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Adding iodine to crops: "You don't see a lack right away, but adding it is beneficial”

Last July, a new potassium nitrate/iodine fertilizer was introduced on the Dutch market: Ultrasol®ine K Plus from producer SQM. For many growers, iodine is a new micronutrient, But the Wageningen Agricultural Service Laboratory for Fertilization Research named it a plant nutrient as early as the 1960s. Originally found in Chilean sodium nitrate, iodine has, however, disappeared into the background over time.

Ultrasol®ine K Plus was tested on tomato crops in Morocco in December and January. Adding iodine resulted in better-developed heads and increasingly uniform fruits per cluster. The grower in question could deliver nine percent more export-quality fruits. In the photo: Roy Cornelis, SQM sales manager North, East, and West Africa.

That is why SQM again made a case for iodine that, since 2022, has been internationally listed as a 'useful nutrient,' grouped with, among others, selenium, and silicon. Royal Brinkman (the Netherlands) and Anorel (Belgium) distribute Ultrasol®ine K Plus.

Iodine is extracted after caliche sediment minerals are dissolved. That is also the source of the nitrate in potassium nitrate. The nitrate - sans iodine - is then purified of elements, and combined with potassium from salt sources, to form potassium nitrate.

Most of the iodine SQM extracts goes to industry, for example, to make contrast media. Only three percent is used for human food production. Nine global production sites mean water-soluble NPK recipes can also be tailored to local markets.

Salt pans in Chile's Atacama Desert, where the sun crystallizes potassium or nitrate.

A 2021 scientific article stirred things up again. Katja Hora, SQM International's Nitrates and Potassium Business Unit Research Manager, and Harmen Tjalling Holwerda, Senior Director of Business Development, co-authored the piece, published in Frontiers in Plant Science. The study, conducted by researchers at PlantLab in Pisa, Italy, used new techniques to demonstrate how iodine works in plants.

"Iodine is considered of global importance for groups including humans, animals, microbes, and fungi, but not crops," Katja explains the reason for the study. "At SQM, we challenge that dogma."

Ten years ago, iodine's role in plant nutrition was analyzed. In 2021, that resulted in a scientific publication; in 2022, another (Evidences for a Nutritional Role of Iodine in Plants, Kiferle et al., 2021; 2022) followed. SQM did its own research, leading to another 2021 publication (Examples of iodine as a micronutrient in plants, for improvement of the leading commercial greenhouse crops cultivation, Hora and Holwerda, 2021).

Process accelerator
The studies showed iodine's effect on crops. They grow faster, realize higher yields, and quality increases. Iodine also positively contributes to crops' resistance to biotic and abiotic stress. However, scientists are not yet 100% clear on exactly how iodine acts.

"Plants can bind iodine to many vital proteins. In roots, those proteins are mainly involved in growth; in leaves, mainly in photosynthesis. Iodine lets plant processes run more efficiently. It seems to be especially involved in communication between the plant's cells and organs and between the plant and the environment. Plants thus respond more quickly to external signals," says Katja.

Pie chart of Kiferle et al. 2021 research with thale cress.

Not loose, with potassium nitrate
Introducing Ultrasol®ine K Plus to the market means growers are deliberately being introduced to iodine for the first time. This substance is always naturally present in very low concentrations in the air, soil, and water. Measuring iodine was and is still often not regularly done.

Is an iodine shortage problematic for plants? An ever-louder 'yes' scientists answer now. Studies have repeatedly shown plants can make good use of iodine, which does them no harm. "Plants recognize iodine quickly, within 48 hours of absorption, and it doesn't cause any stress. Adding iodine has positive effects."

SQM deliberately chose to not offer iodine as a loose product but in combination with Chilean potassium nitrate. "That makes it easy for growers to apply and is most cost-effective compared to offering and administering iodine as a loose product. Growers don't have to change their nutrient solution; just use our potassium nitrate with iodine instead of potassium nitrate without iodine," Katja continues.

Scientific literature shows iodine's limits. In in vitro plant cultivation, for example, a standard 5 micromol/L of iodine is mixed into the medium in the so-called Murashige-Skoog micronutrient mix. "Growers who stay between 5 and 10 micromol/L are fine. The crop can use such levels well."

No 'sodium danger'
Nor need growers be concerned about rising sodium levels or EC. "I believe those sodium limits are still often applied too tightly. But apart from that, adding Ultrasol®ine K Plus does nothing to sodium levels. There's always some sodium in all types of potassium nitrate, even without adding iodine. We add that as potassium nitrate, which has no extra sodium. Irrigation water is usually the main source of sodium," adds Katja.

Cherry tomato tests in Spain, without (left) and with (right) extra iodine added.

Substrate's role
No crop is truly entirely iodine-free. "You'll always encounter very low concentrations of iodine. Probably at a level high enough to keep plants alive but not high enough to meet those plants' needs optimally."

Growers might wonder what switching from, say, stone wool to coir does to iodine levels. Do other substrates contain iodine, and is there a risk of iodine overdose in an organic substrate?

No, says Katja. She knows growers often do not measure iodine, nor do laboratories routinely test for it, but growers need not worry. "Natural substrates like coir contain iodine, but in a very bonded form. The organic compounds trap the iodine, and it's not easily released to the plant unless you start burning the product."

Tests with Ultrasol®ine K Plus in Mexico and Hungary in tomato and bell pepper crops grown on coir confirm that. There, drain water contained no substrate-derived iodine, regardless of the levels intentionally added via Ultrasol®ine K Plus.

SQM found that in strawberry trials in California, where iodine was used as a micronutrient, it did not significantly increase the fruit's iodine levels.

Quantity, quality, and resistance
In recent years, experiments where adding potassium nitrate with iodine have been done at 100+ growers in more than 20 countries. In most cases, those resulted in crops, including tomatoes, having an average ten percent yield increase. Katja reverses that, showing what plants lack when iodine is absent. "The trials show that costs them, on average, nine percent," she says.

Now, it is certainly not always about volumes. Adding iodine also appears to contribute to product quality, such as storability, fruit size, °Brix levels, fruit uniformity on a cluster, or cut flowers' stem length and thickness. "Growers report fewer misformed fruits when using Ultrasol®ine K Plus. Also, better quality means they can suddenly export their product, whereas before they couldn't go because of inferior quality."

One test further found less nose rot occurred in tomatoes and bell peppers. "That is a deficiency symptom. To me, less nose rot primarily means the plant can absorb calcium better, for instance, through a larger root system," Katja explains.

In a trial in Brazil, the lettuce had larger heads and improved root development (photo right) after Ultrasol®ine K Plus was added.

Measuring iodine separately
Market introduction in Europe began later than in other regions. That was due to new European fertilizer regulations. SQM chose to wait to implement the latest regulations before entering the market with the correct labels. Eurofins Agro can already measure iodine in nutrient solutions, water, drains, and crops, Katja says.

Research is not focused on the Netherlands. The reason is simple. "Targeted nutrition is far more profitable outside the Netherlands. Though, it must be remembered that iodine is not a cure-all. It works in such a way that you don't see a deficiency immediately, but when you add extra, you start to see the benefits," Katja concludes.

For more information:
Katja Hora
Tel.: +32 (0) 320 39 737
Mob.: +32 (0) 476 883 351
Email: [email protected]

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