Comfrey Mulch Enriches Soil, But Does Not Improve an Indicator Crop within One Season

Comfrey (Symphytum spp.) is thought to accumulate plant nutrients such as potassium (K) in its leaves and is consequently used widely as a green mulch. We sought to investigate the efficacy of comfrey as a soil amendment by measuring its nutritional composition and the effects of mulching with comfrey on soil nutrients, soil microbial communities, and growth and quality of an indicator crop (kale) over one growing season in a small garden plot. We found that comfrey was rich in K and plots mulched with comfrey had higher concentrations of elemental K, as well as higher concentrations of available nitrogen, compared to plots mulched with paper. Diversity and composition of soil bacterial communities was similar between comfrey- and paper-mulched plots, but began to show a trend toward divergence by the end of the growing season. Overall, comfrey mulch did not enhance the yield or nutritional content of the kale, but perhaps could improve crop performance over a longer period of time or in K-limited soils.