Excess Fertilizer Causes a New Challenge: Low Crop Yields During Drought

For decades, soil scientist Rick Haney watched farms—those he worked on, and those he lived near in Oklahoma and Texas—struggle to balance debt and drought, profits, and yields. “The farmers I’ve been around all my life are trying to hit a home run every year,” says Haney, who now researches fertilizer use with the U.S. Department of Agriculture’s (USDA) Agricultural Research Service, of the annual, high-risk drive to get a bumper crop.

“They only hit it one or two times a decade. I was watching these guys borrow a million dollars for cattle and plants that can die,” says Haney. “They can’t really control how much money they spend on equipment, or almost anything, but they still control how much fertilizer they apply.”

On most conventional farms, fertilizer is a necessity that allows farmers to maximize their yields year after year. But historically, their use has also created problems: fertilizers have facilitated large-scale monocultures, disrupted ecosystems, and largely minimized the need to monitor long-term soil health. And, on a practical level, over-fertilizing crops can lead to unintended consequences.

A growing body of research suggests that in addition to downstream “dead zones” and algae blooms, excess fertilizer can often also hurt crop yields, especially grass crops like wheat and corn, during droughts because of soil eutrophication—a buildup of nutrients that lets plants grow so quickly, they can choke themselves out.

Now, a new study published earlier this year in Global Change Biology provides further evidence that excess fertilizer applied during a drought year can create a boom-and-bust season because of the crops’ struggle for water. The study is part of an international collaborative experiment called the Nutrient Network (NutNet for short), which comprises a patchwork of 130 carefully monitored and managed grasslands across six continents. The project allows hundreds of ecologists, soil scientists, and agronomists to track ecosystem responses to climate change under different conditions around the world.

Does Fertilizer Help or Hinder Crops During Drought?

Globally, droughts and heatwaves are becoming more frequent and more severe, and the North American Great Plains are no exception. Already, the drought this year is shaping up to be one of the most intense in the High Plains since 2013, with 2018 as a close second. With grasses contributing a significant portion of the world’s food supply both directly and through grazing pastures, accurately predicting these plants’ responses to climate change is essential to global food security. One of the big questions at hand is how fertilizer affects grasses’ resilience to drought.

Soil scientist Kevin van Sundert is one of the experts on the case. He researches how grasslands respond to fertilizer and drought and was lead author on the study, which took advantage of NutNet’s existing monitoring stations to track grasslands’ response to the severe drought that baked much of Europe in the summer of 2018. The drought led to significant crop losses in Europe, but it also provided van Sundert an unexpected opportunity to compare how farmland with different fertilizer regimes fared in a real-world drought.

Because grasses have long been thought to be more drought-resilient than other plant types such as forbs (flowering herbs and legumes), van Sundert was surprised to find that fertilized grasses struggled to keep up when the drought hit. “This result came out that grasses are more sensitive to drought, so we need to figure out why this might be,” he told Civil Eats.

Grasses’ dampened response to fertilizer during drought is likely due to a lack of soil moisture. This may seem straightforward—of course there’s less water available during a drought—but here, fertilizer plays an important role. Fertilizers effectively remove nutrient limitation, allowing productivity to boom and plants’ biomass to increase. That increase in plant size is the effect farmers are counting on, but it can also lead to increased competition for water.

Just like a growing city drawing from a limited aquifer, a field with high biomass will increase its water demand, depleting soil moisture reserves more quickly than a field with smaller plants. During a drought, this phenomenon creates increased competition for limited water resources. That can be disastrous, particularly for a monoculture field, where the failure of one plant type means a serious financial loss.

Agricultural biologist Amber Churchill at the University of Minnesota, who was not involved in the study, put it simply. “Those bigger plants need more water,” she says. “That’s one reason why we might see this negative interaction between fertilization and drought.”

This negative effect has been demonstrated by research from NutNet before. A U.S.-based team in 2020 found a similar effect—biomass loss during drought, exacerbated by fertilization—in temperate grasslands in the U.S. Philip Fay, a soil scientist with the USDA, was involved in the previous study. Both studies, he said, suggest that over-fertilization during drought is a risk.

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