Aeration with Large Bark and Peat Particles in Growing Media: Methodological Issues.

Aeration limitations in growing media have been identified in several studies and methods based on an assessment of air filled porosity have been proposed and standardized. Additionally, a simplified method for estimating gas diffusivity has been shown to provide additional relevant information for predicting plant performances but has never been tested and compared to other approaches. This study uses results obtained with different peat and bark substrates of different sizes, in the 1-2 to 10-20 mm diameter, presenting limitations for Poinsettia and Impatiens growth and compares the performances of the two indices (air filled porosity and gas relative diffusivity) for assessing plant performances in a greenhouse experiment using a standardized or a simplified methodological approach. The aeration properties of the mixes were measured prior to potting and directly in the pots, both at the beginning and at the end of the experiment. The results show that air-filled porosity remained relatively unaffected by substrate treatments while gas relative diffusivity differed significantly between treatments at the beginning and at the end of the experiment. Root and shoot growth parameters were significantly and positively correlated to gas relative diffusivity and negatively correlated to the pore effectiveness coefficient. The growth reduction observed in the bark/peat mixes relative to pure peat was most likely linked to limited gas exchange. Air-filled porosity assessments performed in situ (in the pot itself) or prior to potting, in a cylinder, gave inconsistent results or were not significantly correlated to plant growth.