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close this bookHandbook for Agrohydrology (NRI)
close this folderChapter 5: Soils and soil moisture data
View the document(introduction...)
View the document5.1. Soil classification and soil textures
View the document5.2. Soil moisture
View the document5.3 Infiltration
View the documentEquipment costs
View the documentAppendix C: Soils and soil moisture

5.1. Soil classification and soil textures

5.1.1 Soil Horizons and Their Characteristics

The soil profile, as exposed by the side of a pit is usually divided into 3 horizons which are frequently further divided into sub-horizons:

A horizon constitutes the top soil, where any organic matter is found and within which cultivation is initiated. B horizon is the subsoil, without organic matter. C horizon which is composed of weathered rock, usually the parent material.

Soil pits, dug to give an exposure of the soil to the C horizon where possible, provide a great deal of information which is used in the classification of the soil types. From the agrohydrological viewpoint however, it is the practical effects on farming and hydrology of such factors as the effective depth of soil (that is the depth that can provide a medium for roots) that are important. In most cases the effective depth is limited by the nature of parent material and the manner in which it has weathered; climatic influences are often strong. In other cases, gravel bands may be present and if tightly bound, will restrict the development of crop roots. Such bands should be noted as the limit of the effective depth. Roots may be evident in partially weathered parent material but it is unlikely that they contribute much to the intake of crop water and nutrients. Information on parent material, erosion, formation history and climate, indicate past periods of waterlogging and other aspects of the nature of the soil moisture reserve.

Topsoils

Of particular importance is the character of the top 20 cm or so of soil. This is the soil layer that influences soil surface/rainfall relations by its texture and aeration, and represents the approximate depth of cultivation. The top soil layer also determines structural stability, fertility, and the tendency for a soil to cap or erode. There are obvious limitations to digging large numbers of pits in order to determine soil characteristics; the job is a long and arduous one and pits must usually be filled in after examination. The textural definition of surface soils is therefore more commonly assessed by working the soil by hand, when wet. Where an accurate textural analysis of soils is needed, samples are taken and analysed in the laboratory (see chapter 3). Table 5.1 below lists the characteristics of soil textural types when manipulated.

Sandy soils have high rates of infiltration and percent runoff is usually low. They tend to be infertile, relatively acid and prone to leaching. At the other extreme, clay textured soils give high percent runoff in general, though cracking vertisol soils may absorb water until the clay particles swell, the cracks close and runoff results from later rain. Fine textured soils normally have a higher water holding capacity than coarse, sandy soils and their chemical mix is more varied and nutritious for plants.


Table 5.1: Soil Textures According to Manipulation When Wet

Soil depths are also important; whatever the inherent water holding capacity of soils on a unit volume basis, the absolute volume of water available to crops will be small if soils are shallow. This is an important consideration when the viability of water harvesting opportunities is being assessed, as it will be a critical factor in determining how frequently water must be added to the soil moisture reserve.

Soil textures are determined precisely and classified most rigorously in the laboratory as described in chapter 3. The FAO has now adopted the USDA soil classification triangle which categorises soils into textural types according to the percentage of silt, sand and clay components, and is shown in Figure 5.1. The "International" classification (Figure 5.2) is now used in few countries. Relatively small differences exist between them. Several approaches can be taken to the selection and collection of soil samples. Spatially, soil textures can be highly variable, so that when top soil samples are collected to assess the general textural type of a large area (for instance a whole field), samples are taken at individual points and combined well before submission for analysis. If the spatial variation of soil textures is in itself a characteristic under investigation, samples should be taken systematically on a marked grid basis, with each sample given a point reference number accordingly. This avoids subjective sampling.

Where microtopographical features are under study, sampling should take place along defined transects at every one or two metres, according to transect length. Again, the samples are referenced to the sample points and also to the elevations above a base level (a levelling survey will be necessary). Wash-ins, ploughing, crops, vegetation and faunal activity may be recorded. Loose samples (not cores) are collected and sealed in polythene bags and the depths to which they are taken are noted. See chapter 3 for details regarding the dispatch of soil samples.


Figure 5.1: FAO/USDA Soil Classification Triangle


Figure 5.2: International Soil Classification Triangle

Subsoils

Subsoils affect soil water permeability and thereby runoff. In the field, permeability is usually assessed by the observation of soil physical characteristics rather than direct or laboratory measurement of hydraulic conductivity. Common terms used in descriptions are:

Compacted: Firm or hard consistency, close packing of particles resulting in a dense material with reduced pore space.

Cemented: Hard and brittle, soils which do not soften with prolonged moistening.

Deflocculated: Soils in which sodium has entered the exchange complex and dispersed the colloids. This leads to reductions in pore space, aeration and permeability. High levels of pH and electrical conductivity are found. Columnar horizons which are hard and dense may be found.

The colour of soils may give information on aeration and drainage and are described according to the standard Munsell notation. Colours may vary between and within horizons, for example:

Drainage:

Reds

Well drained


Yellows, Greys

Poorly drained

Organic Matter:

Browns & Blacks

High in organic matter

Leaching:

Paler

Moreleached


Darker

Less leached and higher mineral fertility

Wet soil colours are usually darker and there may be the presence or absence of mottles. The colours on Munsell charts that provide the standard reference (described in detail below), are arranged to give the three variables used to define all colours and are recorded in a standard order:

Hue: The dominant spectral colour (increase in redness or yellowness).

Value: The lightness of colour and total amount of light reflected.

Chroma: The purity or strength of colour (increases with a reduction of greyness)

On each card the colours are of a constant hue. The colours increase in lightness vertically and in equally visible steps. The colours increase in chrome to the right and become greyer to the left. In the field, a 1 cm fragment is selected from the sub soil, untainted by organic matter. After deciding whether it is predominantly yellow or red, a colour chart is selected and the sample compared through the most appropriate hole in the chart. Intermediate matches are not uncommon. Check that the hue is correct. Avoid sweat on the colour charts (not always easy).

Mottles, very pale and very dark colours indicate reduced permeability or groundwater near the surface. Rust-coloured mottles along root channels suggest periodic waterlogging, as does an abrupt change from reddish to greyish colouration. Grey mottles in an otherwise reddish weathered rock zone indicate a seasonal water table.

Bulk Density

Soil texture is largely responsible for the bulk density of soils, that is the weight per unit volume, most commonly expressed as g cm-3. Imperial units of lb ft-3 may be seen. Bulk densities are found by comparing the oven dry weight of samples and their volume. Samples are taken from soil pits using standard soil sampling cores, driven into the exposed face below the top soil when the soil is neither very wet nor completely dry. The sample must not be disturbed, so as to maintain its original volume. The sample should be oven dried at 105 °C to 110 °C and weighed to the nearest 0.1 gram.

The bulk density (sometimes called the "specific weight") dry weight of sample/ volume of sample The bulk density in g cm-3 can be converted to lb ft-3 by multiplying by the factor 62.4.

Soils with high bulk densities have a paucity of pore space, impede root penetration, make cultivation difficult and promote runoff.

5.1.2 Pedological Classification

The pedological classification of soils, although basically created with agriculture in mind, is described only briefly here. It is relatively complex and includes an extremely wide range of soil types. Many of the terms and names derive from the Russian language. Soil surveys and maps use the orthodoxy of pedological classification, but in developing countries soil mapping is usually at an early stage or restricted to localities of special interest. Map scales are commonly 1: 250,000 to 1:1,000,000 and cannot be expected to depict the variability of soil types with accuracy.

The pedological classification of soils is broken into two main groups: Higher and Lower categories. Of the higher categories, the nature of Zonal soils depends greatly on the prevailing climate at the time of formation. Intrazonal soils not only are influenced by climate, but also localised conditions, for instance poor drainage, and therefore cross the boundaries of zones. Azonal soils such as lithosols (rocky) and regosols (dry sandy) are not zonal.

In arid and semi-arid regions, regosols, lithosols and lateritic soils (which are red and have a high iron oxide and aluminium hydroxide content) are commonly found. Variation in soil types is wide and intrazonal soils may commonly occur due to changes in local conditions of geology and drainage. Calcareous bands may be common at depth.

Glei are indicative of impeded drainage and a rising and falling of the water table. These mottled colourations may be red, yellow or brown when the water table is low, or grey or blue when it is high, resulting from the oxidisation or non-oxidisation of iron and manganese.

Higher Categories

There are three main orders of soils (Zonal, Intrazonal and Azonal) which are sub-divided into Suborders and Great Soil Groups:

Zonal Soils Suborder Great Soil Groups

1. Cold zone

Tundra,

2. Light coloured arid zone

Desert, Red desert, Sierozem,


Brown/Reddish-brown soils

3. Dark coloured soils of semiarid sub humid and humid grasslands

Chestnut, Reddish-chestnut, Chernozem,


Prairie, Reddish prairie soils

4. Forest grassland transition

Degraded Chernozem and Noncalcic brown soils

5. Light coloured timbered regions

Grey wooded or grey podzolic soils

6. Laterite soils of forested warm temperate and tropical regions

Reddish brown and Yellow brown lateritic and Laterite soils

Intrazonal soils

1. Halomorphic (saline and alkali)

Saline, Soloth, Solonetz soils soils of imperfectly drained arid regions


2. Hydromorphic soils of marshes

Humic-glei, Low humic glei, Bog, and swamps

Groundwater packed soils

3. Calcimorphic soils

Brown forest soils


Azonal soils


Lithosols, Regosols, Alluvial soils

Further classification is beyond the scope of this book. The Unified System, developed in the USA, is concerned with the engineering aspects of soil classification, rather than agriculture.

Lower Orders

The Great soil groups are subdivided into Soil Series and then Types. Series are soils developed from the same parent material and soils within a series have the same profile characteristics except for the texture of the surface layer. Types are determined by the texture of the A horizon. Soil Phases are determined by deviation from the norm, for example a stony phase.