Soil Chemical Methods - Australasia

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 The now superseded Australian Laboratory Handbook of Soil and Water Chemical Methods (Rayment and Higginson 1992) made an important contribution to both components.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. These are reported herein on an air-dry (≤ 40–45°C) basis.

As the great majority of laboratory soil tests described commence with air-dry samples, the tests to be reported on an oven-dry weight basis need to be adjusted, using the air-dry moisture to oven-dry moisture ratio. As an example of non-compliance, results for tests on the Vertosols of Australia, (these are widely distributed across the continent; see Figure 1.3) could be up to 12–14% lower if reported on an air-dry basis rather than on an oven-dry basis. Obviously, inter-laboratory proficiency programs for soils need to pay close attention to the moisture status of results they report on behalf of participating laboratories.

Figure 1.3. Distribution of Australia’s Vertosols (from Isbell et al. 1997).

Note

1.  Empirical methods generally arise via experience and speculation, combined with intuitive ability by the developer, rather than from theoretically based expectations. Typically, they follow the collection and interpretation of large amounts of experimental data and observations, combined with trial and error and meticulous analytical repeatability; i.e. the same soil particle size, a defined extractant and soil/extractant ratio, a set extraction time and temperature, and a robust analytical finish.

References

Isbell RF, McDonald WS and Ashton LJ (1997) Concepts and Rationale of the Australian Soil Classification. ACLEP, CSIRO, Australia.

Standard atomic weights (2007) of selected elements, updated from Pure and Applied Chemistry 78, 2051–2066 (2006), with 2007 changes. IUPAC Commission on Atomic Weights and Isotopic Abundances. http://www.chem.qmul.ac.uk/iupac/AtWt/

Rayment GE and Higginson FR (1992) Australian Laboratory Handbook of Soil and Water Chemical Methods. Inkata Press, Port Melbourne.

SSL (1996) Soil Survey Laboratory Methods Manual. Soil Survey Investigations Report No. 42, Version 3.0. National Soil Survey Centre, Natural Resources Conservation Service, US Department of Agriculture, Washington DC.

2

Sampling, preparation and moisture content

This chapter combines and extends former Chapters 1 and 2 of Rayment and Higginson (1992). Superseded Chapter 1 from that Handbook dealt with sampling and sample preparation, while Chapter 2 dealt with soil moisture contents. Topics sequentially described are soil sampling, sample transport, laboratory preparation and disposal of samples. In addition, there are four methods for soil moisture content, coded as 2A1, 2B1, 2C1, and 2D1. Summary details of these are provided in Table 2.1.

Soil sampling, transport and preparation

Soil sampling is not the main focus of this book but must never be overlooked. All care and attention in the laboratory is devalued if the sample does not truly represent what was intended.

Soils are formed and are continually modified by the actions over time of climate, topography (or relief), vegetation, man, and other biota acting upon parent rocks and on the soil itself. Soils vary in their appearance and in their ability to supply nutrients, water, air and anchorage for plant roots. While individual soils exhibit unique characteristics, they also have characteristics in common. For example, all soils comprise solids (minerals and OM), water and air.

Table 2.1. Summary detaÕs of soÕ moisture methods in this chapter.

The organic fraction includes living plants, micro-organisms and animals, and the remains of these in various stages of decay. Accordingly, they are not sterile, unless they have been chemically fumigated, subjected to sustained high temperatures (e.g. ≥105°C for several hours), autoclaved, or γ-irradiated at from 10 kGy to >70 kGy, the latter to ensure the death of radio-resistant bacteria (Bowen and Rovira 1961; McNamara et al. 2003). Mineral matter consists of particles of sand, silt and clay, formed from chemical and physical weathering of parent rock, minerals, ash, coral, shell and bone. Pore spaces between individual soil particles or aggregates (clusters) of soil particles contain water, air, plant roots and the like. The amounts and kinds of OM, mineral particles, plant nutrients and pore spaces vary both within and between soils.

Prominent amongst reasons for natural, within-soil variation is the soil profile, which commonly stratifies abruptly or gradually into discernable horizons. A hypothetical example is shown and briefly described horizon by horizon in Figure 2.1. Refer to Isbell et al. (1997) and McKenzie et al. (1999) for examples of major soils from Australia and their distribution, and to Fitzpatrick et al. (1999) for associations between soil morphological characteristics and soil fertility.

Soil sampling strategies need to take account of the existence of soil horizons and to the reality that cultivation, cropping, grazing, fertilisation, waste disposal and other land-management activities affect soil chemical, physical and biological properties significantly. Some information on sampling is included for convenience. For more information, readers are referred to detailed reviews on soil sampling for soil chemical analysis, including sample handling by Brown (1993, 1999). See also Etchevers (1986), Anon (1992) and Rayment (1993).

Figure 2.1. Hypothetical soil profile showing principal horizons, with brief explanations.

Field sampling – soil and land use surveys

Beattie and Gunn (1988) provide details on the collection of samples associated with soil and land-use surveys in Australia. Similar strategies apply across Australasia. When tests for micro-nutrients are proposed for inclusion in the analytical suite of soil chemical methods, uncontaminated, heavy duty polyethylene bags are preferred to those made from cloth, which were popular in earlier times. When there are no compelling reasons to do otherwise, it is convenient for the computerization of records to sample common depths such as 0–10, 10–20, 20–30, 50–60, 80–90, 110–120, 140–150, and 170–180 cm (Beattie and Gunn 1988). Sampling depths, however, may need to be varied to best achieve survey objectives and to reflect soil variability adequately. Deeper sampling may be needed on occasions, such as for tracking the movement of nitrates, and to confirm or eliminate the likelihood of sulfides associated with ASS.

Field sampling for diagnostic purposes

Sampling protocols relevant to the crop, soil test/s, and interpretative criteria should be followed. For example, it is common to sample soils at either 0–7.5 or 0–10 cm for the assessment of the P and K fertiliser requirements of field crops and pastures. The usual soil sampling depth for vegetables and for fruit tree crops is 0–15 cm, while a 0–25cm sample is commonly used diagnostically for sugar cane. Deep sampling down to 60–100 cm may be necessary to better assess soil salinity, acidity, S-status, and mineral-N status. Since soil fertility varies down the profile, the sampling depth must be recorded. As tests such as extractable P typically decline with increasing depth, a sampling plan based on a 0–7.5 cm depth may suggest the site is more fertile than might be perceived by sampling the same field at 0–10 cm depth. Swelling of surface soils following recent cultivation and/or due to wetting also affects the accuracy and apparent bulk density of so-called fixed soil sampling depths.

As soil fertility can vary widely even over short distances, the area to be assessed must be divided into units of acceptable variability. This usually involves exclusion of small atypical areas such as fertiliser dumps, eroded gulleys, burn areas, etc. Sampling soon after applications of fertilisers and soil amendments should also be avoided. Oil used for lubricating soil sampling tubes and other equipment can be a direct source of contamination, especially during analyses for OC (Dowling et al. 1985).

Soil sampling errors can be minimised by using sampling equipment/sample containers known to be free of relevant contamination. An appropriate number of sub-samples is also essential. In practice, this usually involves making a composite from around 15 to 30 sub-samples from the area in question. One method of calculating the preferred number of sub-samples follows (Rayment 1985; Rayment and Higginson 1992).

Soil variability can be expressed by a 95% confidence interval:

where

M = mean value from n sub-samples

t(n – 1) = Student t value for (n–1) degrees of freedom at the 5% level of probability

s = standard deviation for n sub-samples

Proceed as follows:

1   Set value for the acceptable confidence interval; e.g. ± 4 mg/kg, i.e.

2   Estimate from a separate study (or accept from past experience) an expected value for the standard deviation (s). For example, analyse 10 sub-samples and use the formula:

3   Find the value of n by substitution; e.g. assume the estimate of s is 6 mg/kg, so by substitution,

  Taking the value of t(n – 1) as 2.0

  giving n = 9

4   For greater accuracy, consult statistical tables for the correct value of t(n – 1) for n = 9 (in this case t = 2.31) and recalculate a new value for n. Continue until a constant result for n is obtained.

5   Recalculate for each element/soil property to be tested and come to an appropriate compromise if multiple tests from the one sampling are intended.

Sample transport, laboratory preparation and disposals

Laboratories that accept unprocessed or non-sterile samples from other countries typically require prior accreditation from quarantine authorities, and need to obtain permits for introduction and subsequent handling of risky materials. In Australia, for example, many intrastate and interstate biosecurity restrictions and conditions apply, some prohibiting the movement or entry of soil samples. Important examples of biosecurity regulations for sample transportation and disposal on a state-by-state basis are provided by Rayment (2006).

IMPORTANT: Laboratory managers and staff should ensure, for bŠsecurity reasons, that their sample acceptance and sample disposal procedures are lawful.

In addition to attention to biosecurity requirements, soil samples should be kept cool or cold between field sampling and receipt at the laboratory (see Note 1 at end of this chapter). This is to minimise biological transformations and other chemical reactions that may result in changes in components of soil chemical fertility, such as the concentration of nitrate-N. Alternatively, soils may be air-dried remote from the laboratory (max. 40–45°C), when an estimate of field moisture content is not required.

After breaking up any large cores or peds on a clean surface, selectively remove by hand or by sieving any ‘rock fragments’ including any obvious concretions. When numeric data are required on >2 mm fractions, determine the weight percentage (oven-dry basis) of the >75 mm and 20–75 mm fractions, relative to total sample weight. Retain a representative portion of the soil in a sealed polyethylene bag or ‘moisture container’ if an estimate of field moisture content is required.

If the sample size remains too large, reduce by ‘coning and quartering’, or by a sample divider. Next (if specified in the method/s to be undertaken) spread the soil samples on drying trays and air-dry at ≤40–45°C. If expedited by applying a forced draft, the air supply should be naturally clean or otherwise filtered (or scrubbed) to remove dust particles and/or chemical vapours.

When the soil is thoroughly air-dry, mix, roll, and/or grind, using equipment similar to examples shown in Figure 1.1 of Chapter 1 (also see Note 2 at end of this chapter). Retain the <2 mm fraction, preferably in an airtight plastic or inert container, for subsequent laboratory analyses. When required, determine the weight percentage (oven-dry basis) of the residual >2–20 mm size fraction. The coarse fractions, which can be discarded, should not slake in either deionised water or in 10% sodium tripolyphosphate (Na5P3O7) dispersant.

When grinding to particle sizes <2 mm is specified (e.g. <0.5 mm) for one or more of the proposed tests, take a truly representative sub-sample (usually around 30 g) from the <2 mm portion. Pass the entire sub-sample through the required mill and store in a small, contamination-free, air-tight container. These samples must be well mixed prior to soil analysis, especially when there are visual signs of segregation. Also, follow grinding specifications as surface area and results can be affected.

Finally, every laboratory should develop and follow clear protocols on the retention of samples following analysis and on how samples should be discarded. As earlier indicated, all non-sterile samples should only be discarded in accord with biosecurity regulations. Similar regulations may also apply to the safe disposal of soil extract solutions, filter papers, and other laboratory wastes.

Effects of drying

Following a review, Etchevers (1986) concluded that temperature-controlled air-drying of soils is appropriate for routine testing where speed and consistency are necessary. Australian practice has been to dry at about 40°C (Rayment and Higginson 1992), whereas drying temperatures of from 30–35°C emerged in New Zealand (Metson 1961).

Air drying invalidates results for nitrite in soils (Keeney and Nelson 1982) and can affect – in variable directions – the results for other tests (e.g. Nelson 1977; Bartlett and James 1980; Shuman 1980; Leggett and Argyle 1983; Menzies et al. 1991; Rechcigl et al. 1992). Tests such as the pH of a hydrogen peroxide extract (Ford and Calvert 1970, Method 4E1) specify field-moist soil. Also, incubation of re-wetted air-dry soils for periods longer than one day are reported to cause an artificial elevation of ionic strength of soil solutions, through mineralisation of OM in some surface soils (Menzies and Bell 1988). Expected changes in analytical values as a consequence of air-drying – or freeze-drying, which has given similar results (e.g. Harris and Safford 1992) – are summarised in Table 2.2, with more details provided in later chapters. Sample storage may also affect results of tests such as phosphate-extractable S (see Chapter 10).

Soil moisture content

The moisture status of soil test results was introduced in Chapter 1. It is often overlooked or ignored that soils do contain sometimes quite significant quantities of moisture, following the attainment of constant weight at drying temperatures of ≈30–40°C. For example, soils high in OM and/or expansible 2:1 phyllosilicate minerals in the smectite family (montmorillonite, beidellite, saponite, nontronite, and several less common examples) can retain over 10% of their oven-dry weight as moisture, when ‘dried’ at relatively low temperatures. In contrast, siliceous sands low in OM may retain as little as 1–2% moisture, following drying at 40°C. Many of the methods described in this book specify corrections to account for residual soil moisture.

Table 2.2. Expected chemical changes in analytical results due to air-drying of soÕ (sourced from Etchevers 1986, Rayment 1993 and Brown 1999).

Method codes for air-dry moisture content, as received moisture content, moisture content – 10 mm tension, and moisture content – approximate saturation paste (Methods 2A1, 2B1, 2C1, and 2D1, respectively), are unchanged from those in Rayment and Higginson (1992).

2A1 Air-dry moisture content

Use this method when it is necessary to correct soil chemical results based on air-dry samples to an oven-dry basis.

When the air-dry moisture content (M%) is known, the correction from air-dry result to oven-dry result is as follows:

Apparatus

Calibrated laboratory oven, fan-forced and set at 105°C.

Moisture tins (with lids) of known weight.

Desiccator.

Procedure

Confirm the weight of each clean, dry, weighing container (W1 g). Weigh accurately from 10 to 50 g air-dry soil (<2 mm) into each container and record weight (W2 g). With lids removed, dry at 105°C to constant weight then quickly transfer to a dry desiccator (no desiccant) to cool. When cool, replace relevant lids and reweigh (W3 g) to determine weight of moisture [(W2 – W1) – (W3 – W1)] = W4 g.

Discard soil on completion in a responsible manner.

Calculation

Report as air-dry moisture content (%).

2B1 As received moisture content

When there is a need to know the moisture status of soils received at the laboratory, take a representative sub-sample and proceed as for Method 2A1.

Report as ‘as received’ moisture content (%).

2C1 Moisture content – 10 mm tension

The subjectivity of visual estimation of a saturated soil paste can be overcome by wetting-up the sample on a capillary saturation table (Longenecker and Lyerly 1964). The procedure given is a modification of that technique (Beatty and Loveday 1974).

The moisture content at 10 mm tension is obtained by calculation, following equilibration of air-dry soil (<2 mm) on a blotting-paper suction plate set at 10 mm water tension.

Apparatus

Blotting paper tension table

A base, constructed from flat, clear acrylic plastic, as described by Beatty and Loveday (1974), is shown in Figure 2.2. The surface slots are cut approximately 60 mm apart; leg height is not critical – up to 300 mm can be used.

Cut strips of blotting paper to fit between the slots in the acrylic plate. Fold these strips so as to pass up from the base, across the top of the acrylic plate, down through the next slot to the base, up through the next slot, etc. Place blotting paper the same size as the flat plate on top of the blotting paper strips and put the apparatus in a container fitted with a lid and an external constant water level device. A separate internal water level indicator is desirable to allow accurate calibration of water height. This should be 10 mm below the top of the acrylic plate (base of soil sample). Wet the strips of blotting paper sheet to equilibrium by capillarity from water added to the container, or by watering the strips and sheet and allowing them to drain to the free water surface.

Procedure

Place a numbered set of rings, e.g. 45 mm ID × 10 mm ht. brass, on individual filter papers (Whatman No. 1; dia larger than the rings) on the blotting-paper tension table. Adjust water level (deionised water) to 10 mm below the top surface of the tension table.

Using a standard scoop (16 cm³ for the ring described) place a sub-sample of each air-dry, ground (<2 mm) soil into the rings so that the levelled-off height does not exceed 10 mm. A level surface of soil and good soil-to-paper contact are necessary for reproducible results.

Figure 2.2. Acrylic base for blotting paper; portion of tension table for saturation extracts.

Replace the lid of the container to minimise evaporation and allow soils to wet by capillarity for 48 hours (h). During this period the water level must be accurately maintained; check after about 24 h; add deionised water as necessary if an automatic water level device is not available.

Remove individual samples of soil by sliding a broad spatula of thin cross-section between the blotting and filter paper. Remove the ring and invert the filter paper over a previously weighed dish and lid (M1). Do not wipe the filter paper on the side of the dish as this will give a high apparent water content. Weigh the wet soils plus dishes with lids to ±0.001 g (M2). Remove lids and dry soils at 105°C to constant weight. Remove from oven, cool in a desiccator, then quickly replace lids on correct containers. Weigh dry soil plus dish and lids (M3).

Calculation

Report result as moisture content – 10 mm tension (%).

2D1 Moisture content – approximate saturation paste

Many soil saturation pastes are based on visual estimations of the appropriate moisture content (Richards 1954). Saturation can normally be assumed when the soil paste glistens as it reflects light, flows slightly when tipped, and slides freely and cleanly from a spatula, except when soils contain much clay.

This method is used to determine the percentage moisture (oven-dry basis) at the point of visual saturation. It is applicable to air-dry soils ground to <2 mm, when a 10 mm tension table and moisture rings are unavailable. Once prepared, the saturation extract is removed for subsequent analyses.

Procedure

Initially determine the air-dry moisture content of each sample by Method 2A1. Next weigh the equivalent of X1 g oven-dry soil (<2 mm) into a container of known weight; weight of container + soil = X2 g. The preferred minimum weight of oven-dry soil equivalent is 100 g. A sample weight of 250 g, however, is convenient to handle and provides sufficient saturation extract for most purposes.

Prepare the saturated paste by slowly adding deionised water to the soil while stirring with a spatula; use a separate spatula for each soil to avoid cross-contamination and soil loss. Tap the container on the bench from time to time to consolidate the soil/water mixture. Note that there are difficulties in preparing ideal saturation pastes with some soils. For example, to minimise puddling in soils of high clay content, water should be added slowly with a minimum of stirring, especially in the early stages. Dry, peaty soils, especially if coarse-textured or woody, usually require overnight wetting-up, followed by a second wetting-up and remixing (SSIR 1984).

Allow to stand overnight. Saturation can be assumed if free water does not collect on the surface and the paste does not stiffen markedly or lose its glistening appearance on standing. If either of these conditions occurs, add more pre-weighed dry soil or water, respectively, and remix until an appropriate saturation paste is obtained. Record the weight of container + soil + added moisture = X3 g. Make necessary adjustments to X1, X2 and X3 for those samples that required further additions of air-dry soil to obtain appropriate saturated pastes.

Calculation

Report result as moisture content of approximate saturated paste (%).

Notes

1.  There is ample evidence that storage of moist soil below 4°C for days to a few weeks – or at freezing temperatures for longer periods – minimises biologically induced transformations in nutrient cations and anions. As air-drying brings soils towards a general equilibrium (Etchevers 1986), changes during storage, not due to contamination from the storage container, are likely to be small and slow.

2.  Piper (1944) recommended crushing rather than grinding but accepted the need to grind heavy-clay soils. The New Zealand designed Rukuhia roller-mill, fitted with a stainless-steel screen (Waters and Sweetman 1955; Metson 1961), is locally popular but performs poorly on many hard-setting Australian soils. Consequently, corn-crackers and stainless steel hammer mills have been successfully adapted for such soils. These mills are efficient, although they have the potential to artificially increase the surface area exposed to subsequent chemical reactions (Etchevers 1986). Unlike the Rukuhia soil grinder, grinding and hammer mills require prior removal of gravels and concretions.

References

Anon (1992) Soil Survey Laboratory Methods and Procedures for Collecting Soil Samples. Soil Survey Investigations Report No. 1. Soil Conservation Service, US Department of Agriculture, Washington DC.

Bartlett RJ and James BR (1980) Studying dried, stored, soil samples – some pitfalls. Soil Science Society of America Journal 44, 721–724.

Beattie JA and Gunn RH (1988) Field operations of soil and land resource surveys. In Australian Soil and Land Survey Handbook – Guidelines for Conducting Surveys. (Eds RH Gunn, JA Beattie, RE Reid and RHM van de Graaff) pp. 113–134. Inkata Press, Melbourne.

Beatty HJ and Loveday J (1974) Soluble cations and anions. In Methods for Soil Analysis of Irrigated Soils. (Ed J. Loveday) pp. 108–117. Technical Communication No. 54. Commonwealth Agricultural Bureaux, England.

Bowen GD and Rovira AD (1961) Plant growth in irradiated soil. Nature 191 (26 August), 936–937.

Brown AJ (1993) A review of soil sampling for chemical analysis. Australian Journal of Experimental Agriculture 33, 983–1006.

Brown AJ (1999) Soil sampling and sample handling for chemical analysis. In Soil Analysis: An Interpretation Manual. (Eds KI Peverill, LA Sparrow and DJ Reuter) pp.35–53. CSIRO Publishing, Melbourne.

Dowling AJ, Shaw RJ and Berthelsen S (1985) The influence on soil properties of mould oil used in sample cores. Australian Journal of Soil Research 23, 655–659.

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Fitzpatrick RW, McKenzie N and Maschmedt DJ (1999) Soil morphological indicators and their importance to soil fertility. In Soil Analysis: An Interpretation Manual. (Eds KI Peverill, LA Sparrow and DJ Reuter) pp. 55–69. CSIRO Publishing, Melbourne.

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Rayment GE (1985) Calibration and interpretation of soil chemical analyses. In Identification of Soils and Interpretation of Soil Data. (Ed GE Rayment) pp. 81–101. Australian Society of Soil Science Inc., Queensland Branch, Brisbane.

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3

Electrical conductivity, related attributes and redox potential

The soil solution plays a major role in the supply of nutrients to plants. It also contributes to the cycling, transformation and transport of nutrients and pollutants in soils and ecosystems (Agbenin 2003). Its key components are soil water and dissolved electrolytes, gases, and water-soluble compounds (Adams 1974). While it is possible to measure all constituents, this is usually impractical. Moreover, the composition of the soil solution is affected by plant nutrient uptake, root exudates, microbiological activity, fertilisation, leaching and other soil properties that vary in space and time.

An indication of the nature and ionic strength of the soil solution can be obtained from the EC of a soil/water suspension. This soil test provides a quick estimate of the concentration of electrically-charged water soluble salts able to enter and persist in the soil solution. These consist predominantly of the cations Ca²+, Mg²+ and Na+ and the anions Cl–, SO4²– and HCO3–. Fertilisers and animal manures often contribute other inorganic ions such as K+, NH4+ and NO3-. Soil EC values are unaffected by non-ionic solutes such as sugars, and ions that combine to form neutral ion pairs. The predominant mechanisms causing the accumulation of soluble salts in farmlands are heavy use of fertiliser, and (more commonly) the loss of water through evaporation and evapotranspiration, leaving ever-increasing concentrations of Cl salts in the remaining soil-water.

Visible effects of elevated levels of soluble salts include loss of stand, reduced plant growth, reduced yields, and in severe cases, crop failure and salt crystallization on the soil surface. High soil salinity may also cause specific-ion toxicities or upset the nutritional balance of plants. In addition, the salt composition of the soil-water influences the composition of cations on the exchange complex of soil particles that in turn affect soil permeability and tilth (Corwin and Lesch 2005).

There is no internationally agreed method for measuring EC for routine soil testing purposes, the main variant being the soil/water ratio. Common ratios include 1:1, 1:2.5 and 1:5, in addition to saturation extracts. A 1:5 soil/water ratio (EC1:5) has wide acceptance in Australia, as other determinations such as pH, water-soluble Cl and water-soluble NO3 can be made in the same aqueous extract. The 1:5 soil/water extract represents a dilution above field water content of from five to >40 times, which typically results in an overestimation of salinity when soils are light textured and/or when they contain sparingly soluble salts such as gypsum (Shaw 1988; Tolmie and Biggs 2000).

Based on the interpretative criteria of Bruce and Rayment (1982), most horticultural species, field crops and pastures prefer soils with very low (<0.15 dS/m) to low (0.15–0.45 dS/m) values of EC1:5. In contrast, high (0.90–2.0 dS/m) to very high (>2.0 dS/m) values of EC1:5, which correspond to high concentrations of soluble salt in the soil, reflect soil conditions unfavourable for all except salt-loving plant species. More detailed interpretative criteria that take account of soil clay content are summarised in Table 3.1.

Table 3.1. RelatŠnships (updated by Tolmie and Biggs 2000, from Shaw et al. 1987 and Shaw 1988) involving expected plant salt tolerance, soÕ salinity ratings, soÕ EC1:5 values and soÕ clay percentage.

There is increasing use of in-field EM mapping of ECa. This non-destructive technology indicates a variety of soil profile properties in addition to soil salinity. Included are spatial patterns of leaching fractions, irrigation and drainage patterns, compaction patterns due to farm machinery, etc. It is usual to calibrate EM data with laboratory measured soil EC levels. For example, Lesch et al. (1995) reported a relationship between relative responses of their EMh sensor and average EC1:5 (units of dS/m) of EC1:5 = 0.554 EMh, with EMh values in the range 0–7 and EC1:5 values in the range 0–5.

Four methods are described in this chapter. The first (method 3A1) is for EC1:5, involving use of a conductivity cell and a conductivity meter. The second (3B1) provides an estimate of soluble salts where these are dominated by Cl–. The third (3C1) provides a quick estimate of soil ionic strength. It is based on the linear relationship described by Gillman and Bell (1978). The fourth (3D1) is a field-based method for redox potential. Summary details on each are provided in Table 3.2. Information on accuracy and precision for Method 3A1 is provided in Appendix 2, Table App. 2.1.

3A1 Electrical conductivity (EC) of 1:5 soil/water extract

This method, updated from Rayment and Higginson (1992), is based on a 1:5 w/v soil/water extract with air-dry (40°C) soil. This soil/water ratio has been widely used in Australia and considerable data have accumulated. While it is realised sparingly soluble salts will contribute to a greater extent at this ratio than at more concentrated ratios, the values of EC are satisfactory for most purposes. When the same suspension is to be used for the determination of Cl– and pH, the EC should be determined first so there is no risk of Cl– contamination from the calomel reference electrode (or similar). Air-dry soil is preferred to oven-dry (105°C) soil, as the latter may convert at least part of any gypsum present to plaster of paris, which has higher water solubility (Rhoades 1982).

If soils contain more than about 1% of gypsum, the soil suspension will approach saturation and have an EC of about 2.2 dS/m. When much gypsum is present it will not be dissolved completely in a 1:5 soil/water suspension. A precise indication of soluble salts, however, loses significance in such soils.

Table 3.2. Summary detaÕs of laboratory and field methods described in this chapter.

EC values increase with increasing temperature and must be corrected if not measured at 25°C. An approximate correction can be made by increasing the values by 2% for each degree that the ambient temperature is below 25°C, and decreasing them by a similar percentage when the temperature is above 25°C. The EC1:5 is reported on an air-dry basis because the conversion to an oven-dry basis cannot be readily calculated. See Method 14B1 for the determination of EC/SE.

Reagents

Deionised Water

The water is to have an EC of <10-4 dS/m, and have a CO2 concentration no more than that in equilibrium with the atmosphere (refer to Note 1).

Acid-Dichromate Cleaning Solution

To 32mL of a saturated water solution of sodium dichromate (Na2Cr2O7) add 1.0 L sulfuric acid (H2SO4; 18 M). Handle with caution and care as this solution is both corrosive and a strong oxidant.

0.01 M Potassium Chloride Reference Solution

Dissolve 0.7455 g potassium chloride (KCl; previously dried at 110°C for 2 h) and make volume to 1.0 L with deionised water that is free of CO2. This solution has an EC of 1.413 dS/m at 25°C.

Procedure

Prepare a 1:5 w/v soil/water suspension. For example, weigh 20.0 g air-dry soil into a suitable bottle or jar and add 100 mL deionised water. Mechanically shake (end-over-end preferred), at 25°C in a closed system for 1 h to dissolve soluble salts. Allow around 20–30 min minimum for the soil to settle.

Calibrate the conductivity cell and meter in accordance with manufacturer’s instructions, using the KCl reference solution at the temperature of the suspensions.

Dip the conductivity cell into the settled supernatant, moving it up and down slightly without disturbing the settled soil. Take the reading with the cell stationary when the system has stabilized (see Notes 2 and 3). Rinse the EC cell with deionised water between samples and remove excess water. Complete EC measurements within 3–4 h of obtaining the aqueous supernatant. Reference soils should be included in each batch of unknown samples.

Report EC (dS/m) at 25°C on an air-dry (40°C) basis.

Notes

1.  High quality RODI that contains inconsequential traces of soluble OM is preferred for reagents and standard solutions. This equates to ASTM Type 1 grade of reagent water.

2.  If EC readings become erratic, clean the EC electrode by soaking it in Acid-Dichromate Cleaning Solution overnight, followed by thorough rinsing with deionised water. If the platinum black has flaked, recoat according to the procedure outlined in APHA (1998). Rinse electrodes thoroughly and keep immersed in water when not in use.

3.  The depth of insertion of the EC electrode should be checked against the 0.01 M KCl Reference Solution to determine locations where no effect on the correct reading occurs. With unshielded electrodes, small containers may be unsuitable for use.

3B1 Estimated soluble salt concentration

An approximate indication of the concentration of soluble salts in soil can be obtained by calculation from EC1:5. Assumptions include (a) that Cl– dominate the soluble anions in the soil, which is not always the case, and (b) the gram-equivalent weight of the soluble salts is around 51 or greater (Jackson 1958). In addition, the presence of gypsum can upset the relationship with EC1:5, as can the different ionic conductivities of particular soluble salts and the influence of soil surface properties.

Rayment and Higginson (1992) suggested the approximate percentage of TSS could be obtained by multiplying the EC1:5 value by 0.34, which derives from the relationships reported by Jackson (1958) and US Salinity Laboratory Staff (1954), with allowance for a 1:5 soil/water ratio. Earlier, Piper (1944) used a factor of 0.375, derived from actual correlations of specific conductivities (at 20°C) with the amounts of soluble salts determined gravimetrically in a large number of Australian soils.

While the value 0.336 is used in the following calculation, it is noteworthy that more refined estimates are available for particular Australian locations. For example, Williams and Semple (2001) reported the following relationship for saline seepage scalds from Central Western New South Wales, where measured soil salt concentrations ranged from 0.19–1.8%. Specifically, TSS (%) = 0.165 + 0.225*EC1:5, where EC1:5 has units of dS/m. This relationship gives higher values (relative to the calculation provided below) at EC1:5 values around 0.15 dS/m and similar values in the EC1:5 range 0.9–2.0 dS/m.

Calculation

Soil soluble salts (% air-dry soil) = [EC1:5 (dS/m) × 0.336]

Report approximate soluble salts (% of air-dry soil).

3C1 Estimated soil ionic strength

Most chemical soil tests give the total concentrations of all species of a particular ion. The effective concentration is the measured concentration only when there is no ion-pairing, hydrolysis or disassociation. Ultimately, the activities of major ions rather than their concentrations per se drive most chemical processes in the soil solution. As an example, ≈28.3% of the soluble Ca and SO4 in a 10 mM CaSO4 solution are paired as neutral CaSO4⁰, whereas there are 10 mM Ca²+ and 20 mM Cl– in a 10 mM CaCl2 solution (Adams 1974).

Soil solution ionic strength must be known in order to calculate ionic concentrations, ion-pair concentrations and ionic activities. Knowledge of soil ionic strength also assists in the development and choice of methods for assessing the cation and anion exchange capacity of soils, particularly those that are highly weathered. Ionic strength also influences the types of Al species in soil solution and their proportions, which influences whether or not the root environment may be Al toxic.

Gillman and Bell (1978) measured/calculated the ionic strength (I) and other soil solution characteristics of a range of highly weathered soils from North Queensland. They found that I at 0.1 bar soil moisture (across an ionic strength range of zero to 14 mM) was strongly correlated with EC1:5. That relationship provides the basis for this useful soil test.

Calculation

Soil ionic strength at 0.1 bar (I0.1) = [0.0446*EC1:5 – 0.000173]

where I0.1 has units of mM, and EC1:5 has units of dS/m @ 25°C.

Report estimated soil ionic strength as mM.

3D1 Redox potential (Eh; field)

Redox potential is an electrical measurement that indicates the oxidation-reduction status of natural and custom-made soils. It is expressed numerically as Eh, with units of mV. Soil Eh is a valuable but difficult-to-measure attribute that helps when assessing soil health and genesis, in addition to providing guidance on soil fertility and the status of soil contaminants (Liu and Yu 1984). Eh reflects the tendency of a soil solution to transfer electrons to or from a reference electrode; i.e. it is a measure of electron pressure/availability in the soil solution (Vorenhout et al. 2004). From this, an estimate of whether the soil (particularly sub-soil) is aerobic, anaerobic, or a mix of the two is obtained. These conditions influence whether chemical compounds such as Fe and Mn are chemically reduced or mostly present in their oxidised forms (Vepraskas and Faulkner 2001; Table 3.3).

Soil redox values are dependent on measurement conditions and other soil properties. Slow electrode response in poorly poised (redox capacity) soils (e.g. Ponnamperuma 1972; Fiedler et al. 2007) is a further challenge. Usually, measurements are made in the field. Here, apparent redox potentials are influenced by drainage, the presence of reactive chemicals, and/or exposure of the soil to atmospheric interactions. Measures of both oxidation and reduction are required in Soil Taxonomy (Soil Survey Staff 1999) for ‘aquic’ conditions, although the duration of reduction is unspecified. Hydric soils are able to support the growth and regeneration of hydrophytic vegetation.

Redox reactions involve the transfer of electrons from reductors (e– donors or reducing agents) to oxidants (e– acceptors or oxidising agents). James and Bartlett (2000) provide many examples.

The capacity factor in redox is referred to as poise (pe) and is defined as the change in added equivalents of reductant or oxidant to bring about a one unit change in pe (or an Eh change of 59 mV). The concept is similar to that of buffer capacity for pH (Stumm and Morgan 1996).

The field method described for quantifying electron activity detects the potential difference between a Pt indicator electrode and a Ag/AgCl reference electrode, both connected to a millivoltmeter (e.g. Jackson 1956; James and Bartlett 2000; Vorenhout et al. 2004). The method assumes the Pt electrode is inert and does not react chemically, while achieving equilibrium with the electro-active species in the soil. Diurnal fluctuations are common and changes in Eh with soil depth are the norm.

Table 3.3. Examples of approximate Eh values for soÕs of different oxidatŠn status and at points of transitŠn for important redox pairs at pH 7.0.

† From Table 14.1 of Jackson (1956)

†† From Table 1 of Vorenhout et al. (2004)

Reagents

Deionised Water

The water is to have an electrical conductivity of <10-4 dS/m, and have a CO2 concentration no more than that in equilibrium with the atmosphere. See Method 3A1 for more details.

≈ 4 M Hydrochloric Acid (HCl)

Prepare by adding ≈195 mL concentrated (ρ = 1.16 g/cm³; 31.5–33%w/v) HCl to about 200 mL deionised water slowly and with stirring. Cool, then dilute to 500mL.

1 M Hydrochloric Acid (HCl)

Aqua Regia

To 3 volumes of HCl slowly add 1 volume of HNO3 with stirring. Store the mixture in a safe location.

Electrode Platinizing Solution

Dissolve separately 1.0 mg platinic chloride [Cl6H2Pt.6H2O; also called chloroplatinic acid hexahydrate] in deionised water and 7.0 mg of lead(II) acetate [Pb(CH3COO)2] in deionised water, combine, mix well and make to 30 mL. This solution suffices for up to 100 electrodes each consisting of a 1–2 cm length of Pt wire.

≈ 0.05 M Potassium Hydrogen Phthalate pH 4.0 Buffer

Prepare an ≈0.05 M solution (actually 0.0496 M) by dissolving 10.12 g potassium hydrogen phthalate (KHC8H4O4; previously dried for 2 h at 110°C) and make to 1.0 L with water described for use with buffer solutions (Alvarez 1984). Exclusion of CO2 is unnecessary, but protect against evaporation and contamination. Store for up to one month but replace solution if mould appears.

3 M Potassium Chloride

Dissolve 223.7 g potassium chloride (KCl) and make to 1.0 L with deionised water.

Ferrous-Ferric Solution (Light’s Solution, for redox measurements and testing of electrodes)

This aqueous buffer solution of known and stable redox potential (+476 ± 20 mV with an Ag/AgCl reference electrode) was described by Light (1972). Prepare by dissolving/diluting each of the following three reagents in deionised water. Combine, mix thoroughly and dilute to 1.0 L in a volumetric flask. The three analytical grade reagents and weights/volumes of each are as follows:

1   0.100 M Ferrous ammonium sulfate [Fe (NH4)2 (SO4)2.6H2O]: 39.21 g/L

2   0.100 M Ferric ammonium sulfate [Fe (NH)4 (SO4)2.12H2O]: 48.22 g/L

3   1.0 M Sulfuric Acid [18 M H2SO4]: 56.2 mL/L (cool before combining with the other two reagent solutions).

Apparatus

Millivolt–pH–Eh Meter

Ideally, the meter should be portable, sensitive and reliable, able to operate reliably across the range +800 to –600 mV or wider.

Figure 3.1. An example of one type of fabricated Pt-electrode for redox measurements. A Pt-wire length of 10–15 mm is ideal.

Platinum blackened platinum electrode/s

Purchase commercially or manufacture (see Figure 3.1) by sealing a short length of clean Pt wire in a glass tube, aided by means of lead glass melted around the Pt wire (or an equally effective, inert bonding agent). Further up, the Pt wire should be spot-welded (connected)