Method | Method |

Soil bulk density

URI: http://linked.data.gov.au/def/ausplots-cv/48ef72ad-f863-42df-9c16-962a0d0d6e36

Also known as D) Soil bulk density

Date created: 2020-07-31
Date modified: 2021-01-14

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Purpose

Soils descriptions i.e. information recorded, number of recordings and coverage of locations, are generally poor across the rangelands region of Australia. Details of location and data available from current descriptions should be obtained prior to the surveys to direct the choice of survey location. The plot descriptions and soil characterisations collected will substantially alleviate this paucity of information.

The data collected can also be used to increase the reliability of the rangelands component of the Soil and Landscape Grid of Australia, produced by the TERN facility consistent with the Global Soil Map specifications. Analyses of the collected samples will greatly enhance the level of knowledge (e.g. nutrient and carbon levels) and hence understanding of rangelands soils and how they will respond to climate change and management options. The nine soil observations can be analysed by a number of different methods e.g. wet chemistry, MIR or NIR (mid infrared spectrometry or near infrared spectroscopy) either individually to provide a measure of variation of the parameter being measured across a plot or bulked together and a sub-sample extracted and analysed to provide a mean value for that parameter across a plot. For AusPlots, keep the samples separate.

Knowledge of bulk density values of rangelands soils and likely variability across the plot is largely unknown. This needs to be considered in deciding number and location of samples across the plots. 

Soil samples down the profile.

Scope

An indicative measure of the bulk density of the soil is necessary to quantify important soil parameters, particularly carbon content. Soil bulk density values are needed to calculate soil properties per unit area e.g. tonnes of carbon per hectare. From a preliminary investigation, it is acknowledged that to ensure the measure is reliable, multiple samples will need to be taken across each plot in different surface substrate classes and at different distances from trees or shrubs. To achieve this is beyond the scope of the initial survey process, so the intention is to obtain an indicative value only for each plot.

Soil bulk density (g/cm3) = mass of oven dried soil (g) / total volume of core (cm3)

The core method is recommended for taking bulk density measures. This involves inserting a metal cylinder (bulk density ring) of known volume into the soil at three depth increments: 0-10 cm; 10-20 cm; and 20-30 cm and removing a soil core from each. These samples are sealed and then taken back to the lab where they are oven dried and weighed.

The accuracy of this method depends on the precise collection of undisturbed soil in the bulk density ring, ensuring the soil occupies the total volume of the ring without overflow, hollows or voids. Care is also needed to ensure all soil is included when weighing the sample.

Accuracy is also influenced by the proportion of coarse fragments in the soil and where significant compaction occurs as a result of inserting the corer. Coarse fragments (soil particles greater than 2 mm diameter) need to be considered in determining soil bulk density (see point 12).

Definition

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Equipment

d. Soil Bulk density

Time required

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Instructions

Time requirements

Depends on the soil type and method used. Can take from 1 hour (where shallow hard pans stop the soil characterisation and soil observations) to 6 hours where a pit is dug and analyses undertaken in the field.

Full steps

See the fine earth bulk density and gravel bulk density parameters.

See the soil ring volume and soil ring weight parameters.

See the gravel volume and gravel weight parameters.

See oven dried soil weight in bag and bag weight parameters.

For a soil sample at a depth of 0-10 cm:

  1. Clear litter and vegetation from a 30 cm x 30 cm area of the soil surface without disturbing the soil to any degree.

  2. Remove the plastic caps from a bulk density ring and put the ring, bevelled edge down, into the soil corer frame.

  3. Place the frame over the cleared area. Using the mallet gently tap the frame into the soil until the top edge of the ring is a few millimetres below the soil surface. Depending on the presence and size of any soil coarse fragments the ring may need to be inserted further. If the soil surface is uneven, an estimate of the mid-point should be used. In order to avoid chipping or bending the cylinder on rocks or other unseen hard material (and thereby changing the volume of the ring) do not use excessive force when tapping the frame into the soil.

  4. Using a trowel or spatula, dig out the soil adjacent to one half of the ring.

  5. Depending on the cohesion of the soil, either,

    • using a spatula or knife carefully trim the soil until it is level with the top rim of the ring, cap the top of the cylinder in situ, insert a trowel a few centimetres under the cylinder and with your other hand on the top cap, remove the ring and excess soil ensuring the soil in the cylinder is not disturbed.
    • or
    • insert the trowel a few centimetres under the cylinder and with your hand on the top of the ring for stability, remove the ring and excess soil ensuring the soil in the ring is not disturbed.

      

  6. Using a spatula or knife, carefully trim the soil at the exposed end of the ring until it is level with the rim. Ensure there are no hollows i.e. the soil level does not go below the rim of the ring, or that the soil does not protrude beyond the rim of the ring. 

    Slight wetting of clay soils using a spray bottle and deionised/distilled water may assist in the trimming process to get the soil flush with the ring ends. Care should be taken to not over-wet the soils as this may affect the volume through filling voids or expanding shrink/swell clays.

  7. Without disturbing the soil, cap that end of the ring.

  8. If the soil sample is not level with the end of the ring due to the presence of rocks, gravel or air spaces (meaning the volume of the soil is not identical to the volume of the ring), reject the sample and repeat the process until a suitable sample is obtained.

    If an accurate volume cannot be obtained after three attempts in an area, then abandon this area and try elsewhere within the plot.

  9. Carefully trim the soil at the other end of the ring ensuring it is level with the rim of the ring (as above).

  10. Cap that end of the ring or tip the entire volume of soil into a sealable (i.e. zip lock) plastic bag for storage. (Soil remains in this vessel for transport to the drying oven, so if the soil has high moisture content or is reactive and is likely to interact with the ring, then a plastic bag should be used).

  11. Ensure that whatever vessel is used, it is suitably labelled with the appropriate voucher number, sample number, depth, date, plot ID, GPS coordinates and collector.

  12. If coarse fragments contained within any sample are estimated to constitute greater than 30% of the volume of that sample, then accuracy levels are compromised and the sample should be discarded. Try again in an adjacent area but if, after three attempts, coarse fragment levels are still >30% of the volume of the sample, then try somewhere else within the plot.

    If the whole site is gravelly and coarse fragment levels consistently exceed 30%, then take double samples from each depth i.e. use two rings at each depth. These can then be combined during the drying process providing acceptable accuracy levels.

  13. Pack rings or plastic bags into a suitable container to protect them during transport back to the lab.

  14. At the local lab where suitable facilities are available, samples will be processed to determine soil bulk density. Where facilities are not available, samples can be re-packed and forwarded to AusPlots in Alice Springs for drying and processing (see Chapter 11e for Alice Springs address).

  15. Fill in the hole when all samples from all depths have been collected.

  16. Laboratory analysis involves drying the soil samples in an industrial/scientific oven (i.e. non-domestic) at 105°C for 48 hours and then weighing the samples. Samples should be weighed on scientific scales in grams accurate to three decimal places.

If the soils are very dry or have limited clay content the process may be enhanced by slight wetting of the soils and leaving them for a while to increase the adhesion of the sample to ensure it remains in the ring. However, over-wetting can alter soil properties (see point 6), so should only be used as a last resort. If the soil is very adhesive and cannot be easily removed from the ring into a plastic bag for storage and transport, then moisten the inside of the cylinder with distilled water from a spray bottle or with vegetable oil.

For sub-surface soil samples (depths of 10-20 cm or 20- 30 cm), dig a pit to the required depth (10 cm or 20 cm) of sufficient size to accommodate the soil core frame, and sample as for steps 2 to 13. Ensure that no soil falls into the pit from the upper layer/s. When removing the ring (step 5 above) ensure that no soil material from upper layer/s falls into the ring.

Where suitable local facilities are available (or the TERN Alice Springs lab), soil samples are oven dried (105°C for 48 hours) to a constant weight and then weighed. If the procedure is followed rigorously, the volume of all the samples will be identical (as the volume of all rings is identical), so sample volume will only need to be determined for an initial sample. However, where coarse fragments are present in the soil these need to be given due consideration in calculating soil bulk density. Two main methods exist, either leaving the coarse fragments in the samples that are dried and weighed, or sieving the sample to remove the coarse fraction (particles >2 mm), and using only the mass and volume of the fine earth fraction. The appropriateness of the method will be dependent on the use of the soil bulk density measures. It is recommended that coarse fragments be removed by sieving, but their mass and volume be determined and made available for any future calculations. (For greater detail see Chapter 11e, Soil samples.) All sieved coarse material should be retained and sent to the National Soil Archive in a separate plastic bag within the container.

Collecting Soil Bulk Density samples using the AusPlots Rangelands Survey Protocol 

Additional notes

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Source

White, A, Sparrow, B., Leitch, E., Foulkes, J., Flitton, R., Lowe, A.J., Caddy-Retalic, S. (2012). AusPlots Rangelands Survey Protocols Manual. Version 1.2.9 page 51-55. Terrestrial Ecosystem Research Network and The University of Adelaide Press. https://doi.org/10.25901/5f2ca309cc9c2

Parameter

TERN is supported by the Australian Government through the National Collaborative Research Infrastructure Strategy, NCRIS.