Chapter 14

Carbon Chemistry

All living organisms from viruses to whales have the element carbon at the centre of their chemistry. The study of this element’s chemical activity is called organic chemistry. Originally, it was thought that all organic compounds came from living organisms, but modern chemistry has brought synthetic urea fertilizer and DDT, which are organic. 

Carbon atoms are able to join to other carbon atoms. Pure carbon compounds take up very different forms; two of the better known ones are graphite (Figures 1a and b) and diamond (Fiures 2a and 1b) which have very different physical properties.

Carbon atoms are able to combine with other atoms e.g. with oxygen to form carbon dioxide (Figure 3) and with hydrogen to form methane (Figure 4). They also combine with hydrogen and oxygen to form families of organic acids (e.g. acetic), alcohols (e.g. methanol, ethanol) and the carbohydrates (e.g. sugars, starch, cellulose). One of these sugars, glucose (C₆H₁₂O₆) is the end product of photosynthesis. This can be represented as a simple ring structure (see Figure 5) which forms the basis of all the organic compounds in plants and animals (see Figure 6).

Measuring Soil pH

pH can be measured using a pH meter or by the use of indicators; both are used for testing soils. Soils sent to laboratories for analysis would normally be tested with pH meters. Colour indicator methods are most commonly used by gardeners who do their own testing.

Colour indicator method (see Figure 1)

Risk assessment. No harmful chemicals involved; barium sulphate used as a powder so eye protection advised.

Required (see Figure): 

• 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 envelope to prevent fading)
• Distilled water
• Barium sulphate
• Clean spatula or similar.

Figure 1 Soil pH testing equipment

The soil to be tested should be a representative sample of the area being analysed. Before the pH test is started, its texture should be found. The procedure for ‘loams’ is as follows (see below for heavier and lighter soils):

•  Soil, preferably dried and sieved, is now added to the test-tube to a depth of 2 cm.
• 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 BDH soil pH indicator; for most soils only a few drops are required (more can be added later if the colour does not develop well enough to see).
• 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 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 weak, add a few more drops of indicator; dilute with a little distilled water if the colour is too concentrated to see the hue.

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, causing it to form groups of particles, it sinks quicker to the bottom). If the soil texture indicates a very light soil (low clay) or a very heavy one (high clay), then the proportions should be altered as follows:

Peats and soilless composts do not require any barium sulphate and the ingredients will float to the top, but the colour should develop in the water below.