Co-amelioration of red soil acidity and fertility with pig manure rather than liming

Lime (calcium oxide), animal manure and crop straw soil treatments have been shown to ameliorate soil acidity, yet their effectiveness at concurrently enhancing soil fertility status and improving crop yields is less well understood. In this study, an acidic nutrient deficient red soil (Ferralic Cambisol) received these treatments at various dosage rates (% of DW soil) in pot experiments with maize plants. Lime was applied at four dosage rates (0.05%, 0.10%, 0.15% and 0.20%), pig manure at three rates (0.50%, 1.00% and 1.50%), maize straw or milk vetch at two rates (0.50% and 1.00%) and combinations of lime (0.10% or 0.15%) with maize straw (0.50%) and/or pig manure (0.50%). Soils treated with and without chemical fertilizers were also included as controls. Measurements of soil pH, exchangeable acidity, plant available nutrients and maize shoot biomass were recorded. Maize shoot biomass increased by 4.7–7.6 times under pig manure treatments, 1.1–1.6 times under milk vetch, 0.4–1.5 times under lime and 1.1–6.2 times under combination treatments, compared with the control. Soil pH increased by 0.5–0.9 units under lime, by 0.2–0.4 units under pig manure and by 0.7 pH units under the combination treatment relative to the control. Variance partitioning analysis showed that on an individual basis, soil acidity amelioration (pH, exchangeable H⁺ and Al³⁺) or nutrient input (C, N, P, K, Ca, Mg, Zn) explained only 4.3% and 5.6% of improved maize growth, respectively. Whereas, their interaction explained 85.9% of the variation. We also report that the over-application of pig manure could lead to P saturation and negative impacts on aquatic systems in the wider environment. Therefore, we recommend a combination of lime, pig manure and straw provides an optimal solution for addressing soil acidity and limiting P saturation in acid soils.