Chemical Fertilisers are Feeding the World - But at What Cost?

Thanks to synthetic fertilisers, we produce more than enough calories to feed 8 billion people. But there is more to this story. Ocean dead zones are expanding, air pollution is rising, and soil health is degrading. Read on to understand the benefits and drawbacks of synthetic fertilisers in agriculture.

What are fertilisers?

Fertilisers are substances that farmers can use to make their crops grow faster and more abundantly. They can be natural (e.g. compost, manure, fermented nettles) or synthetic (e.g. ammonium sulphate, superphosphate, potassium chloride).¹

The most applied synthetic fertilisers contain nitrogen, phosphorus, or potassium - three of the six nutrients plants need in the highest amounts to grow.² But just like a person couldn’t thrive on potatoes and carrots alone, plants need at least 17 essential nutrients to function at their best.³ Plants get a few essential nutrients (hydrogen, oxygen, and carbon) from air and water. The remaining nutrients come from the soil.⁴

Why don’t we fertilise the rainforest?

Have you ever noticed that nobody needs to add fertiliser to the rainforest for it to flourish? It’s one of the most productive ecosystems on earth, but it doesn’t stop growing if nobody applies synthetic nitrogen to it.

That’s because nutrient cycling naturally occurs in healthy ecosystems. Plants capture sunlight and water to create energy in a process called photosynthesis. The plant uses some of this energy directly but converts the rest to carbohydrates and proteins, which the roots pump into the soil to feed microorganisms like bacteria and fungi. Why feed the little guys? In return, those microbes protect plants from disease, help them access water, and retrieve nutrients locked up in rock particles within the soil.⁵

In a wild setting, the waste from plants (e.g., fallen leaves and dead branches) and wildlife (e.g., manure and antlers) would fall to the ground, releasing nutrients back into the soil. This nutrient cycling isn’t so simple in a farm setting. When we harvest crops, we also remove the vitamins and minerals inside that plant material. And most farmers would prefer to prevent deer from eating and sleeping in their crops. That’s a good thing for anyone who doesn’t grow all their own food. When we go to the supermarket, we benefit from the nutrient-dense foods farmers provide. But for soil and the billions of organisms that call it home, interrupting the nutrient cycle can take a toll. And if soil health and fertility decrease, we undermine our ability to produce healthy food in the long run.

Luckily, microbes can extract nutrients from rock particles in the ground so long as they have good living conditions. So, it is possible to maintain nutrient levels without synthetic fertilisers if we take care of soil health with techniques such as minimising disturbance (avoid ploughing that can kill soil life) and “keeping a living root in the ground”. That could look like planting a cover crop we don’t intend to harvest to ensure food for microbes flows year-round into the root zone. (Instead of leaving the soil bare in the months we’re not growing a crop). Additionally, the manure of grazing animals is a nutrient-rich, natural fertiliser. Grazing animals must be managed sustainably so they don’t cause nutrient pollution, overgraze or compact the soil. But natural fertilisers like manure can have a lower environmental impact than synthetic alternatives when livestock are sympathetically integrated into a healthy farm.

The benefits of synthetic fertilisers

Humans have been around a lot longer than nitrogen fertilisers. So, we clearly didn’t need synthetic inputs to produce food before they were invented. What’s different today?

We need to feed an enormous population. At the same time, the percentage of the European population working in agriculture has steadily fallen since the 1930s.^6,7 Fertilisers are a valuable tool in a world where fewer people need to grow more food and do it cheaply to stay competitive in the global market.

Some estimates suggest that nearly half of people alive today are dependent on synthetic fertilisers.⁸ This is largely because of fertilisers' incredible success in boosting critical crop yields.

Increased yields mean we can produce more food in a smaller surface area, preventing land use change like deforestation. (At least in the short run). We have seen a decline in 69% of measured wildlife populations in the last 53 years alone.⁹ So, preserving remaining wild habitats is essential. And it’s not just a matter of ethics; it’s about human survival. Biodiverse habitats like forests, wetlands, and wood meadows sequester carbon, regulate our climate, shelter communities from natural disasters, and protect our most precious resource – water.¹⁰ Some data suggests that over 1.5 billion hectares of land have been spared from agricultural conversion thanks to increased crop yields.¹¹

Artificial fertilisers can also be cheaper than organic soil amendments and often act more quickly on the plant than natural choices.¹² So synthetic fertilisers might be a sensible choice if a farmer is concerned with shorter-term crop health, e.g., if they are desperate to make it through the next harvest. In my home country, France, more than 50% of farms had a debt of 350,000 euros or more in 2020. So, for many, short-term survival isn’t a given.¹³

Another common argument for synthetic fertilisers is that they can be easier for farmers to handle. It’s supposedly less time-consuming and physically challenging to spread synthetic options in the form of concentrated pellets or liquid fertiliser than to collect, store, compost, and spread vast quantities of natural fertilisers, like manure or grass clippings. In my own experience, having married into a farming family, I have yet to meet a farmer who doesn’t work hard. Whether using regenerative organic or conventional methods, I watch my relatives and neighbours working from dawn to dusk, 365 days a year. Some spray fertilisers, and some move cattle frequently so the land can benefit from their soil-boosting manure. I’m not sure that it’s ever easy to be a farmer. But it definitely is true that maximising yields, partly through synthetic fertilisers, has been aggressively pushed on farmers for decades since the hungry times of World War II. Farmers were told to prioritise high yields, and they did.¹⁴ So, it’s not surprising that the use of synthetic fertilisers has increased so much over the previous decades. But those high yields have come at a price.

The drawbacks of synthetic fertilisers

Most countries dramatically over-apply synthetic fertilisers. For example, only 35% of nitrogen fertilisers applied globally are actually used by plants. The remaining 75 million tonnes of nitrogen runs off the land as pollution, and the consequences can be deadly.⁸

1. Ocean dead zones

When fertilisers aren’t used by the plant, they can run into rivers and eventually reach the ocean. In the water, they trigger hypoxia or “dead zones”. Fertiliser runoff can occur when soil is too compacted, so rainfall just runs over the top of the land instead of being absorbed, carrying fertilisers into the nearest watershed. It can be worsened when fertiliser is sprayed directly before a heavy rainfall or when over-spraying means that the crops can’t absorb all the nutrients.

Dead zones occur when too many nutrients enter the water, causing an algal bloom and bacteria feeding frenzy. With algae consuming a large share of the available oxygen and blocking sunlight from aquatic plants and animals, many cannot photosynthesise and will quickly die. When the algal bloom finishes, the dead algae and bacteria also fall to the ocean floor. As they decompose, oxygen is depleted, and hydrogen sulphide is released. At high concentrations, hydrogen sulphide is lethal. But even lower concentrations can make aquatic life more susceptible to disease.¹⁵ This deadly combination of oxygen depletion and elevated hydrogen sulphide creates vast biological deserts.^16,15 As well as the direct impact on ocean life, these dead zones deeply affect the artisan fishers and vulnerable communities that rely on healthy oceans to make a living and to eat.

During an algae bloom, cyanobacteria, commonly known as blue-green algae, develop at the surface of the water and prevent access to light and oxygen for other organisms.

2. Soil health

While adding Nitrogen, Phosphorus, and Potassium (N, P, K) to a crop can make plants grow faster and greener in the short run, synthetic inputs can disturb soil life and prevent soil from functioning correctly in the long run.¹⁹ Microbes don’t just help plants get nutrients. They also help create pores and build a spongy structure in the soil, which can absorb and retain water - pretty handy in extreme weather events, which are only going to get increasingly common as the climate continues to destabilise.^8,20 If fertilisers can kill soil life and make the land less resilient to climate change, there’s a reasonable argument that synthetic inputs undermine crop yields in the long run.

3. Climate change

Synthetic fertilisers contribute to global warming. For example, to make nitrogen fertilisers (the most common fertiliser used in agriculture), fossil fuels produce hydrogen gas, which is later combined with nitrogen to make ammonia - a key ingredient in many synthetic fertilisers.²¹ This process requires a vast amount of fossil fuel energy. Additionally, when too much nitrogen fertiliser is spread, the resulting pollution can thin the ozone layer.²²

Nitrogen fertilisers also cause nitrous oxide pollution. This is a big deal because nitrous oxide is a greenhouse gas nearly 300 times more potent than carbon dioxide over 100 years.²³ Nitrogen fertilisers are a major reason nitrous oxide emissions have increased by 30% over the past 40 years.23 But it’s not the only culprit. Other agricultural sources of nitrous oxide include burning crop residues and livestock manure.²³

4. Human health

Nitrogen pollution contributes to acid rain, pollutes our drinking water, and reduces the quality of the air we breathe.^22,24

Globally, 90% of ammonia emissions come from ammonia-based fertilisers and animal manure, but it’s not just people living in the countryside who are at risk.²⁵ In the UK, farming causes over a quarter of city particle pollution, mainly from ammonia emissions.²⁶ Ammonia pollution can contribute to severe respiratory problems such as asthma in children and lung cancer.²⁷ Nutrient pollution in our drinking water can increase the risk of certain cancers, congenital disabilities, and thyroid disease.²⁸

Even if synthetic nitrogen isn’t the only cause of ammonia emissions or water pollution (manure and human sewage also play their part), we know that most nitrogen sprayed isn’t even taken up by the plant. So, eliminating any wasteful use of synthetic fertilisers is an essential piece of the jigsaw puzzle for purifying our air and water.

5. Sustainability

Even if synthetic fertilisers weren’t so problematic, they simply aren’t sustainable. While we’ve mainly focussed on the impacts of nitrogen fertilisers, an overreliance on phosphorus fertiliser is also a significant problem. Because it’s a non-renewable resource, coming from phosphate rock. We are using up Earth’s phosphorus so quickly that we could run out within 50 to 100 years, according to a study that is already 15 years old.²⁸

Learn how our use of phosphorus impacts the future of food security

It shouldn’t be radical to say that we must find ways to produce food without relying on nonrenewable resources. Because, well, they run out.

Reducing the use of synthetic fertilisers

Luckily, there is plenty we can already be doing to reduce our reliance on synthetic fertilisers. Technology can help farmers spray the right amount of fertiliser at the right time and place. This more precise approach reduces nutrient pollution without impacting the harvest. For example, modern sensors and spraying equipment can identify and deliver precisely how much fertiliser crops need, which varies significantly throughout a single field.³⁰

By spraying more precisely, farmers can save a lot of money. After all, fertiliser prices are a significant cost for farmers. Between September 2021 and 2022, Europe saw a 149% rise in the price of nitrogen.³¹ The more farmers can distance themselves from expensive inputs that wildly fluctuate in price, the better their bottom line.

Nature-based solutions to fertiliser pollution

Nature has been cycling nutrients for millennia. By working with those natural processes, we can reduce the impact of synthetic fertilisers. For example, leaving a buffer zone of wilderness near rivers and streams can prevent runaway fertilisers from reaching the water, as the vegetation between the farm and the water can absorb some of the nutrients.³²

Farmers can also choose not to spray fertiliser before heavy rain, preventing fertilisers from being washed away – a waste of money and a source of pollution. Spraying with the weather in mind can become problematic during extended rainfall or if the job belongs to a contracted company working on its own schedule.³³

Regenerative agriculture

Regenerative farmers use a wide range of techniques to build healthy soil. For example, they might limit the impact of grazing, avoid heavy machinery and synthetic chemicals, and keep a living plant in the ground all year round, so plants are constantly feeding the soil life.³⁴ When the soil is healthy, the need for synthetic fertilisers can be reduced.^35,36

Regenerative farmers look for ways to work with nature. For example, they might plant nitrogen-fixing crops to increase nitrogen levels in the soil naturally. Peanuts, soybeans, and fava beans all boost nitrogen thanks to nitrogen-fixing bacteria colonising their roots.³⁷

Herbal leys

Allowing the land to rest is an essential part of sustainable agriculture. Recovery time can prevent soil from being overly depleted in nutrients and enable soil life to recover from any heavy machinery, pesticides, or fertilisers that farmers could not avoid using.

As part of the rest and recovery stage, farmers can plant an ‘herbal ley’ - a temporary grassland made of legumes, herbs, and grass species.³⁸ This diverse plant life can build soil fertility, attract pollinating insects and other wildlife, and protect the soil from drought and flooding. As livestock graze these grasslands, farmers can produce animal protein and an income during resting years, and the livestock benefit from a diversity of plants to eat.³⁶

Like anything in life, the matter of artificial fertilisers isn’t simple. Synthetic fertilisers have been essential to food security for a rapidly growing urban population. However, overreliance on artificial inputs is harmful to people and the planet. Combining modern technology and sustainable farming practices can help us reduce our dependence on synthetic fertilisers, benefiting people and the living world.