Analytical Methods:Field Texture

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Questions
Particle size analysis is used primarily in studies of depositional conditions. For example was this deposit water-lain or wind-lain, and was this deposit deposited from moving water or stagnant water. More information on water-lain and wind-lain sediments can be found here.

Particle size analysis may also be used to identify sources of materials in anthropogenic deposits and to identify whether there is a mixture of materials.

Particle size analysis may also be used to ascertain whether sub-soils which are thought to have been affected by clay illuviation do actually have higher clay contents.

Sampling
Bulk samples are all that is usually required for particle size analysis. Samples should be taken from a representative part of a single deposit. The sample should not bulk together more than one deposit. For hand texturing a mass of soil ca. 2.5 cm in diameter is required; the soil should be moistened till it just sticks to the fingers and any gravel or stones removed.

Analysis
Hand texturing is the simplest method of determining particle size (texture) in the field. Analysis involves working the soil between the fingers to form balls and ‘worms’.

A flowchart for field hand texturing is provided here.

Also in the field, estimates of grain sizes in sediments can be told from the use of a Grain-size comparator, and a hand lens for the smaller-sized grains.

If more accuracy is required for coarse soils, air-dried soils can be wet or dry sieved. The weighed soil is lightly pounded in a mortar and pestle to break up any aggregates and is then passed through a series of sieves. In the fields this is only really feasible for sandy or gravely soils as it is difficult to sieve accurately to below 60 μm.

Data and interpretation
Soil texture classes are determined by the mixture of sand, silt and clay. The soil texture triangle describes the range of particle sizes that describes each texture class. Humified organic matter can also give a soil or sediment a silty feel. In such cases the organic nature of the silt component should be noted, e.g. organic silt.

Grain-sizes in sediments are classed according to the Udden-Wentworth scale as:

Related techniques
How to describe soil texture Laboratory particle size analysis

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