Effect of elemental sulfur and nitrogen form on substrate pH and growth of Calibrachoa in growing media containing compost high in carbonate
Calibrachoa 'Superbells Imperial Purple' and 'Superbells Cherry Star' were cultivated in growing medium consisting of 30% compost (16.3% CaCO3 on dry weight basis), 50% coco dust and 20% coco fibre volume-1, without sulfur or amended with 2 and 4 g finely ground elemental sulfur L-1, respectively. For estimating the appropriate amounts of sulfur test series with acidification by sulfuric acid were conducted. In addition to the sulfur treatments of the growing media the source of nitrogen was varied in the fertilization during the cultivation period (100% nitrate N, 50% nitrate N and 50% ammonium N, 100% ammonium N). Compared to the different nitrogen forms the effect of the sulfur treatments on pH was more pronounced. Within three weeks a reduction by 0.5 and 1.0 units could be achieved with 2 and 4 g sulfur L-1, respectively. Caused by the reaction of sulfuric acid with calcium carbonate a considerable rise of water soluble salts in the growing media was observed. In contrast, moderate levels of salt were extracted with a saturated solution of calcium sulfate, confirming the observation, that there were no negative consequences by the sulfur treatments on plant growth. On the other hand, chlorosis caused by iron deficiency and growth impairment could be observed in all treatments without sulfur. The most negative impact resulted after additional N fertilization exclusively in the form of nitrate. The improved availability of phosphorus at lower pH could be noticed in the results of P analysis following CAT extraction. There was a negative correlation between nutrient contents in the growing media at the end of the experiment and plant growth.
Amberger-Ochsenbauer, S., Jauch, M., Schmitz, H.-J. and Meinken, E. (2017). Effect of elemental sulfur and nitrogen form on substrate pH and growth of Calibrachoa in growing media containing compost high in carbonate. Acta Hortic. 1168, 359-364
compost, alkalinity, pH, sulfur, nitrate, ammonium