Optimization of solid phosphate sludge composting by integration of horticultural waste

Phosphate sludge (PS) is a by-product of phosphate extraction industry and yet, still contain phosphorus bound to calcium what makes it unavailable. Here, we characterized phosphorus fractions in phosphate sludge and their evolution with other plant nutrients during 60 days aerobic composting with tomato plant residues. Two samplings at initial and final stage (5 and 60 days) were performed and analyzed for water-soluble phosphorus (P) fractioning (WSP), P associated with aluminum and iron oxides (Fe/Al-P), P associated with organic matter (OM-P), and P associated with calcium (Ca-P) along with water-soluble potassium (K), Ca, and sodium (Na) that were also analyzed. Results showed that at the initial stage of composting (5 days), the concentration intervals of each P fraction were 0.3 to 0.34 g kg−1, 0.66 to 0.68 g kg−1, 0.29 to 0.55 g kg−1, and 93.21 to 94.87 g kg−1, respectively, for WSP, Fe/Al-P, OM-P, and Ca-P. After 60 days, results revealed an increase in each fraction as follows: 1.36 to 1.40 g kg−1; 3.11 to 3.29 g kg−1; 2.04 to 2.44 g kg−1; and 91.49 to 94.04 g kg−1, respectively, for WSP, Fe/Al-P, OM-P, and Ca-P. Extractable K, Ca, and Na increased whereas total organic carbon decreased after composting. The dominant fraction Ca-P has decreased during composting period and transformed into WSP, Fe/Al-P, OM-P fractions. The slow release in available forms of plant nutrients is a valuable attribute of a mature organic amendment. This confirms that composting could be a promising approach for application of PS and P recovery to arable soils.