Soil sulfur is not a very well known element, but it is part of the macronutrients. Sulfur or S, is absorbed in the same amounts as phosphorus. Uptake of S by grasses is about 10 kg/ha, leguminous crops 25-30 kg/ha and cabbage about 40-45 kg/ha.
The table below compares sulfur and phosphorus in the % dry material of various crops:
So why do plants need sulfur?
Plants take up sulfur in the form of S2- and SO2 through the leaves from the atmosfeer.
Sulfur is used in various functions in the plant such as:
- It is a component of egg whites and amino acids such as cysteine and methionine.
- It is a component of vitamin B1 and coenzyme A.
- It activates various poteolytic enzymes.
- It is an important part of the aroma in cabbage, turnips and onions.
- In conjunction with magnesium it helps with plant oil production.
- It helps with cold tolerance.
- It helps with nitrogen fixation in the soil
- It prevents nitrate accumulation by converting excess N to protein.
Deficiency symptoms of sulfur
The whole plant is light green.
Light chlorotic spots appear between the veins of leaves.
In some plants, a reddish colour may appear.
The chlorotic leaves do no dry out and don’t fall off as in the case of a fungus attack or nitrogen deficiency.
S is less mobile in plants than N.
High leaching under irrigation can cause S stress in plants without the grower noticing. The first commercial identification was done in Malawi where S deficiency was misdiagnosed as Tea yellows. In South Africa some S deficiency occurs in Kwazulu Natal.
Conditions that are conducive to S deficiency are:
- High rainfall or high volume of irrigation.
- Coarse of rough soil texture throughout the soil profile promoting leaching.
- Warm climate that promotes biodegradation of organic material.
- The application of additional organic and non-organic material to the soil that results in wide C:S and/or N:S ratio.
- The use of fertilizer that does not contain sulfur such as double super phosphate.
- The use of crops with very high S uptake over a long time.
In intensive crop production S deficiencies are rare because most of the fertilizers and sprays already contain enough sulfur for the plant to grow optimally.
Featured image obtained from Pixabay. Image taken by Ildigo