Phosphorus Availability from German Sewage Sludge Ashes to Plants Cultivated in Soilless Growing Media of Contrasting pH

Sewage sludge ash (SSA) as a secondary phosphorus (P) source may help to conserve global rock phosphate deposits if used in the base fertilization of soilless growing media. Plant availability of SSA-P was studied in a pot experiment with marigolds in a peat-based growing medium of contrasting pH (4.5, 6.0). Six SSAs were included in the study and compared to water-soluble monocalcium phosphate (MCP) and rock phosphate (RockP). In addition to the efficacy of SSA-P, calcium chloride + diethylenetriaminepentaacetate (CAT) and calcium-acetate-lactate (CAL), as commonly used extractants in Germany to analyze plant-available P in growing media, were tested in terms of their ability to predict plant P uptake. No SSA reached the relative P use efficiency (rPUE) observed for MCP. However, at a growing medium pH of 4.5, the average rPUE for SSA was 70% of the MCP rPUE. Compared to RockP, at a growing medium pH of 6.0, SSAs resulted in an rPUE of nearly the same low level. In contrast, at pH 4.5, the rPUE from SSAs was mostly lower than that from RockP, and there were differences in rPUE among SSA origins. Therefore, in addition to a variation in particle size distribution, there might be differences in the P-bearing phase. Sewage sludge ashes represent a promising substitute for primary P sources only in crops requiring a low pH in the growing medium. The extractants CAT and CAL, on their own or in combination, did not reflect plant P uptake from SSA. To improve the prediction of plant-available SSA-P, solvent modifications or additional extractants might be promising and should be subjected to further research.