ON AN OVERCAST day, Roger Sylvester-Bradley walks along a hawthorn hedge, collecting a thick rind of mud on his leather boots, before stepping into a gently sloping field of barley.
He stoops to pluck an ankle-high seedling from the ground and examines its healthy mop of fine white roots. Turning them in his hands, he says, “when you see a plant that’s deficient in phosphorus, it doesn’t look like this.”
That’s something of a surprise to Sylvester-Bradley, a crop scientist at ADAS, an agricultural consulting company in Cambridge, England. Phosphorus occurs naturally in soil and is a critical nutrient for plant growth. For centuries, farmers have added extra to their fields to boost harvests, but Sylvester-Bradley and his colleagues are studying ways to produce food using less of it./
The reasons are twofold: First, phosphorus runoff from farms contributes to widespread water pollution. Second, we don’t have phosphorus to waste.
Nearly all of the phosphorus that farmers use today—and that we consume in the food we eat—is mined from a few sources of phosphate rock, mainly in the United States, China, and Morocco. By some estimates, those could run out in as little as 50 to 100 years. Geologists know of other deposits, but they are harder to access and contain less phosphorus. Thus, the price will likely rise, making it harder for growers to afford fertilizer and for people to afford food.
Here and at other experimental sites in England, Sylvester-Bradley and his colleagues have taken a first commonsense step toward addressing the problem: They stopped adding phosphorus fertilizer to half the barley field to see how the plants would fare. Eight years later, they have only just started to observe the first effects on crop size and yield. The plants have survived on the excess nutrients in the soil—so-called legacy phosphorus—which some say represents a key piece of the phosphorus puzzle.
Researchers have calculated that, in countries like the United Kingdom and the United States, there is already billions of dollars’ worth of fertilizer in the ground that could help offset demand for mined phosphorus. Using it up would also curb phosphorus runoff.
To Paul Withers, a soil scientist at Lancaster University and one of Sylvester-Bradley’s collaborators, tapping into legacy phosphorus is a no-brainer and continuing with the status quo is a recipe for both ecological and humanitarian disaster. “We can’t have agriculture polluting the environment and using resources the way we are,” Withers says. “It’s just going to cause a meltdown in the end.”
A devious nutrient
Phosphorus is a non-negotiable requirement for life. It’s the backbone of DNA and the P in ATP—the molecule that carries energy around cells. Plants need phosphorus to grow, which is why farmers have been feeding it to their crops for millennia.
At first, and without understanding the chemistry, people used manure and human waste as fertilizer. Then in the 1800s farmers recognized that phosphorus-rich bones and rocks worked too.
In 1842 an Oxford University dropout named John Bennet Lawes patented a process for treating these new mineral forms of phosphorus with acid, making the nutrient more accessible to plants, and soon began selling the world’s first human-made fertilizer.
Lawes plowed his considerable profits back into research at his family’s country estate, which later became the Rothamsted Research center. And there, scientists discovered that phosphorus was a somewhat devious nutrient.
The fertilizer Lawes manufactured contained a soluble, inorganic form of phosphorus that plants can readily use. But as soon as the phosphorus hit the soil, a large fraction of it reacted with soil minerals, forming compounds that crops can’t access. Some also got locked away in equally unavailable organic forms.
From those observations, scientists concluded that farmers shouldn’t scrimp on phosphorus. They should heap it on, especially as they raced to feed the world’s growing populations during the 20th century.
In fact, it was once Withers’ job to spread the word. As a government farm advisor in the 1980s, he drove a red Volvo station wagon around the winding roads of rural England telling farmers to make sure their crops got plenty of key nutrients.
This method, which Withers calls “insurance-based farming,” still prevails in many parts of the world. In Europe, farmers apply roughly 4 kilograms of phosphorus for each kilogram that we consume in food. For U.S. diets, that ratio is about 9 to 1, and in China, it may be as high as 13 to 1. (There are crucial exceptions in places where farmers have never had adequate access to phosphorus fertilizer, like many parts of Africa and South America.)
Phosphorus is lost at many stages of food production and processing. But these inefficiencies pose a problem as looming changes in phosphorus availability and price threaten to destabilize the world’s food system, Withers says. “We’ve sort of gone over the top and we’ve come back to vulnerability.”
To make matters worse, some unused fertilizer builds up in the soil, which causes environmental problems long after it’s applied, says Helen Jarvie, a hydrochemist at the Centre for Ecology and Hydrology in Wallingford, U.K. Her research shows that it slowly leaks into the environment for decades, confounding well-intentioned efforts by landowners to reduce nutrient pollution.
Even small amounts of phosphorus runoff from farms and sewage are enough to fuel algal blooms that fill waterways with festering green scum. Sometimes, like in Lake Erie, they produce toxins that can foul drinking water and use up dissolved oxygen, killing fish and other aquatic life.
According to one study, phosphorus pollution affects nearly 40 percent of Earth’s land areas. And the damage adds up. By one estimate, the impacts of excess phosphorus and nitrogen—another key nutrient—on water quality and ecosystems cost $2.2 billion per year in the U.S. alone.
A slam dunk for plants?
If legacy phosphorus is an environmental liability, it is also a tremendous opportunity, according to Withers and other scientists. He and his colleagues calculated in a 2015 study that fields in the United Kingdom contain more than $10 billion worth of phosphorus, enough to meet the country’s fertilizer demand for up to 54 years.
Many other nations possess similar reserves. A 2012 analysis found that global soils contain enough legacy phosphorus to cut the projected demand for new fertilizer in half by 2050.
“The plants can use our mistakes from the past,” says Sheida Sattari, lead author of the study.
By the numbers, legacy phosphorus looks like a slam dunk. But can plants actually live on it? Studies suggest that, in places with long histories of phosphorus overuse, like the U.K., crops can thrive for 10 years or more on the stores built up in the ground. The most extreme example comes from Saskatchewan, where researchers haven’t added phosphorus to plots of wheat since 1995. Twenty-five years later they still haven’t seen problems.
Conventional measures of soil chemistry suggest they should apply more fertilizer, says Barbara Cade-Menun, who oversees the experiments at the Swift Current Research and Development Center in Canada. “But our yields aren’t changing.”
Scientists think that as plants use up the readily available phosphorus in the fields, soil minerals and organic matter release more of the nutrient. Cade-Menun doesn’t yet know whether changes in soil chemistry, soil microbes, or plants themselves can explain what’s happening in her plots. Regardless, the results suggest that those inaccessible forms of phosphorus that the Rothamsted researchers fretted about aren’t quite as off-limits as scientists once thought.
And that means just cutting back on fertilizer could go a long way to meeting phosphorus demand and reducing runoff without jeopardizing harvests.
At some point, however, soil phosphorus drops low enough that crops become stressed. That’s partly because some of it really is out of reach for plants, but also because many modern crops cannot get ahold of what is there.
The scarcity of phosphorus in nature forced wild plants to develop strategies for securing an adequate supply. Many evolved extensive root systems that search out phosphorus. Some can also excrete chemicals to liberate the nutrient from the soil.
But most commercial crops don’t have those abilities. Scientists cultivated them in well-fertilized soils that didn’t require plants to spend energy deploying such tools. And, in a world of plentiful resources, breeders didn’t select for varieties with strong phosphorus-harvesting traits. The result, says Phil Haygarth, a soil scientist at Lancaster University, is “a load of fast-growing, dumb plants” that struggle to extract phosphorus from the soil.
Researchers now want to create smarter crops. In 2012, scientists identified a gene in an ancient variety of Japanese rice that enhanced the plant’s ability to find phosphorus by growing fine roots. Researchers then bred the trait into modern rice plants, and in 2019 farmers in Madagascar—which has naturally nutrient-poor soils—started testing some of the most promising varieties.
Sigrid Heuer, a researcher at Rothamsted who helped with the rice study, is searching for a similar gene in wheat as part of the International Wheat Yield Partnership. Other scientists are developing crop varieties that don’t need as much phosphorus in the first place.
Besides breeding, no-till farming could help by preventing soil compaction and encouraging good root development to help plants access more legacy phosphorus. Adding symbiotic fungi that spread through the soil may extend a plant’s underground reach, and growing crops alongside legumes and other plants that secrete phosphorus-releasing compounds can free up more of the nutrient.
Withers and Sylvester-Bradley have been running down the phosphorus levels in their test fields for the exact purpose of exploring these kinds of approaches.
The researchers had to abandon the barley field in Cambridge because of changes in farm ownership. But at the remaining sites, phosporus levels have finally dipped low enough for them to start conducting experiments on how to help plants access as much legacy phosphorus as possible. The first will compare the performance of existing commercial wheat varieties.
The researchers had to wait longer than expected—nearly a decade—for phosphorus levels to drop back to natural levels. But that fact alone should reassure growers that they can safely cut back on the nutrient, Sylvester-Bradley says.
“The take-home for farmers, as far as I’m concerned, is they can relax.”
nationalgeographic.com, 14 October 2020