Why have we fertilized the world? Global drivers of NPK fertilization in major crops since 1961

Since the second half of the 20th century, NPK (nitrogen, phosphorus, and potassium) inorganic fertilizers have been pivotal in boosting global crop yields. These fertilizers have supported the growth of the global population, facilitated dietary shifts towards protein-rich foods, and expanded industrial applications, such as biofuel production. However, the extensive use of fertilizers has disrupted natural biochemical cycles, leading to environmental impacts and raising social and economic concerns.

Four crops—maize, rice, soybean, and wheat—currently occupy over 50% of global croplands, account for more than 60% of global agricultural fertilizer inputs, and produce two-thirds of the proteins consumed by humans. Rice and wheat are primarily used for direct human consumption (food), whereas maize and soybean are also used for livestock feeding (feed) and other industrial applications. The debate over food versus feed versus other uses has typically centered on land occupation, labour, and water usage. However, the regional and temporal drivers of fertilizer use among these major crops remain poorly understood. We hypothesized that, in recent decades, larger fractions of fertilizers have been allocated to feed and other uses compared to food. Furthermore, we aim to discern whether the changes in fertilizer consumption for each use have been driven by expansions in crop areas, increased fertilizer intensification, or a greater proportion of crops being allocated to each use.

To investigate the temporal changes in total fertilizer use across the three main nutrients and four key crops, while distinguishing between their final uses (food, feed, and other uses), we adopted a comprehensive approach: First, we integrated national-level temporal data from 1961 onwards concerning crop fertilization, production, consumption, and trade, and second, we accounted for uncertainty in our estimates using Monte Carlo simulations. Finally, we performed a multiplicative factor decomposition to analyze the drivers behind the variations in total nutrient consumption for each nutrient and use category.

We found a significant increase in fertilizer use among the four main crops for the three nutrients and across all use categories over the past six decades. Globally, increases in fertilizer use for food and feed purposes are relatively balanced. However, at the national level, most countries have shifted towards a higher proportion of fertilizer use for feed compared to food. These shifts were driven by different crops: increases in fertilizer use for food were primarily linked to rice and wheat, for feed with maize and soybean, and for other uses predominantly with maize. Notably, for soybeans, the allocation of fertilizers between feed and food uses more than doubled during the studied period. Across all nutrient-use combinations, changes in total fertilizer consumption were mainly driven by increased fertilizer intensity rather than expanded cropped area or crop usage. However, for feed use, changes in total phosphorus and potassium consumption were equally influenced by increases in cropped area. The increase in fertilizer use for other uses, primarily driven by maize since 1990, appears closely linked to bioethanol production, especially in the United States, the leading producer.