Phosphorus from fertilizer builds up in soil for decades, study shows

Algae blooms that can kill lakes and rivers have long been a problem in Canada. One of the biggest culprits in producing that algae is phosphorous, which can come from many sources — including the heavy use of fertilizer.

But the scale of the problem is only now being fully understood. A new study has revealed some bad news about the phosphorus cycle, which explains how the essential element moves through the ecosystem. CBC Radio science columnist Torah Kachur explains what the study tells us.

Why is the phosphorus cycle important to understand?

As a nutrient, phosphorus is a key element of life. It gets taken up by plants and microbes from the soil into DNA and other molecules in the cell. The phosphorus is now available to go up the rest of the food chain.  When organisms die, they return that phosphorus to the soil or to the ocean where it can be reused again.

Phosphorus is often in short supply in soil, and therefore is supplied by fertilizers, along with nitrogen.

This use of fertilizer has greatly increased our capacity to supply food for the planet, but it has come at a cost.

Nature can't cycle that much phosphorus each year. Instead it accumulates in soil and in the water. Because of human activity, the phosphosphorus "cycle" becomes more like a one way dead end.

The heavy use of fertilizer, and the runoff of phosphorus in particular, has caused a major issue in lakes and streams.

• Lake Erie's algae explosion blamed on farmers

Just like phosphorus can help plants grow, it also does a great job of helping algae grow. This causes algal blooms, and eventually can have a very negative impact on water sources.

Take, for instance, Lake Erie. All the runoff of agricultural activities — both fertilizer and animal waste — helped contribute to massive algal blooms during the 1960s and '70s. 

There was so much algae that it effectively starved the rest of the life in the lake for essential nutrients like oxygen. Fish stocks collapsed, partly because of the phosphorus from agriculture. 

Major algal blooms were reported again as recently as 2014 and 2015.

• Stunning NASA photos spotlight algal blooms in Lake Erie, Lake St. Clair
• Scientists warn of large, toxic algae bloom on Lake Erie

What does this new study tell us about the phosphorus cycle?

Partly, the study paints an even worse picture than we previously had of dying lakes and collapsing fisheries.

That's because it found phosphorus doesn't just accumulate in lakes and streams, but also in the landscape. And that, previously, was extremely difficult to measure.

This new study, published in Nature Geoscience, was led by researchers at Arizona State University. They looked at 70 years of records for three of the worlds major river basins — the Thames in the U.K.; the Maumee, which is a tributary of Lake Erie; and the Yangtze in China.

Part of what they added to the equation is determining the effects of simple population growth. Human waste, food waste and animal waste all are big sources of phosphorus accumulation.

For two of the rivers — the Thames and Maumee — there was a big accumulation of phosphorus about 20 years ago.

But since then the legacy phosphorus, as it's called, has been slowly depleted — which is a good thing. That's because nature is catching up to the amount of available phosphorus, and using it all up.

Fertilizer use has decreased in those areas, and — importantly — state-of-the-art sewage treatment, especially in London, England, has limited the amount of phosphorus getting into the environment.

The Yangtze River in China is a different story. The researchers found a continual accumulation of phosphorus that shows no sign of stopping.

What can be done about phosphorus accumulation?

Mostly, it's about slowing down the release of it into the environment. If you take agriculture out of the picture, there's a natural balance of available phosphorus in the environment. But of course, we need crops to eat.

The problem then is there isn't so much a phosphorus cycle as a one-way trip into the lakes, said phosphorus expert Barbara Cade-Menun.

She works with Agriculture Canada's Swift Current Research and Development Center.

"We put it on the crops, we take those crops off the land, we feed people," she said.

"The phosphorus that's in our manure — where does that end up? It doesn't end up back on the land feeding the crops. So the cycle is open-ended."

This one-way trip needs to be avoided, or at least reduced.

And the easiest and best way to limit release of excess phosphorus is to reduce fertilizer use. Which doesn't sound like an easy way to still feed the planet, but you'd be surprised.

One study done by Barbara Cade-Menun showed that wheat crops grow just as well on land fertilized with just nitrogen than on land fertilized with both nitrogen and phosphorus, because there's a lot more leftover phosphorus in the soil from previous years than many people think.

So that might be one way forward — if we understand more about how much phosphorus is actually available to crops, the amount applied can be reduced.

Should we be limiting fertilizer use in our yards?

In short, yes. Fertilizers certainly make your lawn look lovely, but they are not great for your local watershed.

But with a few precautions, you can fertilize your lawn in a more responsible way.

For starters, use a nitrogen-only fertilizer. Nitrogen still is a problem in water supplies, but if the natural cycle is overloaded with nitrogen from human activities, some of it can leave by going into the air. So there's a lot more wiggle room with the nitrogen cycle to accommodate fluxes in amounts.

And to get your nice green lawn, all you really need is nitrogen. But still, make sure you don't spread any fertilizer pellets on natural drainage areas close to runoff — you want to keep those pesky little pellets from running onto hard surfaces where they rapidly enter the water supply. And definitely don't wash them away if you spill any.

But the biggest way to reduce your need or use of fertilizer is to recycle your grass clippings. The grass contains the perfect ratio of nitrogen and phosphorus already, so leaving clippings on your lawn to feed the next crop grass can actually halve the amount of fertilizer needed — and that also makes yard work a lot easier.