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Summary SOIL 1 (chapters 3-4)
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Summaries of chapters 3 and 4 of Introduction to Soil Geography 1.
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SOIL3 SOIL-FORMING PROCESSES
3.1 COMPOSITION OF SOIL
Soil differs from its parent material as a result of all kinds of changes to the original properties that have taken place over time and
under the influence of the atmosphere, water, plants and animals.
We distinguish between three soil phases (Fig.1):
• The solid phase contains mineral particles (derived from the parent Figure 1: approximate
composition of soil (the
material) and organic matter (mostly humus). It is the source of nutrients
jagged boundary
and provides a habitat for soil life. between water and air
When the volume% of organic matter in a soil is less than 15–20% we speak indicates that their
of mineral soils, if the volume% is larger (e.g.in peat soils) it’s organic soil. contributions can vary
greatly).
Between 0.5 and 2% of the volume is occupied by roots, and other soil life
occupy barely the 0.5%.
• The liquid phase is normally an aqueous solution called the soil solution (water with all
kinds of salts dissolved in it) but can also partly consist of organic liquids.
• The gaseous phase contains air.
Soil particles have spaces between them called pore spaces, that contains water and air. The total pore space can be calculated
by subtracting the volume% of the solid phase from the total soil volume: 100% - volume% of the solid phase. à POROSITY.
The arrangement of soil particles is called soil structure.
Soils can differ greatly from each other and from their parent material. In some very young soils, the solid phase consists almost
entirely of the minerals that you may find in various igneous and metamorphic rocks, so, the primary minerals, usually quartz and
silicates. These primary minerals are mainly present as sand and gravel. In most soils with particles much finer than sand
aluminium and manganese occur. These secondary minerals are formed from primary minerals.
3.2 SOIL PROFILE DESCRIPTION
Over time a soils changes: morphology. Every soil has a distinct morphology, that is determined by the five soil-forming factors:
climate, organisms, relief or topography, parent material and time.
Soil formation processes operate on parent material, over time turning it into a soil profile. A soil profile is the result of soil-
forming processes occurring under certain conditions.
The physical, chemical and biological conditions are not the same throughout the soil profile.
There is a vertical succession of soil horizons: soil layers parallel to the soil surface that have morphological, chemical and often
also physical and biological characteristics that differ from those in the overlying and underlying layers.
The properties of young soils are largely determined by the geological substrate: there has been insufficient time for soil
formation processes to produce a profile typical for the climate and other conditions. In many young soils, the soil properties
reflect the geological processes, or geogenesis, more than the soil-forming processes.
Table 1:
example of a soil profile description form
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,3.2.1 Texture
The term texture refers to the relative proportions of differently sized particles in a soil.
Particle diameters are in millimetres or micrometres.
• Gravel, cobbles, stones, boulders (everything >2 mm diameter)
• Sand (0.05 to 2 mm diameter)
• Silt (2 to 50 μm diameter)
• Clay (<2 μm diameter)
Sand (0.05 to 2 mm) Silt (2 to 50 μm) Clay (<2 μm)
The mass ratios in which sand, silt and clay occur in the mineral part of the soil (i.e. after removal of gravel and organic matter)
determine the textural class. A triangle diagram is usually used to determine the textural class.
Soil texture triangle indicating the
12 soil textural classes according to the
USDA classification system.
For example, soil sample A contains
30% sand, 20% silt and 50% clay, and is therefore a clay soil.
The soil texture triangle with its three fractions
gives only a rough indication of the grain size
distribution. More elaborate laboratory methods
can provide the relative amounts of soil particles
for several size classes. The results of such analyses
may be depicted in a particle size frequency
curve, where the logarithms of the particle size are
shown on the horizontal axis (with the size
increasing towards the right), and the cumulative
percentages of occurrence of the particle size on
the vertical axis.
3.2.2 Colour
The colour of a soil particle, a soil sample, or a soil horizon is a function of the type, abundance and distribution of pigments.
Three major factors influence soil colours: 1) organic matter content, 2) water content, and 3) the presence of iron and manganese.
Dark colours usually indicate organic matter coatings, also soil is dark when wet. Shades of red, brown, yellow, and grey in soils are
means the presence of iron.
Spots of a colour in a matrix of another colour grey are called mottles and are typically found at the depth at which excess water
accumulates because it cannot drain away any deeper.
The colour of a soil sample should be determined under uniform, bright light, preferably sunlight, and using samples that are “field
moist”. Soil scientists compare soil samples against standardized colour chips in special Munsell colour charts.
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, 3.2.3 Structure
The way individual particles are assembled is the soil structure. The resulting structural units are called aggregates or peds. Soils
with structural units have a pedal structure, and soils without structural units have an apedal structure.
Soil structure creates a range of different-sized pores, and the circulation of water and gases depends a lot on this.
Soil structure is best described in terms of grade (degree of aggregation), class (average size) and type of aggregate (shape).
A structureless soil has no aggregation. Fine-textured soils (dominantly clay) would have massive chunks, and coarse-textured
soils (dominantly sand) would be like single-grained beach sand. A soil with a strong structure has well-developed and distinct
aggregates that are durable and obvious in undisturbed soil.
The class of structure is defined by the average size of the individual aggregates and may be either fine, medium or coarse.
The type of structure is described by the shape of the individual aggregates.
In general, four main types of soil structure are recognized: spheroidal, plate-like, block-like and prism-like.
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