Steel Slag Raises pH of Greenhouse Substrates

Abstract

Dolomitic lime (DL) is the primary liming agent used for increasing pH in peatmoss-based substrates. Steel slag (SS) is a byproduct of the steel manufacturing industry that has been used to elevate field soil pH. The objective of this research was to determine the pH response of a peatmoss-based greenhouse substrate to varying rates of DL or SS. Two experiments were conducted with an 85 peatmoss : 15 perlite substrate. In the first experiment, the substrate was amended with 0, 2.4, 4.8, or 7.1 kg·m−3 of either DL or SS. Half of the containers remained fallow and the other half were potted with a single sunflower (Helianthus annuus L. ‘Pacino Gold’). In the second experiment, fallow containers were only used with the substrate amended with 0, 2.4, 4.8, 9.5, or 14.2 kg·m−3 DL or SS. Sunflower were measured for relative foliar chlorophyll content, shoot mass, root ratings, and foliar nutrient concentrations. Substrate electrical conductivity (EC) and pH were measured weekly using the pour-through procedure. All sunflower plants grew vigorously, although nonamended controls had less shoot dry weight than those amended with DL or SS. There were minor differences in foliar concentration of N, Ca, Mg, and Mn; however, these differences did not adversely affect plant growth. Summarizing across both experiments, EC was affected by treatment and time, although all substrates had EC readings within the range recommended for floriculture crop production (1.0–4.6 mS⋅cm−1). Substrate pH differed slightly in Expt. 1 between fallow and planted containers. Substrate pH increased exponentially with increasing rates of either DL or SS. Maximum pH in fallow DL and SS amended substrates was 6.57 and 6.93, respectively, in Expt. 1 and 6.85 and 7.67, respectively, in Expt. 2. The SS used in this experiment resulted in a greater pH response than DL with higher application rates. SS is a viable material for raising pH of soilless substrates.