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Sulphur deserves equal billing with NPK

When it comes to essential nutrients for plant growth, nitrogen, phosphorus, and potassium (NPK), usually take the starring roles. However, there’s another element that deserves its moment in the spotlight, and that’s sulphur. Since sulphur is found in plants in similar quantities as phosphates, it shouldn’t be left back stage, but given equal billing.   

Abundant, healthy pasture with good sulphur levels.

Abundant, healthy pasture with good sulphur levels.

About sulphur

Sulphur occurs naturally in the earth’s crust and is the 13th most common element. Apart from fertiliser, it is used to manufacture a range of everyday items like paint, rubber, medicines, sugar, detergents, plastics, and paper.

Although it used to be mined, elemental sulphur is now mostly sourced as a co-product from the oil and gas industries, and is traded all around the world.

Sulphur in Plants

All plants need sulphur. They absorb it through their roots as sulphate (SO42-), although they can absorb small amounts through their leaves as sulphur dioxide (SO2).

Without sulphur, plants can’t form the amino acids cysteine, cystine, and methionine that are among the key building blocks of proteins.

If enough sulphur isn’t available, plants can’t successfully produce the chlorophyll that they need for photosynthesis, so their production of starch, sugars, oils, fats, vitamins, and other compounds is compromised.

A lack of sulphur also affects the ability of plants to make an enzyme that is required for nitrogen uptake. For legumes like clover, sulphur is needed to form the healthy nodules used in nitrogen fixation.

Sulphur Deficiency

Not so long ago, sulphur deficiencies in New Zealand agriculture were rare. This was due to the widespread use of superphosphate which contains sulphur along with phosphate. However, more sulphur-free phosphate and nitrogen fertilisers are now being used, so sulphur deficiencies in our soils are more common.

Sulphur shortages are more likely to occur with high rainfall in winter because sulphates are easily leached. Deficiencies are also more likely with low temperatures, and with soils which are acidic or low in organic matter.  

Sulphur deficiency in plants can look very similar to nitrogen deficiency with the same yellowing of leaves and stunted growth. However, it is the young leaves which are most affected when sulphur is short. As always, the best way to check for a deficiency is to soil test regularly.

Addressing sulphur deficiency

Plants require a continuous and regular supply of sulphur during their growth, so the ideal sulphur fertiliser would deliver its nutrients over an entire growing season. Unfortunately, there is no single sulphur fertiliser that works like this.

While sulphur is included as a secondary constituent of some NPK fertilisers like Fertco’s KingPhos (4.3% sulphur), the sulphur is in sulphate form. This form is good because the sulphur is immediately available to plants. But sulphates are easily leached so plants won’t get the regular sulphur source they need for long.

On the other hand, Elemental Sulphur, a high analysis sulphur (90%), is not in sulphate form so does not leach. But it takes some time to be oxidised by microbes to become plant available sulphate.

Another option is to apply Fertco’s Super S. Super S is a high analysis elemental sulphur which is extremely fine (75% sulphur). The tiny size of the particles allows the sulphur to be oxidised quickly, even in winter or spring when temperatures are as low as 10°C.

Which is the best fertiliser?

So which fertiliser should you apply? The ideal solution would be to apply Elemental Sulphur with some Super S. This would supply both quicker and slower acting forms of sulphur which would provide the vital elements for protein production and photosynthesis, but also ensure that nitrogen uptake is more efficient.

One advantage with sulphur fertilisers is that they are less expensive than other key nutrients like nitrogen and phosphates. So it’s not a costly exercise to include some sulphur with your fertiliser. Given its importance, it’s just sound practice to give sulphur star status alongside NPK.

This article was published in the Coast & Country News.


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