Can We Reduce Fertilizer use Without Sacrificing Food production?

Fertilizers help us to achieve higher crop yields. This is an obvious net positive for humans: farmers can produce and earn more, and the world has more food.

Fertilizers can increase crop yields. This not only offers important benefits for farmer incomes and food security, but also produces environmental benefits by reducing our demands for farmland. Many countries would benefit from using more fertilizer.

When they’re over-applied, they can also become an environmental pollutant. We might assume that there is nothing we can do: that to achieve higher yields we need more inputs and therefore necessarily cause more pollution. But the research shows that this is not necessarily true. Farmers in many countries can reduce fertilizer use without sacrificing food production.

One of the world’s biggest and most impressive studies shows us that simple interventions can produce large results. In a decade-long trial, researchers worked with 21 million smallholder farmers across China to see if they could increase crop yields while also reducing the environmental impacts of farming. They were successful.

In the decade from 2005 to 2015, average yields of maize, rice and wheat increased by around 11%. At the same time, nitrogen fertilizer use decreased by around one-sixth. By producing more crops and needing less fertilizer, this experiment provided an economic return of US$12.2 billion. This wasn’t achieved through major technological innovations or policy changes: it involved educating and training farmers on good management practices.

It’s often assumed that fertilizer use – alongside the pollution it creates – and crop yields present an inevitable trade-off. To increase yields, you need more and more fertilizer. This large-scale study suggests this trade-off is not always as extreme as we might think.

To be clear: fertilizers are vital for global food production. There are few innovations that have transformed the world as much as synthetic nitrogen.

For most of human history, food production was limited by the amount of reactive nutrients that were available for crops. This all changed with Fritz Haber and Carl Bosch. Rather than relying on the scarce nitrogen that exists naturally within the world’s soils, we could produce our own. Their innovation (the Haber- Bosch process) at the beginning of the 20th century enabled the lives of billions of people. Estimates suggest that every second person reading this has them to thank for being alive today.

Fertilizers help us to achieve higher crop yields. This is an obvious net positive for humans: farmers can produce and earn more, and the world has more food. What’s less obvious is that this has a large environmental benefit. Higher crop yields mean we need to use less land for farming. This means we can protect forests and maintain natural habitats.

But it’s true that alongside the environmental benefits, there are also some downsides. Not all of the nitrogen we use is used by the crops. The rest runs off the soils and into the natural environment: fertilizing the rivers and lakes and thereby upsetting the balance of ecosystems and causing biodiversity loss.

We might assume that there is nothing we can do: that to achieve higher yields we need more inputs and therefore necessarily cause more pollution.

Nitrogen use efficiency:
balancing yields and the need for nutrient inputs Using lots of fertilizer wouldn’t necessarily be a bad thing if all of it was used by the crops. Unfortunately, most of it isn’t.

To capture this, we can look at the ratio of nitrogen in harvested products (our crops) compared to our inputs (fertilizers or manure); this ratio is called the ‘nitrogen use efficiency’ (NUE). A NUE of 60% would mean that the amount of nitrogen in our crops was 60% of the nitrogen that was added to them as inputs. The remaining 40% of nitrogen was not used by the crops.

A low NUE is bad. This means very little of the nitrogen we add is taken up by the crops. A NUE of 20% would mean that 80% of the applied nitrogen became a pollutant.

Some countries have a very high NUE – greater than 100%. You might assume that this is good news. In fact, it’s often the opposite. This means they are undersupplying nitrogen, but continue to try to grow more and more crops. Instead of utilizing readily available nutrients, crops have to take nitrogen from the soil – a process called ‘nitrogen mining’. Over time this depletes soils of their nutrients which will be bad for crop production in the long-run.

Globally, NUE has been stubbornly low, at 40% to 50% since 1980. This is surprisingly low. It means that less than half of the nitrogen we apply to our crops is actually taken up by them. The rest is excess that leaks into the natural environment.

But there are very large differences in NUE across the world, as shown in the map. Some countries achieve low NUE – less than 40%. Both India and China, for example, have an efficiency of only onethird. Some countries, though, do much better. France, Ireland, the UK, and the US, have an efficiency greater than two-thirds.

How nitrogen use efficiency has changed over time
We can reduce nitrogen pollution without a decline in yields.

So, nitrogen efficiency rather than just fertilizer use seems like a better sustainably metric for us to benchmark. We might assume that all countries could achieve the same high NUE. But, maybe it’s still unfair to compare countries across the world in this way. Differences in climate, vegetation, and soil types mean we can’t achieve the same yields with the same inputs everywhere. Some countries might have more favourable environmental conditions than others.

How can we better understand which countries are doing well in these yieldfertilizer trade-offs?
An interesting way to tackle this question is to look at the discontinuities of yields and nitrogen pollution at international borders. This is the approach that David Wuepper and his colleagues took in a recent study, published in Nature. By looking at the discontinuities of yields, nitrogen balances and inputs across borders the researchers investigated the role that each country’s agricultural policies play. This is because the environmental conditions, climate and soil qualities should be very similar just across the border. Technically they should be able to achieve a similar level of NUE, and similar yields. If there are large differences in yields or pollution between one country and its neighbour, we would therefore assume there are important country-specific effects playing a role. It mimics a ‘natural experiment’ where the environmental conditions are held constant, and policy decisions are the changeable variable.

Across this large global dataset, researchers have found that the discontinuity in nitrogen pollution across borders was much larger than the discontinuity in yield gaps. Their results suggest that globally there is massive potential to reduce nitrogen pollution without impacting crop yields.

They conclude that nitrogen pollution could be reduced by around 35% if polluting countries became as efficient as their neighbours. This would have little impact on crop yields – increasing yield gaps by only 1%.

There are a couple of important points we need to keep in mind. All of these values are measured relative to a country’s neighbours. A country might have a good score because their neighbour gets very low yields. Or a country scores well because its neighbour uses nitrogen inefficiently. These are the countries that are overapplying nitrogen the most: they could probably reduce fertilizer use significantly without affecting their crop yields.

How can we use nitrogen more efficiently?
Most of the largest polluters are middleincome countries. During the 1960s and 1970s, many of today’s middle-income countries kick started their ‘Green Revolution’ and achieved large increases in food production. Governments offered subsidies for farmers to use fertilizers and other inputs. This made fertilizers cheap and reduced the incentives for farmers to use it efficiently. This cheap fertilizer is one of the reasons that these countries massively over-apply nitrogen today.

One way that governments can therefore reduce nitrogen pollution is to adjust the ratio of fertilizer prices to the return on agricultural products. They can adjust subsidies to make it costly for farmers to overuse fertilizers. Instead, they could reallocate these financial resources towards practices that have positive environmental impacts.

Another option is to invert the financial incentives: rather than subsidizing fertilizers, you could tax them.

We might want to make fertilizers more expensive for countries that overuse them. But we actually want to do the opposite for countries with large yield gaps. Many countries across Sub-Saharan Africa use barely any fertilizer at all. They achieve very poor yields as a result. Providing subsidies for fertilizers and other inputs would be of massive benefit.

One of the challenges of putting fertilizer on your crops is that it can be hard to know where it is needed. Some parts of your field might be lacking in nitrogen while others have more than enough. Often the easiest and quickest solution is to apply it everywhere, especially if fertilizers are heavily subsidized and cheap.

But with emerging technologies, we can do better. Thanks to information from drones or satellite imagery, we can implement ‘precision farming’, which allows us to see exactly where fertilizers are needed the most.

Plant breeding technologies could also offer new opportunities. We can try to improve how efficient we are at using nitrogen, but there’s an opportunity to improve how efficiently plants use it too.

Let’s not forget that one of the most promising solutions and one we often overlook is the simplest and oldest of all. Legumes (crops such as beans, peas and lentils) perform their own magic when it comes to nitrogen. They have the ability to capture nitrogen in the atmosphere and transform it into reactive nitrogen on their own. This is called ‘biological fixation’. Unlike most other crops where we have to add additional nitrogen, they create it by themselves. Growing more legumes – either on their own, or alongside other crops – is one of the easiest ways that we can bring nitrogen into the soil.

Large policy changes and technological advancements are often needed to make a large difference, but we shouldn’t underestimate the impact that education can make. Many view crop yields and environmental pollution as an unavoidable trade-off. It doesn’t have to be. We can reduce pollution a lot without reducing crop yields. Less pollution, more food, higher farmer returns, and less farmland make this a problem with multiple wins if we can implement the right solutions.