ARS | Publication request: The Effect of Nitrogen Form on pH and Petunia Growth in a WholeTree Substrate

Submitted to: Southern Nursery Association Research Conference
Publication Type: Proceedings
Publication Acceptance Date: January 16, 2009
Publication Date: June 9, 2009
Citation: Witcher, A.L., Fain, G.B., Blythe, E.K., Spiers, J.M. 2009. The Effect of Nitrogen Form on pH and Petunia Growth in a WholeTree Substrate. Southern Nursery Association Research Conference vol.54:428-433.

Technical Abstract: The objective of our research was to investigate the effect of nitrogen form and proportion on peat-lite (PL) and WholeTree (WT) substrate pH and petunia growth. Chipped whole pine trees (consisting of needles, limbs, bark, wood and cones) were obtained from a commercial fuel wood chipping operation in western Georgia. The WT chips were further processed with a swinging hammer mill to pass a 3/16-in screen. The WT substrate was compared to a PL substrate (8:1:1 peat moss:perlite:vermiculite). Each substrate was amended (per cubic yard) with 5 lbs dolomitic limestone, 1 lbs gypsum, and 1.5 lbs Micromax. On October 11, 2007, 5.5-in containers were filled with substrate and each planted with two petunia plugs (from 288-cell flats), placed on elevated benches and hand irrigated as needed with a nutrient solution. Nutrient solutions contained nitrogen (N), phosphorus (P) and potassium (K) at 300, 150 and 300 ppm, respectively. Five N treatments were supplied as different proportions of ammonium (NH4+N) and nitrate (NO3–N). The N treatments were 100% NH4+N (100NH4), 75% NH4+N:25% NO3–N (75NH4:25NO3), 50% NH4+N:50% NO3–N (50NH4:50NO3), 25% NH4+N:75% NO3–N (25NH4:75NO3) and 100% NO3–N (100NO3). At 36 days after planting, 100NO3 treatments had the highest pH, while 100NH4 treatments had the lowest pH, regardless of substrate. Substrate pH decreased as NH4+N proportion increased, while an increasing NO3–N proportion resulted in a rise in substrate pH. A wider range of WT substrate pH, in comparison with the PL substrate, occurred and is most likely due to a lower buffering capacity. Optimum plant growth occurred in the mixed N-form treatments for each substrate. Within N-form treatments, WT substrates produced lower SDW than PL substrates. Lower SDW was expected in the WT substrate, due to higher fertilizer rates required to produce comparable plants.