Interpreting Soil Test Results: What Do All the Numbers Mean?
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About this ebook
The precursor to this book, What Do All the Numbers Mean?, known as The Numbers Book, was widely used and accepted for interpreting soil test results. This new edition has been completely updated and many sections have been expanded, particularly those on acid sulfate soils and soil salinity. It is a handy and straightforward guide to interpretation of the numbers associated with a wide range of soil tests.
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Interpreting Soil Test Results - CSIRO PUBLISHING
Introduction
Soils are a valuable resource and a critical component in many of the environmental and economic issues facing today’s society. Understanding soils and interpreting soil data is especially relevant for many environmental and land management issues facing the community. These issues include urban development, control of salinity, clearing of native vegetation, prevention of land degradation, control of water and wind erosion, irrigation development, the management of effluent disposal and the management of acid sulfate soils.
However, soil science is a specialised field and can be complex. When writing or examining land assessment or environment reports it is often difficult and time consuming to find interpretation of the soil data. These guidelines were compiled to assist in overcoming this problem and are designed for workers in all categories of land use management. The information in this book was collated from a wide range of reference material.
The interpretations and values provided in this text are not intended for specific advice on particular problems or issues, but provide a general background on the variety of soil tests available and how the results from these tests may be interpreted. They are not intended to be used as a replacement for specific professional advice.
1
Soil sampling
1.1 Where, how and when to sample
To characterise soils at a site, a suitable sampling design is required. Sampling design depends on:
the landscape or location from which samples are being taken
the purpose for which samples are being taken
the resources available to take and test the samples.
The basic questions to be considered are:
When is the best time to sample?
Where should the soil be sampled?
How many samples are required?
What spatial pattern should be used to take the samples?
What depths should be sampled?
For general sampling purposes, the basis for making these decisions is discussed in Petersen and Calvin (1986), Beattie and Gunn (1988), Rayment and Higginson (1992), McBratney (1993), Brown (1999), and Chapman and Atkinson (2007). If contaminated sites are to be sampled then there are some special problems in sampling soils. These need to be taken into account to ensure that the samples are representative of the site. These problems are discussed in Laslett and McBratney (1995).
However, there are some general sampling patterns that are frequently used, including:
regular grid
regular transect (often used when there is thought to be a trend in a particular direction)
completely random
stratified sampling where samples are taken on the basis of:
landforms
sections of a paddock, especially if a paddock has different soils or landform elements within it
areas of different management histories
some other stratifying factor
stratified random sampling, where the samples within each stratifying unit are randomised
herring-bone grid (for contaminated sites).
Once collected, samples for nutrient analyses may be bulked to give a composite sample. Generally, bulking should be done only when the samples come from a relatively uniform area, or what is thought to be a relatively uniform area. Petersen and Calvin (1986), Tiller (1992), McBratney (1993), and Laslett and McBratney (1995) discuss the limitations of bulking samples and the recommended procedures to follow.
Another difficulty in sampling is the problem of temporal viability, where results for samples or measurements taken at one time may be different to results for samples or measurements taken at another time. Some obvious examples are:
Measuring infiltration in a tilled paddock. The infiltration is much higher before rainfall packs the soil down and crusts the surface.
Measuring salinity on a site after a large amount of rainfall. After rainfall the soil solution may be diluted, compared with the solution measured after a dry period.
Sampling soils immediately after adding fertiliser or soil ameliorants such as lime or gypsum.
These difficulties need to be considered when sampling soils or making measurements on soils and when interpreting the results of any tests carried out. Sampling through time is required, or, alternatively, the conditions when measurements are made or samples are taken should be standardised (or at least recorded). For example, some soils require specialised sampling techniques such as the following:
For best practice sampling procedures for acid sulfate soil refer to Ahern et al. (1998, 2004) and subsequent version updates as they occur.
Sampling soils for rapidly metabolised chemical species such as nitrate (NO3–) (see Peverill et al. 1999).
1.2 The number of samples required to produce a soil map
The number of samples or ground observations needed to produce a soil map or to do an investigation will vary with the local characteristics of the site. Factors that influence the number of samples required include:
geology
landform
land use history
purpose for which the investigation is being done.
There are general guidelines on the number of samples required to produce a reliable map (Reid 1988). It should be remembered that these are general guidelines and their applicability will be influenced by the factors outlined in Section 1, Soil Sampling. The number of samples required is usually expressed as the number of samples or observations per cm² of the map (see Table 1.1).
Table 1.1. Number of observations for 1 km²
(1 km² = 1000 × 1000 m)
Source: Reid (1988).
Tables 1.2–1.3 are a general guide to the minimum number of samples required for 1 km² of land at different map scales. Different criteria apply for investigations of areas less than 1 km².
The relationship between soil survey effort and map scale has been derived by Dent and Young (1981). It estimates the effort required in days in the field to develop a soil map at different scales. The actual effort will vary depending on such factors as existing information, the complexity and predictability of the soil patterns and difficulties of access. Nor do these estimates consider the purpose for which a soil map is being developed. Nor do they take account of modern methods of soil survey using remote sensing and geographic information systems (Gunn et al. 1988; Gessler et al. 1995). They are intended to indicate the relative effort required for a detailed soil map at the scale and thus represent a maximum value. Therefore, these are only broad guidelines.
Table 1.2. Recommended intensities of investigation based on map scale (DLWC 2000)
Table 1.3. Soil survey effort as minimum number of days in the field for different scales
2
Soil physical properties
Soil physical fertility or soil structure can have as large an impact on plant growth as chemical fertility. Several suggested critical values for soil physical properties are presented in this section, but further discussion of soil physical fertility and soil structure is presented in Cass (1999) and Geeves et al. (2007a).
2.1 Particle size distribution
2.1.1 The relationship between field texture and particle size
Particle size distribution describes the relative amounts of gravel, sand, silt and clay within the soil. These are the building blocks for the soil and can have a large effect on the soil properties (see Table 2.1). Clays have a high surface area of 5–750 m²/g depending on clay type and can have a high amount of chemical and physical activity. Sands have a smaller surface area (0.100–0.01 m²/g) and tend to be less chemically and physically active (McKenzie et al. 2004).
Table 2.1. Particle size
This particle size scale is equivalent to the International Scale for particle size description.
Note that in the US system, silt is defined as 0.02–0.05 mm and fine sand as 0.05–0.20 mm.
Table 2.2 is a guide to the description used for the proportion of each type of particle in soil.
Table 2.2. General levels ot different particle size contained in a soil
Examples: <10% clay = very low level of clay: >50% silt = very high level of silt.
There is also an approximate relationship between field texture and particle size distribution, as shown in Figure 2.1. (See McDonald el al. (1994) for method for field texturing.)
Fig. 2.1. Soil texture triangle (adapted from Soil Science, University of Sydney, 1991). This triangle is effectively equivalent to that in McDonald era/. (1994) but is easier to use because there are only two variables. This simplification is possible because clay ÷ silt + sand always add up to 100%. Example: 20% clay and 67% sand will give a loam texture.
To estimate field texture grade from laboratory