Association of Arabica Coffee Quality Attributes with Selected Soil Chemical Properties

Abstract: Coffee (Coffea arabica L.) bean quality attributes differ based on the origin of the produce. Several agro-ecological conditions influence coffee bean quality attributes. Soil chemical properties may be some of the factors affecting the quality attributes. However, no study has so far been conducted to elucidate the association of coffee bean qualities with soil chemical properties in both major and minor coffee growing regions of Ethiopia. Thus, this research was conducted with the objective of establishing association of chemical soil properties with coffee cup quality attributes. Coffee beans as well as soil samples from which the beans originated were subjected to chemical analysis. The coffee beans and the corresponding soil samples originated from large scale coffee plantations (Bebeka, Gemadro and Goma), districts from southwestern major coffee growing region (Gore, Jimma, Lemkefa), West (Gimbi), East (Badano, Chiro, Darolebu, Habro and Melkabelo), South (Yirgacheffe) and northwestern minor coffee growing districts (Ankasha, Bure, Mecha and Jabi). The soil samples were collected from the depth of 0 - 50 cm near the coffee trunks and samples of ripe coffee cherries were picked up from the trees during the 2010/11 harvest season. Selected chemical properties of the soil, namely, available potassium, cation exchange capacity, exchangeable acidity, exchangeable bases, available micronutrients, available phosphorus, total nitrogen, soil pH, electrical conductivity, and percent organic carbon were determined from 53 soil samples in Jimma University soil laboratory and Wolkitie Soil Testing and Soil Fertility Improvement Centre using the established procedures. The sampled red coffee cherries were carefully subjected to the dry processing methods and the separated beans tested for quality attributes in accordance with Ethiopian Commodity Exchange (ECX) and Specialty Coffee Association of America (SCAA) coffee quality test procedures and standards. Correlation and stepwise regression analyses were done to establish the association of the selected soil chemical properties with the coffee bean quality attributes. The correlation analysis revealed that coffee quality attributes were positively and significantly associated with CEC (r = 0.36**), available soil Mg content (r = 0.28*), exchangeable acidity (H⁺) (r = 0.35*), and soil pH (r = 0.30*). However, the coffee quality attributes were negatively and significantly associated with soil available Cu (r = - 0.35*), available Zn (r = - 0.40**), and total N (r = - 0.40**). The regression analysis showed that coffee quality attributes were more profoundly dependent on available Fe content (R² =0.22) and CEC (R² = 0.13) in the soil. The soil CEC and available soil iron (Fe) accounted for 13 and 21.9%, respectively of the observed variation in the overall coffee quality attributes that determines the final coffee grade and consumer preferences. Therefore, it could be concluded that coffee quality attributes improved with increase in the levels of soil CEC, Mg, H1+, and pH, while decreasing with increase in the levels of available soil Cu, Zn and total N. However, enhanced soil CEC and available iron content led to improved grade and overall specialty coffee quality attributes whilst enhanced soil available zinc and copper as well as total soil nitrogen led to reduced grade and overall specialty coffee quality attributes.

Keywords: Available soil iron; Coffea arabica L.; CEC; Coffee grade; Specialty coffee quality attributes; coffee cup quality