Soil Micro Morphology Principles

Soil micromorphologyprinciples

Lalitha. MS

Soil is the living heart of agriculture

Introduction  Soil micromorphology is concerned with the description,

measurement of soil components and pedological features at a microscopic to sub-macroscopic scale.  The study of micromorphology began with Kubiena (1938), who was primarily interested in using knowledge of soil microstructure to interpret soil genesis  Some pedogenic process may not be sufficiently advanced for their effects to be seen macroscopically. Translocation of OM,Fe,clay particles can be identified by the presence of micro-scopic scale coatings on ped surfaces and pore walls

Applications Thin sections have and are being used in the study of  Agriculture (effects on soil structure and porosity)  tillage practices, cultivation, irrigation  Pesticide movement  effects of additives such as fertilizers, organic matter, swadge  Environmental quality  waste disposal, land reclamation  water quality and conservation  soil erosion and degradation  interactions of soil material, water, air and organisms  Soil structure and porosity  influence of tillage  soil crusting  water and gas movement

 Soil mechanics and soil strength  Soil-root environments and interactions  Wetlands investigations and the morphological indicators of

wetness  Soil biology, zoology, and microbiology  Mineral synthesis, weathering, degradation and transformations  Identification of diagnostic soil horizons  Soil genesis, geomorphology, classification and taxonomy  Archaeological investigations, paleopedology, and paleoclimatology  Soil-materials, environment and interactions

Sampling principle •size of the samples •number of samples •orientation of the samples; i.e. vertical, horizontal, flat •time of sampling •method Size: A unit may, macroscopically, look completely uniform, while microscopically there may exist important differences. The only way to find out is by systematic sampling. Number of samples: One sample from one unit. Orientation: Whether a sample is taken vertical, horizontal or flat depends on what one wants to sample. In a vertical sample the sample long-axis is vertical, in a horizontal sample it is perpendicular to this. A flat sample has been collected from a horizontal surface.

Time of sampling: Very wet conditions may make it difficult to get samples to the lab without disturbance. It can then be advisable to wait for dryer conditions. Method: For micromorphological research undisturbed and oriented samples are needed, which requires specific sampling techniques and precautions. The common methods used to obtain micromorphological samples are reviewed by Murphy (1986). He provided much practical advice on sampling techniques. A). Cemented and cohesive material Undisturbed specimen of cemented and cohesive materials can be obtained by carving or chiseling pieces out of the soil or sediment. All such samples should be wrapped tightly in protective material, such as aluminum foil or plastic, with marked orientation and properly labeled. B). Loose material In case of very loose or brittle material the sediment may have to be pre-impregnated in the field, before sampling and transport. The sediment is covered and partially impregnated with a cementing agent. After the cement has hardened, the sample can be removed, labeled and wrapped as previously described. Cementing agents are, for example, a colourless varnish (cellulose acetate solution in acetone), polyester resin, or an epoxy resin.

C). Non-coherent material Non-coherent material is mostly sampled with the help of metal or cardboard boxes. The box is 8x6x4 cm, the mammoth-size box is 15x8x5 cm. The boxes have two loose lids and a body that can be opened, so that samples easily can be removed in the laboratory without disturbance. Preferably the body has a cutting edge on one side. Procedure in the field: •Smooth, the face of the pit or of the exposed profile wall •Cut the sample tin into the wall with a knife •Take the tin out of the wall by cutting away enough of the host sediment •Cover the side with a lid •Open the lid on the other site of the tin to check if the tin is completely filled up, if not fill up to prevent collapse during transport; use different Material for filling and make a note of it •Indicate the top and front •Note the number of the sample tin, with all the relevant information concerning depth, orientation, etc.

Thin sections  In order to observe a

sample it is necessary to previously prepare it.

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The method depends on whether we are dealing with a coherent material (rock) or a loose material (soil and sand).

Preparation of slide from rock  Cut  The first step is to cut the original rock in order to obtain a fragment with a

flat surface similar in size to that of the preparation which we require.  Polish  Once the flat surface is achieved, it must be polished to remove the roughness

of the cut and make it as smooth as possible.  Cementation  It is cemented onto a glass slide with resin or Canada Balsam.  Cut  It is then recut, trying to make the second face parallel to the first and as thin

as possible.  Lapping  Next, the sample is lapped until it is only 50µm thick, so that with a final

polish its thickness is between 20 and 30µm.  Cover  The last stage is to cement a cover glass on top with the same

material used as before, trying to ensure that no air bubbles remain trapped in between.

Making thin sections of soil requires three basic operations  Drying of samples  Samples can be dried at room temperature, followed by drying in an oven at 40

0C or less.  Instead of removing the water by evaporation, moist samples can be submerged in an acetone bath where the water will diffuse out of the voids and into the acetone solution surrounding the sample. Acetone replacement minimizes shrinkage and crack formation. Lee & Kemp (1992).  Freeze-drying  Jongerius and Heintzberger (1975) recommended that freeze-drying can be used on samples that will shrink considerably with air or oven-drying, and which contain Organic materials that may dissolve in acetone.  The samples, less than 2 cm thick, are dipped into liquid Freon 12, so that the water in the sample is quickly frozen at a temperature of -158 0C. After freezing, the samples are transferred to an ice condensator where the ice can be removed from the sample by sublimation.

 Impregnation with a resin  Samples of soils and sediments are hardened bymimpregnation with an

unsaturated polyester resin, Frencken Synolite,, with monostyrene as thinner.  The samples are impregnated under vacuum. After impregnation a pressure

of 6 atmospheres is exerted on the samples with nitrogen gas, to improve the impregnation.

Sawing and grinding  After hardening of the resin a slab is sawn from the block, using a diamond

studded sawing blade.  The slab is ground with a grinding machine and finally by hand with korund powder or papers or on diamond platen (Lee & Kemp, 1992), until scratches are removed.  The slab is mounted with the resin on an object glass, 1 mm thick. When fully cured, the slide is placed in a special jig and the slab is sawn to a thickness of 1-2 mm.  After this the slide is ground and polished, first with a machine, and finally by hand to a thickness of 20 micron. The final thickness is reached when the quartz grains are white to grey in colour, in cross-polarized light. After cleaning, the slide is covered with a cover glass (0.17 mm thick).  During all the treatments oil should be used as lubricating agent, instead of water, to prevent swelling of clays and dissolution of salts.  Most common thin section sizes are: 28 x 48 mm (petrographic size), 80 x 60 mm (Kubina size) and 150 x 80 mm (mammoth size), depending on the type of research.

The petrographic microscope  A petrographic microscope is used to

observe a series of characteristics in a mineral which reflect its properties and allow us to identify it.  The petrographic microscope is a compound microscope which can work with plane polarised light  The size of minerals that allows for optical identification is not samaller than 0.010 mm.  Identification of cryptocrystalline and amorpous materials can be achieved using submicroscopic techniques such as a scanning electron microscope.

Natural light and plane polarised light • Natural light (from the sun) vibrates in all directions in space (infinite directions of vibration) • Polarised light vibrates in a single plane at any one moment in time, but the direction of the plane of vibration changes with time. • When it always vibrates on the same plane, it is called plane polarised light (which we shall simply call polarised light).

What is relief When we observe different colourless and transparent mineral grains, some stand out more than others, although their thickness may be very similar. This is the phenomenon known as relief. In the outline of the minerals, there are some darker lines which, according to their width, make the grains stand out to differing degrees. Relief is an optical property that describes how well a material can be seen and distinguished from its surroundings.

Why does relief appear? When a ray crosses the surface separating two mediums with a different refractive index with a certain degree of inclination, its path diverges. In this case N>n, and the rays bend away from their optic normal (which is perpendicular to the surface which separates the two mediums) and are concentrated in the mineral, leaving a space which causes a dark area to appear on its edge.

Why are there different degrees of relief? The greater the difference between the refractive indices, the greater the refraction of the waves. This means that their divergence is greater, thus leading to a wider dark area surrounding the mineral. Relief is a consequence of the difference between the refractive indices of the two mediums in contact. When there is only slight relief, the difference is small, while when the relief is sharp, there exists a significant difference between the indices of the mineral and the medium.

COLOUR The colour white •When different radiations, capable of activating the eye sensors, come together in the same beam, the result is a sensation which we know as white light or white colour. •When a beam of white light crosses a prism, the radiations present, which have different frequencies, wave lengths and velocities, make the crystal show different refraction indices for each one of the components, meaning that their paths diverge unequally and thus the different colours are separated. A primitive beam can be reconstructed by interposing a lens which acts in the opposite direction to the prism.

Complementary colours •Not only can white light be constructed with the sum of all the visible radiations but also with some pairs of "colours". These colours, which together give white light, are known as complementary colours. •Green and red are complementary, and when superimposed, they generate the colour white. By interposing an object which prevents one of these radiations from passing through it, there is no sum and the colour which has been able to pass through appears. •Blue and orange are another pair and the third pair is made up of yellow and violet.

Opaque and transparent crystals •As crystals refract light, they absorb part of the incident radiations. The absorption is always proportional to the thickness of the crystal. •When absorption is total, no light can pass through and the crystal is opaque (1), whereas if part of the light can pass through, the crystal is known as transparent (2).

Colour of transparent crystals •When the absorption of light is homogenous in all the wave lengths and with a very low value, the crystal appears to be white (1). As the absorption increases, the crystal gradually appears to be greyish (2). When the absorption is total for a certain wave length, the crystal appears to be black colour •when we see a mineral with a certain colour, it is because the radiations corresponding to its complementary colour have been absorbed. •In part 4 of the figure, the mineral absorbs the radiations corresponding to red and allows those corresponding to the other five basic colours to pass through it. •Orange mixes with its complementary colour blue to give white light. Yellow together with violet also give white light. The only colour which is not nullified by its complementary colour, as this has been absorbed by the crystal, is green. Consequently, the light that comes out of the crystal is green.

Pleochroism •Pleochroism is the ability of a mineral to absorb different wavelengths of transmitted light depending upon its crystallographic orientations. •This is the property which some minerals present by which they appear to be a different colour depending on the direction in which they are observed, i.e. according to the direction of vibration of the waves that cross them.

Cleavage •This is the tendency of minerals to split in a uniform fashion along a series of parallel planes. •These planes correspond to the most dense reticulated planes which are known as cleavage planes.

How is it seen? Under the microscope, the cleavage planes are translated into one or more surfaces of parallel straight lines, as we can see from the figure.

1 cleavage direction •Only one preferential direction is appreciable for a crystal which splits into very thin plates of the mineral. This is very common in the minerals which have a layered structure, such as phyllosilicates. •In the figure, examples are shown of biotite (1) and muscovite (2)

2 cleavage directions Two plane surfaces appear. These surfaces split according to particular angles for each kind of mineral. The figure shows that these are perpendicular for a feldspar (1)

HABIT This is the tendency of some minerals to appear in certain geometric shapes, which correspond to simple or combined crystallographic shapes. The habit of minerals is the result of the internal structure of the crystals. Habit can be seen macroscopically or with the help of a stereographic microscope