This article appears in the March 2021 issue of Potato Grower.

NPK rules, right? Well, not exactly. If you’re focused only on the importance of these three macronutrients, it’s likely your potato crop is not reaching its full potential.

Few people would find reason to criticize efforts to improve air quality. But to some extent, farmers have a legitimate gripe: Cleaner air has created nutritional deficiencies in America’s crop fields. Sulfur, specifically.

For years, the fumes belching from our power plants carried with them so much sulfur that crops never displayed any deficiency. Much like that other vital input in crop production, sunlight, we never thought to supplement it because there was always enough to go around. It became the “invisible” macronutrient.

As we’ve begun to recognize the deleterious effects of excessively burning fossil fuels—acid rain, particulates, excess carbon dioxide and so on—it has of course been the right course of action to clean up and reduce emissions; no one wants to repeal the Clean Air Act. But, as is so often the case, the law of unintended consequences has come into play. Farmers now need to make up for the sulfur deposits they were previously getting “for free.” Our soils are no longer receiving the annual sulfur bounty; data from the EPA shows that sulfur emissions fell by up to 94 percent between 1990 and 2019.

Unless we take steps to replenish them, our fields are now running a sulfur deficit as successive crops take their fill.

Why is Sulfur So Important?

It’s often referred to as the fourth macronutrient. Whatever crops you’re growing, sulfur’s role in crop biochemistry can’t be ignored and is of similar importance to nitrogen or phosphorus, supporting plant functions that can affect not just yield but also quality and marketability.

In potato production, adequate sulfur levels are vital for nitrogen uptake, chlorophyll production, tuber development, stress and pest resistance, carbohydrate generation, amino acid formation and vitamin synthesis. Decreased efficiency in any one of these areas becomes a limiting factor on crop yield and potential, while also playing a significant role in how crops taste and smell, as well as how they perform in subsequent use. For example, the gluten that’s so important in wheat—giving bread its chewy, soft texture—is dependent on sulfur-sulfur bonds. For good bread-making wheat, we need good sulfur uptake.

What’s more, across all crops, sulfur deficiencies rarely present with visible symptoms. Hidden deficiency is often more damaging than acute deficiency, as it’s usually only discovered when it’s too late to remedy.

Sulfur also has another role—as a fungicide. Indeed, sulfur-based fungicides were among the first available to farmers, and they remain important today, whether applied to cut seed or as a supplement in conjunction with modern blight control chemistry.

What’s the Solution?

It’s important to understand how plants absorb sulfur. Their preference is for sulfates; this is the only form in which plant roots can access the nutrient. There’s also a limited role for leaves: Not only can they absorb small quantities of sulfur dioxide direct from the atmosphere, they also help to regulate nutrient absorption while combating disease. When applied as a foliar spray, the nutrients slowly enter the plant tissue as required. But sulfur’s disease-suppressing abilities ensure that its presence on the leaf surface, particularly through the early stages of growth, act in synergy with later applications of fungicide and insecticide. That’s because a healthy plant can stave off infections itself, becoming less reliant on the fungicide to do all the work of disease suppression.

It’s no coincidence that incidences of diseases such as early and late blight are increased in potato plants where even a moderate sulfur deficiency exists. Yet during the growing season, a crop’s sulfur requirements can often greatly exceed the soil’s inherent capacity to supply it, particularly as levels are now in decline.

Choosing the right product is obviously important. OMEX Agrifluids focuses on products that are immediately bioavailable, either because they’re unaffected by soil microbial activity or because they take advantage of specific plant characteristics. SulpHomex Ultra is OMEX’s solution for potatoes. It’s a co-formulation of sulfate—for immediate root absorption and use within the plant—and nitrate, the form of nitrogen most readily absorbed by plants.

As for sulfur “philosophy,” i.e., application, take the little and often approach. This way, you’ll not only reduce leaching (environmentally unsound but also representing an immediate loss to your bottom line) but maintain sulfur at sufficient levels both in the soil and on the leaf, such that it’s always available to the potato crop during periods of rapid growth. In this way, lack of sulfur should never become a limiting factor in your pursuit of optimizing yield potential.

Integrated into early-season insecticide and fungicide sprays, SulpHomex Ultra will not only build sulfur content within the plant, but also provide a potato crop with a different mode of suppressive action, building the foundation for a healthy, high-yielding and disease-free crop.