Many alternatives to synthetic fertilizer exist. The USDA lists "compost, cover crops, plant by-products, animal manure, and other biological materials" as common choices used in organic farming to avoid synthetic fertilizer.
Synthetic fertilizer increases short-term crop yields by adding limiting nutrients such as nitrogen to the soil. However, it also contributes to global warming via nitrous oxide emissions and disrupts ecosystems via runoff.
In addition to cover crops that protect the soil and organic fertilizers such as compost and manure, farmers also employ crop rotation to enrich soil, utilizing nitrogen-fixing crops such as legumes.
There is disagreement about whether organic farming can feed the world. A widely cited Nature study from 2017 says it is possible, but assumes more land is made available for agriculture, more people adopt a plant-based diet and less food is wasted.