This paper compares fertilizer requirements for vegan and omnivore diets. Mass balance models are established and used to estimate that amount of fertilizer required to supply one unit of a nutrient from plants and from animal food products (AFPs). Two critical parameters are the nutrient inverse feed efficiency (IFE, the ratio of nutrient content of animal feeds to the nutrient content of animal food products, varying from 2.5-450) and the fraction of nutrients in animal manure that is recycled into new feed crops (faw-crop), varying from 0.15-0.70. For an AFP share of 20%, fertilizer requirements vary from 1.0-14 times that of a vegan diet as IFE increases from 2.5 to 250 with faw-crop = 0.70, from 1.1-29 times that of a vegan diet for faw-crop = 0.35. Thus, shifting from animal to plant sources of protein can substantially reduce fertilizer requirements, even with maximal use of animal manure. This paper adds a new and important dimension to the ongoing efforts, motivated by climate and health concerns, to transition more fully to plant-based diets. All rights reserved, Elsevier.