Colour indicator method
- Clean test-tubes; ideally ones that have stoppers that can be removed top and bottom for easy cleaning;
- BDH soil pH indicator;
- BDH pH colour chart (kept in an envelope to prevent fading);
- Distilled water;
- Barium sulphate;
- Clean spatula or similar.
The soil to be tested should be a representative sample of the area being analysed? (see p379). Before the pH test is started, its texture should be identified (see p307).
- The soil, preferably dried and sieved, is now added to the test-tube; for 'loams' this should be to a depth of 2 cm (see below for heavier and lighter soils).
- Add to this the same depth of barium sulphate (2 cm).
- Shake together.
- Half fill the test-tube with distilled water (some specially designed tubes have a mark to fill to).
- Add soil pH indicator; for most soils only a few drops are required
- Place bung in top of tube and shake.
- Leave to stand.
- Find the soil pH by matching the hue of the colour with one of the colours on the chart. This should be done by having the light coming over the shoulder whilst holding the tube against the white background alongside the colour square on the chart.
- If the colour is too concentrated to see the hue add a little distilled water; a few more drops of indicator if too weak.
This procedure is suitable for most soils, but it should be noted that the barium sulphate is added to help clear the water in order to see the colour clearly (by flocculating the clay so that it sinks more quickly to the bottom). If the soil texture indicates a very light
(more can be added later).
- Figure 20.3 Soil pH testing. Equipment required includes test tubes, spatula, barium sulphate, pH indicator, colour chart. 1 Barium sulphate added to soil. 2 Distilled water added. 3 Shake. 4 Add indicator and leave to stand. 5,6,7,8 Check colour against the chart to find soil pH
soil (low clay) or a very heavy one (high clay) then the proportions should be altered as follows:
'Loams' Light soils Heavy soils
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