Effects of Household Waste Compost and Wood Ash on the Physico-Chemical Properties of A Tropical Sandy Soil: Case of A Ferralitic Soil from Brazzaville (Congo)

The ferralitic sandy soils of peri-urban Brazzaville (Republic of Congo) are acidic, poor in organic matter (OM) and have a low cation exchange capacity (CEC). This study evaluated, after a 14-day incubation and before any crop cultivation, the effects of seven treatments on the physico-chemical properties of such a soil: T0 (control); T1-T4, wood ash at 125, 250, 500 and 1,000 g per 5-kg pot (2.5-20% of soil mass); T5, household waste compost (100 g/pot); T6, a 1:1 (w/w) mixture of T1 and T5 soils (≈ 61 g ash + 49 g compost per 5 kg). Pots were arranged in a randomized complete block design with four replicates, and 18 variables were analysed by ANOVA (Tukey HSD) and principal component analysis (PCA). The unamended soil was moderately acid (pH 5.52), with low OM (2.17%), total N (994 mg/kg) and CEC (6.37 cmol(+)/kg). Wood ash (pH 11.0; Ca 15.5%) raised soil pH to 10.21-10.28 (T3-T4), strongly increased Ca, Mg and K, and raised EC up to 1,816 µS/cm, while TME contents decreased, mainly through dilution. Compost increased total N (up to 1,159 mg/kg in T5) and MWRC. T6 showed the most balanced measured profile: pH 7.86, MWRC 28.8% (highest value), OM 4.40%, CEC 12.11 cmol(+)/kg and N 1,093 mg/kg, with EC (1,311 µS/cm) lower than in T3-T4. All TME remained below the NF U 44-041 soil limit values. PCA retained three components (89.17% of variance): PC1 (62.85%) opposed the ash-driven mineral enrichment (Ca, Mg, K, pH, EC) to native Fe, Al and TME; PC2 (19.27%) reflected the compost-driven organic gradient (N, MWRC, OM). T6 lay between both gradients, consistent with a complementary effect of the two amendments; this should be validated by crop-response trials.