one or more of the major soil testing laboratories in Australasia for soil fertility assessments, for natural resource management, for environmental protection, and/or for the characterisation of modal profiles collected for land-use survey purposes;

2 inclusion in ASPAC inter-laboratory proficiency programs for soil chemical tests, and

3 publication in the scientific and/or methodological literature, particularly when relevant to the Australasian region.

All methods have been described in reasonable detail, including guidance on the elemental composition of reagents. To save space, reagent preparation and operational procedures, once described, are not usually repeated but are cross-referenced. In addition, the preferred form for expressing results is provided for each code number. Appendix 1 should be consulted to obtain the recommended numbers of decimal places for the reporting of results. Factors to convert results from an element to a species or vice versa are provided when warranted.

Given the empirical nature of many of the tests described, analytical conditions such as sample particle size, moisture status of the test sample, the extractant, the soil/extractant ratio, and temperature/s should not be changed, unless there are experimental data to show what tolerances are permitted on each of these parameters. Concentrations of primary and/or working standards, however, can be tailored to suit particular sample types and analytical instrumentation, despite the inclusion of suggestions. All calculations for primary standard solutions are based on 2007 standard atomic weights (Standard atomic weights 2007) and assume 100% purity of the proposed chemical/s; adjustments based on actual assays should be made. Likewise, adjustments to segmented flow and flow-injection analysis flow-sheets and operating conditions may be necessary to account for different types of equipment and/or reagent quality, although changes to the specified wave lengths for associated detectors are less likely. Such adjustments, if made, must not significantly change the sample/reagent combinations, as this could affect the resultant concentration of the element or species under test. Laboratory participation in inter-laboratory proficiency programs is recommended to help benchmark contemporary measurement performance against others who participate in the same programs.

An assumption throughout, unless otherwise specified, is that all chemicals are dry and of analytical grade, and that all mineral acids are concentrated (typical molarities are given). What is referred to as reagent water or deionised water should be of high and consistent quality, although distilled water of similar quality is acceptable and in a few cases may be superior. One example is in the preparation of the molybdenum-blue reagent associated with colorimetric determinations of orthophosphate-P, which can be adversely affected by organic impurities in some deionised waters.

To limit opportunities for in-house errors, weights and volumes are consistently provided to guide the preparation of reagents. No particular significance, however, should be placed on the weight/volumes specified. These can be varied proportionately (with care), depending upon the number of samples to be analysed, in conjunction with the expected shelf lives of the particular solutions. Analysts must ensure all such solutions, if subjected to periods of storage, are in good condition prior to use. If not, the old solutions should be disposed of in an environmentally sensitive manner and fresh solutions prepared. Shelf life guidance is often provided, but these times may not apply if storage conditions are unfavourable.

All specified dilutions, solution transfers, and volumes should be made as accurately as warranted. Descriptions requiring the use of pipettes, burettes, and volumetric flasks imply that accurate volumes are required. Moreover, while preparation of working standard solutions from a single primary standard solution are commonly described, there is a lower risk of systematic error if three or more weighings are involved in preparing a range of primary reference solutions. The use of commercially available standard stock solutions is acceptable: preparation time can be reduced and errors minimised if the quality of the commercial product is assured and stated instructions for mixing and dilutions are followed carefully.

Importantly, analysts using the methods described should be appropriately trained and practiced in the safe use and handling of laboratory chemicals and equipment, and should apply this training at all times. Moreover, risk assessment profiles should be developed for all methods involved with hazardous substances and temperatures outside of ambient conditions. It is further assumed that all laboratories are equipped with normal laboratory equipment, glassware, fume cabinets, well-calibrated balances, refrigeration, and at least one constant-temperature room at 25±2°C (or better) for the extraction of soils and equilibration of extracting solutions and reagents. If the constant-temperature room is not within the 25±2°C specification, the actual temperature used (e.g. 20±2°C) should be recorded, as temperature can affect apparent results.

There should be facilities to receive soil samples from clients and to dry and grind/sieve soils to the moisture status and particle sizes specified (see Figure 1.1). These activities must not introduce chemical or physical changes to the samples or add measurable contamination via the materials used in construction of the equipment or from the air or surroundings. Airborne contaminants include particulates and chemical vapours, such as ammonia fumes. Subsequent storage of prepared samples in inert containers should be clean, cool and dry.

Figure 1.1. Examples of soil preparation and soil grinding equipment. The mills shown in the centre and right-hand side are located in a laminated enclosure with a stainless steel screen at the base to draw dust particles away from the sample and the operator. The left-hand side photograph shows simple equipment used for crushing soils (e.g. hard setting clays) if required before grinding.

Where end-over-end extraction is specified, the equipment should ensure good, gentle and regular contact between all soil particles and extracting solutions. A drum diameter of around 560 mm revolving at about 15 rpm is most satisfactory. There should be a minimum of about 20% free airspace within each extraction bottle to facilitate good mixing, while moments of inertia for all extracting bottles in the one shaking machine should be as near as possible to equal (see Figure 1.2 for an example).

Figure 1.2. Part of an end-over-end soil shaking machine, showing the location of a rack of 10 soil extraction bottles prior to closure of the opening with a stainless steel lid. There is provision in the drum for 10 such racks, all with the same radius from the centre-point of rotation. The plastic-coated wire tray shown above the drum assists loading and unloading of the shaking machine.

When a laboratory routinely performs tests for a particular element (e.g. potassium), and also uses soil extracting solutions containing the same element for other purposes, it is recommended that separate glassware and/or plasticware be used to minimise the opportunities for cross-contamination. All other apparatus, disposable equipment, filter papers, etc., should either be well cleaned before use or periodically tested for freedom from relevant contamination.

Analysts should periodically seek independent verification of their analytical results. For example, certified and/or secondary reference materials should be used frequently. Acceptable values for empirical soil tests need to be established in collaboration with laboratories known to be proficient at performing such tests.

Moisture status of test results

As indicated, method codes identify often quite complex soil analytical procedures. The codes also cover the type of sample, method of extraction, the analytical ‘finish’, and what is reported. It follows that laboratories who report results against these codes infer full compliance, inclusive of the moisture status of reported results. Those in this book comply broadly with international best practice (e.g. SSL 1996).

Specifically, characteristic soil properties generally are expressed on an oven-dry weight basis (oven drying to constant weight of 105–110°C), where oven-dry refers to a soil without free water but inclusive of crystal water in the case of gypsiferous soils. Exceptions are results for electrical conductivity and pH, which cannot be readily adjusted to a moisture-free basis by simple calculations using the air-dry moisture to oven-dry moisture ratio. Other important exceptions are soil tests performed for ‘fertility’ or ‘diagnostic’ purposes. Thes...

Table of contents

• Cover
• Title
• Copyright
• Contents
• Preface and acknowledgements
• About the authors
• Abbreviations and acronyms
• 1. Introduction
• 2. Sampling, preparation and moisture content
• 3. Electrical conductivity, related attributes and redox potential
• 4. Soil pH
• 5. Soluble chloride
• 6. Carbon
• 7. Nitrogen
• 8. Carbon/nitrogen ratio
• 9. Phosphorus
• 10. Sulfur
• 11. Gypsum
• 12. Micronutrients
• 13. Extractable iron, aluminium and silicon
• 14. Saturation extracts
• 15. Ion-exchange properties
• 16. Lime requirement
• 17. Total miscellaneous elements
• 18. Miscellaneous extractable elements
• 19. Alkaline earth carbonates
• 20. Acid sulfate soils
• Appendix 1. Reporting of results
• Appendix 2. Accuracy and precision of methods
• Appendix 3. Acids and guidelines for their dilution
• Appendix 4. Table of selected atomic weights (2007)
• Appendix 5. Units and conversion factors
• Index