Responses of Contrasting Tropical Soil Orders to Silica Enriched Biochar Application: A Systematic Review of Soil Fertility Implications

Silica enriched biochar has emerged as a promising amendment for improving fertility in highly weathered tropical soils; however, its effectiveness varies considerably among soil orders due to differences in mineralogy, weathering intensity, and nutrient retention characteristics. This systematic review evaluated the responses of contrasting tropical soil orders, particularly Ultisols, Oxisols, Inceptisols, and Entisols, to silica enriched biochar application and synthesized the dominant mechanisms influencing soil fertility improvement. The review followed the PRISMA framework using literature retrieved from Scopus and ScienceDirect databases published between 2016 and 2026. A total of 875 records were initially identified, of which 32 studies fulfilled the inclusion criteria and were analyzed qualitatively. The findings demonstrated that silica-enriched biochar improved multiple soil fertility parameters through acidity neutralization, enhanced cation exchange capacity, nutrient retention, and silicon-mediated phosphorus mobilization. In Oxisols, available silicon increased from 36 to 209 mg kg⁻¹, while water retention improved by up to 30%. In Ultisols, soil pH buffering capacity increased by more than 67%, accompanied by significant increases in soil organic carbon, total nitrogen, and available phosphorus. Entisols showed substantial improvements in nutrient availability, with soil P, N, and K increasing by approximately 72%, 52%, and 33%, respectively, while maize yield increased by up to 93%. Overall, silica enriched biochar demonstrated strong potential to enhance soil fertility, nutrient use efficiency, and crop productivity in tropical agroecosystems, although its effectiveness remained highly dependent on soil-specific characteristics and biochar properties.