Optimizing the Use of Organic Amendments and Cover Crops in Certified Organic Vegetable Production Systems of the Pacific Northwest.

Composts, manures, and cover crops increase nutrient recycling within agricultural systems and help maintain long-term soil fertility and quality. However, their effect on soil fertility and greenhouse gas (GHG) emissions are typically harder to predict than those from processed fertilizers. Furthermore, they can favor phosphorus (P) over-fertilization due to P enrichment relative to crop nitrogen (N) requirements. Thus, optimizing the management of composts, manures, and cover crops is complex, especially in vegetable production systems that have high nutrient demand, low nutrient use efficiency, and high risks of nutrient loss, a risk aggravated in areas with heavy wintertime precipitation, e.g., the Pacific Northwest (PNW). We addressed this unique challenge to improve the sustainability of vegetable production systems with three experiments established in British Columbia.

First, we monitored nutrient losses and GHG emissions from chicken, turkey, and horse manures protected or not from wintertime rainfall. Exposure to wintertime rainfall decreased nutrient concentrations substantially (e.g., 90% for potassium) while increasing GHG emissions, especially in poultry manures.

Then, we measured the effects of fall-applied chicken and horse manures on crop productivity, soil fertility, and GHG emissions. Chicken manure increased cover crop biomass, squash yields, and N availability relative to horse manure, but it also increased environmental impacts (e.g., N leaching risk, GHG emissions).

Finally, we compared how relying on different fertility sources affected nutrient cycling, GHG emissions, and crop productivity. Using municipal compost instead of manure or processed organic fertilizers resulted in similar yields of crops with low N requirements, and lower N leaching risks and GHG emissions. However, compost reduced yields of cover crops and crops with high N requirements, and it increased P surpluses.

Overall, our research identified management practices that will help increase the benefits of using composts, manures, and cover crops in organic vegetable farms of the PNW.