A Soil Fertility Status and Nutrient Interrelationships in Alkaline, Sandy Soils of the Maknossa Agriculture Project, southwest Libya: Implications for Cereal and Fodder Production

Abstract

Abstract

Sandy, alkaline soils under center-pivot irrigation are increasingly relied upon to expand cereal and fodder production in arid regions, yet their capacity to supply phosphorus (P) and micronutrients is poorly characterized at the farm scale. This study assessed soil fertility status and nutrient interrelationships across 20 georeferenced topsoil samples (0–20 cm) collected from center-pivot fields at the Maknossa Agriculture Project, Wadi Utba, Fezzan, Libya, an area cultivated with wheat, barley, and fodder crops. Soils were analyzed for pH, electrical conductivity (EC), and macro- and micronutrients using standard extraction procedures (Olsen-P, 1 M ammonium nitrate for exchangeable cations, 0.05 M EDTA for micronutrients) with inductively coupled plasma analysis. Descriptive statistics, Nutrient Index (NI) and Soil Fertility Index (SFI) classification, Spearman correlation, and principal component analysis (PCA) were used to characterize fertility status and its underlying drivers. Soils were consistently alkaline (mean pH 8.10 ± 0.14) with excessive calcium (2304.6 ± 347.98 ppm) but only marginal available phosphorus (11.55 ± 4.38 ppm). Soil pH correlated negatively with both available P (rs = -0.776, p < 0.001) and Zn (rs = -0.763, p < 0.001). Three principal components explained 79.0% of total variance, representing an alkalinity axis (pH, Ca, K), a trace-metal/salinity axis (Cu, Mn, Na), and a mixed nitrogen–carbonate axis. Despite these constraints, the SFI classified 70% of samples as having High overall fertility and the remaining 30% as Moderate, indicating that productive potential is present but chemically constrained. An operational phosphorus fertigation model incorporating a pH-scaled alkaline-fixation correction factor is proposed to guide site-specific triple superphosphate (TSP) application, replacing an earlier, methodologically flawed version of the model. These findings support a shift from broadcast phosphorus application toward acidified fertigation and foliar micronutrient management at this and comparable arid, calcareous, center-pivot sites.