- Soil ‘type’ is quite a vague term as it can be used to mean several things, for example it may mean differentiating soils by ‘series’ or ‘association’, which are the technical terms used by soil scientists to define classes of soils; or it may refer to a specific mineral texture, e.g. sandy or silty; or it may refer to the organic matter content of a soil, e.g. peaty or ‘organic’; or the calcium carbonate content, e.g. ‘calcareous’. Usually the meaning should be clear from the context.
- Within agriculture, soils are often referred to as ‘coarse’/’medium’/’fine’ (predominantly sandy/loamy/silty or clayey respectively); or ‘light’/’heavy’, (sandy or silty/clayey). These terms may be used in a qualitative sense by a worker who knows the land but has not seen results of a laboratory analysis – and the terms are no less relevant for that.
- In the PRG potassium and phosphorus nutrient models from 2019, soil ‘types’ are defined in a manner specific to each of those particular models. The classifications used, rely on the available literature information for each of those nutrients, and the information provided in the publications used to form the models. Therefore the ‘types’ are not relevant to wider situations and should not be used for other applications. Furthermore, when making decisions about fertiliser applications to specific soils using the models, the soils should be classified as described in the models. This may not describe the soil using terms which are usually used for that soil, however please remember that the classifications are intended only for that specific use and are a means to categorise soils appropriately according to the available information.
- The information in this section can be applied to UK soils and used for general descriptions and understanding.
Soil texture is determined after removal of the organic portion of soil and then the stones, and therefore refers only to the (finer) mineral fraction; hence the percentages of sand, silt and clay must add up to 100 %, even in an organic-rich soil. The difference would be that an organic soil would be described as such, whereas a soil dominated by the mineral fraction (i.e. low in organic matter) would be classified by texture (see ‘useful definitions’ below).
- If a soil has 35 – 70 % stones by volume, it’s ‘sandy/loamy/clayey skeletal’; with > 70 % stones the classification is ‘skeletal’. If the stones are rounded (as opposed to angular), ‘skeletal’ is substituted for ‘gravelly’.
- ‘Stones’ are classified as mineral particles of size > 2 mm.
Mineral soils: non-peat, non-chalk, non-stony
- This refers to soils with < 35 % stones.
- Soil texture can be determined either by using the soil texture triangle (Figure 2.2) after laboratory analysis for particle-size distribution; or by following an in-field guide: see Assessing soil texture by hand.
- Particle-size distribution (PSD) can be analysed in the laboratory (after removal of organic matter and any stones, which should be recorded and reported) by either dispersal in solution, or laser diffraction. Laser diffraction is the standard method used by Lancrop (checked 2015).
- In the UK and Europe (USA uses slightly different categories) standard classification of particles by size are as follows (1 µm is a micrometer, 1 x 10-6 m):
- sand 2 – 0.06 mm (2000 – 60 µm); this may be further subdivided into coarse and fine sand;
- silt 0.06 – 0.002 mm (60 – 2 µm);
- clay < 0.002 mm (< 2 µm).
- Figure 1 shows some classifications which can be used to describe soil with different PSDs.
Figure 1. Texture triangles from Clayden and Hollis (1984) showing different categories of soil textures. This refers to the < 2 mm size mineral fraction of soils with < 35 % stone content by volume.
Using a texture triangle
The texture triangle shown in Figure 2 shows standard categories used for description in the UK. Be aware that in the USA a slightly different triangle is used.
Figure 2. Standard UK texture triangle from the LandIS website.
- The letters used refer to sand (S), silt (Z) and clay (C). In other texture triangles, sand (Sa), and silt (Si) may be used instead.
- Soils are described in the same manner as standard English language descriptions, e.g. a vehicle designed for carrying lots of people, which happens to be large, would be described as a big bus. Similarly, soil which is predominantly clay but which contains a reasonable proportion of silt, would be described as a silty clay (ZC).
- Soils with more even proportions of sand/silt/clay are described as loam, with the dominant fraction mentioned, e.g. silty loam (ZL), or sandy clay loam (SCL – think ‘big, blue, bus’).
- To use a texture triangle, the <2 mm mineral fraction of soil sample should first be analysed using a standard laboratory method, to determine the percentages of each of the three categories of particle.
- Then, plot the point on the three-sided graph in Figure 2. To do this, take each of the percentages in turn and move along the relevant axis until the correct value is reached, moving in the direction shown by the arrow on that axis – see examples below – and the point at which the lines cross will show the classification of soil.
- Alternatively, the tool on the LandIS website may be used to determine texture for you.
Example 1: a hypothetical soil with 40 % sand, 24 % clay, 36 % silt.
1. Move along the sand axis from right to left until you reach 40 % – see the blue lines in Figure 3.
2. Move along the clay axis from bottom left to top right until you reach 24 % – see the red lines in Figure 4. The point where the lines cross shows that the soil is a clay loam (CL).
3. The clay content can then be plotted to confirm that the total percentages of each component add up to 100 % and that no mistake has been made in plotting the points – see yellow lines in Figure 5.
Example 2: where the soil does not have any particles of one size class, e.g. sand 50 %, clay 50 %, silt 0 %.
Note that the method is exactly the same, independent of the proportions; this example is included to clarify a situation which may be confusing.
- Move along the sand axis from right to left until you reach 50 % – see the blue lines in Figure 6.
2. Move along the clay axis from bottom left to top right until you reach 50 % – see the red lines in Figure 7. The point where the lines cross shows that the soil is a sandy clay (SC). The points cross on the outside line of the triangle. This is fine.
3. The clay content can then be plotted to confirm that the total percentages of each component add up to 100 % and that no mistake has been made in plotting the points – see yellow lines in Figure 8. In this case there is 0 % silt, so the silt ‘line’ (shown in yellow in Figure 8) must be on the silt axis itself, confirming the position on the class boundary.
Soils on texture boundaries
- Note that example 2 resulted in a category that was on a boundary, as would occur, for example in a soil with 20 % sand, 35 % clay, 45 % silt.
- In that case, the LandIS website categorises the soil automatically as a silty clay loam (ZCL). In practice, of course, the percentages are less likely to come out exactly on a boundary than they are within a group, and even a 0.1 % difference (e.g. 20 % sand, 35.1 % clay, 44.9 % silt) would result in a reclassification to clay. Of course this is due to the necessity of defining absolute boundaries; in practice the difference between soils would not be categorical, but rather would be gradual.
- In boundary cases it may be necessary to undertake an in-field hand assessment to determine the group of the soil for practical/management purposes.
- The areas within the triangle described by each ‘type’ (e.g. clay, silt loam) are not even, and the boundaries are not evenly spaced. This is because the categories rely on physical characteristics which affect how they behave, and thus their workability in agriculture, rather than a strict mathematical categorisation.
- Furthermore, the names rely on the characteristics of the soil, rather than the strict proportions of each component. For example, a soil with 60 % sand and 40 % clay would be described as a sandy clay, NOT a clayey sand, as may be expected.
- Clay particles have a pronounced effect on soil texture, therefore the quirks described above often relate to clay-rich soils. To have the main properties of a clay, a soil only has to have 40 % clay, whereas to behave like a sand it requires 80 – 90 % sand.
- Note that texture determined according to laboratory analysis and the texture triangle may not be exactly equivalent to an in-field method, due to the difference in particle shapes, chemical components of the mineral fraction, and other soil constituents including organic matter, which would be present in the field sample but not in the sample analysed by the laboratory.
- As usual when considering soil analysis, it is wise to consistently use one method; to get to know your soils; and to appreciate the reasons behind different results that different methods may give and therefore be able to interpret findings accordingly.
This section defines certain terms commonly used to refer to soils.
- Information and diagrams from Clayden and Hollis (1984).
- Peats are at least 40 cm deep within the upper 80 cm; or more than 30 cm of organic material directly on bedrock or skeletal material.
- Classifications of peat are shown in Figure 9: with 20 – 50 % OM (by LOI), the type of peat is determined by the proportion of sand in the mineral fraction.
Figure 9. Categories of peat a determined by sand and organic matter proportions.
General determination of ‘calcareous’ soils in the UK is defined by the amount of CaCO3 (chalk) present in the soil as shown below. Chalk content may be described as a percentage or in ppm.
Information from LandIS website; also shown in 2018 PRG Notes Calcium and Magnesium.
Further details from Clayden and Hollis (1984) Criteria for Differentiating Soil Series, Soil Survey of England and Wales.:
- Soft, extremely calcareous material (> 40 % CaCO3 in ‘fine earth fraction’) was previously called ‘marl’ or ‘chalky’; now known as ‘carbonatic’ (as long as it contains < 36 % stones).
- If the sand-sized fraction of a carbonatic soil comprises ≥ 50 % of mineral fine earth, it’s described as ‘carbonatic sandy’.
- If the sand-sized fraction of a carbonatic soil comprises < 50 % of mineral fine earth, it’s described as ‘carbonatic loamy’.
Assessing soil texture by hand
This guide provides a flow chart to aid determination of soil texture while in the field (Reference: Rowell, D. L. (1994). Soil Science: Methods and Applications).
It is recommend that those who wish to use this guide practise on some samples for which the textural analysis has been determined by a laboratory.
In most cases the guide is pretty straightforward, however there are sections which ask for the ‘predominant’ or ‘significant’ feeling, which is something which must be gained through experience.
Due to the subjective nature of some of the determinations, this should be used very much as a guide rather than a definitive determination.
However, with more practice, the guide will become more useful.
Soil types notes
Organic matter for potatoes 2020 (Different types of organic matter and building up organic matter in soil)
Soils update 2019 (Different sources or organic amendments and their potential effects)
Analysis of soil organic matter 2018 (Appropriate sampling and analysis)
Biochar 2018 (A material used for carbon capture but with disadvantages in potato crops)
Links to external websites
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