Soilless substrates used in U.S. floriculture are comprised primarily of sphagnum peatmoss amended with perlite, vermiculite, pumice, and to a lesser extent sand, compost, and other components. Sphagnum peatmoss does not contain sufficient micronutrients (B, Fe, Mn, Cu, Mo, Zn) to support vigorous crop growth. Plants grown in peatmoss without added micronutrients have been shown to be deficient in Cu (Adams et al., 1986), B and Fe (Smilde, 1975), Zn (Penningsfeld, 1972), and Mo (Cox, 1988). These and other papers have shown that micronutrient deficiencies are often an interacting function of micronutrient application, lime application, and substrate pH (Smith et al., 2004).
Micronutrients are primarily supplied to containerized floriculture crops by one of two methods. Many producers use a water-soluble fertilizer that provides macronutrients (N, P, and K) as well as all secondary nutrients (Ca, Mg, and S) and micronutrients. This fertilization method provides low concentrations of water-soluble micronutrient salts or chelates with each irrigation event. Depending on the irrigation system, micronutrients in these water-soluble fertilizers could be applied to the substrate (via drip irrigation) or to the foliage and substrate (via overhead irrigation booms). Micronutrients can also be incorporated into the substrate using a granular formulation. These products include varying concentrations of each micronutrient, usually in a water-soluble salt or chelate, but sometimes as finely ground minerals with limited solubility that presumably render the micronutrients available slowly over time. Frost et al. (2003) showed that either water-soluble or granular incorporated fertilizers are effective in providing micronutrients; however, geranium (Pelargonium ×hortorum ‘Orbit Red’) relative growth rate was higher with water-soluble micronutrients in the irrigation stream.
Steel slag is a by-product of the steel industry with high mineral nutrient content. As steel scraps and iron ore are melted in a basic oxygen furnace, calcium oxide (CaO) and dolomitic lime are introduced as fluxing agents to remove impurities from the molten steel. Mineral impurities removed by the fluxing agents, along with the CaO and dolomitic lime, form a molten slag. The slag is poured off from the steel, cooled, and processed into particle size fractions ranging from dust to gravel. Steel slag has been shown to be an effective liming agent for soilless substrates in container culture (Altland et al., 2015) as well as field soils (Ali and Shahram, 2007; Rodriguez et al., 1994).
The impurities removed from the molten steel by the fluxing agent include varying concentrations of elements considered to be plant micronutrients. Properties of steel slag and the elemental content of the impurities vary by the type of furnace in which steel is produced (Yildirim and Prezzi, 2011). Despite differences, most steel slags are similar in that they are composed primarily of CaO, SiO2, and FeO, with CaO making up more than 35% of steel slag mass (Yildirim and Prezzi, 2011). Although micronutrients are present in steel slag, the mineral form of the micronutrients may or may not render them available for plant uptake. Furthermore, the high CaO content of the steel slag causes a rapid increase in substrate pH (Altland et al., 2015), which could render many of the micronutrients less available for plant uptake (Peterson, 1980). Therefore, the objective of this research was to determine if steel slag could be used as the sole micronutrient source for container-grown floriculture crops in a peatmoss-based substrate.
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