AERATION IN GROWING MEDIA CONTAINING LARGE PARTICLE SIZE

Large particle sizes are often added to growing media for improving their aeration properties (storage and gas exchange). This is most often sound but recent researches have shown that when those large fragments are very coarse, platy or disk shaped and gas impermeable (like coarse bark), they may create barriers that restrict gas diffusion even if they improve air storage.
An experiment was carried out to check whether conclusions previously obtained with coarse bark (impermeable) could be extrapolated to coarse peat.
Several mixes were made of a 1:1 fine peat (< 10 mm): coarse peat milled and sieved to obtain 1-2, 2-4, 4-6, 6-10 or 10-20 mm particles.
Two additional mixes were made of a 2:1 fine peat (< 10 mm): coarse 10-20 mm bark milled and sieved to obtain 2-4 or 10-20 mm particles.
Once limed and fertilised, these substrates were used to grow Euphorbia pulcherima (Poinsettia) for 17 weeks in a greenhouse fall production.
Aeration properties of those mixes were measured directly in the pots 5 and 17 weeks after potting.
Results showed that air filled porosity remains relatively unaffected by fragment sizes but for the coarse bark size which doubled the air filled pore space.
Gas relative diffusivity was shown to reach maximum value with the 2-4 mm sizes and to rapidly diminish as fragment sizes were increased from 4 to 20 mm or decreased to 1-2 mm.
Bark based mixes showed the same trend, having clearly lower diffusivity than peat based mixes, and diffusivity being the highest for the 2-4 mm.
Root growth parameters were significantly and positively correlated to gas relative diffusivity but showed no correlation with air filled porosity.

International Symposium on Growing Media and Hydroponics

J. Caron, PH. Morel, L.M. Rivière

air content, air-filled porosity, gas relative diffusivity, gas diffusion, peat substrates, bark, Euphorbia pulcherima

https://doi.org/10.17660/ActaHortic.2001.548.25