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.
BackgroundUsing secondary phosphorus (P) sources recovered from waste streams for base fertilization of soilless growing media may help conserve the global phosphate resources. However, little is known regarding the plant availability of P from secondary sources for soilless plant production.AimThe objective of this study was to investigate the plant availability of P from secondary sources using a peat‐based growing medium and standard extraction methods.MethodsSeventeen secondary P sources differing in terms of feedstock and processing were tested, and their fertilization efficacy was compared with that of water‐soluble monocalcium phosphate (MCP‐P) in a pot experiment using marigolds. The correlation of plant P uptake with calcium chloride + diethylenetriaminepentaacetate (CAT) and/or calcium–acetate–lactate (CAL) extractable P amount in the growing medium (PCAT and PCAL, respectively) was examined.ResultsOnly three of the 17 tested secondary P sources (all struvite forms) exhibited comparable fertilization efficacy to MCP‐P. The remaining sources showed P uptake varying between 4% and 91% of MCP‐P. For precipitates from liquid waste streams and untreated organic waste, except for sewage sludge and compost, the P uptake ranged from 71% to 100%. The highest variability (4%–86%) was observed for thermally and thermochemically treated secondary P sources. Plant P uptake was closely correlated with PCAT (R2 = 0.76) and PCAL (R2 = 0.78), and this correlation became even stronger when PCAT and PCAL were combined (R2 = 0.89).ConclusionsMost struvite forms meet the requirements of base P fertilization for soilless plant production and can substitute water‐soluble fertilizers obtained from primary P sources. The combination of PCAT and PCAL may be a promising approach to characterize the plant availability of P from secondary sources in soilless growing media and warrants further consideration.
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