How many people does synthetic fertilizer feed?

November 07, 2017

Predictions of a global food crisis — that the world’s food production would not be able to keep pace with population growth — have a long history. In the 18th century the English cleric Thomas Robert Malthus hypothesized that gains in per capita resources would inevitably be outstripped by population until food supplies finally acted as a barrier to further growth.¹

Such predictions have continued well into the 20th and 21st centuries; in his controversial 1968 book The Population Bomb, Paul Ehrlich argued that global population would continue to grow until the point of mass starvation.²

Both Malthus and Ehrlich should be surprised to see the current state of the world. Today, we can support a global population of around 7.4 billion (and growing), with many consuming far in excess of requirements.³

There are a number of scientific and technological innovations which have allowed for rapid growth in crop productivity, particularly in the second half of the 20th century. None of these had a more dramatic impact than the ability to produce synthetic nitrogen fertilizer.

In fact, it’s estimated that nitrogen fertilizer now supports approximately half of the global population. In other words, Fritz Haber and Carl Bosch — the pioneers of this technological breakthrough — are estimated to have enabled the lives of several billion people, who otherwise would have died prematurely, or never been born at all.⁴

It may be the case that the existence of every second person reading this attributes back to their 20th century innovation.

Atmospheric alchemy: making fertilizer from air

Understanding the significance of nitrogen fertilizer requires a brief explanation of its role in global crop production. In addition to water and sunlight, crops need three key nutrients to grow: nitrogen, phosphorous and potassium. Nitrogen is often the nutrient that is limiting to further crop production, despite Earth’s atmosphere containing more than 78 percent. This is because in the atmosphere, nitrogen exists in its largely unreactive N₂form, rather than in a reactive form which plants can utilize.

For millennia, agricultural crop had to rely on the limited quantity of reactive nitrogen which was naturally occurring in soils and ecosystems.⁵

This remained the case until 1908 when the German chemist Fritz Haber developed a process by which atmospheric N₂ could be converted into ammonia (NH₃) – a form of reactive nitrogen which plants can use.⁶

Carl Bosch, another German chemist and engineer, was able to take Fritz Haber’s laboratory-scale process and develop it at an industrial scale. The combined “Haber-Bosch process” remains the primary industrial method for the production of synthetic nitrogen fertilizer.

How many people does nitrogen fertilizer feed?

In simple terms, crops typically respond positively to nutrient inputs. As we explore in detail in our entry on Crop Yields, crop yields and fertilizer application typically show a strong positive relationship. Fertilizer application, combined with other productivity factors such as improved crop varieties, genetic breeding, irrigation and mechanization led to a significant inflection in crop yield trends across the world in the 20th century. It should be noted that this growth in agricultural output — from both industrial and organic farming — has undoubtedly resulted in important ecological and resource pressures. However, as we cover in detail in a recent blog post, organic farming (that is, agriculture without synthetic inputs) can often have a greater environmental impact than conventional agriculture.

So, how many people does synthetic nitrogen fertilizer actually feed? Below we draw upon several published estimates, which tend to converge on a similar share of the global population. Results published by Erisman et al. (2012) in the scientific magazine Nature are shown in the chart.⁷

These results also tie closely with Vaclav Smil’s widely-quoted estimates, which we discuss later.⁸

In the chart we see the actual global population trend in blue — growing from around 1.65 billion in 1900 to almost 7.4 billion in 2015.
The line in grey represents estimates of the number of people fed by synthetic nitrogen fertilizers. As we see, nitrogen fertilizers only became available following the commercialization of the Haber-Bosch process from 1910 onwards. Since then, Erisman et al. estimate it has supported 42 percent of global births over the past century. This amounts to 44 percent of the global population in 2000 being fed by nitrogen fertilizers, rising to 48 percent in 2008. Here we have extended this estimate to 2015 with the continuation of the assumption that 48 percent of the global population are fed by nitrogen fertilizers. Since the share supported by the process continues to rise, this may in fact be a conservative estimate. This means that in 2015, nitrogen fertilizers supported 3.5 billion people that otherwise would have died.
The red line represents the size of the global population which would therefore be supported without the use of nitrogenous fertilizers. This is shown simply as the actual population minus the number of people reliant on them for food production. Without this innovation, global population may have been reduced to only 3.5 to 4 billion people.

World population with and without haber bosch nitrogen v2 850x600

How can we estimate the number of people fed?

Firstly, it’s important to note that these estimates are understandably difficult to derive with a high degree of certainty. This difficulty arises for several reasons. Notably there have been a number of additional contributors to productivity gains in agriculture throughout the 20th and 21st centuries, including crop breeding, irrigation, mechanization, and farm management techniques — untangling the individual role of nitrogen fertilizers alone is challenging. Secondly, the global food system is complex and geographically highly unequal: high-income countries have moved beyond the stage of aiming to meet basic nutritional requirements from food production, and now dedicate a large share of food production to meat (which is a much less efficient nitrogen converter) and bioenergy production.⁹

Nonetheless, general estimates tend to converge on a figure in the range of 40-50 percent of the population. Let’s look at three widely cited estimates:

Utilizing a range of long-term evaluations, spanning a total of 362 seasons of crop production, of crop yields and nutrient budgets across the world, Stewart et al. (2005) concluded that between 30-50 percent of yield increases could be attributed to synthetic fertilizer inputs (and typically even higher in the tropics).¹⁰

Smil (2004) reached similar conclusions, suggesting with high confidence that global crop harvests would be approximately half of current levels without nitrogen fertilizer inputs.¹¹

Vaclav Smil derives his calculations based on the use of nitrogen crop, livestock and human protein (of which the building blocks are nitrogen) balances. His nitrogen balance concluded that 85 percent of all nitrogen in food protein for humans is derived from cropland; the remainder came from seafood or livestock on grazing land. Since nitrogen fertilizer provided around half of the nutrient in this harvested crop, he estimated that it provided 40 percent of dietary protein in the mid-1990s. He concluded that 40 percent of the global population in 2000 were dependent on food production from synthetic fertilizers.

Erisman et al. (2012) have since updated these estimates to the year 2008, estimating a similar (but slightly higher) share of 44 percent in 2000 and 48 percent in 2008.

The complexities of the global food system make it challenging to provide a firm figure, however, it’s likely that just under half of the global population is dependent on synthetic nitrogen fertilizers. This is further shown in the chart. As a result, the Haber-Bosch process is likely to have enabled the lives of at least 3 to 3.5 billion people today.

World population supported by synthetic nitrogen fertilizers v3 850x600

Additional discussion:

Could we have achieved the same without synthetic nitrogen?

If Fritz Haber had not discovered how to make synthetic nitrogen, would we have found alternative solutions to support a population as large as today’s?

Addressing this question partly relies on retrospective guesswork about whether, in the absence of synthetic nitrogen fertilizer, we would have managed to supply nitrogen via other methods. If Fritz Haber or Carl Bosch (or any scientist to follow) hadn’t developed a method for transforming inert atmospheric nitrogen into reactive nitrogen plants could utilize, what is the likelihood that other solutions would have filled the gap?

One solution would have been to greatly increase the production of nitrogen-fixing legume crops. As noted earlier, leguminous crops (i.e. peas, beans and other pulses) possess a unique ability to transform (or ‘fix’) atmospheric nitrogen into reactive nitrogen in the soil. Growing these crops can therefore increase soil nitrogen sources over time. Whilst this might have credibly increased the total availability of reactive nitrogen to some extent, it’s unlikely to have been a widely-scalable solution for several reasons.

Firstly, legumes tend to be lower-yielding relative to cereals and other staple crops. This is true of yields today, but also true of historical yields — in the late nineteenth century, Western European legume yields were typically less than half that of staple cereals.¹²

Farmers in the 19th and early 20th century would have had little incentive to widely adopt these crops. Secondly, despite the many nutritional and environmental benefits of legumes, they form only a small component of most peoples’ diets. Furthermore, preferences for pulses and legumes tend to decline as incomes rise and food choices widen. The overall appetite for legumes — both from a farmer’s and dietary preference perspective means it would have been high unlikely that they would have been able to supply nitrogen at a scale close to that of the Haber-Bosch process throughout the 20th century.

Another potential nitrogen source is that of organic wastes — nitrogen is supplied in many organic farming systems today in the form of animal manure. Couldn’t we have relied on these non-synthetic nitrogen sources instead? In fact, we did; prior to Haber-Bosch and the creation of synthetic nitrogen inputs, most agricultural systems relied on the recycling of manure, wastes and other biomass back into the soil to maintain nitrogen balance. The issue is that these existed in limited supply: recycling nutrients, by definition, means you have a limited supply. This allowed societies to sustain moderate levels of nitrogen but did not allow them to create more. As Smil (2004) discusses in detail, previous societies could typically support only small numbers of domesticated animals, and as a result, had very limited supplies of manure and animal wastes.¹³

Synthetic nitrogen not only increased crop yields, but also enabled an expansion in livestock numbers. Increased productivity and excess crops allowed farmers to allocate an increasing share of output to livestock — particularly grain-fed animals. Overall, this has increased the amount of reactive nitrogen which can be recycled through our agricultural systems; with more livestock, we also have more manure to recycle. But it’s important to recognise that such levels of organic nitrogen sources are only available because of previous synthetic nitrogen inputs. The creation of synthetic nitrogen delivered reactive nitrogen to the soil which could then be recycled in the form of organic wastes and biomass. If organic nitrogen could today support a large share of the global population then it is because synthetic nitrogen has enabled it to do so by adding reactive nitrogen to our agricultural systems. Without Fritz Haber’s discovery, this would never have been a possibility.