Comparing how compost and manure affect soil organic matter using a complete factorial design

Background The aim of this study was to determine the influence of physico-chemical factors on soil organic matter by comparing two types of soil improver: one produced from olive mill waste cake residues (CRPM) and the other from manure (CF). This was achieved using Full Factorial Design, the main objective of which is to measure precisely the impact of each factor on a given response, to analyze the interactions between the various factors and to identify the optimum conditions for achieving a specific objective or improving the performance of a process. Methods Four independent factors were studied: pH, electrical conductivity, humidity and carbon/nitrogen (C/N) ratio. Using design, based on their significance, coefficient of determination, analysis of variance and Pareto charts, the interactions between these variables were assessed to identify those with the most positive impact on soil fertility. Results The results show that, for CRPM compost, the most positive interaction is between pH and ratio C/N. For manure, the interaction between pH and moisture has the greatest beneficial effect. These findings underline the importance of controlling all three factors − pH, moisture and ratio C/N to optimize soil fertility. Conclusions The analysis confirmed the reliability of the models used, with p-values below 0.05 and coefficients of determination (R2 and adjusted R2) close to 1, indicating the robustness of the models. Pareto diagrams were used to precisely identify the most relevant interactions for improving soil amendment management.