How to lower the pH of your soil

By Fredalette Uys | 8 January 2018
In healthy soil, there is a balance between biological, chemical and physical processes. Agriculture disturbs this balance and it is up to the farmer to restore it.

The pH level of your soil influences the availability of nutrients to plants in the soil. If the pH levels move beyond the optimal range of about 6 (slightly acidic) and 7.5 (slightly alkaline), your plants will have difficulty to absorb certain nutrients and deficiencies will occur, ultimately affecting the yield and quality of your produce.

If the pH levels in your soil are outside of this range, you can either alter the pH level or you can choose plants that are adapted to the pH range of your soil.

Before you add anything to your soil to change its pH level, it is important to test it to establish its exact pH reading. Altering the pH of your soil should not be a guessing game.

The first visual symptoms that can be observed in plants when the pH is too high (or alkaline) are the yellowing of leaves due to the weak absorbability of iron (Fe), manganese (Mn), copper (Cu) and zinc (Zn). These elements play important roles in processes like photosynthesis and enzyme activation.

If your soil is alkaline (above a pH of 7), you can use several products like organic matter, ammonium fertilisers and sulphur compounds to lower the pH level of your soil.

These substances react with the soil particles to lower the pH. The reactions can be either chemical or biological. Biological reactions take place when micro-organisms metabolise the added substances into more acidic substances to change the pH of the soil. Biological reactions can be slower and according to the season, since microorganisms don’t respond well to cold and dry conditions.

Also read: Do it yourself: Making your own basic soil pH test


Take the following factors into account when you plan to acidify your soil:
1. The pH level you want to reach.
2. Will the amendments provide extra beneficial nutrients?
3. Your time frame – when do you want to have the pH changed
4. Budget.

If your soil contains high levels of calcium carbonate (CaCO3), it will be difficult to lower its pH. CaCO3 will neutralise the acidifying agents you add and you will have to add a lot of acid just to neutralise the CaCO3. It would be better to rather choose plants adapted to more alkaline soils.


Organic matter

Adding organic matter to your soil can lower its pH, but you need substantial amounts to do so. This can be very costly needs a lot of effort and works best in a small piece of land.

Ammonium fertilisers

Ammonium fertilisers like urea, ammonium nitrate and ammonium sulphate serve a double action by also fertilising the soil and acidifying. Bacteria in the soil convert the ammonium into acidic compounds. Ammonium fertilisers should however only be used to make gradual changes.

Sulphur compounds

Elemental sulphate, iron sulphate and aluminium sulphate are used to dramatically decrease the pH of soil – by 1 or more pH units.

Acidification of elemental sulphur happens when the soil bacteria converts sulphur into sulphuric acid by combining it with water and oxygen. The bacteria need warm, moist, aerated soil for this process. Incorporating the sulphur into the soil is more effective than applying the sulphur on top of the soil.

The process can take up to a year to complete. Applying too much sulphur could lead to acidification of your soil, which can be detrimental to plant health.

Iron sulphate and aluminium sulphate lowers the soil pH through a chemical reaction and is much faster. However, this could lead to excessive iron and aluminium levels in the soil.

Also read:
How to test soil life and health
How to use fertiliser to restore degraded soil


In our next article in this series, we’ll write about increasing your soil’s pH level.