Storage Time of Pine Wood Chip Aggregates Affects Phytotoxicity and Plant Growth

Chemical composition of organic alternative substrates and substrate components may influence toxic and phytotoxic affects on plant growth during culture. In response to phytotoxicity reports in some substrates and substrate components, investigators have developed multiple evaluation procedures to predict a substrate’s potential for affecting plant growth due to any phototoxic effect. The use of fresh pine wood (as a substrate or substrate component) in horticultural crop production has increased in recent years and some data suggests that when used fresh and at high percentages, pine wood can decrease herbaceous plant growth and inhibit seedling germination. Pine wood chips (PWC) are one such new component being evaluated as a replacement for perlite in greenhouse substrates.  To screen for phytotoxicity, seedling germination tests and plant growth trials were conducted on fresh (pine trees freshly harvested) and aged (pine tree harvested, chipped and stored for five months) PWC. Both fresh and aged PWC were produced by chipping freshly harvested loblolly pine trees (Pinus taeda) and hammer milling the wood chips through a 6.35 mm screen. Fresh and aged PWC were both amended to peat moss at 20 and 30% ratios. An 80:20 peat:perlite control was also used in this study. Separate containers of each substrate were sown with five radish (Raphanus sativus) or cucumber (Cucumis sativus L.) seeds. At 14 d after sowing, germination percentage and germinated seedling dry weights were determined. Additional containers of each of the five substrates were planted with celosia (Celosia plumosa) or impatiens (Impatiens walleriana) plugs and grown for four weeks in a greenhouse.  Radish germination was highest in both the 20% and 30% aged PWC compared to the fresh PWC or perlite seedlings.  Radish dry weights were similar in perlite and aged PWC substrates and lowest in fresh PWC. Cucumber germination was similar in aged and fresh PWC at both percentages and lowest in the perlite control substrate, however dry weights of germinated seedlings was highest in the perlite control. In the growth trials, celosia plants grown in aged PWC at both rates had higher dry weights compared to plants grown in the fresh PWC substrates. Dry weights for impatiens were similar for the control and in both aged and fresh PWC substrates. These results indicate crop variability to fresh PWC phytotoxicity. Overall, the use of aged PWC seems to improve plant growth and therefore is recommended.