Phosphorus availability of sewage sludges and ashes in soils of contrasting pH

Camilla Lemming, Martin Toft Simmelsgaard Nielsen, Lars Stoumann Jensen, Charlotte Scheutz, Jakob Magid*

*Corresponding author for this work

Research output: Contribution to journalJournal articleResearchpeer-review


Background: Little is known about the relationship between plant availability of P in different sludges and their derived ashes, its development over time and its interaction with soil pH. Aim: This study addresses this knowledge gap, by investigating the P availability and fractions of six sludges produced under different conditions and the incineration ashes derived from five of these, plus the effect of time (84 days) and soil pH on P availability. Methods: These materials were applied to two similar soils with contrasting pH (5.8 and 7.8), based on the assumption that lower pH would favor plant availability of P from these sources. Triple super phosphate and a control (no P addition) were included as references. Results: The relative P fertilizer availability (RFA) in soil of both sludges (18–77%) and ashes (0.1–25%), measured as water extractable P, was found to be highly dependent on their origin. While the P availability of sludges was strongly affected by soil pH, ash P availability showed either no or little soil pH effect, in contrast to our expectations. For the sludges, RFA was negatively correlated to the ratio (Fe+Al)/P in both pH soils and positively correlated to sludge water extractable P in the higher pH soil. For the ashes, the ratio Fe+Al to P and the pH reaction of ashes in water were strongly negatively correlated with RFA in the lower pH soil. Ca/P ratios did not predict RFA in either sludges or ashes. Nevertheless, we ascribe the comparably higher average RFA in sludges of 58% found in the low pH soil compared to 25% in the alkaline soil to the greater solubility of Ca-P species under lower pH. For ashes, the availability increased over time, while rather stable for sludges. Conclusion: In conclusion, the P availability of both sludges and ashes was highly dependent on their origin, and for sludges highly dependent on soil pH.
Original languageEnglish
JournalJournal of Plant Nutrition and Soil Science
Publication statusAccepted/In press - 2020

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