Effect of Microbial Inoculants on Peat-based Substrate Microbial Activity and Bedding Impatiens Growth with Conventional or Organic Slow-release Fertilizers

As the number and availability of microbial inoculants for use in soilless production systems increases, their effects on substrate microbial activity and plant growth need to be evaluated. In a greenhouse experiment, Impatiens walleriana ‘Super Elfin XP White’ plugs were grown in a peat-based substrate with 13 treatments consisting of variations of three microbial status (autoclaved peat, no innoculant, and two rates of SubCulture^TM with bacterial and/or mycorrhizal inoculants) and three pre-plant fertilizers (conventional Osmocote^TM, and organic feathermeal or blood meal).  Substrate CO₂ respiration, pH and electrical conductivity were measured throughout the production cycle, and impatiens growth was evaluated at mid- and end-crop.  Organic blood meal treatments resulted in larger plants than feathermeal or Osmocote. Use of microbial inoculants did not result in more plant growth when used in conjunction with bloodmeal or Osmocote, but contributed to increased plant dry weight within feather meal treatments when a 5X rate of inoculants was used (p=0.026). Autoclaving peat generally resulted in decreased plant dry weight compared to non-autoclaved treatments. Seven and 14 days after transplant (DAT), substrate microbial CO₂ respiration was greater with the two organic fertilizer sources, regardless of the presence of inoculants. By 21 DAT, microbial CO₂ respiration was similar across treatments and increased dramatically at days 30 to 50 DAT.  Presence of inoculants did not significantly increase microbial CO₂ respiration compared to uninoculated treatments.  These results provide information about the function of microbial inoculants in soilless production systems.