A farmer spreads organic fertilizers of bone meal pellets and rock
phosphate before planting spinach in the Harmony garden in Golden, Colorado.
PHOTOGRAPH
BY JOE AMON, THE DENVER POST/GETTY IMAGES
Farmers are facing a phosphorus crisis. The
solution starts with soil.
Overuse of fertilizer has led to phosphorus
shortages and water pollution. But farms might not need so much to grow healthy
crops.
BY JULIA ROSEN
PUBLISHED OCTOBER
14, 2020
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.
Roger Sylvester-Bradley inspects the roots of a healthy barley
plant for signs of phosphorus deficiency. The field has had no added fertilizer
for almost a decade, and the plants are only now starting to show a slight
lack.
PHOTOGRAPH
BY JULIA ROSEN
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.
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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.
A front end loader moves granules of monoammonium phosphate into a
storage warehouse at the PhosAgro-Cherepovets fertilizer plant in Cherepovets,
Russia, on Aug. 9, 2017.
PHOTOGRAPH
BY ANDREY RUDAKOV, BLOOMBERG/GETTY IMAGES
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.
Smarter crops
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.”
This story
was supported by a science journalism fellowship from the European Geosciences
Union.
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