Developing soil quality indices for predicting site classes in Pinus patula stands of Sao Hill and Shume Forest Plantations, Tanzania

Soil quality indices (SQIs) are comprehensive measures of soil function, integrating physical, chemical, and biological properties, which are used globally to predict suitable sites for agricultural and forest productivity. However, a lack of information on the SQIs in East Africa, particularly in Tanzania, impacts its implementation in the forest sector. Therefore, this study analyzed soil properties and developed SQIs for predicting site productivity of Pinus patula stands in Tanzania. Specifically, we aimed to (i) develop SQIs under different site classes across two forest plantations and (ii) test if SQIs can predict site classes across two forest plantations and which soil physio-chemical variables inherent in the SQI contributed most to the prediction. Principal component analysis with varimax rotation was used to develop SQIs. Orthogonal partial least squares were used to test whether SQIs and soil variables can predict the site classes. Our findings show that SQIs were SCII (0.68) > SC III (0.57) > SC IV (0.56) at SHFP. Similarly, for the SFP, the values were SC I (0.67) > SC III (0.59) > SC II (0.57). The highest SQI values indicate better quality of the site class. We found that the SQIs of studied plantations fall under the intermediate soil quality (0.55 < SQI < 0.70) class. Furthermore, SQIs and some soil variables, including available phosphorus and magnesium, were identified to be the most influential variables for predicting site productivity.