Soil chemical properties of smallholder avocado intercropping systems in northern Tanzania

Avocado production is expanding in northern Tanzania; however, information on the spatial variability of soil fertility across major production areas remains limited. This study assessed soil chemical properties in smallholder avocado agroforestry systems across six districts in the Tanga, Kilimanjaro, and Arusha regions. Composite soil samples (0–30 cm depth) were collected from 54 villages and analysed for pH, electrical conductivity (EC), organic carbon (OC), total nitrogen (TN), available phosphorus (P), exchangeable potassium (K), calcium (Ca), magnesium (Mg), and copper (Cu). District-level differences were evaluated using one-way analysis of variance, while Pearson correlation, principal component analysis (PCA), and linear mixed-effects models were used to examine nutrient relationships, spatial fertility patterns, and associations with elevation. Soils across all districts were non-saline (EC: 0.13–0.25 dS m −1 ), and pH values (5.9–6.7) fell within the range generally considered suitable for avocado cultivation. Meru and Karatu generally exhibited higher concentrations of OC, TN, and exchangeable base cations. In contrast, Lushoto, Korogwe, and Rombo showed lower concentrations of some nutrients, particularly K and P. Elevated Cu concentrations were observed in Moshi and may be associated with the long-term use of copper-based fungicides. The linear mixed-effects model indicated that only OC and TN increased significantly with elevation. PCA showed that inter-district variation in soil fertility was primarily associated with differences in exchangeable base cations and organic matter-related nutrients. These findings provide baseline information on soil fertility patterns in northern Tanzania and a foundation for future studies integrating soil properties with topographic, climatic, and management factors to support sustainable avocado production.