ARS | Publication request: Substrate Acidification by Geranium (Pelargonium x Hortorum) I: Temperature Effects

Submitted to: Journal of the American Society for Horticultural Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 2, 2008
Publication Date: October 1, 2008
Repository URL: http://hdl.handle.net/10113/19239
Citation: Taylor, M., Nelson, P., Frantz, J. 2008. Substrate Acidification by Geranium (Pelargonium x Hortorum) I: Temperature Effects. Journal of the American Society for Horticultural Science. 133:508-514.

Interpretive Summary: Some crops cause a pH decline over a period of one or two weeks and the cause of this decline is not known. Geranium is one such crop. The geranium cultivar ‘Designer Dark Red’ was grown in three experiments to test the effects of temperature on pH decline. The first experiment tested the effect of four temperatures (14/10, 18/14, 22/18 and 26/22 ºC day/night) on pH drop. In this experiment, substrate pH declined from 6.8 to 4.6 as temperature increased. There was also a tendency of the tissue phosphorus concentration to be low (0. 10-0.14% of dry weight vs 0.2% or higer for “healthy” plants) in those low-pH plants. It was not possible to determine if the drop in substrate pH caused the decreased P or of it was a result of low P supply. To resolve this, a second experiment was designed to investigate multiple P supply rates with the three highest temperatures used in the previous experiment. Substrate pH still declined with increasing temperature and the decrease was slightly greater in the low-P-supplied treatments. This indicates that temperature acted independently of P supply (and tissue P concentration) and caused geraniums to acidify the substrate. In the third experiment the amount of acidity produced by roots of plants grown at the two highest temperatures was quantified. Plants grown at the highest temperature had a 28% increase in the amount of acidity produced when standardized for similar amount of root size. Therefore, it is clear from this study that high temperature can cause Sudden pH decline (SPD) by geranium.

Technical Abstract: Sudden pH decline (SPD) describes the situation where crops growing at an appropriate pH, suddenly (1-2 weeks) cause the substrate pH to shift downward one to two units. ‘Designer Dark Red’ Geraniums were grown in three experiments to test the effects of temperature on SPD. The first experiment tested the effect of four temperatures (14/10, 18/14, 22/18 and 26/22 ºC day/night) on substrate acidification. At 63 d, substrate pH declined from 6.8 to 4.6 as temperature increased. Tissue P of plants grown at the highest 3 temperatures was extremely low (0.10-0.14% of dry weight), and P stress has been reported to causes acidification. It was not possible to determine if the drop in substrate pH was a singular temperature effect or a combination of temperature and P. To resolve this, a second experiment with factorial combinations of the three highest temperatures from experiment one and 5 pre-plant P rates (0, 0.065, 0.13, 0.26, or 0.52 g•L-1) was conducted. Regardless of tissue P concentrations, that ranged from very deficient to above adequate, substrate pH decreased with increasing temperature. At 63 days after transplanting (DAT) in the 0.065 and 0.13 P treatments, tissue P was deficient and pH decreased with increasing temperature from 5.6 to 4.7 and 5.9 to 4.7, respectively. In the 0.26 P treatment, tissue P was just at adequate at low temperature and there was no acidification. At the medium and high temperatures, tissue P was deficient and statistically similar, yet pH decreased to 5.2 and 4.7, respectively. In the highest P treatment, all tissue P levels were not statistically different, above adequate, and pH declined with each increase in temperature from 6.5 to 5.0. The results at 63 DAT, once more show that temperature acted independent of tissue P and caused geraniums to acidify the substrate. In the third experiment the amount of acidity produced by roots of plants grown at the two highest temperatures used in the first two experiments was quantified. Plants grown at the higher temperature had a 28% increase in the meq of acidity produced per gram root. It is clear from this study that high temperature can cause SPD by geranium.