ARS | Publication request: Nitrogen form affects pH and EC of whole pine tree substrate and growth of petunia

Submitted to: Journal of Environmental Horticulture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 15, 2011
Publication Date: December 12, 2011
Citation: Witcher, A.L., Fain, G.B., Blythe, E.K., Pounders Jr, C.T. 2011. Nitrogen form affects pH and EC of whole pine tree substrate and growth of petunia. Journal of Environmental Horticulture. 29:213-219.

Interpretive Summary: Substrates containing non-composted wood-based materials have been commercially available in Europe for many years, while whole pine tree substrates have been recognized as a possible peat-alternative in the United States. Some crops grown in wood-based substrates require supplemental fertilizer for optimum growth compared with plants grown in a peat-lite (PL) or pine bark substrate, yet increased fertilizer application rates and nitrogen form can affect substrate pH over time and subsequently affect plant growth. Two experiments were conducted to compare petunia plant growth in a whole pine tree (WPT) substrate and a PL substrate of 8 sphagnum peatmoss : 1 perlite : 1 vermiculite (by volume). Five nitrogen treatments were supplied as different proportions of ammonium (NH4+ N) and nitrate (NO3- N). The pH range of WPT substrate widened considerably over time among the N treatments, while a change in substrate pH was minimal for PL substrate during the same period. Greater shoot dry weight was obtained in PL substrates compared with WPT substrates. Greater substrate air space and total porosity was associated with WPT substrates compared with PL substrates, the latter having greater container capacity. We demonstrated petunias could be produced in WPT using NH4+ N and NO3- N at various proportions, although the mixed proportions resulted in the most commercially acceptable plants. Although WPT substrates are not widely available in the US, the cost of such substrates is estimated to be roughly 50% less than PL substrates. Increased interest in alternative substrates, specifically wood-based materials, from industry professionals will be required to encourage commercial production of such materials.

Technical Abstract: Wood-based substrates are potential alternatives or amendments to traditional peat-based and pine bark substrates. Undesirable changes in substrate pH may result from the application of supplemental fertilizer required by some crops grown in wood-based substrates. Experiments were conducted to evaluate petunia growth and substrate pH in response to nitrogen (N) treatments applied as a nutrient solution to whole pine tree (WPT) and peat-lite (PL) substrates. Nitrogen treatments were applied as 100% ammonium (NH4+ N), 100% nitrate (NO3- N), or a combination of both in various proportions. The pH range of WPT substrate widened considerably over time among the N treatments, while a change in substrate pH was minimal for PL substrate during the same period. Generally, 100% NO3- N and 100% NH4+ N resulted in the highest and lowest substrate pH, respectively, regardless of substrate. Greater shoot dry weight was obtained in PL substrates compared with WPT substrates. Maximum shoot dry weight and flower count with ‘Celebrity Rose’ petunia were obtained with the mixed N-form treatments in both substrates. Greater substrate air space and total porosity was associated with WPT substrates compared with PL substrates, the latter having greater container capacity.