Submitted to: HortScience
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/14/2014
Publication Date: 4/1/2015
Citation: Altland, J.E., Locke, J.C., Zellner, W.L., Boldt, J.K. 2015. Steel slag raises pH of greenhouse substrates. HortScience. 50(4):603-608.
Interpretive Summary: The primary component in greenhouse potting substrates is sphagnum peatmoss. Substrate solution pH of non-amended peatmoss ranges from 4.0 to 4.5. Ideal pH for most greenhouse floriculture crops ranges from 5.8 to 6.2. Dolomitic lime is most often used to elevate substrate pH in peatmoss-based media. Steel slag is a byproduct of the steel industry, recently introduced as a liming agent for agricultural purposes and has potential for use in greenhouse substrates. The objective of this research was to quantify and model the impact of steel slag on pH of a peatmoss-based greenhouse substrates, and compare it to the industry standard dolomitic lime. The ability to raise substrate pH was much greater for steel slag than dolomitic lime. This might have some application for greenhouse or nursery producers where very high pH is desired or there is some need to counteract a very acidic or pH-lowering amendment. Most crops are grown at pH between 5.8 and 6.2, levels that are attainable with dolomitic lime. Furthermore, over-application of steel slag could result in exceedingly high pH whereas even a gross over-application of dolomitic lime would, based on the results of these experiments, result in pH no higher than 6.6. While steel slag offers more immediate and effective liming power when high pH (>7) is desired, dolomitic lime offers an ideal Ca : Mg ratio, more moderate pH increases, and less risk of high pH with accidental over-application.
Technical 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, only fallow pots were 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 with only minor differences in growth and foliar nutrient concentration. 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 to 4.6 mS/cm). Substrate pH differed slightly in Expt. 1 between fallow and planted pots. 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.