Phosphorus speciation, agronomic performance and soil nutrient testing for waste recovered fertiliser use in Northern Irish soil

  • Rebecca Hall

Student thesis: Doctoral ThesisDoctor of Philosophy


Phosphorus (P) runoff from agricultural land is causing P enrichment of surface waters, and is one of the main reasons why surficial water bodies in the United Kingdom (UK) are failing to meet Environmental Quality Standards (EQS) for fresh waters under the European Union (EU) Water Framework Directive. In Northern Ireland (NI), agronomic and environmental targets for soil P are based on measurements of sodium bicarbonate extractable-P (Olsen-P), but this methodology has been shown to underestimate bioavailable P in the iron and aluminium rich basalt soils of the Antrim Plateau, which covers a third of NI. An alternative advanced speciation method, namely DGT (Diffusive Gradients in Thin films), could be used instead to assess P bioavailability in these and other soil types in NI. The method has been successfully used on a range of soils in England and Scotland, but has not yet been trialled in NI. To adhere to the European Commissions aim for ‘ensuring more sustainable application of nutrients’ an evaluation of the status of P in NI is needed.

Fertiliser P used in agriculture is derived from non-renewable rock phosphate resources, or else from organic fertilisers such as manure, bio-solids and compost. Globally, the demand for fertiliser P is increasing by 400,000 tonnes per year and only 17% of this ends up in our food supply. The additional 83% is lost from ‘farm to fork’ through runoff into surficial watercourses, soil erosion, and food waste which is not composted. Within NI, total deliveries of P fertiliser to farms decreased dramatically between 2003 and 2012, but has been rising slowly again since 2014, thus increasing agriculture’s reliance once more on non-renewable P resources.

Phosphorus recovery as struvite (MgNH4PO4.6H2O) from high nutrient waste water and subsequent use as a slow release P fertiliser, is now being explored. However, the mineral composition, solubility and hence bioavailability of this P can differ markedly between waste waters from different treatment plants. Furthermore, in NI 2.38 million tonnes of organic waste is collected annually in NI, which is not all recovered as fertiliser. Of this reusable organic content 260,000 tonnes of chicken waste is sent to be anaerobically digested to reduce pathogen content and waste volume. The digestate end product is a nutrient rich organic source which has the potential to supply local agriculture with P.

In this PhD, experimental platforms have been developed to assess the most effective way to measure soil test P (STP) in the unique soils of NI and how to accurately predict the behaviour of recycled P in soils. This was done by (i) surveying chemical properties in relation to STP in a 65 grassland soils in NI (ii) assessing the bioavailability of struvite P from a range of waste waters and (iii) comprehensively evaluating the P speciation, chemical behaviour in soil and water, and agronomic performance of chicken manure digestate. The main findings of this work are;

• In Basalt soils Olsen P underestimates plant available P supply
• Estimating plant P uptake rate(s) is uncertain given variation in soil type and other nutrient deficiencies such as Potassium (K) and Sulphur (S)
• Variability in P status, depending on STP method used (Olsen P, Morgan P or DGT), has implications for management decisions and fertiliser recommendations
• Across all samples none of the STP methods strongly correlate with herbage P, but this is not the case in plant pot trials
• Strongest relationship between DGT and herbage P was in basalt parent material (R2 = 0.52)
• The Morgan P soil test which used in the Republic of Ireland (ROI) correlated with the soils collected in southern border regions of NI.

• Struvite differed markedly in colour/appearance, texture, metal content and solubility • Tested struvites all had similar P-speciation, but minor impurity phases were detected • There was no significant difference between the struvite samples and triple-superphosphate (TSP) with respect to either plant growth performance or P-bioavailability
• The struvite products are produced from different wastewater streams/ processes, yet possess a uniformity in overall fertilising performance within our experiments
• Encouragingly, no appreciable agronomic difference in terms of yield was observed between the struvite and chemical fertilisers, which provides further evidence in support of struvite as a sustainable alternative to mined rock-phosphate derived products
• Importantly the struvite samples analysed within this study contained both significantly fewer trace metal impurities (e.g. Aluminium, Al, Copper, Cu, Calcium, Ca, and Zinc, Zn) than TSP and lower concentrations of toxic metals such as cadmium.

• Orthophosphate (Pi) was the dominant P fraction in chicken digestate from NMR analysis
• 74% of Pi was redox sensitive Iron (Fe) bound P in the digestate sample
• P is highly soluble in water and hydrochloric acid extractions, and more extractable than chemical fertiliser and struvite
• However, chicken digestate waste provided the lowest P plant offtake out of the three fertilisers as Fe in the chicken digestive limits plant P uptake
• Potential for developed innovative fertilising products/amendments from waste streams that optimise plant uptake and reduce loss to the environment.

There needs to be an integrated Nutrient Management Plan and market for recovered nutrients in NI which should include incorporation of DGT methodology. STP and fertiliser input is dependent upon the soil chemistry, biology and physical composition, as well as the physiological ability of the plant to sequester legacy P. A more holistic P management approach is needed to achieve sustainable nutrient application, which could be by using struvite products generated from different waste sources (chicken digestate) precipitation technologies and wastewaters.
Date of AwardDec 2020
Original languageEnglish
Awarding Institution
  • Queen's University Belfast
SponsorsNorthern Ireland Department for the Economy & Environmental Protection Agency
SupervisorPaul Williams (Supervisor) & John McGrath (Supervisor)

Cite this