Changes in Chemical and Physical Properties of Soilless Potting Mixes Amended with Composted Dairy Manure of Varying Maturities

Horticultural grade bark and peat moss traditionally comprise the primary components of soilless potting mixes used for nursery stock production, but due to their increased costs and questions of renewability, growers have shown interest in using composted dairy manure as a medium component.  To address concerns about its suitability, dairy manure was collected during 9 months of composting, with samples taken at 10 days (0), 3, 6 and 9 months. Soilless potting mixes consisting of bark (90, 60, or 30% softwood bark by volume) and sand (10% for each mix) were amended with 0%, 30% or 60% compost, respectively, and these three media were made for each age of composted manure.  Initial physical and chemical properties of these different potting mixes were determined.  Initial air capacity of the mixes was unaffected by the amount of compost in the mix, regardless of the compost maturity.  In contrast, initial water-holding capacity and total porosity were significantly affected by the amount of compost added, but remained within or very close to the acceptable ranges for a growth medium. Initial available K, P, B, extractable Cl^-, NO₃^--N, and SO₄^--S increased from 1.2 fold to 43 fold, depending on the nutrient and age of the compost.  Initial electrical conductivity (EC) of the mixes increased from ~ 2.2 dS∙m^-1 in control mixes to ~ 25 dS∙m^-1 in the 60% compost amended mixes for the 3, 6 and 9 month composts.  Initial pH of the mixes containing 60% compost also averaged ~ 8.1 for these three ages of compost.  In contrast, initial C:N ratios of control mixes was ~71, whereas the mean initial C:N ratios of 60% compost media made with 3, 6, or 9 month composts was ~ 17.6.  These results indicated that dairy manure composted 3, 6, or 9 months can have minimal effects on potting mix physical properties but can drastically increase mineral levels and alter salt and pH properties of the mixes, depending on the amount of compost added to the mix.