8
TESTING METHODS
8.1 Introduction
Testing procedures for soils are often complex and plagued by interferences. This is because there is
such an enormous difference between soil types, and much lower levels of homogeneity than is found
in air or water. This means that both the sampling and the analysis procedures can often be unreliable
unless strict guidelines are followed. Sometimes even with careful scrutiny, and adherence to
procedures good results are still not obtained – and many replicates must be obtained to provide a full
picture of the soil.
This chapter examines the common methods used for testing of soils. It is impossible to cover
all of the analysis used for soils, but instead the most common techniques are examined. These vary
from country to country (and sometimes from region to region) as soil profiles can change
dramatically, even in a small area.
8.2 The measurement of soil properties
Our use of soils changes its properties. It also influences both the soils interaction with the
environment and it’s ability to produce crops. It is important that these properties should be measured
and the measurements understood to allow better understanding of environmental issues and more
effective management of agricultural activities.
Much useful information can be obtained by observation of soils in the field, but this
observation is often highly subjective. This chapter deals with the measurement of soil properties as a
means of obtaining an objective understanding of our use of soils and the environmental implications.
There are three approaches to the measurement of soil properties:
•
test pits – normally semi-quantitative assessments
•
in situ – use equipment inserted into soil, without significant disturbance of the soil (especially
for water-related measurements
•
in the laboratory – done on soil samples taken from the field.
This chapter deals with soils using these three approaches. It gives guidance on:
• the choice of site for a test pit, digging procedure and sampling methods
• describes methods that can be used to assess texture, stone content and porosity.
• discusses soil variability and describes procedures to obtain representative samples from fields or
plots.
• the measurement of basic soil properties and nutrient and water availability
• provides methods to assess soil fertility
• examines problems resulting from our use of soils such as salinity and pollution of soils by
pesticides and metals
Ideally erosion should be included but we do not have sufficiently simple methods which can be used
to give meaningful data, and so this topic has been omitted.
8.3 Analysis in the field
As is the case with most analysis, field data often minimises sampling error, but it does not guarantee
reliable or even consistent data. In the case of soils where homogeneity is very poor (a common
occurrence) great variation may occur even within sampling points which are only a few meters apart.
None the less much of the physical data from soil analysis is best (or can only be) obtained in the field.
Some of the more important field analysis techniques are discussed in the following pages.
8. Testing Methods
Soil survey reports and land descriptions
A Soil Survey Report tells what kinds of soil exist in an area. The soils are described in terms of
location, profile characteristics, relationship to each other, suitability for various uses, and needs for
particular kinds of management. Each report contains information about soil morphology, soil origins,
soil conservation, and soil productivity. This information allows the user to determine the suitability of
a soil for such uses as crop production, growing pasture or trees, septic tank installation, road
construction, pond sites, wildlife, etc.
There are two major parts in a soil survey report: the soil map and the narrative.
The soil map usually consists of several sheets in the back of the report and is printed on aerial
photographs. A boundary line surrounds each soil area on the map. Some soil boundaries
approximately follow contour lines, but these two kinds of lines should be very carefully
distinguished. A contour line connects points of equal elevation and cannot intersect other contour
lines except where there is a cliff. Soil boundaries, on the other hand, are drawn wherever there is a
significant change in the type of soil and may cross contour lines at any angle. Soil boundaries
intersect each other in complex patterns somewhat akin to the design of a jigsaw puzzle.
The narrative gives meaning to the soil map. This portion of the report gives names to the soil
symbols placed within each soil area on the map and discusses the nature of the soils. Thus, the user
can consider the potential and limitations of specific soil areas.
In NSW, there is a major soil survey database called SALIS (Soil and Land Information
Service) run by the Department of Natural Resources1.
USING A SOIL SURVEY REPORT
A user interested in an overall picture of the soils in an area county turn first to the soil association
section in the narrative portion of the report to find a discussion of the general soil pattern. A user
interested in the soils of a particular area must first locate that area on the map and determine what
soils are there. Index sheets are included to help find the right portion of the map. The map legend
gives the soil names for each symbol.
Each report contains a section of soil descriptions that serves the needs of those who are
interested primarily in the nature of the soils. Another section covers the use and management of the
soils and is most helpful to those who use the soil or who give advice on and assistance in its use
(farmers, graziers, soil conservationists, economists, real estate agents, mining companies, etc.).
Management needs and estimated yields are included in this section. Most reports have tables of
engineering properties of soils that are useful to highway engineers, sanitary engineers, and engineers
who design water storage or drainage projects. Information is included regarding the suitability of a
particular soil for use as topsoil and which soils are underlain by sand and gravel that might be used
for road construction. Limitations are listed for many uses, including camping sites, playgrounds, golf
course fairways, septic tank filter fields, and building sites. A soil survey report can be useful to
people in many different professions.
Soil Pits
If soil samples are required from deeper than about 20 cm, either an auger can be used or a pit must be
dug. The former will provide only limited information because the samples are taken below a small
area of the surface (a few cm2) and are disturbed. The exposure of a soil profile in a pit allows
information to be obtained both on the vertical arrangement of soil material into horizons and their
horizontal variability. Soil structure, porosity and other features, some of which would have been
disturbed by an auger, are displayed for observation and measurement. The choice of sampling method
therefore depends on the purpose of sampling.
Before going into the field to investigate or sample soils it is important to gain as much
information about the area and its soils as possible. Books and maps produced by both local and
national surveys may be available which deal with the topography, geology and soils of the area.
1
The SALIS website is http://www.dnr.nsw.gov.au/soils/about_salis.shtml
49
8. Testing Methods
CHOOSING SITES FOR SOIL PITS
It is a good idea to do a full walk around the area in order to view the site from more than one position.
An understanding of the landscape aids understanding of the distribution of soils. Refer back to the
topography, geology and soil maps of the area. Use a screw auger to establish the general properties of
the soils, making a number of borings to find the 'most representative' site for your purpose.
It is impossible to give guidance on exactly where to sample because of the wide range of
reasons for sampling, but it is sensible to exclude some sites on the basis of excessive disturbance.
These include old roads, where old buildings once stood or where river or field drain dredgings have
been dumped. For most purposes the following should also be avoided:
•
headlands of arable fields (the outer 10 m)
•
areas close to gateways, paths and tracks
•
sites where straw or fertiliser have been stored;
•
sites used for localised burning of crop residues or hedge trimmings;
•
old field boundaries where a hedge or bank has been removed and the land levelled.
Note that although the 'most representative' site may be chosen in terms of spatial variability over an
area, temporal variability (changes with time) means that some observed and measured properties are
only representative of the soil at the time of sampling. Most obviously, soil water content varies from
day to day, and this and root distribution vary seasonally. Less obviously, biological activity varies
seasonally, causing changes in aeration and the amounts of available nutrients. Agricultural
management also causes seasonal changes particularly in the topsoil through cultivation and
fertilisation, and in the longer term liming and subsequent re-acidification change soil pH values.
Other properties are more permanent, for example horizon depth, texture, stone content and ion
exchange capacity.
DIGGING A SOIL PIT
Equipment
Spade, auger, pickaxe, trowel, small knife, wooden shoring and props, polythene sheets,
25cm paint brush.
METHOD
Observe and record features of the site and the soil surface following standard procedures
for a Soil Survey.
Pit size and orientation
The area extent of the pit depends on the required depth of observation and sampling. A pit
to 1 m depth should be about 1 x 1.5m in width.
The face of the pit to be used for observation, photographing and sampling should face the
sun.
Excavation
Mark out the area of the pit. To preserve the characteristics of the upper few cm of soil avoid
treading the soil on top of the face to be examined. Lay a polythene sheet alongside the pit. If
the soil is covered with grass or other vegetation, cut square turf sections and place them on
the sheet maintaining their relative position. Excavate the soil keeping the topsoil and subsoil
in separate piles. Record the conditions encountered when digging, for example dense or
stony layers.
If the soil pit is greater than about 1m in depth it may be necessary to support the walls with
shoring to prevent collapse. If there is a danger of collapse, there must always be two
persons at the site during excavation. It may be necessary to dig out steps to facilitate entry
to and exit from the pit.
50
8. Testing Methods
Cleaning the profile
The faces of the pit will have been cut and smeared by the spade. Using a trowel or knife
pick soil from the face from the surface downwards to remove contamination and expose the
features of the soil horizons. In a dry soil finally clean the face with a small brush.
Profile description Follow standard procedures (see section on soil profiles).
Leaving the site
A pit should only be left unattended during the day if there is no possibility of people or
animals falling into the hole. A warning rope should be placed around the site. If the pit is to
be left overnight it should be fenced or covered with boards. When work is complete the soil
should be replaced in the correct sequence. Tread the soil occasionally to compact it into the
hole. Finally replace the turf sections and tread them into place, leaving the site as you found
it.
Study of soil profiles
Soil profiles (see Chapter 3 for detailed description) may be examined in pits, in road banks, or in
mounted sections called soil monoliths. The soil profile should be as well exposed and as undisturbed
as possible.
The best way to begin examining an exposed soil profile is to locate the major horizons. Look
for a dark-coloured A horizon (Ap if plowed) at the surface. See if there is a light coloured E horizon
below that. Look for a brighter colour in the subsoil (B horizon). At the bottom there will be either
hard rock (R) or unconsolidated material not much affected by soil-forming processes (C horizons).
Some profiles have organic layers (0 horizons); some have transitional horizons or subdivisions within
major horizons. The horizons present are an important characteristic of any soil. For example, the
presence of an E horizon probably indicates either a very old soil or one developed under forest
vegetation.
After the horizons are located, the study of the soil profile becomes a study of the individual
horizons. Each horizon has texture, structure, colour(s), reaction (pH), etc. that can be identified and
described. Some of these characteristics can be seen from a distance; some can be determined only by
detailed examination and/or laboratory work.
The kind and sequence of soil horizons determine the soil series to which the soil belongs.
Official soil series descriptions tell what horizons a soil must have to belong to a particular series and
set limits on some of the important characteristics of horizons within the soil profiles.
Soil users learn to recognise their properties and becomes aware that soils differ greatly from
one another. These differences influence the productivity of the soils and their suitability for specific
uses. A single land area may have several soil series, each with a different potential productivity for
crops and different suitability for other kinds of uses.
8.4 Sampling
Soil samples can be taken either with disturbed or undisturbed stratification. Disturbed strata samples
are taken without consideration of the natural texture (structure). Undisturbed samples are taken from
the earth by means of a cutting cylinder (core sampler or auger) in such a way that their original
condition (structure) is maintained.
The soil sample must be representative of the whole area to be examined. This condition is not
easy to satisfy even on mixing many single samples since heterogeneity can vary enormously. For
collection, a random distribution of sampling points would be ideal but in relatively homogeneous soil
pooled samples of almost equally good quality can be obtained with less effort by reducing the
sampling area. Such sampling techniques are illustrated in Figure 8.1.
51
8. Testing Methods
random
diagonal line
cross line
test lot
FIGURE 8.1 Some typical soil sampling techniques
For the sampling of soil at least 20 single samples per 10 000m2 must be taken with an earth boring
tool (or spade) and combined to a mixed sample. The usual sampling depth is up to 20 cm in arable
land or 10 cm in pasture. Undisturbed soil samples are obtained with a cutting cylinder with minimum
capacity of 100 mL. For special examinations (e.g. testing of nutrient penetration), samples are taken
down to 1m depth after a soil pit has been dug out to reveal a smooth vertical earth profile.
Another widely used sampling strategy for agricultural purposes is to walk along a W shaped
path taking at least 25 samples which are bulked, as shown in Figure 8.2 for a field. A smaller number
of samples may be adequate. Sampling of arable fields can be by auger normally to 15 cm depth
(screw or bucket depending on the amount of soil sample needed). Grassland should be sampled using
a cheese-type corer or tubular corer to avoid losing the top few cm of soil, and samples are normally
taken from the top 7.5 cm.
FIGURE 8.2 W shaped sampling strategy for a field
Samples for nutrient testing should be taken where possible at the same time of year at best after
harvest and before application of fertiliser. Sampling is also possible during the main growth period.
For sampling to determine the water profile of soil, a thorough profile description is necessary
(e.g. structure, presence of roots, layer changes). The cutting cylinder is pushed steadily in the vertical
or horizontal direction in order to avoid compression.
After collection, the samples are packed in plastic bags. When tests for micronutrients are likely to be
included in the analytical suite of soil chemical methods, uncontaminated, heavy duty polyethylene
bags are preferred for sample collection.
Each sample is provided with an identification tag. For longer storage, disturbed samples are
air-dried, milled (stones removed) and sieved. The fine earth (particles < 2 mm) is then used in tests.
Sampling from a field or plot
Frequently the aim of sampling a field or a plot is to obtain a 'representative' value for a soil property
such as salinity or organic content. Occasionally however, it may be important to obtain information
about the nature of the spatial variability of that property. Both purposes can be achieved by taking a
number of samples over the area. In the first case the samples are bulked and sub-sampled to give a
52
8. Testing Methods
representative sample, and in the second case the samples are analysed separately, in a similar manner
to the characterisation of a horizon in a soil pit.
Because of the need to take many samples, it is not normally feasible to dig pits at each
location, and a soil auger or corer is therefore used.
Sampling using augers
Augers vary in size and design (see Figure 8.3).
These augers are rotated and pressed into the soil
to take samples from depth increments of 15-20
cm. The samples are inevitably 'disturbed' to varying degrees and so the observations that can be
made on the samples will be restricted: colour,
texture, stones, roots and horizon depth can be
recorded but soil structure cannot. Special coring
equipment is required to obtain 'undisturbed'
samples.
To avoid contamination between successive
vertical samples care is needed in placing the auger
back into the hole and in extracting it and the auger
should be cleaned between samplings.
Field sampling: For laboratory testing
Sampling protocols relevant to important crop and
soil tests should be followed. For example, it is
common to sample soils at either 0-7.5 or 0-10 cm FIGURE 8.4 A range of augers: A – screw; B –
for the assessment of the P and K requirements of bucket; C – sampling tube; D – Dutch mud; E –
field crops and pastures. The usual soil sampling peat (from http://soils.usda.gov )
depth for horticultural and tree crops is 0-15 cm.
Deep sampling down to 60-100 cm may be necessary to better assess soil salinity, acidity, S, and
mineral N status. Since soil fertility varies with soil profile depth, the sampling depth must be
recorded.
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, erosion gullies, 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 organic carbon and
as such should be avoided where possible.
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.
Laboratory preparation of samples
Soil samples should be kept cool or cold between field sampling and receipt at the laboratory. This is
to minimise biological transformations and other chemical reactions that may result in changes in
apparent soil chemical fertility. Alternatively, soils may be air-dried remote from the laboratory (max.
40°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
sieving any rock fragments. If required, 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 (see next section), or by a
sample divider. Next spread the soil samples on drying trays (if applicable) and air-dry at up to 40°C.
53
8. Testing Methods
When the soil is thoroughly air-dry, mix, roll, and/or grind. Retain the <2 mm fraction, preferably in
an air-tight 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.
When fine grinding is specified, take a representative sub-sample (usually around 30g) from the
<2 mm portion. Pass the entire sub-sample through the required mill and store in a small air-tight
container.
SAMPLE SIZE REDUCTION
Often the size of a field sample is large as many smaller samples may be pooled to try to get the
overall picture of soil quality (this does not apply if spatial differences in a soil are being examined).
In this case sample size reduction techniques need to be employed. The most common of these are the
use of riffles, coning and coning and quartering, which you should have covered in an earlier
laboratory subject.
See Practical Exercise SA01
8.5 Physical testing methods
Many soil properties can be described once a soil profile has been exposed. Physical testing actually
means testing of physical properties, those characteristics of the bulk material, such as colour, texture,
porosity and conductivity.
Some properties are assessed qualitatively, for example colour, using standard Munsell colour
charts. Others are assessed semi-quantitatively: for example, the presence of roots may be recorded as
'many coarse roots, few fine roots' with 'many' and 'few' being based on the approximate number of
roots per unit area of the exposed profile, and ' coarse' and 'fine' based on root diameter. The
assessment of quantity is often aided by the use of standard charts: for example, the quantity of stones
exposed in the profile can be compared to standard stone charts.
Soil colour determination
Colour is described by using a Munsell Colour
Chart. Munsell notations describe colour in terms
of hue, value, and chroma.
Hue refers to the colour of pigment that
must be mixed with black and white (or the proper
shade of grey) to produce the colour to be
matched. Soils range in hue from red (R) through
yellow-red (YR) to yellow (Y) with some spots of
green-yellow (GY) or even green (G). The
continuous range of hue that exists between these
major colours is subdivided by number prefixes
ranging from 0 to 10.
Numbers ranging from 0 for absolute black
to 10 for absolute white designate value.
Quantitatively, value is equal to the square root of
the percentage of light reflected. In the colour
book, the values range from 2 at the bottom to 8 at
the top of each page.
Chroma is an indication of the amount of
pigment that must be mixed with the proper value
of grey to produce the particular colour. Pure grey
colours have 0 chroma. Increasing brightness is
indicated by chromas up to about 8 in soils or to 20
for the total range of colour. Chroma is scaled
across the colour book page with grey colours (0
54
FIGURE 8.5 A page from a typical Munsell colour
book. The rectangles represent coloured chips that
are compared to the soil colour.
8. Testing Methods
chroma) on the left side of the page to brighter colours (8 to 10 chroma) on the right side of the page.
Figure 8.5 illustrates the arrangement of colour chips on a given page (hue MYR) showing the
value and the chroma.
The first step in determining a soil colour is to locate the closest hue by turning the pages of the
colour book. Think in terms of "yellower" or "redder" while doing this. The most common hue in
temperate region soils is lOYR. Warmer climates commonly have redder soils. Wet soils often have
yellower colours. Next, the soil is held near the chips on the chosen page and moved up or down to
match value and left or right to match chroma. The nearest colour chip is taken as the soil colour.
Soil colour is best determined in sunlight with the light coming over the shoulder. Dry soil is
usually about 2 units higher in value than the same soil moist. There may be a difference of 1 unit in
the dry and moist chromas of a soil; the hue generally remains the same.
Soil Texture
This is probably the most commonly used descriptor of a soil (see Chapter 3 for coverage of the
theory). A soil is allocated to a textural class, depending on its content of sand-, silt- and clay sized
particles. In the field, the textural class can be determined subjectively from the feel of a moist soil
moulded between the fingers and thumb because the particle-size distribution influences the
mechanical properties of the material. With experience, a soil scientist comes to know the feel of each
textural class and can accurately allocate a soil to a class. Over the years schemes have been developed
to enable scientists to 'learn the trade', and have been brought together to give the Texture Triangle
(Figure 3.1).
EXERCISE 8.1
Assessing soil texture by finger feel.
Equipment
Wash bottle, old cloth, spatula.
Method
As far as possible use only one hand for the soil sample, keeping the other hand clean or at
least reasonably dry for writing down your results.
1.
2.
3.
4.
5.
6.
7.
Take about half a handful of soil from the profile face.
Remove 'foreign bodies' such as roots, seeds and insects.
Remove stones to leave the fine earth fraction. Although all particles >2 mm should be
removed, in practice very small stones may remain.
Add a little water from a wash bottle, allow the soil to absorb the water and then work
(mould) the moist soil in the hand and then between the thumb first two fingers until the
soil is uniformly moist and has been broken down into its individual particles. Clay soils
are initially dry and need much more working to satisfy these requirements.
Add more water or more soil, working the soil until the sample is at its sticky point, i.e.
the condition in which the soil being wetted just begins to stick to the fingers. Clay soils
may seem to become drier as they are worked due to the continued absorption of
water. More water may need to be added until the condition of the soil is stable.
Follow the guidelines in Figure 8.6 and record a textural class.
Wipe residual soil off the hand thoroughly before taking another sample of soil for
texture assessment.
55
8. Testing Methods
FIGURE 8.6 Flowchart for assessing soil texture (from http://www.mt.nrcs.usda.gov/about/lessons/soil.html)
Results
56
8. Testing Methods
There are also more objective means by which the proportion of the various particle sizes can be
determined. You might expect sieving to be the means, but it is too time consuming, so an indirect
way using the different rate of settling in water of different sized particles.
THE SETTLING RATE METHOD
It has been found that for the silt-size particles plus some of the coarse clay, the rate of settling is
proportional to the square of the diameter of the particles, and the constant relating the two measures
varies only with the temperature of the water. For room temperature, it is equal to 6000, and this gives
a relationship as shown in Equation 8.1.
v = 6000d2
Eqn 8.1
where v is the settling velocity in cm/minute and d the particle diameter in mm (it is an assumption of
this method that the particles are spherical).
The two sizes needed for determining soil texture are the limits between sand and silt (0.05 mm)
and between silt and clay (0.002 mm).
EXAMPLE 8.1
Calculate how far a 0.05 mm diameter particle would travel in one minute.
v = 6000 x 0.052
= 15 cm/min
Therefore, in 1 minute, particles of this size would have travelled 15 cm.
EXERCISE 8.2
Would larger particles than 0.05 mm travel more or less than 15 cm in this time?
What does this mean about the sand fraction?
Calculate how long a 0.002 mm diameter particle would take to travel 1 cm.
57
8. Testing Methods
How to apply this in the laboratory
If a uniform suspension of soil in water is allowed to settle for one minute and a sample is then
withdrawn from a depth of 15 cm, that sample will contain no particles larger than 0.05 mm diameter.
Larger particles will have settled beyond that depth even if they started at the surface.
For every particle smaller than 0.05 mm that has settled from the 15 cm depth, another similar
particle should have arrived from above. Thus, the smaller particles are still at their original
concentration at that depth. It is, therefore, possible to determine the concentration of particles smaller
than 0.05 mm – silt and clay – as it was in the original suspension by measuring the total concentration
in this sample. This measurement is made by evaporating the water and weighing the soil.
After a longer time period, the same procedure gives the concentration of particles smaller than
0.002 mm.
EXERCISE 8.3
A 110g sample of moist soil was weighed for analysis and a 55g sample was placed in Basin
1 in the oven to dry. The 110g sample was dispersed and placed in a settling cylinder.
Distilled water was added to make a total volume of 1000 mL.
The cylinder was inverted several times, shaken, placed on the table, and a 25 mL
sample was withdrawn one minute later from a depth of 15 cm. This sample was placed in
Basin 2 and dried in the oven. This contains the silt and clay particles (< 0.05 mm).
0.002 mm particles settle 3 cm in 2.08 hours (2 hrs and 5 min). A 25 mL sample was
taken at that time from the 3 cm depth and placed in Basin 3 which was dried in the oven.
This contains the clay particles (< 0.002 mm).
After the three samples dried for 24 hours, the weights of the soil residues were:
Basin 1 = 49.10g (oven-dried soil)
Basin 2 = 2.04 g (silt + clay)
Basin 3 = 0.40 g (clay)
Calculation starter hints
If 55 g of soil oven dried to 49.1 g, then the 100 g sample analysed must weigh 98.2 g oven
dry, which is the basis for the %composition calculations.
The sample proportion was 25 mL from 1000, so a scaling of factor of 40 is needed.
(a)
Calculate the mass of silt + clay in the total sample.
(b)
Calculate the mass of clay in the total sample.
(c)
Calculate the mass of silt.
(d)
Calculate the % of silt and of clay, and use this to determine %sand.
(e)
What is the soil texture?
58
8. Testing Methods
Several factors complicate soil texture determinations, most of which have already been mentioned.
1.
The procedure cannot be used for subdividing the sand fraction into separates because the
settling velocity of sand is too fast, and the relationship in Equation 8.1 does not apply. This
limitation is overcome by measuring the sand separates with sieves.
2.
The shape of the particles influences their settling velocity. Clay particles tend to be flat instead
of spherical, and what we actually calculate is an equivalent diameter (the diameter of a sphere
that would settle at the same rate) that is smaller than the width but larger than the thickness of
the flat particle.
3.
Clay particles tend to stick together and settle as larger groups at faster rates than calculated.
Two causes for this are:
• organic matter which should be removed by treating the soil with hydrogen peroxide for
several hours.
• flocculating cations, especially Ca2+, Mg2+, and H+; these can be replaced with Na+ (a
dispersing cation) which is added to the suspension with a suitable anion to unite with the
problem ions; suitable reagents include sodium silicate (water glass), sodium oxalate, and
sodium hexametaphosphate (Calgon).
See Practical Exercise SA02
Water
The amount of water present in the soil at any given time is most commonly expressed as a percentage
of the oven-dry weight of the soil (which is the weight of the soil solids for all practical purposes).
This percentage is commonly determined for the field capacity of the soil and for air-dry soil.
See Practical Exercise SA03
Conductivity
This is best determined quantitatively by a conductivity meter. Conductivity (EC) is an important
parameter when assessing salinity.
This method is based on a 1:5 soil/water extract. This soil/water ratio has been widely used in
Australia for many years and a considerable database has accumulated. While it is realised that
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- concentration and pH values, the EC should be determined first so that there is no
risk of contamination from the calomel reference electrode.
Should soils contain more than about 1% of gypsum, the soil suspension will approach
saturation and have a conductivity of about 2 dSm-1. When much gypsum is present it will not be
dissolved completely in a 1:5 soil/water suspension. However, a precise indication of soluble salts
loses significance in such soils.
Electrical conductivity values increase with increasing temperature and must be corrected if not
measured at 25ºC. An approximate correction can be made by increasing the values by 2% for each
degree that the ambient temperature is below 25ºC, and decreasing them when the temperature is
above 25ºC.
There is no clear relationship between EC (1:5 soil/water) and total soluble salts due to the
different ionic conductivity of the various salts and to the influence of the soil particles. An
approximate value for percentage total soluble salts may be obtained by multiplying the EC at 25ºC
(dS m-1) by 0.34.
The EC is reported on an air-dry basis because the conversion into an oven-dry basis cannot be
calculated easily.
See Practical Exercise SA01
59
8. Testing Methods
Stone Content
Stones are for most purposes an inert component of the soil. Although of interest to the geologist and
pedologist as an indication of the history of the site and soil, in the context of these notes, they affect
soil fertility by taking up space which would otherwise be occupied by fine earth. Thus the ability of a
given volume of soil to hold water and nutrients is reduced. Stones are also a hindrance to cultivation.
For these reasons they are included in soil fertility and land use capability classifications.
ASSESSMENT OF STONE CONTENT
After cleaning the profile, estimate by eye the
percentage stone content using the charts in
Figure 8.7. Essentially an area percentage is
determined which is assumed to be equal to a
volume percentage in the soil, this being the
measure that is normally required for assessing
the effects of stones on soil fertility.
MEASUREMENT OF STONE CONTENT
Normally the stone content is expressed as a
percentage by volume. The stones in a soil
sample taken from a measured volume in the
field are weighed and their volume determined
after measuring their density in the laboratory.
Differences in stone porosity have to be taken
into account. A percentage by mass can also be
determined.
Soil pH
Changes in soil pH have significant impact on
the physical and chemical properties of the soil.
pH measurements are normally made using a
suspension of soil in water, which is tested with
a calibrated pH meter.
Soil solutions are well buffered. When
distilled water that has a pH of about 5.6 is
added to a soil having a different pH, the pH of
the water changes to be close to that of the
FIGURE 8.7 Field assessment of stone content
solution in the moist soil. This is brought about
(from DL Rowell, Soil Science Methods and
Applications)
in acid soils by Ca2+ and Al3+ in soil solution
+
moving on to the exchange sites to displace H
in response to the dilution of the soil solution.
In calcareous soils pH is controlled primarily by the dissolution of CaCO3 so again the solution is
buffered. By using a standard procedure for the measurement of pH, comparisons between soils can be
made with confidence even though absolute values are difficult to interpret. Differences in pH caused
by adding different volumes of water to soil are known as suspension effects.
Measurements can be made in the surface layer of a moist soil provided sufficient water is
present to make liquid contact between the electrodes.
Under some circumstances where buffering capacity is not adequate, soils may be suspended in
0.1M KCl for pH determination.
See Practical Exercise SA01
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8. Testing Methods
CALCIUM CARBONATE CONTENT - FIELD METHOD
The field estimate of calcium carbonate (CaCO3) content is based on the reaction of soil with dilute
acid giving both visible and audible effects using Table 8.1. The method is only approximate and not
sensitive to differences in CaCO3 contents above 10%.
TABLE 8.1 Field estimation of calcium carbonate content
Field description
%CaCO3
Audible effects
Visible effects
Non-calcareous,
< 0.5%
None
None
Very slightly
calcareous
0.5-1
Faintly increasing to
slightly audible
None
Slightly calcareous
1-2
Slightly increasing to
moderately audible
Slight effervescence confined to
individual particles, just visible
2-5
Moderately to distinctly
audible; heard away from
the ear
Slightly more general
effervescence visible on close
inspection
Calcareous
5-10
Easily audible
Moderate effervescence; obvious
bubbles up to 3 mm diameter
Very calcareous
>10
Easily audible
General strong effervescence;
obvious bubbles up to 7 mm
diameter
See Practical Exercise SA01
Organic Content
A few relatively methods for determining this exist, which do not require expensive specialised
equipment.
A wet combustion (chemical oxidation) technique can be used to determine the organic matter
content in soil. The reaction of K2Cr2O7 with H2SO4 forms a strong oxidising agent, chromic acid,
which can oxidise the carbon in organic matter to CO2. The chemical reaction produces a green colour
directly dependent upon the amount of organic matter in the soil. Measuring the intensity of the green
colour with a colorimeter gives a reading that is proportional to the percentage of organic matter
present. A standard curve, developed by using samples of soil with known quantities of organic
matter, relates colour intensity with organic matter content.
A back titration method, based on the same acid dichromate decomposition, can also be used.
See Practical Exercise SA04
8.6 Chemical testing methods
There are an enormous number of chemical testing methods for soils. It is impossible to cover all of
them in this subject, hence only a selection of the more important methods will be covered. Most
methods are modified according to soil type. For example a different method of phosphorus
determination would be made on a rainforest soil with high organic content, than would be used on a
dry sandy loam.
The other issue is what part of the soil is to be analysed: the solid material or the species in the
water in the soil. The latter is relatively standard; after all, it is simply water, and once extracted from
61
8. Testing Methods
the soil, is analysed like any other aqueous solution. Table 8.2 shows the typical levels and methods of
analysis for the commonly analysed soil solution species.
TABLE 5.2 Soil solution analysis
Analyte
pH
Typical level (mg/L)
n/a
Method
pH meter
Na
11
ICP
K
15
ICP
Mg
3
ICP
Ca
84
ICP
Al
n/a
ICP
NO3
12
ISE, IC
Cl
57
ISE, IC
SO4
11
IC or ICP
H2PO4
2
UV/VIS or ICP
HCO3 & CO3
n/a
titration
It should be noted that in many cases, the extracting solution is not just water, but a pH-controlled
ionic solution.
Nitrogen
Nitrogen exists in the soil as organic N in plant material, and three inorganic forms: ammonium,
nitrate and nitrite. The latter two are best done by ion chromatography or ion selective electrodes,
while the others, and total nitrogen use the Kjeldahl method.
This is based on wet oxidation and has found wide acceptance for many different sample types,
eg food, coal, soil. Nitrogen in the sample under analysis is converted into NH4+ by digestion in
boiling concentrated sulfuric acid in the presence of a catalyst. Either distillation/titration or
colorimetric determinations of the NH4+ present are commonly used to complete the measurement of
soil total N.
Unless special precautions are taken, Kjeldahl digestion does not ensure quantitative recovery of
all forms of soil N, especially those forms such as nitrate. Adequate recoveries are usually achieved
but when complete recoveries are essential, particularly when soil NO3- nitrogen is known to exceed
0.002%, the sodium thiosulfate-salicylic acid modification of the Kjeldahl digestion procedure should
be employed (this converts nitrate into ammonium).
There are also questions about the ability of Kjeldahl digestion to fully recover indigenous,
clay-fixed NH4+ nitrogen in some soils. As Australian soils examined are generally low in this N
fraction, errors from this source should be minimal.
The measurement of NH4+ nitrogen and NO3- nitrogen in soils must be undertaken with caution,
since rapid transformations can alter their concentrations. Fast cooling or drying below 55ºC can
minimise these effects, but air-drying completely invalidates results for NO2- nitrogen.
Water or salt solutions are suitable extractants for NO3- nitrogen (and NO2- nitrogen) in the great
majority of soils. As an exchangeable base, NH4+ nitrogen must be displaced from the surface of
negatively charged soil colloids with another cation, commonly by K+ or Na+. Once extracted, total N
can be determined conveniently by steam distillation/titration or by automated colorimetric
procedures.
See Practical Exercise SA05
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8. Testing Methods
Phosphorus
Phosphorus is an essential element for all living organisms. Most unfertilised Australian soils contain
small amounts of phosphorus, usually less than 0.2%. Much of this is immobilised in forms not readily
available to plants, such as organically bound phosphorus and insoluble mineral phosphorus. Thus the
quantity of phosphorus available to plants is seldom related to the total soil reserves of this essential
element.
There are a range of sample preparation methods for phosphorus, which differ in the type of
phosphorus being targeted:
•
total – all P is analysed, regardless of form and availability; complete decomposition and
solution of the solution (mineral and organic) is required
•
available – this attempts to simulate the conditions in the soil by the use of synthetic extracting
solutions e.g. the Olsen method: extraction of air-dry soil with 0.5M NaHCO3, adjusted to pH
8.5 for 30 minutes
Once the desired amount of P is released by the sample preparation, the principal methods of analysis
are:
•
visible spectrophotometry – using one of the standard colour forming reagents for phosphate
•
ICP emission – P emits in the far UV below 200 nm, so some extra instrumental adjustments
(eg vacuum) are required
•
X-ray fluorescence – not as sensitive, but can be done directly on the soil itself without need for
decomposition
See Practical Exercise SA06
micronutrients
In recent years there has been increasing use and acceptance of micronutrient tests in soil fertility
assessment. Major objectives are to separate deficient from non-deficient soils, and to indicate when a
profitable response to applications of specific micronutrients might be expected. Micronutrient soil
tests are also used to indicate possible nutrient and heavy metal toxicities.
Because micronutrients can exist in soils in water-soluble, exchangeable, adsorbed and
complexed forms, as well as in secondary clay minerals, insoluble metal oxides, and primary minerals
a wide range of methods have been used for their determination. Of these those which involve
extraction with DTPA (diethylenetriaminepentaethanoic acid) followed by analysis with ICP are very
popular because of the speed and accuracy of ICP analysis, and the extraction procedure which
simulates plant roots.
See Practical Exercise SA07
Sodium absorption ratio and salinity
The sodium absorption ratio (SAR) for a soil is a useful measure of predicting the likely occurrence of
soil salinity. Essentially it relates the ratio of sodium ions to calcium plus magnesium ions using a
nomogram (see Figure 8.8) or by calculation using Equation 8.2.
SAR =
[Na + ]
Eqn 8.2
([Ca 2+ ] + [Mg 2+ ])
2
where the concentrations of the three ions are in mmole/L.
63
8. Testing Methods
How to use this
Draw a line connecting the
two measures – Na and Ca
+ Mg.
Where it crosses the
middle line is the SAR
value.
FIGURE 8.8 Nomogram for the determination of SAR value of soil extracts
(from http://www.kgs.ku.edu/General/Geology/Sedgwick/gw07.html )
EXERCISE 8.4
A soil is tested for leachable Na, Ca and Mg, and the results are (in mmole/L: 15, 3 and 2
respectively. What is the SAR? Use both methods.
It may be determined in many ways, but one of the easiest is to leach the cations from the soil, then
determine the concentrations of Na+, Ca2+ and Mg2+ using an ICP spectrophotometer.
See Practical Exercise SA08
64
8. Testing Methods
Measurement of CEC
There are a variety of methods for measuring CEC, each of which gives somewhat different answers.
Each relies in the removal of ions with a concentrated solution of an ionic substance intended to drive
off the adsorbed ions. Some of the techniques used to analyse the released ions include:
•
titration with EDTA
•
flame AAS
•
ICP emission
•
Kjeldahl N analysis – all adsorbed ions are replaced by ammonium, which are then released by
excess potassium; the ammonium is then analysed
The simplest procedure to determine the amount of exchangeable cations in a soil is to release the
adsorbed ions with ammonium ethanoate solution and titrate the soil extract with EDTA. This directly
measures the combined levels of magnesium and calcium, but doesn’t detect Na & K. If the pH of the
soil is also determined then the soil’s cation exchange capacity may also be estimated, using a simple
correction factor to account for the sodium and potassium.
See Practical Exercise SA09
pH buffering capacity
The buffer capacity is measured in each direction – acid and alkaline – and is done by adding known
amounts of acid (as HCl) or alkali (as NaOH or lime) to soils and allowing a equilibrium period before
measurement of pH. A graph of amount added (per kg of soil) vs pH is then plotted, and the buffer
capacity is the slope of the graph. The buffer capacity will be quoted as an amount of acid or alkali
(typically millimoles H+ or g CaCO3) per kg of soil per pH unit.
Pesticides
One of the greatest problems with soils used for agriculture is the contamination with pesticides –
particularly those that are organochlorine based. The latter are known as residual because they remain
in the soil for many years after application and may accumulate in some plants and be incorporated
into the food chain.
The standard method for determination of organochlorine pesticides in soil (and water) is the
gas chromatograph-mass spectrometer (GCMS) method. This involves extracting the soil sample with
an organic extracting agent (such as hexane) or using CO2 as a supercritical fluid extractant, then
placing the extract in the GCMS which separates out any organic substances by chromatograph, then
identifies them by spectral matching using a mass spectrometer. The USEPA has developed a standard
method for this procedure that all laboratories around the world follow.
What You Need To Be Able To Do
•
define important terminology
•
describe sampling procedures for soil
•
explain how the important physical and chemical tests are performed
65