Responses to potassium (K) fertilizer are reported in different parts of Ethiopia in recent years, contrary to the general perception that K fertilizer was not necessary on the soils. However, use of adequate K level for maximum economic yield is possible only when soil test data are calibrated against response curve for a given crop and soil type. This pot trial was conducted to calibrate K soil test with the response of teff (Eragrostis tef (Zucc.)) at application of increasing levels of K. Random soil samples (0-20 cm) were collected from 60 locations having Mehlich-3 (M-3) K soil test values ranging from 16 to 910 mg kg −1 across 20 teff growing districts in the highlands of Ethiopia. The relative biomass yield of teff was significantly correlated with Mehlich-3 extractable K, suggesting that the extraction method fairly estimated the available soil K. Soil test M-3 extractable K values were categorized, based on relative biomass yield, as "very low" (< 55 mg kg −1), "low" (55-210 mg kg −1), "medium" (210-280 mg kg −1), "high" (280-500 mg kg −1) and "very high" (> 500 mg kg −1). And K application rates of 139 and 54 kg ha −1 for the very low and low K categories respectively would suffice for optimum teff yield. The critical limit of K in soil (M-3) using the graphical method was 210 mg kg −1 for the 85% relative biomass yield level. An inverse relation of yield response to the soil K status was observed suggesting that application of K fertilizer increases yield of teff in low status soils. However, field verification is required to draw sound conclusions.

Nutrient depletion and imbalance are among the major attributes that contribute to declining soil productivity in the highlands of Ethiopia. The blanket fertilizer recommendation, which considered only urea and Di-ammonium phosphate (DAP), was used over the past four decades to improve soil fertility for enhancing crop production. Nevertheless, the average national yields of small cereal including teff were low, despite application of nitrogen and phosphorus (NP) fertilizers. On-farm trials were conducted in the 2015/16 and 2016/17 cropping seasons at 18 locations on Ethiopian highland Vertisols to determine the response of teff to potassium (K) fertilization along with other limiting nutrients. Five K levels (0, 30, 60, 90 and 120 kg ha-1) in the form of murate of potash (KCl) were used in randomized complete block design with three replications. Separate analysis of variance was conducted for each sites and year. Least Significant Difference (LSD) test at P ≤ 0.05 was used to separate means whenever there were significant differences. Analysis of variance revealed a highly significant difference (P < 0.01) between treatments in both straw and grain yields and tissue nitrogen (N) and K concentrations of teff over the twocropping seasons in 67% of the test locations. Additionally, responses to K were obtained on soils with available K test ranging between 166 and 282 mg kg-1. The Ca: K and Mg: K ratios were strongly and negatively correlated with relative yield and the correlations suggest that soil with Ca: K > 50:1 and Mg: K >15:1 are likely to respond to potassium fertilization. The yield advantage accrued due to K application ranged from 30 to 77% in 2015/16 and 8 to 51% in 2016/17 seasons. The economic optimum K fertilizer application rates varied between 60 kg K ha-1 in 44% of the sites to 90 kg K ha-1 in 23% of the sites. The findings highlighted the need for revisiting fertilizer program to enhance the yield and nutrient uptake of teff in K responsive soils and developing critical levels for K in the study sites.

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