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American Society for Horticultural Science


Denver, CO

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Consistent delivery of up to 1 mM silicon (Si) to the root-zone is beneficial for plant health, but the maximum solubility of silicon is only 1.7 mM between pH 4 and 8. This makes Si difficult to concentrate and add through a proportioner in liquid fertilizer systems. The solubility of Si begins to increase with increasing pH above 9. We added potassium silicate (K2SiO3) and fumed silica (SiO2) to RO water that was pH adjusted to pH 5, 8, 9, 10, or 11 with either KOH or NaOH. Samples were filtered and dried to obtain the mass of undissolved solids in each treatment. The measured optical turbidity decreased with increasing pH and reached zero at pH 11. We found that this pH is necessary to maintain solubility in a concentrate tank. Below this pH, the concentrated silicon was not completely soluble and the precipitate damaged the injection system of proportioners. Moreover, increasing the pH prior to adding the potassium or fumed silica was necessary to achieve Si solubility. Increasing the pH after adding Si did not result in complete dissolution. The calcium in tap water promoted precipitation at all pH levels and it was necessary to use RO water. KOH and NaOH were equally effective in increasing solubility. By increasing pH of the concentrate tank to 11 prior to adding the Si we maximized solubility and minimized damage to proportioners from precipitate accumulation. The tank concentrate is optically clear with 15 mM Si, or 1700 mg per liter of K2SiO3. With 1 to 50 dilution from proportioners we now add 0.3 mM Si at each watering in our liquid fertilizer system.

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