Yield benefits and environmental costs of different compost sources on urban garden plots

Background/Question/Methods

Increasing food demand is placing unprecedented stresses on agricultural and natural systems. Approaches are needed to increase production on existing lands while reducing impacts on natural systems. Given the global rise in urbanization, urban agriculture will make important contributions toward these goals. However, research is needed to improve production efficiency in urban environments and to engage citizens in urban agricultural efforts.

This project examined effects of different compost treatments on crop yield, crop nutrition, and nutrient runoff in an urban setting. Research sites were located at two community centers in St. Paul, Minnesota. Each site had 32 4-m² elevated plant beds. We had eight soil treatments: six locally produced composts (1. barley mash/wood chip compost, 2. cow manure compost, 3. yard-waste “leaf” compost, 4. a manure/leaf compost blend, 5. a manure/barley compost blend, and 6. a barley/cow/leaf blend), a standardized amount of 24N-8P-16K synthetic fertilizer, and no-fertilizer. Heirloom and conventional crops were planted in each bed: eggplants, potatoes, tomatoes, bell peppers, jalapeno peppers, and squash were arranged in identical formation for all sixty-four plots. We collected yield data twice per week. We also assessed CN content of selected crops, quantified N and P concentrations in runoff sampled with lysimeters, and assessed soil nutrient availability and microbial diversity.

Results/Conclusions

We found that compost can lead to higher crop yields than synthetic fertilizer, and the yield benefits of compost vary considerably among compost types. Most notably, 1) yields from cow manure and barley compost were ~2x higher than synthetic fertilizer yields, 2) barley compost (which contained no animal waste) had yields that did not differ significantly from cow manure compost, and 3) yard waste compost yielded only ~12% of the most productive composts, but did have ~4x higher yields than the no-fertilizer treatment. High-quality compost may also be associated with greater nutrient run-off. We found some evidence that dissolved nitrogen (N) and phosphorus (P) concentrations in run-off collected in June 2014 were particularly high in the most productive treatments.

Together, these results suggest that compost practices can have large impacts on yields and nutrient pollution associated with urban agriculture. This work will help inform policy decisions aimed at maximizing the benefits of urban agriculture without increasing environmental impacts from urban areas. Moreover, our work was connected to youth and senior education programs at community centers to maximize the social impact of the project.