Balancing nitrogen and phosphorus on vegetable farms in southwest BC
Introduction
To better understand the tradeoffs farmers face between nutrient management strategies, we evaluated three different approaches in two years of field trials across 20 mixed vegetable farms located in Pemberton, Vancouver Island, and in the Fraser Valley.
Like many crops, vegetables require relatively large amounts of nitrogen (N). Applying compost at rates to meet crop N needs can provide large amounts of carbon to build soil organic matter, however, this practice leads to over-applying phosphorus (P), which can build up in the soil and become an environmental risk. High nitrogen fertilizers, like feather meal, blood meal, and alfalfa meal, can provide N with minimal P. However, these fertilizers can be expensive and lack the carbon benefits of compost, which may result in declining soil organic matter.
For this study, we evaluated the following nutrient management approaches:
- ‘High Compost': We applied a large amount of compost to meet crop N needs. Over time, repeated applications of this quantity of compost can lead to P buildup in the soil and increase environmental risks.
- 'Low Compost + N': We applied a smaller amount of compost to meet crop P needs, and also applied feather meal, a high-nitrogen fertilizer, to meet crop N needs. This management strategy prevents excess P from building up in the soil while also meeting the N requirements of the crop.
- 'Typical': This method follows each farmer's usual nutrient application practices, varying by farm.
About this Brief
This brief was prepared by the BC Food Web team with the help of Amy Norgaard and is based on the following scientific journal article:
Norgaard, A. E., Lewis, D., Borden, K. A., Krzic, M., Carillo, J, & Smukler, S. M. (2022). Trade-offs in organic nutrient management strategies across mixed vegetable farms in Southwest British Columbia. Frontiers in Sustainable Food Systems, 6. https://doi.org/10.3389/fsufs.2022.706271
Key Findings
- Phosphorus and nitrogen can contaminate water resources if these nutrients are lost from farm fields through runoff or leaching.
- Applying compost to meet crop nitrogen needs is a common practice on organic farms, particularly in areas where compost and manure are abundant. However, this practice typically results in over-applying phosphorus beyond crop requirements.
- We found that applying compost to meet crop phosphorus needs and a high nitrogen fertilizer to meet crop nitrogen needs is most likely to achieve both environmental and economic goals of vegetable farmers using organic nutrient sources.