Global study finds fertilizer response remains hard to predict despite precision tools

A global study examining fertilizer responses in grasslands across six continents has found that predicting where and when nutrients will boost plant growth remains highly uncertain, despite advances in precision agriculture.

The research, published in Scientific Reports and led by Oliver Carroll of the University of Guelph’s Arrell Food Institute, analyzed 61 grassland sites that are part of the international Nutrient Network. Fertilizer applications increased average biomass by 43 percent, but 26 percent of test plots showed no measurable response.

“All sites responded at least once, but only four of the 61 showed consistent biomass increases across all years and plots,” said Andrew MacDougall, professor in the Department of Integrative Biology at the University of Guelph and one of more than 40 study co-authors.

According to MacDougall, this variability underscores the difficulty of predicting fertilizer effectiveness, even with detailed environmental data. “If you’re just interested in yield, it’s always safer to add nutrients everywhere,” he said. “There’s so much variation—wet years, dry years—it’s very hard to predict.”

Grasslands were selected for the study to minimize the influence of prior fertilization. Sites ranged from reindeer pastures in Finland to dry grasslands in Africa and diverse ecosystems in China and North America.

MacDougall noted that moisture levels are the dominant factor determining nutrient response, with drought conditions often nullifying the benefits of added fertilizers. “If you’ve got a drought in Australia or the Canadian West, you can add all the nutrients you want—plants are not going to get bigger,” he said.

The findings build on MacDougall’s earlier research suggesting that fertilizers fail to influence yields about 25 percent of the time. Attempts to identify those conditions precisely, however, have proven difficult.

While full predictability remains elusive, the study suggests that reducing fertilizer use slightly could cut costs and environmental impacts without significant yield loss. “If you’re willing to sacrifice a bit of yield, there’s money to be saved and more profit for the farm,” MacDougall said.

As fertilizer prices rise and the availability of key nutrients such as nitrogen and phosphorus tightens, researchers expect precision tools—such as microdosing and tissue analysis—to remain vital for optimizing fertilizer use.

“I thought for sure we’d see the golden path forward,” MacDougall said. “But all it did was reinforce that this is hard to predict, and the safest thing is just to add fertilizer everywhere. But as costs go up, we clearly aren’t going to be able to keep doing that.”