Land Use Impacts on Phosphorus Sorption Isotherms: A Critical Assessment for Site-Specific Fertilizer Management in the Highlands of Southern Ethiopia

Phosphorus (P) deficiency limits crop productivity in tropical acidic soils, yet information on P sorption dynamics across land uses is scarce in southern Ethiopia. This study evaluated P sorption and estimated external P requirements (EPR) for four land uses (coffee, enset, cultivated, eucalyptus) in Bule Hora district. Twenty-four composite soil samples (0–20 cm) were analyzed, and P sorption data fitted to Langmuir and Freundlich isotherms. EPR was defined as the theoretical P required to maintain 0.2 mg P L−1 in soil solution under laboratory conditions, as a comparative reference, not a field fertilizer recommendation. Soils were clayey, acidic (pH 4.99–5.68), with very low soil available P (<5 mg P kg−1). The Langmuir model best described sorption (R ² = 0.89–0.98). Sorption maxima ranged 164.8–264.6 mg P kg ⁻¹ , and EPR 97.0–192.9 mg P kg ⁻¹ ; highest in cultivated and eucalyptus lands. The current DAP recommendation (100 kg ha ⁻¹ , 18-46-0) supplies ∼20.1 kg P ha ⁻¹ , equivalent to ∼8.4 mg P kg ⁻¹ . This is only 4.6–6.4% of the laboratory-derived EPR for cultivated land (131–183 mg P kg ⁻¹ ) required to maintain 0.2 mg P L ⁻¹ . Xm correlated positively with clay (r = 0.75) and negatively with organic matter (r = −0.75) and available P (r = −0.73). We conclude that Bule Hora soils have high P fixation. These EPR values require field validation before revising recommendations. Site-specific management combining validated EPR-based rates, organic matter amendments, and targeted liming may enhance P use efficiency and sustain productivity in these strongly sorbing soils.