Summary
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 overapplied, 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.1 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.2 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.3 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. In this article I show that farmers in many countries can reduce fertilizer use without sacrificing food production.
There are large differences in fertilizer use across the world
Crops, like any organism, need nutrients to grow. When particular nutrients are lacking, they fail or grow at a much slower rate. These are called ‘limiting nutrients’. What nutrient is the most limiting varies across the world: some soils lack nitrogen, while others lack phosphorus or potassium.
If a soil is lacking nutrients naturally we can add our own. This can be in the form of synthetic fertilizers, or organic additions such as manure. There are very large differences in how much fertilizer is applied across the world. We see this in the charts below: first as a map of average fertilizer use per hectare of cropland; and second, with the breakdown by nutrient in the bar chart.
There are 100-fold differences between countries. In many of the world’s poorest countries – particularly across Sub-Saharan Africa – farmers apply only a few kilograms of fertilizer per hectare. For context, one hectare is about 1.5-times the size of a football pitch.4 Contrast this with countries such as China, Brazil, the UK or Egypt, where farmers apply hundreds of kilograms per year. They apply as much in a few days as some farmers do in an entire year.
This has led to a divided world:
- In many poorer countries we need more fertilizers. Improvements in crop yields have been slow, and large yield gaps could be closed through more and better management of inputs.5 This is not only good for farmers, but also for the environment: for the reasons above, closing yield gaps is one of the best ways we can prevent habitat loss across the tropics.
This is why it’s damaging for agencies, such as the United Nations Development Programme to continually promote the message that the less fertilizer, the better. It’s not good for humans, or the environment. - But, as we will see, many countries are overapplying nitrogen. They could cut back without negatively affecting their crop yields.
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.
Soon we will see that 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 1980.6 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 one-third. 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 favorable environmental conditions than others.
How can we better understand which countries are doing well in these yield-fertilizer 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.7 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 neighbor, 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.
The contrast at the border between Kazakhstan and China; and Turkey and Syria provide good examples of this. We can see this in the aerial shots. The conditions for growing crops on either side should be similar. But China and Turkey have much more vegetation than their neighbors as a result of nutrient inputs and how they manage agriculture.
Using satellite imagery and geospatial datasets these researchers could measure four key metrics at high-resolution across hundreds of thousands of cross-country borders: cropland nitrogen balances, nitrogen pollution, yield gaps (the amount that yields could be increased with better management of nutrients), and the natural vegetation potential. They found cross-border differences in the first three metrics, but not in natural vegetation potential. This is important because it means our assumption that the environmental conditions on either side of borders is similar, is a valid one.9
Across this large global dataset, the researchers 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 neighbors. This would have little impact on crop yields – increasing yield gaps by only 1%.
Their results also allow us to understand what countries are using nitrogen inefficiently. The map here shows how countries compare in levels of nitrogen pollution versus their yield gains relative to their neighbors. Positive values – shown in orange and red – mean a country causes more pollution than necessary for the yields that it achieves. Negative values – shown in blue – means a country causes less.
There are a couple of important points we need to keep in mind. All of these values are measured relative to a country’s neighbors. A country might have a good score because their neighbor gets very low yields: South Korea is a good example. Or a country scores well because its neighbor uses nitrogen inefficiently: Mongolia is a good example.
China has the highest score of 170%. This means it causes 170% more nitrogen pollution than is necessary to achieve its level of crop yields. Brazil, Mexico, Colombia, and Thailand also create a lot of pollution. These are the countries that are overapplying nitrogen the most: they could probably reduce fertilizer use significantly without affecting their crop yields.
We often assume that more pollution is an unavoidable cost of trying to close yield gaps. But this trade-off does not always exist.
How can we use nitrogen more efficiently?
You might notice that most of the largest polluters are middle-income countries. During the 1960s and 1970s, many of today’s middle-income countries kickstarted 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.10 This cheap fertilizer is one of the reasons that these countries massively overapply 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 re-allocate 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. As we saw earlier, 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.11 Plant breeding technologies could also offer new opportunities.12 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.
Finally, there’s a lot that we can do by training farmers to adopt sustainable management practices. The 21-million-farmer study in China makes this clear. 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.
