Differing Phosphorus Crop Availability of Aluminium and Calcium Precipitated Dairy Processing Sludge Potential Recycled Alternatives to Mineral Phosphorus Fertiliser
- 1. Teagasc
- 2. University of Ghent
Description
The European dairy industry generates large volumes of wastewater from milk and dairy
food processing. Removal of phosphorus (P) by complexing with metal (e.g., aluminium, calcium)
cations in P rich sludge is a potential P source for agricultural reuse and P recycling. However, there is
a significant knowledge gap concerning the plant availability of this complexed P in comparison to
conventional mineral P fertiliser. The current absence of information on plant P bioavailability of
dairy processing sludge (DPS) limits the ability of farmers and nutrient management advisors to
incorporate it correctly into fertiliser programmes. The present study examined the most common
types of dairy sludge—(1) aluminium-precipitated sludge (“Al-DPS”) and (2) calcium-precipitated
lime-stabilised sludge (“Ca-DPS”) at field scale to assess P availability in grassland versus mineral
P fertiliser over a growing season. The experimental design was a randomised complete block
with five replications. Crop yield and P uptake were assessed for 4 harvests. The initial soil test
P was at a low level and the experimental treatments were super phosphate at 15, 30, 40, 50 and
60 kg P ha1, two dairy sludge applied at 40 kg P ha1 (comparison was made with mineral P at
same application rate) and a zero P control applied in a single application at the beginning of the
growing season. Results showed a significant positive slope in the relationship between P uptake
response and mineral P application rate indicating the suitability of the experimental site for P
availability assessment. The P bioavailability of Al- and Ca-DPS varied greatly between treatments.
The P fertiliser replacement value based on the 1st harvest was 50 and 16% increased to 109 and 31%
cumulatively over the four harvests for Al- and Ca-DPS, respectively. The Al concentration in Al-DPS
did not limit P bioavailability, but low P bioavailability from Ca-DPS can be associated with its high
Ca content that can lead to formation of low soluble Ca-P compounds at alkaline pH conditions with
a high Ca/P ratio. These findings show that P availability from dairy sludge can be quite different
depending on treatment process. Consequently, it is critical to have P availability information as well
as total P content available to ensure the application rate meets crop requirements without creating
environmental risk by over application.
Files
Ashekuzzaman et al., 2021. Differing P availability of Al and Ca dairy sludge.pdf
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