Investigating the effect and efficiency of steel slag as a source of silicon for plant uptake.
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Silicon (Si) is the second most abundant element on the planet, after oxygen, making about 25% of the Earth's crust. Since it exists in the Earth’s crust, many plants can accumulate it in large concentrations, in amounts similar to macronutrients. Si enhances growth and yield of some annual and vegetable crops, promotes upright growth (stronger and thicker stems, shorter internodes), prevents lodging, promotes favourable exposure of leaves to light, provides resistance to bacterial and fungal diseases and decreases the effects from abiotic stresses such as: high or low temperatures, salinity, heavy metal and aluminium toxicity and water deficiency. Si treatment effects on plant growth under disease and drought stress were analysed to determine the effectiveness of steel slag as a source of Si for plant uptake. Four Si treatments: slag products (SP1.7 and SP5.0); Agri-sil granular (AGS); and potassium silicate (Pots) were tested on rye grass, maize, zucchini, green pepper, broccoli and beans under disease and drought stress. Energy Dispersive X-ray (EDX) microanalysis was performed to determine the Si content within the leaves resulting from the different treatments over time. All treatments provided a positive Si uptake into the plant leaves. Maize had the highest rate of Si levels taken up into the leaves over a period of 180 days for all Si treatments, when compared to the other crops under disease stress. Si-treated plants accumulated Si into their leaves at a higher rate under disease stress than drought stress. Si treatments improved the growth of all test crops. Steel slag was an effective treatment for providing Si for the uptake in plants and to improve plant growth. The effect of pre-harvest Si application to inhibit Colletotrichum capsici on post-harvest pepper fruit (Capsicum annuum L.) was analysed. Pepper fruit were harvested from pepper plants (Capsicum annum L. cv. Revelation) that were Si treated to provide Si for plant uptake. Si treatments used were: Pots, which was used as a positive control; SP1.7; SP5.0; and AGS. The area covered by infection (%) on the fruit was recorded every seven days for a period of 21 days, to determine the disease progress. All Si treatments significantly reduced the rate of infection by the pathogen. By day seven, the disease progress was inhibited, with a recorded area of infection being below 3.5% compared to the control, which was at 8.4%. By day 14, it was inhibited from 33.6% (control) to below 16% and by day 21, it was inhibited from 57.4% (control) to below 31% for all Si treatments. Area under the disease-progress curve (AUDPC) value (%days) was the lowest for the SP1.7 treatment, which means it enhanced post-harvest disease resistance by the greatest amount. SP5.0 had the highest AUDPC value from all Si treatments. Pre-harvest application of Si reduced post-harvest anthracnose disease in green pepper fruit. The efficiency of steel slag as a source of Si for citrus and avocado uptake was analysed. Three different species: Citrus sinensis (Orange cultivars: Valencia and Navel); Citrus limon L. (lemon); and Persea Americana L. (avocado) were used for this study. Five Si treatments were tested: Pots; AGS; Agri-sil liquid (ASL); SP5.0; and SP1.7. EDX was performed to determine the Si content within the leaves resulting from the different treatments over time. All Si treatments provided a positive uptake of Si into the citrus leaves. Valencia trees treated with the SP1.7 had the highest rate of Si taken up into the leaves, with an area under curve (AUC) value of 210.24%days, followed by SP5.0 with an AUC value of 195.48. SP1.7 and AGS provided the highest rates of Si uptake into navel orange leaves with AUC values of 187.02 and 187.92, respectively. Lemon trees treated with SP1.7 and AGS had the highest rates of Si taken up into the leaves. Citrus trees treated with SP1.7 had higher rates of Si taken up into the tree leaves, with the exception of the AGS treatment having the highest rate of Si taken up in lemon. Avocado trees treated with SP1.7 had the highest rate of Si taken up into the leaves, with an AUC value of 28.29. Steel slag was an efficient and effective source of Si for the uptake in citrus and avocado leaves.