| Summary: | Low-temperature (350 °C) vitrification in a KNO<sub>3</sub>-NaNO<sub>3</sub>-KHSO<sub>4</sub>-NH<sub>4</sub>H<sub>2</sub>PO<sub>4</sub> system, containing various additives to improve the chemical durability of the obtained material, was investigated. It was shown that a glass-forming system with 4.2–8.4 wt.% Al nitrate admixtures could form stable and transparent glasses, whereas the addition of H<sub>3</sub>BO<sub>3</sub> produced a glass–matrix composite containing BPO<sub>4</sub> crystalline inclusions. Mg nitrate admixtures inhibited the vitrification process and only allowed obtaining glass–matrix composites with combinations with Al nitrate and boric acid. Using ICP and low-energy EDS point analyses, it was recognized that all the obtained materials contained nitrate ions in their structure. Various combinations of the abovementioned additives favored liquid phase immiscibility and crystallization of BPO<sub>4</sub>, KMgH(PO<sub>3</sub>)<sub>3</sub>, with some unidentified crystalline phases in the melt. The mechanism of the vitrification processes taking place in the investigated systems, as well as the water resistance of the obtained materials, was analyzed. It was shown that the glass–matrix composites based on the (K,Na)NO<sub>3</sub>-KHSO<sub>4</sub>-P<sub>2</sub>O<sub>5</sub> glass-forming system, containing Al and Mg nitrates and B<sub>2</sub>O<sub>3</sub> additives, had increased water resistance, in comparison with the parent glass composition, and could be used as controlled-release fertilizers containing the main useful nutrients (K, P, N, Na, S, B, and Mg).
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