The 2012 ASHS Annual Conference

Evidence of the potential beneficial effects of supplemental silicon in fertility programs for crop production is plentiful. Recent acceptance of silicon as a fertilizer in organic practices increased the profile of the nutrient, so the potential sources of silicon must be determined. Slag is a granular by-product of iron processing, and depending on the smelting process, can vary greatly in its composition. Within any given smelting process, the resulting slag is predictable and consistent. We sought to determine if different slag types could be used as a supplemental silicon source in containerized crop production, and sought to determine if any heavy metals were also leached from the materials. Extractions (water and/or mild acetic acid solution) were made of slag and slag-sphagnum peat mixtures for predictions of Si availability. Additionally, the ornamental crop zinnia (Zinnia elegans) was grown in containers with slag and compared to zinnia grown in conditions receiving no supplemental Si or supplemental Si from different sources. Silicon availability differed greatly among slag types, with extraction methods not always able to predict silicon bioavailability. There appeared to be little change in silicon supply over time among the slag types. Some slag types caused pH to rise out of recommended range for plant growth, behaving like a liming agent. Perhaps most significantly, we detected no heavy metals in the leaves of zinnia, even after supplying doses of slag that were 30x above rates that would be applied in the field. Overall, some slag types appeared to have promise as a silicon source, but caution must be used to avoid pH rise.