Why does ferrous sulfate fail as a soil acidifier and iron supplement for potted plants?

Why does ferrous sulfate fail as a soil acidifier and iron supplement for potted plants?
The core issue is chemical instability. Ferrous sulfate oxidizes rapidly in air, under strong light, and in alkaline water or soil, converting from plant-available ferrous iron (Fe2+) to ferric iron (Fe3+), which plants cannot absorb. Many gardeners apply it incorrectly by dissolving crystals in water and immediately watering plants, but the solution often turns rusty yellow before uptake, leaving useless ferric deposits in the soil. Over time, these accumulations can coat root surfaces, block nutrient absorption, and even cause root burn. The product also has limited acidifying power when used alone. For acid-loving plants like gardenias, azaleas, and camellias, the recommended approach is preparing a fermented fertilizer liquid, mixing water, oilseed cake, and ferrous sulfate, then allowing several months of anaerobic digestion before use.