Objective The aims were to improve the soil nutrients and aggregate structure of acidified purple soil and to ensure the suitable application amounts of wood ash in Southwest China.

Method Based on a field experiment, seven different application rates of wood ash residue were set (0, 3000, 6000, 9000, 12000, 15000, and 18000 kg hm⁻²). The physical and chemical properties of the soil, nutrient stoichiometric ratios, soil microbial characteristics, and changes in soil aggregate structure in the surface layer (0 - 20 cm) and sub-surface layer (20 - 40 cm) of purple soil were analyzed 30 days and 180 days after application.

Result The application of wood ash increased the pH of the surface and subsurface layers of purple soil, and the effect was more significant with increasing application rate and time. The soil pH in treatment (18000 kg hm⁻²) increased by 0.67 - 0.93 units compared to the CK treatment. In terms of available soil nutrients, wood ash application initially (30 days) significantly increased the alkali-hydrolyzable nitrogen (N) content in both soil layers, but with increasing application time, there was no significant difference in alkali-hydrolyzable N content in the subsurface soil among wood ash treatments. Wood ash application significantly increased the available phosphorus (P) and available potassium (K) contents in the surface soil, but high application rates (>15000 kg hm⁻²) of wood ash had a reduced effect on increasing available P content. The application of wood ash significantly increased the organic carbon content in both the surface and subsurface soil layers (P < 0.05), especially after 30 days of application, the organic carbon (C) content in the surface soil increased by 1.77 - 4.83 g kg⁻¹ compared to the CK treatment. The application of wood ash for 30 and 180 days significantly increased the C∶N ratio in the surface soil, especially at low application rates (3000 - 6000 kg hm⁻²). The C∶P and C∶K ratios in the subsurface soil were mainly significantly increased after 30 days of wood ash application, and only medium application rates (9000 - 12000 kg hm⁻²) of wood ash could effectively increase the C∶P and C∶K ratios in the subsurface soil. The application of wood ash, especially at the treatment (12000 kg hm⁻²) significantly increased the number of bacteria and fungi in the soil, and the application rates of 9000 - 12000 kg·hm⁻² effectively increased the soil microbial biomass carbon (SMBC) and soil microbial biomass nitrogen (SMBN) contents, but wood ash application significantly decreased the SMBC∶MBN ratio. In addition, the 12000 kg hm⁻² treatment effectively increased the mass percentage of > 0.25 mm soil aggregates in the surface soil, and improved the mean weight diameter (MWD) and geometric mean diameter (GMD) of soil aggregates

Conclusion In conclusion, wood ash can effectively improve soil acidity, increase soil available nutrients, and increase soil microbial populations and the mass percentage of soil aggregates (> 0.25 mm). Among them, the application rates of 9000 - 12000 kg hm⁻² of sawdust ash have a better effect on the improvement of purple soil.