Table 2.

Effect of soil application of Al sludge on P availability to plants

Experimental treatments	Soil properties	Performance	Reference
Al sludge vs commercial fertilizer (4 rates), Switchgrass, pot experiment, 3 harvests, 50 d interval. P rates based on Olsen’s available P in sludge	pH 8.3, low avail P (3 mg/g), organic C (25 g/kg)	1st harvest had no difference between sludge and fertilizer. Later harvests had P uptake higher for sludge. Sludge has org P which released P gradually. Soil available P decreased with time for commercial fertilizer due to soil P fixation	Tolofari et al. (2021)
Same as above except the crop was maize, 4 harvest, 45 d interval	Same as above	Same results as above except sludge was better even for the first harvest	Tolofari et al. (2022)
Al/Ca precipitated sludge (40 kg P/ha) compared with mineral fertilizer (5 rates, 15–60 kg/ha). Field expt. in Ireland for 1 year with rye grass	pH 5.6, organic C 1.8%, Morgan’s available P 2.86 mg/L (P deficient soil)	Mineral fertilizer had higher plant P uptake at 1st harvest (P highly soluble initially). Fourth harvest sludge produced higher P uptake. Slow availability of P from sludge. Sludge P less prone to lock-up in soil than mineral fertilizer P	Ashekuzzaman et al. (2021)
11 types of sludges compared in a pot trial on ryegrass. 1st cut at 6 weeks, later 4 weeks apart. Sludges 225 mg P/pot, Mineral P fertilizer 3 rates (75–225 mg P/pot)	pH 7.2, organic C 0.26%, Available P 0.47 mg/kg	Plant-available P lower than that of mineral fertilizer. Plant availability more for Fe sludge than Al sludge because of lower solubility of Al sludge. Liming the sludge increased P availability because of shift from Al/Fe P to CaP− still lower P availability than mineral fertilizer	Øgaard and Brod (2016)
DAP fertiliser (20 kg P/ha) vs Al sludge, 5 rates (124–744 kg P/ha). 2 yr-field trial in Western Australia on barley and wheat	pH 5, organic C 0.74%, Available P (Olsen’s) 7 mg/g	Plant P concentration increased with increased rate of sludge but lower than that for DAP in both years for both crops. Sludge had less P availability than DAP even when applied at higher rates because P was bound to Al	Rigby et al. (2013)
Al sludge applied to two soils at 6 rates (equivalent to 5–500 tonnes/ha). Pot expt. for 4 weeks on lettuce	Sandy soil pH 7.1, Clay soil pH 5.5	P uptake declined with increase alum rate. Plant growth also declined, more with high pH soil due to sludge induced P deficiency. Not due to Al toxicity. pH slightly declined in sandy soil but increased in clay soil	Lombi et al. (2010)
One of the 5 sludges tested in pot trial (73 days) with ryegrass was Al based. Inorganic fertilizer 30–120 kg/ha; Sludge 20 tonnes/ha	pH soils 5.7, 6.74; Organic C (%): 5.61, 5.28	Al sludge produced low P fertility value in this short-term experiment. Sludge from biological treatment produced highest P fertility comparable to inorganic fertilizer. Total P will keep accumulating in soil from Al sludge over long term use	Krogstad et al. (2005)
Al sludge after treatment of alum and lime tested in a 5- week pot trial. 4 rates 1.5–18% moist sludge	pH 4.0 (very acidic), available P 0.76 mg/kg, Organic matter 4.5%	Reduced P uptake due to P adsorption by the increased amounts of alum with Al and Ca. Sludge increased root elongation, soil acidity buffering capacity, hydraulic conductivity, water holding capacity	Kim et al. (2002)