Growth of Four Native Plant Species in Potting Mixes Amended with Anaerobically Digested Cattle Manure

Anaerobically digested cattle manure is a by-product of cattle waste processing for pollution control and biogas production, yet this material has the potential to be an economical component in potting mixes used for sustainable container production in nurseries.  The objective of this study was to determine the effects of anaerobically digested cattle manure on the growth of four native plants species grown in potting mixes amended with this nutrient rich material.  Five potting mixes containing 0%, 15%, 30%, 45% or 60% composted manure (by volume), 10% sand, and 90%, 75%, 60%, 45%, and 30% aged pine bark, respectively, were used in this study.  Anaerobically digested manure was composted for 3 months before it was used in the potting mixes.  The native perennials Mimulus guttatus, Holodiscus discolor, Philadelphus lewisii, and Penstemon strictus were grown in 3-liter containers of these mixes for four months.  Plant heights were measured every three weeks.  At the end of the experiment, plant shoots were harvested for biomass determinations.  At the beginning of the experiment, some leaves on Holodiscus and Philadelphus plants grown in 30% or more manure-amended media became chlorotic, and leaf margins became necrotic on some Holodiscus plants in the 60% manure medium.  Three Holodiscus plants with these symptoms died during the study.  All four species grew as well or better in media amended with up to 30% manure compared to plants grown in the bark and sand (control) mix.  By the end of the study, Penstemon plants grown in 60% digested manure grew 17% taller and produced 62% more shoot biomass than plants in the control mix.  Mimulus plants grown in 30% manure produce about 47% more shoot biomass than control plants, whereas Holodiscus plants grown in 60% manure produce at least 18% more shoot biomass than control plants.  Shoot biomass production by Philadelphus plants was unaffected by the potting mixes.  Final shoot heights for Mimulus, Holodiscus, and Philadelphus plants were similar for all plants in all of the potting mixes.  This study demonstrated that anaerobically digested cattle manure can be a stable amendment for potting mixes and can improve a bark-based medium when incorporated as 30% of the potting mix volume.