The What, Where, and Why of Fertilizer

How does fertilizer drive carbon pollution, and how did we get here?

Fertilizers explained

It takes a lot of food to feed a global population of 8 billion people — and much of it depends on modern agriculture techniques, including the use of fertilizer.

Today phosphorus, potassium, and nitrogen (often referred to collectively as NPK) are widely recognized as macronutrients, substances required by plants in large quantities for optimal growth, but even before these chemical elements were discovered, farmers applied them to their fields in the form of manure and other minerals.

The Industrial Revolution paved the way for the development of synthetic fertilizers, and by the early 20th century, new technologies emerged that now help feed half of the world’s population.

But for all the benefits they bring, fertilizers also have a significant impact on our planet. The modern-day synthetic nitrogen fertilizer sector is responsible for 1.31 gigatons (Gt) of CO₂e each year, which is more than the aviation and shipping sectors combined.

What makes this sector unique is that only one-third of these emissions are associated with production; 84 percent of those fertilizer emissions come from ammonia synthesis, which typically requires natural gas as the feedstock.

Around two-thirds of fertilizer emissions are generated through their application in cropland, when CO₂ and N₂O are directly and indirectly emitted. It is important to note that these field emissions vary significantly around the globe and are also very difficult to accurately measure. But that does not mean mitigation is impossible; we can cut emissions from both production and application with technological advances, cleaner fuels, and better knowhow on the ground.

A history of fertilizer

While today both phosphate and potash (potassium) fertilizers are produced from mined feedstock, in the beginning of the 19^th century they were available mainly from biological sources. The development of chemical phosphate production methods allowed for the construction of the first fertilizer factory in 1842 and industrial-scale production of potash from mineral salts started in Germany in 1860.

Although the benefits of nitrogen were not immediately recognized, the elements of the nitrogen cycle were discovered in stages during the 19^th century, which led to a surge in demand for nitrogen fertilizers by the end of the century. As several methods for producing nitrogen fertilizers on an industrial scale emerged in the early 1900s, the Haber-Bosch process quickly became the most prominent. This process runs on natural gas and entails a chemical reaction between nitrogen and hydrogen in the high-pressure and high-temperature reactor, yielding ammonia. The first industrial-scale Haber-Bosch plant began operations in 1913, marking a new era in nitrogen fertilizer production.

As the technology evolved and the production cost dropped, the use of ammonia expanded beyond high-value crops like cotton and tobacco, becoming a vital fertilizer for food production. At the same time, rapid expansion of global production capacity turned the industry into a major polluter.

Intensive agriculture unlocked exceptional yields

The unprecedented population growth in the 20^th century demanded increased food production and improved agricultural efficiency. Extensive agriculture, when limited labor and fertilizer inputs are compensated by extended farmland, became economically unfeasible by the 20th century; since soil fertility decreases and reaches a plateau over time, this practice required continuous expansion into new lands, making it impractical. Intensive agriculture, when higher yields are achieved through increased inputs, became a standard practice instead. As in previous eras, the increase in land productivity was driven solely by innovations, including crop rotation mechanisms, irrigation and drainage development, and, most of all, the application of fertilizers.

The fertilizer industry developed alongside agriculture. As the role of different nutrients (not limited to NPK) was recognized, application of single nutrient fertilizers became impractical, and farmers needed compound fertilizers. Small-scale regional fertilizer mixers were becoming uncompetitive with large-scale bulk producers, resulting in market centralization, as small companies had to either transform into producers, blenders, and distributors, or go out of business. A combination of mechanical, infrastructural, chemical, and biological innovations, supported by agricultural research and development investments, fueled a productivity surge in the United States starting in the 1930s.

The nuanced role of fertilizers

Though synthetic fertilizers undoubtedly play a vital role in modern agriculture, the topic is more complex than it appears. Significant differences in fertilizer application are observed across different geographical regions, as fertilizer distribution between grassland, food, and nonfood crops varies significantly. Therefore, correlating food production with fertilizer application across geographies can be misleading.

It’s also critical to recognize that land productivity and emissions resulting from fertilizer application depend on numerous factors, many of which cannot be controlled by farmers. For example, yield depends on the physical and chemical properties of soil, weather conditions, crop pests, and diseases.

While fertilizers can contribute to boosting crop yields, their effectiveness is contingent on the specific application case. Soils need very careful balance: both the deficiency and excess of fertilizers lead to a decrease in yield. Farmers calculate which nutrients and in what quantities should be added for economic efficiency using a crop response function. Oftentimes, results indicate that required NPK ratios are unavailable in the market, necessitating application of custom nutrient blends. It has been proven in practice that a comprehensive approach to agriculture, which targets multiple factors simultaneously beyond just fertilizer application rates, is the most effective pathway to increase yields and can triple crop response to fertilizers.

Efficiency of nitrogen use

Nitrogen is a vital nutrient for plant growth, but its uptake and utilization are influenced by complex environmental factors and chemical processes in the soil, which are sensitive to moisture and temperature. This makes it a challenge to calculate the correct amount of nitrogen-based fertilizer to apply, presenting risks to groundwater and our atmosphere when it is not applied properly.

Although nitrogen makes up 78 percent of the atmosphere, most plants cannot absorb it directly from the air. Instead, they absorb it through their roots in the nitrate form, when nitrogen is bonded to oxygen. Nitrification, a process carried out by soil microorganisms, converts ammonium into nitrites and then nitrates, making it digestible for plants.

Nitrates have great mobility in the soil and heavy rains can leach them deep in the ground. If nitrates enter the groundwater, water can become toxic for humans. Fertilizers can also run off the fields and enter surface waters. Excessive nutrient concentrations deteriorate water quality, a process known as eutrophication. Further, excessive soil moisture intensifies denitrification, which converts the remaining nitrates back into nitrogen gas and returns them to the atmosphere. Another type of nitrogen loss after fertilizer application is called volatilization. This process involves the loss of the ammonia gas into the atmosphere. The extent of the loss depends on weather conditions, the type of fertilizer, and the method of application. Nitrate fertilizers can help minimize volatilization. In contrast, higher nitrogen losses are typical for fertilizers containing urea, especially when they are applied on the soil surface in hot weather with insufficient moisture.

Mitigating emissions

It’s possible to reduce emissions associated with the fertilizer industry in both production and application of fertilizers. To minimize nitrogen losses and improve nitrogen uptake by crops, nitrogen fertilizer application practices should be adjusted to weather conditions, cultivated crops and their uptake times, as well as soil properties and soil nutrient availability. Though several synthetic nitrogen fertilizers are available today, their market adoption depends on industry regulations and available infrastructure and machinery. Inhibitors, which slow down nitrogen release, can also help minimize nitrogen losses. Finally, emerging precision agriculture technologies can offer solutions, which improve visibility of fertilizer efficiency.

In addition to reducing emissions, better fertilization allows for lower water consumption and land use in the agricultural sector. Regarding emissions originating from synthetic fertilizer production, green ammonia production technologies are already being deployed. In addition to environmental benefits, a disaggregated ammonia production pathway offers fertilizer price stability and supply chain resilience.

The agriculture and fertilizer sectors have undergone a massive transition. Agricultural productivity in the United States has increased fourfold over the past century. While new disruptive factors, including ammonia utilization in new use cases, are expected to emerge in the coming years, there are also new opportunities on the horizon. As the industry continues to evolve, embracing innovation and sustainability will be key to meeting future challenges and ensuring continued growth in agricultural productivity.