Depending on the agricultural situation, it might be advantageous to lower the soil pH for
citrus trees, growing in very alkaline or sodic soils.
The process begins with testing the soil pH to determine its current pH level and the
amount of adjustment needed. Citrus trees should be grown in soils with a pH of 5 or more
on the KCl pH scale. Soil pH should not exceed 6.5 on the same scale, with the ideal pH
considered being one of 5.5.
When citrus trees are grown in highly alkaline soils the uptake of phosphorous is inhibited,
as well as most micro elements. (Except for Mo). This happens because phosphorous
reacts and binds with calcium and the micro elements bind with hydroxides and
carbonates in the soils. This can be overcome in the short term with yearly phosphorous
applications and regular micro element foliar sprays.
Sulphur-based amendments, such as elemental sulphur, aluminium sulphate, or iron
sulphate, are the most effective and commonly used agents for acidifying soil. Elemental
sulphur is preferred for long-term pH adjustment, as soil bacteria convert it to sulfuric acid
over several months, gradually lowering the pH. A soil temperature of at least 13 degrees
Celsius is needed for the soil bacteria to start converting the elemental sulphur into sulfuric
acid in the soil.
Aluminium sulphate and iron sulphate act faster but are more suitable for smaller-scale
applications or when immediate results are required. It takes 8 times the quantity of iron
sulphate in comparison to 1 part of elemental sulphur to lower soil pH with one pH unit. pH
is a logarithmic scale of 10, which means that a pH value of 4 is 10 times more acidic than
a pH value of 5, but 100 times more acidic than a value of 6, as an example. Clay soils
buffer acidity and alkalinity and thus its pH is naturally adjusted over more time than that of
sandy soils, which means more elemental sulphur is needed on clay soils. In the Eastern
Cape citrus production areas, the average clay content varies of between 18% up to
around 28%. This has implications for irrigation scheduling, as well as fertilizing practises.
Organic materials such as sphagnum peat moss and pine needles can also help acidify soil when incorporated into the planting area or used as a mulch. Applying ammonium-based fertilizers, such as ammonium sulphate or urea, can lower pH over time through the release of hydrogen ions during nitrification. The use of KCl and other Chlorides
(Remember KCl has 46% Cl) should best be avoided as well as nitrate-based fertilisers, in
a situation where soil pH needs to be lowered. The chlorine contained in KCl can easily
damage the clay colloids or “platelets” in clay soils, which holds massive risk for the uptake
and use of normal irrigation water, and are quite detrimental to soil bacteria and other
organisms needed to convert the elemental sulphur and even for the nitrification processes
in the soil. Sulphates such as K2SO4 and ammonium nitrogen are better alternatives in
such cases.
To ensure the effectiveness of soil pH reduction, maintain consistent moisture levels, as
the conversion of sulphur requires active microbial activity. If you smell a rotten egg smell,
after applying elemental sulphur, it means that your soil has too little oxygen in it, and the
microbes are not converting the applied elemental sulphur into sulfuric acid, but rather into
hydrogen sulphide (H2S). Hydrogen sulphide is detrimental to plant growth, and can
damage citrus trees. Avoid over-liming or using materials that raise soil pH, like limestone
or wood ash, in the treated areas. Monitoring the soil pH periodically it is crucial to avoid
over-acidification and to maintain the desired pH(KCl) range of 5 – 6.5, with the ideal Ph
being 5.5.
Ultimately, the choice of amendments and practices depends on the specific soil type,
initial pH level, and the crop being cultivated. Always follow recommended rates based on
soil tests to achieve targeted results without causing harm to the ecosystem.
If you need assistance with your soil pH, kindly contact Johan Vorster on 064 624 8550.
