Silt Clay Fraction

Determination of Silt Clay Fraction

The silt clay fraction, which is too light and small to be successfully distinguished by grading sieves, can be determined by settlement. Essentially a prepared sample is allowed to settle through a column of water. The larger sand and silt particles settle out more quickly than the finer clays so that over a period of time the material is deposited according to size. By measuring the depth of the settlement layer at set times the relative proportions of this fraction can be determined. The formula is relatively simple and easy and can be used for larger size particles with, in the absence of grading sieves, settlement being a method that can be used to determine the entire particle distribution.

In the laboratory procedure outlined in Rowell (1994) the method is described for using a 10g air-dried sample which would typically contain less than 4g of clay and in sandy and weathered soils less than 1g. Whilst a well equipped laboratory has the means to accurately measure such tiny volumes, our margins of error are not so acute so the method here has been greatly adapted.

The method is normally performed on an air dried sample that had been passed through just a 2mm sieve as the sieving process is not particularly thorough in separating out the finer fractions from the sand, and a small proportion is retained as a ‘dust’ layer.

Relative to the whole mass of the soil this layer is insignificant but relative to the total clay fraction, particularly in very sandy soils where the total clay content is small, it is not.

The method similarly requires the organic fraction, particularly the microbial biomass, which causes the binding of soil particles, to be removed by treating with Hydrogen peroxide. This is prepared by diluting 200ml of ‘100 vol. H2O2’ (30g H2O2 per 100ml) with 800ml of water (technically distilled or de-ionized) to make up a litre of solution containing 60g H2O2.

Hydrogen peroxide –safety precautions!

Hydrogen peroxide is highly corrosive and will oxidise skin as easily it does organic matter. Suitable protective gloves and eye glasses are normal recommended safety considerations when working with hydrogen peroxide. In the event skin is splashed wash the affected area immediately with copious amounts of water. If the affected area is the eye or the resulting burn severe then medical assistance should be sought.

Beginning with sufficient air-died 2mm sieved soil (100-500g) pass this though a 0.2 and 0.06mm sieve to generate approx. 50g of silt clay; weigh the three fractions then put the silt clay fraction into a suitable sized and appropriate glass beaker of 500ml to 1ltr capacity. For each 10g of soil being treated add 10ml of H202 solution. This should be added carefully as the reaction begins quite vigorously. The solution will froth and this can be reduced by adding a few drops of Octan-2-ol.

Once the frothing has subsided, add another 10ml of solution per 10g of soil and repeat until fresh applications produce no further reaction. Once the frothing has stopped, stir and apply gentle heat with a Bunsen flame or suitable alternative heat source.

Add more H2O2 periodically whilst stirring over this gentle flame until the reaction stops. You should have used no more than 100ml H202 per 10gram of soil. Now raise the temperature and bring the solution to boiling point then reduce its volume by half before allowing to naturally cool. This should have oxidised all the organic matter leaving approx 250ml of solution and 50g mineral particles.

We could now further treat the solution with a dispersion agent to separate the soil particles further. The dispersion agent is prepared by dissolving 50g sodium hexametaphosphate, which binds to positive sites, and 7g of anhydrous sodium carbonate which causes repulsion between particles, into 1ltr water. For each 100ml of peroxide treated soil solution 10ml of dispersion agent should be added and stirred.

The treated solution should now be poured into a suitably large bottle or sealable jar (500ml or at least double the volume of the solution) to no more than half full. Any soil remaining in the original beaker should be washed out with water and into the bottle which should end up ½ but no more than ¾ full. The bottle should then be sealed and the contents vigorously shaken; ideally with a mechanical shaker over night or several times over several hours by hand.

Given one last vigorous shake the bottle should be opened and the contents ‘stood’ onto the column of water. he intention is to release 250ml of solution containing 50g of soil onto the surface of the column without disturbing the column.

Done correctly the sample separates out as the larger particles fall quicker than the smaller ones; resulting in sorted layers accumulating at the bottom. The first of these will be any sand followed by large silt particles and as time progresses the smaller particles will settle out. From the chart below we can estimate the distribution of the particle size based on settlement velocities over a measured distance. The formula for calculating the settlement velocity is derived from Stokes’s Law for streamlined flow:

V = 2gr2(Ps –Pl)/9w

Where V is the sediment velocity (ms-1), g the gravitational force (9.81Nkg-1), r the particle radius (m), Ps the density of a soil particle ( 2600kgm-3), Pl the density of the water (998kgm-3) and w the viscosity of the water (1.002 x 10-3 Nsm-2 at 200C).

Over a 50cm column the sands should settle out in under 160 seconds. a mark should be made indicating any sediment layer that has formed at this point. The first of the silt fraction, particles > 0.02mm will settle out completely after 24 minutes and the second half of this fraction after 40 hours. At both timings mark the sediment layer. The clay fraction will now take over a month to settle out and should be measured 30-40 days later.

next: soil mineral properties

Free Cultural Works: Malcolm McEwen (2011) (CC BY-NC-SA)

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