Yield decline has been the hallmark of Ethiopian sugarcane plantations. However, the extent and causes of the decline have not yet been empirically studied, making it difficult to manage the problem. This study aimed at analyzing the long-term yield data (1954–2022) with respect to variety and soil type. Thus, 8,923 records of yield data were summarized and sorted into decades, varieties, and soil types and then analyzed by applying Mann-Kendall and Tukey’s tests. The fields were classified and mapped using ArcGIS 10.3. The results revealed that 69% of the plantation fields were classified as “yield declining,” and the overall rate of decline has been 8.4 quintals ha−1 year−1 (R2 = 0.76). The rate of decline was higher for older than newer varieties and for vertisols than cambiols. Therefore, the older varieties should be micropropagated or replaced with improved ones, and the vertisols should be amended through practices such as green manuring, improved fallows, etc.

Over the last 50 years, the sugarcane yield in Wonji plantation has declined by about 40%. Perhaps one of the possible causes for the decline is soil degradation. Thus, the major soil quality indicators were evaluated for the extent of change that might occur due to long-term conventional cropping practices. To that end bio-sequential soil sampling was performed by collecting soil samples from adjacent virgin and cultivated lands of Wonji sugar cane plantation. The samples were analyzed and compared for major soil properties. The result showed that the SOM contents of cultivated land were 53% and 34% lower than the virgin land at 0 cm-30 cm and 30-60 cm depths, respectively. Total N, P Olsen, exchangeable K and soil EC of the cultivated land were also 56%, 84%, 86% and 54% lower than the virgin land at 0 cm-30 cm. The differences were also significant at 30 cm-60 cm. There was no significant change in soil pH at both depths. In general long-term conventional cropping practices depleted the SOM, total N, P Olsen and exchangeable K. However, pH and EC were in the optimum range that soil acidity, salinity and alkalinity were not a problem. As the soil type of the plantation is heavy clay, particularly, the degradation in SOM content might cause the yield decline. In order to fully identify, understand and manage the problems of soil quality deterioration further study is necessary.

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