Particle Size Distribution

Determining Particle Size Distribution (PSD)

Before commencing this method it is advisable the pages on soil sampling are first consulted so that any analysis is done in accordance with a proper strategy and with respect to taking a representative sample. The following method has been adapted from standard laboratory procedures by phasm for use in the field.

EQUIPMENT:

Grading sieves with apertures of 2mm*, 0.2mm* and 0.06mm*,
scales accurate to one gram and an error of no more than +/- 0.2g,
2x 1ltr jugs, plastic sample bags, foil trays
trowel, marker pen and sampling strategy map.

 

Sampling in the field

At each point in the standard ‘W’ or along the transect line, 3 to 5 samples should be taken (3 being the minimum required for statistical analysis and 5 the most economical). The samples should not be taken from the surface, which is often contaminated with organic debris (leaves, twigs, etc) but from just below the soil profile (approximately 8cm/3inch down). Similarly the sample site must not be unique but representative of the entire field.

A sample of approximately 150g should be taken and if necessary passed through a 2mm sieve in the field before being bagged and labeled; a process repeated at each site until 15/25 samples have been collected.

If the stone content is greater than 10% then it should be properly calculated for whilst it still has little influence on the soil processes, at volumes greater than 10% it significantly affects the total soil volume and so needs to be known and accounted for if future management calculations are to be relevant.

Stone content

Stone content is determined by passing a much larger sample of soil through a 2mm sieve. The simplest way to do this is to use a spade and a piece of plywood (20cm x 8cm) as a template.

First clear away any surface litter from the soil then place the plywood on the sample site and use the spade to cut around the plywood template.

Using a trowel carefully remove the soil from under the plywood to a depth of 12cm and pass this through a 2mm sieve to separated approximately 2 litres of soil into two fractions. From this the stone content can be visually estimated or we can measure both the mass, by weighing the stones and the volume by displacement. We do not need the sieved soil anymore and this can be returned: keep only the stones.

Volume by displacement

To measure the volume of stones take two 1 litre jugs. Fill one with water to the 1 litre mark and put the stones from the sieved soil in the other jug. Now pour the water from one jug into the other until it covers the stones. Give the jug a gentle shake to displace any trapped air in the stones and then top up to the 1 litre mark.

The amount of water that now remains in the first jug is equal in volume to the stones in the second and as the sample was approximately 2 litres it is just a case of dividing this figure by 2 to calculate the percentage of stone content.

If the stones are large then increasing the size of the plywood template and or removing soil to a greater depth might prove more practical. Similarly if the stones are caked in soil they can be first washed. Ensure that you adjust your calculations to reflect the new volume of an increased sample area.

Preparation for analysis

Once all the samples have been taken the first thing to do is to air dry them to remove all the freely available water. This can be done at room temperature where the atmosphere is dry however it should ideally be done with some gentle heat assistance that can be provided by a few 60w incandescent light bulbs.

The samples should first be transferred from their plastic bags to open containers, such as the disposable foil trays used for take away foods, the foils should be marked with the sample id and these placed under the heat lamps or in a suitable warm and dry place for 48 hours. Technically the sample should be heated to 400C for 48 hours, but 2 days in a warm conservatory in mid July is just as good. Once air dry the samples are then ready for sieving.

Textural Group and Particle Size Distribution (PSD)

Lab, kitchen or garage analysis is now performed using these air-dried samples from your field site. The analysis takes the form of separating the samples into five fractions based on their approximate spherical diameter using grading sieves. The first of which is 2.0mm and it is used to remove all particle sizes greater than 2.0mm as, with the exception of extremely stony ground, such large particles play an insignificant role in the soil processes relevant to fertility. However whilst stones play an insignificant part in the soils hydrological, biological and chemical processes they invariable originate from the same parent material that the soil originates from and so share a similar mineral profile; thus the mineral composition of the stones can be indicative of the likely availability of plant nutrients in the soil system. That said they should first be removed and if significant their volume measured and recorded.

After passing through a 2.0mm sieve a ten gram sample should first be removed from each replicate and put aside; this will be used later to measure the pH. Either the entire remaining sample or a 100g sub sample should now be passed through a 0.200mm sieve to separate the top fraction, the course sands. which play a significant role in the drainage characteristics of a soil but have no significant water retentive or nutrient properties. The mass of course sands should be recorded.

The sample should now be passed through a 0.060mm sieve to remove the fine sands and leaving the silt and clay fraction. The fine sands similarly contribute to the drainage characteristics but also contribute to the soils water storage capabilities. The mass of fine sands should be recorded.

These two sand fractions are composed of the most chemically inert and resistant material of the soil, quartz. They are easily separated out by first passing the soil through a 0.200mm aperture sieve and then a 0.060mm aperture sieve. For most soils the sand fraction should account for between 15-85% of the textural component with between 20-60% being optimum.

Where these sieve sizes are not available then a size as close to these should be chosen. Bear in mind that the particles themselves do not conform well to being graded; for they are not spherical or uniform and we are assuming that they are! So this method uses arbitrary cut off points, cut off points that differ under different authorities with the USDA using a 0.050mm sieve as opposed to the British 0.060mm size for separating the larger sand fraction from the smaller silt and clay fraction. Similarly, particularly in very sandy soils a case could be made for separating the fine sand fraction further into medium (> 0.100mm) and fine (>0.060mm) sand as this could reveal more about the potential pore distribution and the soils tendency to drought. So whilst being perhaps the easiest most useful property that we can learn about the soil the particle size distribution is still very much an approximation.

Below 0.060mm lie the silts and below 0.0020mm lie the clays. It is within these 2 fractions that water and nutrient storage is primarily maintained and whilst it is possible to use grading sieves to mechanical separate the silts further, it is only possible to separate the clay fraction using the settlement process described next.

This quantitative analysis can be used to confirm a hand analysis and/or determine a textural classification for each sample using the charts below. Ideally the samples should all reflect one another and whilst we should expect some variability between samples and sites they should be within given boundaries.

Soil Textural Classes using the USDA system (colours) and the UK ADAS system (bold lines) to distinguish soil textural classes. Note that under the British system there are only eleven classes but under the American there are twelve. This discrepancy is as a consequence of differences in determining the cut off point between fine sand and silt; which under the British system is at 0.06mm whilst with the American the division is at 0.05mm. Whilst this adds a little confusion and slightly shifts the proportions it really only results in the creation of a pure silt class in the USDA system and a slightly different definition and terms with respect to the loam (USDA)/sandy silt loam (UK) and silt loam (common). One could call it a blessing for it serves as a good example of the difference in measuring and assessing soil properties.

Variations and trends

On undulating, sloping or terraced land, particularly where multiple sampling strategies have been used we may see trends where a given component (i.e. the silt/clay fraction) increases in quantity as we progress down a slope. This may result in different sites having slightly different textural properties and is perfectly normal and acceptable, or it may be indicative of a boundary in soil types existing in the field. Since different soils require different management then any such boundaries should be identified by supplementary sampling.

next: Determining the Silt Clay fraction

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

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