Efficiency of Vivianite from Water Purification Depending on Its Mixing with Superphosphate and Application Method

Vivianite precipitation is gaining attention in phosphorus (P) removal from water purification. It is an iron (Fe)- and P-rich compound that can be used as a slow-release P fertilizer. However, this slow release can constrain P supply to crops in the initial growing stages. This limitation can be overcome by mixing with soluble P fertilizers and with banding application. Thus, the objective of this study was to assess the fertilizer effect of vivianite and superphosphate mixtures and determine the most effective application method for vivianite and its mixture with superphosphate as a soluble fertilizer. A pot experiment was conducted by growing sunflowers in calcareous soil under controlled conditions involving two factors. The first factor was the combinations of vivianite and superphosphate: 100% Vivianite + 0% Superphosphate –T2–, 70% Vivianite + 30% Superphosphate –T3–, 30% Vivianite + 70% Superphosphate –T4–, 0% Vivianite + 100% Superphosphate –T5—at a single P rate of 50 mg P kg⁻¹ and a non-fertilized control –T1–. The second factor was the application method: (i) mixing vivianite powder with the bulk soil and (ii) applying it in bands at three points around the plants. The dry matter (DM) yield in the roots and shoots of the sunflower when all P was applied as superphosphate was higher than when it was applied as vivianite. However, the combination of superphosphate and vivianite in different proportions (T3 and T4) led to a considerably higher DM yield compared to sole vivianite application (T2). The highest plant P uptake was observed in T5, while the lowest was in T1 and T2. The replacement values on a dry matter (PFRV_DM) and P uptake (PFRV_P Uptake) basis and the nutrient use efficiency of T3 and T4 were higher than that of T2. However, the PFRV_DM and the PFRV_P Uptake were in the same range as the proportion of the superphosphate added to the fertilizer mix. Thus, increased P use efficiency could be achieved with mixtures of vivianite and superphosphate. However, the contribution of vivianite to the fertilizer mix is difficult to access in a short growing cycle. Hence, further research is recommended on the residual effect of vivianite in such fertilizer mix on subsequent growing cycles.