Biology Fieldwork

A Level

Soil

Measuring soil pH

2. Soil

Soil

Some of the most useful abiotic factors to record are soil characteristics.

Remove vegetation with a trowel. Take a small sample of soil (a quarter of a trowel full is plenty) from 15cm below the surface of the vegetation and seal it in a polythene bag straight away to prevent it from drying out. Put a label marked in pencil inside the bag to show the location and time. Plastic 35mm soil pots or film pots are a useful alternative to polythene bags.

Collecting soil samples by digging holes is destructive and you must have permission from the landowner before you start. Only remove a small amount from each place - not a trowel-load. Replace any vegetation you have removed. Wash your hands thoroughly after carrying out soils work.

Soil Sampling Harlech Sand Dunes (1) Measuring Soil Infiltration Rate At Mallaghmore
Taking a soil sample in a dune slack at Harlech sand dunes, near Rhyd-y-creuau Field Centre Measuring soil infiltration rate in semi-fixed dunes at Mullaghmore sand dunes, County Sligo, near Derrygonnelly Field Centre

Here are some of the most useful soil factors that you can measure:

Soil pH

A chemical pH test can be carried out in the field, or on the soil samples in the lab.

  1. Put 1 cm of soil into a test tube
  2. Add 1 cm of barium sulphate to the test tube (this attaches to fine particles and makes them sink leaving a clear layer above it)
  3. Add enough distilled water to bring the level half way to the top of the test tube
  4. Add two pipettes full of indicator solution
  5. Seal the test tube with a rubber bung and shake well (make sure that the contents are mixed thoroughly)
  6. Allow the test tube to stand until the soil has settled out leaving a coloured solution
  7. Hold the tube next to the colour chart and decide which pH colour it most closely matches.
  8. Dispose of the soil in the bin.

Alternatively hand held digital pH probe can quick and simple to use both in the filed or in the lab. 

2. Soil texture

The soil texture is determined by the proportion of the soil made up of sand, silt and clay. A quick way of judging soil texture in the field is outlined in the FSC soils types field guild identification chart.

3. Soil infiltration rate

Soil infiltration can be used to understand the soils composition and also how compact it is. 

Infiltration -tube

You will need

  • container of water (keep this constant between experiments - 4 litres is enough)
  • 30 cm ruler
  • metal / plastic tube (to be the 'infitration tube')
  • stopwatch
  • block of wood
  • mallet
  • at least 2 people - one to pour in water, the other to operate the stopwatch and take readings

Infiltration -stopwatches

Bang the infiltration tube into the ground until it forms a seal (using the wood and mallet to bang it in evenly and avoid affecting the results by stamping on the area of land being tested).

One person fills the infiltration tube to a standardised level (e.g. 15cm). The other starts the stopwatch, and then record the water level every 30 seconds. To maintain water pressure, person 1 should top up the infiltration tube if their water level falls below a certain point (e.g. 10cm). Person 2 should make a note of when this is done, so that it can be taken it into account when they are calculating their infiltration rate.

Soil moisture content

Soil water content is measured by weighing the fresh soil sample, drying the sample, then weighing the dry soil. The difference between the two figures is the soil moisture content. It can be expressed as a percentage of the mass of the fresh soil sample.

The most effective way to dry the soil sample is to use an oven (a microwave oven will do), but if this is not available then you could leave the soil sample to dry overnight.

  1. Find the mass of a heat resistant crucible.
  2. Half fill the crucible with soil from the soil sample and then weigh it again.
  3. Place the crucible in an oven at a temperature just above the boiling point of water (ideally around 105°C) until it is dry.

You can tell when it is completely dry by weighing the crucible and soil, returning it to the oven for 10 minutes and weighing again and so on, until there is no change in weight after 10 minutes. Don't be tempted to turn the oven up to make the drying take place more quickly. You will only begin to burn off the humus in the soil and won't then be able to find out either the weight or humus content accurately. Now record the weight of the crucible and dry soil. Find the difference between the dried soil and the fresh soil, and use this to calculate the % moisture content of the fresh soil.

5. Humus content

Soil humus content is measured by weighing a dry soil sample, burning the humus in the soil, then weighing the soil left. The difference between the two figures is the humus content, which can be expressed as a percentage of the mass of fresh soil sample.

It is usual to measure for soil humus content after you have measured for moisture content, so that you have figures for the mass of fresh soil and also samples of dry soil.

The most effective way to burn off the humus is to use a furnace that is capable of reaching a temperature of 550°C. If this is not available, a Bunsen burner can be used instead.

If using a furnace, place the crucible and dry soil into a furnace at 550°C for 15-30 minutes, until all the humus has been burnt off.

Or if using a Bunsen burner Using a tripod and gauze sheet, heat the soil sample in a crucible above a full-flame Bunsen burner for 30 minutes to 2 hours, until all the humus has been burnt off.

Record the weight of the crucible and soil and calculate the percentage of humus in the soil. The mass of dry soil, rather than the mass of wet soil, is used to calculate the percentage humus content. Using dry soil mass allows you to compare soil samples taken on different days.