Synthesis and Optimization of Fly Ash-Derived Silicon Fertilizer and Its Effects on Ryegrass Growth

Stockpiling coal fly ash occupies extensive land and creates environmental management challenges, motivating the use of fly ash as a resource. This study examined the preparation of a fly ash-derived silicon fertilizer and its short-term agronomic effects. Three process factors affecting available SiO2 were examined: fly ash-to-additive mass ratio, calcination temperature, and calcination time. Regression analysis showed that calcination time contributed most strongly, followed by the fly ash-to-additive ratio and calcination temperature. The selected conditions were a fly ash-to-mixed additive mass ratio of 1:1, a Na2CO3:Li2CO3 internal mass ratio of 1:0.14, at 800 °C, with a reaction time of 39 min, under which the experimentally verified available SiO2 content was 21.66 ± 0.23% (mean ± SD, n = 3). In the 30-day perennial ryegrass pot experiment, the treatment supplying 69.3 mg available SiO2 kg−1 soil produced the strongest growth response among the tested prepared-fertilizer rates. Under the tested conditions, the prepared fertilizer did not produce a statistically detectable treatment-related increase in total soil Cr, Hg, Cd, or Pb relative to CK. Because the initial soil Cd and Pb concentrations exceeded the cited GB 15618–2018 risk-screening values, these results do not demonstrate absolute regulatory compliance. Metal leachability, chemical speciation, and plant heavy metal uptake were not determined; the environmental assessment is therefore limited to the total soil concentrations of these four elements in this short-term pot experiment.