Soil pH
The soil pH tells us if it is acid, neutral or alkaline.
Measuring soil acidity
The pH scale is used to measure acids and alkalis. It starts at pH 0, extremely acid, and goes up to pH 14, extremely alkaline, which is the opposite of acidic. At the halfway point, pH 7, the soil is neither acid, nor alkaline, so it is said to be neutral.
A soil of pH 6 is 10 times more acid than a soil pH 7 (neutral).
A soil of pH 5 is 10 times more acid than a soil of pH 6, and 100 times more acid than a soil of pH 7, and so on.
Soil type, climate and organic material all influence pH. If your area has clay soils your soil probably is acid. If you live in a sandy area or near limestone your soil could be slightly alkaline.
The best pH
New Zealand soils range from about pH 3.5 to pH 8. Most plants grow best in a slightly acid soil. At this pH plant roots easily take up all the nutrients that plants need and at this range most nutrients are freely available.
A camellia does not mind acidic soils.
Rhododendrons, camellias, azaleas, daphne and blueberries prefer acidic soils.
Delphiniums will do well in an alkaline soil.
It can be useful to find out the pH of the soil.
This can be is done with a pH meter or pH paper.
A pH meter has a stem that is dipped into a soil solution. The pH is shown on a dial.
When dipped in water containing a soil sample, pH paper changes colour.
The pH is read by matching this colour with the colour on a chart. If the solution is very acid the indicator turns red, less acid is orange or yellow, and neutral is green. In weak alkaline solutions the indicator turns blue and in strong alkaline solutions it is purple.
pH and plant nutrients
Soil acidity can cause nutrients to be changed chemically to forms that can't be used by plants.
- Nutrients become more soluble and are easily leached out of the soil, so they are removed when the soil water drains away.
- Nutrients become more insoluble, won't dissolve, and therefore are unavailable to plants in the soil water.
- Below certain acidity levels, soil bacteria stop decomposing.
Test your understanding so far by selecting True or False.
Drag the correct word into the correct position.
- Below is a diagram that shows the effects of pH on nutrient availability. The width of the bands shows the availability of nutrients.
Drag the correct answer to complete the statements.
Key points
- Use the pH scale to measure soil acidity.
- Soil pH is measured on a pH scale of 0–14 where 0 is very acid, 7 is neutral and 14 is very alkaline.
- Soil acidity affects nutrients availability, soil bacteria activity and plant growth.
- The optimum pH for growing most plants is 6.0–6.5.
What's next?
Go to: 3 Biological properties.