Pine wood chips as an alternative to perlite in greenhouse substrates: Nitrogen requirements

Processed pine wood (Pinus sp.) has been investigated as a component in greenhouse and nursery substrates for many years. Specifically, pine wood chips (PWC) have been uniquely engineered/processed into a nonfiberous blockular particle size, suitable for use as a substrate aggregate. In container substrates, nitrogen (N) tie-up during crop production is of concern when substrates contain components with high carbon (C):N ratios, like that of PWC that are made from fresh pine wood. The objective of this research was to compare the N requirements of plants grown in sphagnum peat–based substrates amended with perlite or PWC. Fertility concentrations of 100, 200, or 300 mg•L⁻¹ N were applied to ‘Profusion Orange’ zinnia (Zinnia ×hybrida) and ‘Moonsong Deep Orange’ marigold (Tagetes erecta) grown in sphagnum peat–based substrates containing 10%, 20%, or 30% (by volume) perlite or PWC. Zinnia plant substrate solution electrical conductivity (EC) was not influenced by percentage of perlite or PWC. Perlite-amended substrates fertilized with 200 mg•L⁻¹ N for growing zinnia, maintained a constant EC within optimal levels of 1.0 to 2.6 mS•cm⁻¹ from 14 to 42 days after planting (DAP), and then EC increased at 49 DAP. In substrates fertilized with 100 and 300 mg•L⁻¹ N, EC levels steadily declined and then increased, respectively. Zinnia plants grown in PWC-amended substrates fertilized with 200 mg•L⁻¹ N maintained a constant EC within the optimal range from 14 to 49 DAP. Marigold substrate solution EC was only influenced by N concentration and followed a similar response to zinnia substrate solution EC. Zinnia and marigold substrate solution pH was influenced by N concentration and generally decreased with increasing N concentration. Plant growth and shoot dry weight were similar when fertilized with 100 and 200 mg•L⁻¹ N. According to this study, plants grown in PWC-amended substrates fertilized with 100 to 200 mg•L⁻¹ N can maintain adequate substrate solution pH and EC levels and sustain plant growth with no additional N supplements. Pine wood chips are engineered and processed to specific sizes and shapes to be functional as aggregates in a container substrate. Not all wood components are designed or capable of improving/influencing the physical and chemical behavior of a substrate the same. On the basis of the variability of many wood components being developed and researched, it is suggested that any and all substrate wood components not be considered the same and be tested/trialed before large-scale use.