Quantifying trade-offs among on-farm and off-farm fertility sources to make vegetable organic farming systems more sustainable

Abstract

A core principle of organic farming is the reliance on internal nutrient recycling, favoring nutrient sources produced on farms (e.g.,manures) as opposed to those that are not (e.g., processed fertilizers, municipal compost). Soil fertility management varies tremendously among organic vegetable production systems, potentially with large effects on crop productivity, the environment, and trade-offs between them. We compared four certified organic systems using manure, municipal waste compost, and/or processed fertilizers to grow a mix of vegetable crops with variable nutrient demand. The four systems added nutrients to meet recommended nitrogen (N) inputs, estimated phosphorus (P) removal, or both, with different trade-offs expected between crop yields and environmental impacts depending on the imbalance in N:P ratio between fertility sources and crop removal. Municipal compost minimized trade-offs between yields and environmental impacts for crops with a low N demand (e.g.,beet). In these systems, we observed higher carbon inputs (2–6 fold), and lower residual soil N (40–50%) and nitrous oxide (N2O) emissions (50–75%) compared to systems based on manures or processed fertilizers. In contrast, the manure-based system had the highest crop yields when N demand was high (e.g., cauliflower), but also the highest environmental impacts. Using processed fertilizers instead of manure typically lowered N2O emissions (45%) and P surpluses (85%), but it also reduced yields (10%). Overall, our results confirm the need to develop high-yielding organic vegetable systems that minimize environmental impacts and the challenge of minimizing farm-scale yield-environment trade-offs without relying on off-farm fertility sources.