Phosphorus Adsorption Characteristics of Soils in the Four Selected Agroecological Zones of Southern Highlands of Tanzania

The Southern Highland Zone constitutes Tanzania’s principal agricultural belt, accounting for approximately fifty percent of the nation’s food supply. Most of the soil of the region’s agroecozones is strongly acidic, causing poor crop yields and inefficient fertilizer utilization. Phosphorus (P) sorption characteristics were investigated across acidic soils to assess their implications for P availability and soil fertility management. Surface soil samples (0-30) cm sampling depth obtained from the unlimed and limed plots of study sites were analyzed for P-sorption dynamics. The P-sorption data were fitted to the Freundlich isotherms to estimate sorption capacities and heterogeneity of adsorption sites. The results on Freundlich P-adsorption capacity (K F ) in mg/Kg of soil recorded 1433.0±131.0 and 1396.6±141.7, 971.0±222.4 and 883.3±269.5, 1644.2±26.3 and 1595.7±43.0, 1055.4±62.3 and 1001.9±105.5; standard phosphorus requirement (SPR) in mg/Kg of soil 223.0±5.8 and 205.1±13.2, 122.8±17.7 and 104.7±18.4, 235.4±5.0 and 221.6±3.0, 113.3±19.0 and 97.9±13 for unlimed and limed experimental plots at Mbimba, Kifyulilo, Igeri and Uyole respectively. These results revealed strong P adsorption trends. Liming substantially reduced P-sorption capacities and heterogeneity of adsorption sites in the soil, increased equilibrium P concentration, indicating improved P availability through charge neutralization and reduced activity of iron (Fe) and aluminum (Al) sesquioxides. The SPR computed values indicated that the external addition of phosphorus according to national recommendations provide suboptimal fertilization of phosphorus in the Southern Highlands of Tanzania. These findings highlight that managing soil acidity through liming is essential to minimize P-adsorption and enhance fertilizer use efficiency in low-P tropical soils.