Overcoming physical limitations in alternative growing media with and without peat
Significant efforts are being made to find alternative to peat. In doing so, chemical, biological and physical limitations have appeared for using some alternative components to sphagnum peat in organic growing media. Aeration limitations may be linked to too fine particles limiting air space or too coarse particles limiting gas exchange in growing media, with a working substrates being made of a combination of less than 4 mm material with a coarse fraction between 2-4 mm. High microbial respiration may be a problem solved by increasing air filled porosity, or through an appropriate choice of container geometry. Too high a proportion of coarse fragments may also result in poor capillary rise properties and too low available water, resulting in temporary water stress being experienced by the crop and subsequent yield drops. Increasing the proportion of fines in substrates and the choice of an appropriate container configuration may solve the problem here also. The wettability of alternatives has however to be considered, even if they generally rewet better than peat, because the risk of hydrophobicity varies with material and may evolve during cultivation, with a strong influence on water retention and flow properties. Too low or too high a bulk density at the beginning of the growth has also been observed to reduce rooting into the substrate with poor consequent root and shoot growth, a problem now possibly solved by adjusting the compaction level of the substrate during mechanical potting. Salt levels in some material requires a close follow-up for some waste already high in salts and in material of low biostability. Finally, loss of structural stability has been observed with some composts, which can be compensated by decreasing the proportion of composted material in the substrate. Attempts made to design substrates based on information on particle distribution only has so far reached limited success. Alternatively, characterizing the growing media properties once potted is one additional step proven to be more successful than using particle size distribution information only, because of the important impact that processing, handling and potting as well as container configuration have on physical properties and the subsequent plant growth.
Caron, J. and Michel, J.-C. 2017. Overcoming physical limitations in alternative growing media with and without peat. Acta Hort. (ISHS) 1168:413-422
growing media, aeration, water availability, wettability, particle size, gas diffusivity, sphagnum peat