Mineral fertiliser equivalent value of dairy processing sludge and derived biochar using ryegrass (Lolium perenne L.) and spring wheat (Triticum aestivum)

As supply chains of chemical fertilisers become more precarious, raw or derived bio-based fertilisers (herein referred to as bio-fertilisers) from the dairy processing industry could be good alternatives. However, their agronomic performance is relatively unknown, and where documented, the method to estimate this value is rarely presented. This pot study investigated aluminium-precipitated and calcium-precipitated dairy processing sludges (Al and Ca-DPS) and DPS-derived biochar as potential bio-fertilisers to grow ryegrass (Lolium perenne L.) and spring wheat (Triticum aestivum). The study aims were to examine how (1) application rate (optimal versus high) and (2) calculation methods (with and without chemical fertiliser response curves) can affect estimates of nitrogen and phosphorus mineral fertiliser equivalence value (N-and P-MFE) and associated agronomic advice. The results from both crops showed that for nitrogen application rates (125 or 160 kg ha-1 for ryegrass and 160 or 240 kg ha-1 for spring wheat) estimates of N-MFE increased for both Al-DPS and Ca-DPS as application rate increased. Dry matter yield response curves produced the highest % N-MFE results (e.g., ryegrass-50% and 70% for Al-DPS and Ca-DPS) with other calculation methods producing all similar results (e.g., ryegrass-20% for Al-DPS and Ca-DPS). For phosphorus application rates (40 or 80 kg ha-1 for ryegrass and 50 or 80 kg ha-1 for spring wheat), estimates of P-MFE did not increase with application rate. Negative P-MFE values obtained for Ca-DPS and DPS-biochar when growing ryegrass and spring wheat grain, respectively, indicated low plant available phosphorus. Overall, Al-DPS had better performance as a bio-fertiliser when compared to the other products tested. There was no significant difference between the two calculation methods of MFE, which suggests that the determination of MFE could be simplified by using one application as opposed to numerous application rates of fertilisers. Future work should focus on elucidating the N-and P-MFE of a wider range of DPS and STRUBIAS bio-fertilisers, and alternative methods should be investigated that enable a comparison across all bio-fertiliser types.

Automated summary

This study investigated the potential of dairy processing sludges and biochar as bio-fertilisers and examined the effects of application rate and calculation methods on estimates of nitrogen and phosphorus fertiliser equivalence value. The results showed that nitrogen fertiliser equivalence value increased with application rate, while phosphorus fertiliser equivalence value did not. Aluminium-precipitated dairy processing sludges performed better as bio-fertilisers compared to other products. The two calculation methods of fertiliser equivalence value did not show significant differences, suggesting that the determination of equivalence value could be simplified.