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Sulphur: Soil Acidity or An Essential Nutrient For Plants?

Updated on June 28, 2013

Sulphur as a Fertilizer

Sulphur as a Fertilizer
Sulphur as a Fertilizer | Source

Soil Acidity

Elemental sulphur powder is traditionally used to acidify soils in order to lower soil pH so that one can grow ericaeous plants on alkaline soils, such as rhododendrons, azaleas, camellias, heathers and blueberries.

Soil bacteria convert the sulphur powder into sulphuric acid which can than be used by plants. Also available as sulphur chips, these take longer to be converted and you need more of them than pure sulphur powder.

But, sulphur should be really be regarded as an essential nutrient for all types of plant in the first instance, rather than as an acidifying agent. After all, Dr Carey Reams, an eminent American soil scientist, states, soils should hold approximately 200lbs per acre of sulphur or 22g per sqm.

When I carry out soil tests for clients, the sulphate form is nearly always non-existent and so I have to recommend the addition of sulphur. As you will see below, I have outlined the reasons why sulphur should be regarded as an essential nutrient.

Sulphur an Essential Plant Nutrient

A much forgotten about nutrient, sulphur is just as essential as nitrogen, phosphorus and potassium, and is found in every living cell. Required in similar amounts as phosphorus, sulphur is found as a component of plant proteins, such as cystine, cysteine and methionine, just three of the 21 amino acids which are the main building blocks of proteins, as well as activating certain enzyme systems and being a component of vitamin A. Plants such as mustard, onions and garlic are high in sulphur giving them their characteristic smell.

The amount of sulphur in the soil has declined in recent years, mainly due to the requirements of government legislations to reduce sulphur emissions from power stations and car exhausts etc into the atmosphere, thus preventing very diluted sulphuric acid falling to earth in the rain. (Of course too much “acid rain” was bad for the environment, as demonstrated in the 1980’s, with large swathes of forest dying).

Plant and animal scientists have found that plant tissue should contain one part sulphur to 20 parts of nitrogen, therefore this nutrient is just as important as any other. Sulphur is vital for the formation of chemical compounds necessary to help resist attack from pest and diseases; plus aids in growth and seed formation. In legumes it plays an important role in the fixation of nitrogen.



Sulphur Deficiency in Plants

Sulphur deficiency in plants is characterized by the yellowing of young shoots, thus making it essential in the formation of chlorophyll vital for photosynthesis. It is not always easily recognizable and are frequently confused with similar looking nitrogen deficiency symptoms.

Sulphur deficiency symptoms are:

Plants are small and spindly with short, slender stalks.

Growth rate is retarded and maturity is often delayed.

On most plants the young leaves are a yellowish-green.

The nodules found on the roots of Legumes are less frequent on sulphur deficient soils.

Fruits may not fully mature and are light green. Spotting may also occur on leaves, as with potatoes.


Sources of Sulphur

Plants actually get their sulphur from three different sources with 95% of the total sulphur is made by microbial activity converting decomposing organic matter, including plant and animal waste, insects, worms and dead microbes, plus airborne particles and the weathering of soil minerals.

Sulphur is present in the soil as organic sulphur compounds, sulphides (S-), elemental sulphur and sulphate Plants cannotabsorb organic or elemental sulphur. For this, it has to be converted into the more available sulphate form.

Sources of sulphur are either organic or non-organic and according to studies made by Dr Carey Reams (as already stated above), 200lbs of sulphur are required per acre or 22g per sqm. Organic is always best, but if your soil is significantly deficient, then emergency applications of sulphur would help to bring the levels up.

If you suspect your plant is suffering from sulphur defcicency, then it may be necessary to get your soil tested for this nutrient. Initial testing will show how deficient the soil is and this is important, because adding unnecessary amounts of sulphur may cause other deficiencies to occur.

Sulphur can be added using the following amendments.

Organic matter varying amounts

Sulphur powder – (100% sulphur)

Ammonium Sulphate

Calcium Sulphate (Gypsum)

Magnesium Sulphate (Epsom Salts)

Potassium Sulphate

Iron Sulphate

Manganese Sulphate

Zinc Sulphate

Copper Sulphate


by Alistair Olver

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