Farmer Focus: Benefits of applying calcium sulphate to land

It is winter in South Africa. Winters here are sunny and dry with frost most nights. While I say that the winters are dry, we recently caught the edge of a weather system giving us 15mm of rain.

That doesn’t sound like much, but my cover crops are very grateful as the last time we received any rain was in April.

As it is the fallow season, now is the time to prepare the fields for the coming summer crops. I have decided to spread 1t/ha of gypsum (calcium sulphate) on all fields that haven’t received any in the past five years.

See also: Read more from our arable Farmer Focus writers

Gypsum contains about 18% sulphur and is the cheapest form of the chemical we can access. We all know about the three macro elements; N, P and K – perhaps S is the fourth.

In the past I have either added sulphur to my planting mix or applied it with the nitrogen top dressing. I feel it will be easier and cheaper to get my fields into a gypsum cycle, perhaps 1t/ha every five years.

It is a by-product of the production of fertiliser from rock phosphate and as a bonus it can contain up to 2% phosphorous. Fortunately, our rock phosphate is low in heavy metals.

I have used it in the past to ameliorate naturally occurring subsoil acidity. This may sound strange, a while gypsum has a high calcium content, it is not a lime and does not neutralise hydrogen ions at all.

However, naturally acidic subsoils contain sesquioxide clays. The sulphate portion of the product combines with the sesquioxide, releasing a hydroxide ion, which can neutralise acidity.

As it is water soluble it can access acidic soil deeper than any lime-incorporating implement can get to.

I see this application cycle as having two main benefits. First the sulphur and second it will help to neutralise acidic subsoil I may have missed.

Not to mention the so-called “softening” effect gypsum is believed to have on the soil.

Bruce farms in Kwa-Zulu Natal, South Africa. He plants 320 Ha rain fed summer crops. He also runs 2,200 weaner oxen on pastures, finishing them in a feedlot with maize grown on the farm.

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