Steel Slag Affects pH and Si Content of Container Substrates

A substrate representing a typical greenhouse potting mix was prepared using 85% sphagnum peat and 15% perlite.  The substrate was filled into 10 cm wide containers.  A pulverized steel slag (SS) from a basic oxygen furnace, and dolomitic limestone (DL) were amended to the base substrate at a rate of 0, 2.4, 4.8, or 7.2 kg/m³.  Pots were planted with a single sunflower (Helianthus annus ‘Pacino Gold’) plug.  All containers were fertilized at each irrigation event with a complete commercial fertilizer (including micronutrients) at a rate of 100 mg/L N.  Substrate pH was measured weekly using the pour-thru method for 6 weeks.  At the conclusion of the experiment, sunflower shoot dry mass and foliar nutrient composition was determined with a Thermo Iris Intrepid ICP-OES.  Substrate pH response to SS and DL amendments were fit to exponential curves, which differed for the two amendment type.  Exponential response curves for DL increased rapidly then leveled off with a maximum pH of 6.5 with 7.2 kg/m³ DL.  In contrast, pH in SS-amended substrates increased more gradually over the range of applied rates but reached a higher pH of 6.9 with 7.2 kg/m³.   Foliar Si in control sunflowers averaged 684 mg/kg Si, while those amended with DL contained 1180 mg/kg Si with no differences across DL rate.  Sunflower amended with SS contained 8900 to 14,700 mg/kg Si over the range of applied rates.  Steel slag can adjust pH of peat-based substrates, although the response curves differ from dolomitic lime.  Steel slag has the additional benefit of supplying high rates of plant-available Si.