Agricultural intensification is deemed necessary to feed the world. It is associated with detrimental emissions of agro-chemicals to air and water, extended use of fossil resources and loss of space for biodiversity. Circular agriculture is put forward nowadays as an answer to these problems. Circularity has the connotation of societal sectors recycling each other’s wastes but also of minimising losses to the environment. Application of these principles to agriculture is challenging, however. The recycling of nutrients in by-products is thus far from perfect due to logistic complications, the presence of contaminants, or lack of plant-availability once these by-products are applied to land. Even if these flaws would be entirely solved, most by-products still contain insufficient plant-available nitrogen (N) because losses of N between fork and field, and back again, are simply inevitable. This holds also in combination with the best available nutrient application methods and optimised soil quality. N supplements, among which manufactured fertilisers, are therefore an absolute necessity, also in a circular agriculture. Unfortunately, many crops respond to N according to the law of diminishing returns. The loss of N per unit produce is then positively related to the N application rate. Agronomically suboptimal N rates can in that case reduce both local and global N emissions. In the northern hemisphere where N application rates are often high, extensification therefore deserves as much consideration as intensification does elsewhere. Reconsiderations should not only apply to how we feed our crops, but also to how we feed ourselves if we take global problems seriously.
22 pages, 5 figures, 1 table, 37 references