Fertilizer
Full explanation
Critics of veganism argue that animals play an indispensable role in agricultural nutrient cycles. Animal husbandry provides natural fertilizer in the form of manure and slurry, contributing to soil fertility. Without animals, it is argued, farmers would have to rely entirely on synthetic fertilizers, which come with ecological and energetic disadvantages.
1. Historical agriculture
Traditionally, crop production and animal husbandry were closely interconnected. Animals consumed crop residues, produced manure, and thereby helped close local nutrient cycles. For a long time, manure was considered a central component of sustainable agriculture.
2. Organic matter and soil life
Animal manure contains organic matter that can improve soil structure, water retention capacity, and microbial activity. Particularly in organic farming systems, livestock manure often plays a significant role.
3. Dependence on synthetic nitrogen
Synthetic nitrogen fertilizers are predominantly produced using the energy-intensive Haber–Bosch process. Critics argue that a purely plant-based agricultural system would depend more heavily on industrial fertilizers and thus rely on fossil resources.
From this, it is concluded that animal husbandry is necessary to maintain soil fertility in a sustainable way.
Full reply
The argument overlooks a central systemic question: Why do we require such large quantities of fertilizer in the first place? A substantial portion of agricultural land is not used to feed humans directly, but to produce feed for animals.
1. Feed-conversion ratio and nutrient losses
Animal production is associated with significant conversion losses. Depending on the species, several kilograms of plant calories are required to produce one kilogram of edible animal product. This so-called feed-conversion ratio means that a large share of the energy contained in plants does not become edible meat, but is used for the animal’s metabolism, movement, and maintenance.
In other words: 100% plant feed does not become 100% animal fertilizer. A substantial portion is lost as energy. Animal production therefore increases the total cropland required — and consequently the overall demand for fertilizers.
2. More animals = more feed = more fertilizer
The more animals are raised, the more feed crops must be grown. This additional crop production increases the demand for nitrogen, phosphorus, and potassium. A plant-based food system would require significantly less total cropland, since the inefficient conversion through animals would be eliminated.
3. Manure is not a closed cycle
Animal manure does not arise from nothing. It is based on previously cultivated plants, which themselves required fertilization. Moreover, nutrients are lost during the conversion process — for example through ammonia emissions, nitrous oxide, or leaching into groundwater. The idea of a perfectly closed nutrient cycle is not empirically sustainable in modern agricultural systems.
4. Systemic reduction instead of compensation
If fewer animals were raised, less feed would need to be produced. Less feed production means less land use — and therefore lower fertilizer use overall. The issue is not that plant agriculture without animals would be impossible, but that an animal-based food system artificially increases total nutrient demand.
Conclusion
Animal husbandry can provide fertilizer, but it is simultaneously a major driver of agricultural nutrient demand. Due to the conversion losses inherent in animal production, an animal-based food system requires more cropland and therefore more fertilizer overall. The fertilizer argument thus illustrates the inefficiency of the existing system rather than its necessity.