The pH or acidity of the soil can be a key factor in ensuring strong and healthy plant growth.
While it is true that specific plants vary in the soil pH they prefer, there are some factors which are affected by soil pH universally across all plants.
We have taken a look at how soil pH affects plant growth and how you can test soil pH levels.
How does soil pH affect plant growth?
The main factor which varies with soil pH is nutrient availability.
- Very acidic – pH 3.0 to 5.0
- Calcium, Potassium, Magnesium and Copper (Ca, K, Mg and Cu) are highly soluble so are easily washed away
- Phosphates are unavailable for absorption
- Bacteria growth is hampered, so there is less rotting of organic material which makes new nutrients available
- Acidic – pH 5.1 to 6.0
- Ideal for certain plants which cannot withstand lime
- At pH 5.3 or below, all of the buffering lime has been used up – this means any small additions of acidic medium will have a larger effect on bringing the soil pH even lower
- Slightly Acidic – pH 6.1 to 7.0
- Usually the best level for most plants (except for specifically “lime hating” plants like Rhododendrons and some others)
- Nutrients are readily available
- Bacteria and earthworm activity is optimum, resulting in good renewal of soil with new matter being quickly broken down
- Alkaline – pH 7.1 to 9.0
- Phosphorous (K) becomes less available
- Iron and Manganese (Fe and Mn) are less available – lime-induced chlorosis can result
- Plants may exhibit symptoms of nutrient deficiencies
Remember that these are general guidelines, and that different plants do have different preferred pH levels, so check what pH is best for the specific crop.
How can I test soil pH?
There are two main options available for measuring soil pH, each with their own benefits and drawbacks.
These all-in-one kits allow simple and low cost measurement of pH
- Simple and low cost
- All in one kit with little to no maintenance needed
- Some kits also allow other parameters to be measured like nitrates, phosphorus, potassium (NPK) etc.
- Lower accuracy of results
- Some rely on manual observation of the results – i.e. the sample changes colour and you compare this by eye to a chart. This can be difficult if the water obtained is still coloured or cloudy from the soil.
An electronic pH meter offers a higher accuracy method, but does require more maintenance and care.
- More accurate results
- Direct digital reading – does not rely on human observation by eye, so all readings are more consistent
- There are now lower cost and simple to use meters available which are suitable for soils – e.g. the Laqua Twin pH meter, or the TRUEscience pH cap paired with the flat pH probe
- You must make sure you use a suitable probe – a double junction flat probe is best for soils, others may quickly fail in soil environments
- Ongoing care for the meter is needed – calibrating it at the start of each use day using pH buffers, and making sure it is stored correctly when not in use
- Other parameters cannot be tested (like NPK etc)