Physical, Chemical and Biological Properties of some Alternative Growing Substrates.

Peat represented the predominant constituent of growing media for soilless production in the last decades. However, due to the high cost of extraction and future availability and sustainability, a worldwide search for alternative substrates has been developed, focusing on secondary processing compounds or recycling wastes. In order to evaluate the suitability of new materials, organic by-products, namely hemp fiber (2 particle sizes) and grape marc were studied and compared with peat and some rising substitutes: Coir (3% fiber) and green compost. Methodology: Some of the main physical (particle size distribution, bulk density, water holding capacity and air capacity), chemical (pH, electrical conductivity, ash, carbon, hydrogen, nitrogen and heavy metals content) and biological properties (phytotoxicity) were analyzed. Results: All materials showed water holding capacity >40% (threshold considered sufficient on the base of peat-related classification), except for coir and hemp with large particle size and marc that registered a poor water-holding capacity (<40% retained water). The pH of the base materials was different with values ranging between 6.21 and 9.05. Compost and marc showed high values of electrical conductivity and copper content (and heavy metals for green compost) and a possible negative influence on germination of cress as resulting from the bioassay. Conclusion: Physical, chemical and biological properties of the materials suggested that coir has the potential to partially or totally replace peat, whereas marc and compost require to be mixed with other materials or properly select the source-of-origin materials before processing.