Corn Growth as Affected by Suspension Fertilizers Containing Fluorosilicic Acid

Fluorosilicic acid (H₂SiF₆) is a suggested additive to suspension fertilizers produced from monoammonium phosphate (MAP) to prevent gelling of the suspension. A greenhouse study was conducted to determine if F added to soil via P suspension fertilizers was detrimental to corn (Zea mays L.) growth. The F sources consisted of reagent-grade NH₄F, NH₄SiF₆, and three P suspension fertilizers. Equilibria constants and measurements with a F⁻ ion selective electrode (ISE) indicated SiF₆²⁻ dissociates to H₄SiO₄ and F⁻ at pH > 4 and total F concentration <1 mM. Therefore, only F⁻ needed to be considered as a potential toxic component in soils. At an initial soil pH of 5.8, an 11% dry-matter weight decrease was observed when a suspension fertilizer with 89 g F kg⁻¹ was added to soil to supply a rate of 170 mg F kg⁻¹ soil. An adequate level of F in P suspension fertilizers is between 16 and 23 g kg⁻¹ and this level did not affect corn dry-matter weight with application rates ≤ 32 mg F kg⁻¹ soil. At high soil F-application rates from 30 to 170 mg F kg⁻¹ soil, F was not suspected to be the phytotoxic agent since F concentrations in corn tops were all <0.4 mg kg⁻¹. There was a significant correlation between soil Al and F extracted with water, which led to a hypothesis that decreased corn growth was due to Al phytotoxicity.