Organic Science Cluster 4 researchers are looking at the potential of replacing animal-based fertilizers with plant-based alternatives in organic greenhouse agroecosystems.
Nutrient supply is a fundamental yet complex aspect of organic farming. Foundational principles focus on a system-based approach to enhance soil health, fertility, and biological activity. This is achieved through strategic crop rotations, cover cropping, and the application of amendments approved for use in organic production. Since the organic standards prohibit synthetic inputs, producers must look for naturally derived inputs; fresh or composted manure is a common choice.
Non-organic manure
The ideal organic farm traditionally aimed to close the nutrient cycle by recycling resources on-site and minimizing reliance on external inputs. With the growing intensification and specialization of agricultural practices, there is a consensus that achieving nutrient circularity should be approached at the food system level rather than focusing solely on individual farms. Today, the majority of organic field and greenhouse vegetable farms rely on nutrient inputs sourced externally. Many of these inputs consist of animal-based commercial fertilizers derived from conventional farming systems. On European organic farms, conventional manure accounts for 17–26% of total external nutrient inputs, while inputs from non-agricultural sources make up 31–41% (Reimer et al., 2023). In Canada, the organic standards, with certain restrictions, allow the use of non-organic manure if organic manure is not available (CGSB, regulation CAN/CGSB-32.310-2020). This is permitted because of the limited supply of organic manure. However, it must be considered that the production of these non-organic inputs is most likely linked to environmental and animal welfare practices that do not align with organic farming standards and principles. Additionally, some of these inputs may contain chemical pesticide residues, although the extent to which these residues persist in the soil or on crops after application remains uncertain (Muola et al., 2021).
The organic sector’s reliance on inputs derived from practices that are inconsistent with organic farming principles highlights a significant contradiction that demands attention. In Europe, field application of animal manure and commercial fertilizers is limited to 170 kg of nitrogen per hectare per year (European Union, regulation 2018/848). Some countries and private certification bodies have adopted stricter standards. For example, in Denmark, the limit for conventional animal-based fertilizers is set at 50 kg of plant-available nitrogen per hectare per year (Lynge et al., 2023). Some European organic farmer associations advocate for a complete ban on the use of conventional manures, as their inclusion is seen as undermining organic certification standards. However, the lack of viable alternatives currently prevents the adoption of stricter regulations.
Limited nitrogen supply
Nitrogen availability has been identified as the primary limiting factor for expanding organic production. One study suggests that a substantial increase in organic farming could lead to a 57% production gap compared to conventional farming, with 77% of this gap attributed to nitrogen deficiency (Barbieri et al., 2021). When considering global supply and demand trends, it also appears that our sector’s reliance on conventional manure may not be sustainable in the long term. Firstly, an increase in demand for conventional manure is to be expected. Many regions world-wide are working towards expanding their organic farming sector. As an example, the European union is aiming to almost triple its 2020 acreage under organic production and reach 25% of total farming area under organic management by 2030. Secondly, a reduction in supply of animal manure could occur. While meat consumption is rising in emerging countries, the opposite trend is evident in North America and Europe. The growing shift towards plant-based diets in response to climate change could further impact the animal production sector, reducing the availability of associated by-products in Global North countries.
Even if the availability of inputs were not a constraint, nitrogen supply for crop production could not rely solely on animal-based fertilizers. Excessive dependence on manure to maintain soil fertility can lead to nutrient imbalances, particularly phosphorus accumulation. To sustain appropriate Nitrogen to Phosphorus ratios across diverse cropping systems, it is recommended that no more than 15–25% of nitrogen be supplied through solid animal manure (Möller et al., 2018).
Biological nitrogen fixation
Biological nitrogen fixation (BNF) has always been a central component of nitrogen supply in organic agroecosystems. It is traditionally harnessed through crop rotations and inclusion of legume-based cover crops. But the net nitrogen contribution of these strategies is rarely sufficient for the higher nutritional needs of many vegetable crops (Lynge et al., 2023). Additionally, cover cropping strategies offer limited control over the timing of nutrient release.
Given that nitrogen provisioning is a critical factor in sustaining the productivity of organic farming systems, greater efforts should focus on enhancing and optimizing the contribution of BNF. One potential strategy is cut-and-carry, which involves repeatedly harvesting cover crops and using the fresh biomass as fertilizer. Another alternative is the use of plant-based fertilizers, such as alfalfa meal, which are more practical for large-scale greenhouse farms and for year-round fertilization activities.
Research on plant-based fertilizers
At Université Laval, our research team led by Dr. Martine Dorais is evaluating the impact of partially or fully replacing animal-based fertilizers with plant-based alternatives in an organic greenhouse agroecosystem. Specifically, we are investigating how different fertilization regimes affect nutrient use efficiency, greenhouse gas emissions, crop productivity, and microbial activity related to the nitrogen cycle. Trials are being conducted at the Institut national d’agriculture biologique (INAB) certified organic greenhouses and at partnering farms.
Through this work, we strive to assist organic greenhouse vegetable growers in adopting fertilization practices that incorporate higher proportions of organic plant-based fertilizers. Plant-based fertilizers, particularly those derived from legumes, are nearly phosphorus-free, making them a valuable option for meeting nitrogen requirements without exacerbating soil nutrient imbalances. This is particularly critical for greenhouse crops, where nitrogen uptake can reach as high as 1250 kg per ha per year for tomato cultivation (Dion et al., 2020).
Expanding the use of biological nitrogen fixation (BNF) is one potential strategy to reduce reliance on conventional animal manure. However, large-scale production of plant-based fertilizers will require careful consideration of land use and allocation. Solely relying on BNF is not a viable solution, as it would lead to the depletion of phosphorus and potassium reserves in the soil (Reimer et al., 2020). Other options, like the recycling and conditioning of food waste and crop residues are also important to consider. Future organic fertilization must combine plant-based amendments and BNF with other organic inputs and cropping practices to fully and sustainably replenish soil nutrient reserves.
Our research on plant-based fertilizers is part of a larger Organic Science Cluster 4 activity, Plant-Based Fertilizers, Biochar and Intercropping to Improve the Sustainability and Resilience of Organic Greenhouse Agroecosystems, which runs from 2023-2028.
References
Barbieri, P., Pellerin, S., Seufert, V., Smith, L., Ramankutty, N., & Nesme, T. (2021). Global option space for organic agriculture is delimited by nitrogen availability. Nature Food, 2(5), 363–372. https://doi.org/10.1038/s43016-021-00276-y
Canadian General Standards Board (CGSB). (2020). Organic production systems: general principles and management standards. CAN/CGSB-32.310-2020.
Dion, P.-P., Jeanne, T., Thériault, M., Hogue, R., Pepin, S., & Dorais, M. (2020). Nitrogen release from five organic fertilizers commonly used in greenhouse organic horticulture with contrasting effects on bacterial communities. Canadian Journal of Soil Science, 1–16. https://doi.org/10.1139/cjss-2019-0056
European Union. (2018). Regulation (EU) 2018/848 of the European Parliament and of the Council of 30 May 2018 on organic production and labelling of organic products and repealing Council Regulation (EC) No 834/2007. Available at: https://eur-lex.europa.eu/eli/reg/2018/848/oj/eng
Lynge, M., Kristensen, H. L., Grevsen, K., & Sorensen, J. N. (2023). Strategies for high nitrogen production and fertilizer value of plant‐based fertilizers. Journal of Plant Nutrition and Soil Science, 186(1), 105–115. https://doi.org/10.1002/jpln.202200031
Möller, K. (2018). Soil fertility status and nutrient input–output flows of specialised organic cropping systems: A review. Nutrient Cycling in Agroecosystems, 112(2), 147–164. https://doi.org/10.1007/s10705-018-9946-2
Muola, A., Fuchs, B., Laihonen, M., Rainio, K., Heikkonen, L., Ruuskanen, S., Saikkonen, K., & Helander, M. (2021). Risk in the circular food economy: Glyphosate-based herbicide residues in manure fertilizers decrease crop yield. Science of The Total Environment, 750, 141422. https://doi.org/10.1016/j.scitotenv.2020.141422
Reimer, M., Hartmann, T. E., Oelofse, M., Magid, J., Bünemann, E. K., & Möller, K. (2020). Reliance on Biological Nitrogen Fixation Depletes Soil Phosphorus and Potassium Reserves. Nutrient Cycling in Agroecosystems, 118(3), 273–291. https://doi.org/10.1007/s10705-020-10101-w
Reimer, M., Oelofse, M., Müller-Stöver, D., Möller, K., Bünemann, E. K., Bianchi, S., Vetemaa, A., Drexler, D., Trugly, B., Raskin, B., Blogg, H., Rasmussen, A., Verrastro, V., & Magid, J. (2023). Sustainable growth of organic farming in the EU requires a rethink of nutrient supply. Nutrient Cycling in Agroecosystems. https://doi.org/10.1007/s10705-023-10297-7
Published March 5, 2025
Organic Science Cluster 4 is an industry-led research and development endeavour co-managed by the Organic Federation of Canada and the Organic Agriculture Centre of Canada at Dalhousie University and supported by the AgriScience Program under Agriculture and Agri-Food Canada’s Sustainable Canadian Agricultural Partnership together with over 80 funding partners.
