Spreading of mixtures including wastes from the mussel shell treatment industry on an acid soil: effects on the dissolved aluminum species and on pasture production

Three mixtures were spread on a pasture land to study their effects on main parameters of both solid and liquid phases in the soil, on Al species present in the soil solution, and on pasture production, then comparing the results with that corresponding to a conventional NPK inorganic fertilization, to a NPK + mussel shell (MS) treatment and to control plots. All three mixtures included shell calcination ash and sewage sludge from the mussel shell treatment industry, two wastes that are difficult to recycle due to problematic characteristics, namely high electrical conductivity and pH values. The combination of those wastes with wood ash from the timber industry, or with valued mussel shell, gives mixtures with overall improved characteristics, which could be recycled in acid soils. The composition of each mixture was (w/w, dry weight): mixture M78 47% sewage sludge, 48% shell ash, and 5% wood ash; mixture M58 47% sewage sludge, 40% shell ash, and 13% mussel shell, and mixture M32 45% sewage sludge, 51% shell ash, and 4% mussel shell. Soil and plant were sampled in 24 plots (quadrupled replications for 5 treatments and 1 control) during July and October, in two successive years. The results indicate that, comparing with the control plots, the NPK + MS treatment caused the highest increase in pHH2O (5.18-5.42 versus 4.26-4.86), pHKCl (4.09-4.62 versus 3.69-3.89), pH in the soil solution (5.90-7.03 versus 5.16-5.56), exchangeable Ca (2.96-39.43 versus 0.25-0.78 cmol(+) kg−1), and soluble Ca (4.92-29.01 versus 0.65-1.52 cmol(+) kg−1), whereas this NPK + MS treatment caused the lowest concentrations of exchangeable Al (0.66-4.15 versus 6.34-7.76 cmol(+) kg−1), Al saturation (1.73-49.42 versus 71.45-87.48%) and total Al in the soil solution (0.15-0.60 versus 0.52-0.73 mg L−1). Further, Al3+ was not found in the mixture-treated and in the NPK + MS-treated plots, where the organically-complexed Al dominates over the monomeric Al, contrary to that occurring in the control and in the NPK-treated plots. The pasture production was similar for the mixture-treated and the NPK + MS plots, and it was higher than for the NPK-treated and for the control plots, particularly regarding sown species. For most parameters, NPK-treated and control plots are not significantly different, whereas the values in the mixture-treated plots are close to those treated with NPK + MS. In view of that all, the three mixtures assayed could be spread on acid soils to increase pasture production, then complementing or even substituting the more conventional treatments with limestone and inorganic NPK.