Growing Food: A Guide to Food Production

If more than about 8 MT/ha is needed, this should be applied in two applications
with the first ploughed in several weeks or months before the second.

pH Sand & loamy Clay loams Peaty soils

sands and clays (25% OM min.)

4.5

5.5

6.0

15 (7) 20 (7) 29 (7)

9 (5) 12 (6) 13 (4)

6 (2) 8 (3) 5 (0)

The figures above show that the quantities involved are large, and lime is not normally
locally available in such large quantities.
The information is included here to show that although lime is one solution to the
problem of soil acidity the remedy is neither cheap nor easy.

Testing for Soil pH

Universal pH Indicator.

several indicator chemicals
A chemical is added to soil that is in suspension in water in a test-tube, and the
change in colour is noted, as indicated below:

pH Colour

Below 5

5–6

7

8–9

Above 9

Red

Pink

Green

Blue

Purple

Litmus paper or litmus indicator can also be used, but this is even less precise than a
Universal pH Indicator.
Litmus turns to red in acidic soil water, and blue in alkaline or neutral soil water.

Also known as micronutrients (or minor elements) these are chemical elements that are
required by plants and animals in very small quantities for the proper functioning of
enzymes, hormones and vitamins.
The most important trace elements for plants are manganese, molybdenum, zinc,
boron, iron, copper and sodium. Magnesium, calcium and sulphur are also known

23

It is very easy for amateurs to discover the very approximate pH of a soil by using a
Chemical indicators change colour in response to the pH of
the solution. Most chemicals used as indicators respond only to a narrow pH range.
A universal indicator, however, is sensitive to a large pH range because it uses

GROWING FOOD – THE FOOD PRODUCTION HANDBOOK

d Trace Elements