Cations and Related Soil Variables as the Basis for Nutrients Management Strategies in Central Ethiopian Agricultural Soils - I

The study was aimed at assessing the fertility status of soils with special emphasis on cation “concepts” and related physico-chemical variables as decision-support tools in making fertilizer recommendations. Thirty six soil samples collected from three representative locations, namely Arsi (Ar), East-Shewa (ES) and West-Shewa (WS) were considered. The results of the study showed that all the studied soils were salt-free or had no sodicity problems. But, the exchangeable aluminum (Al3+) was detected in some 20.8% of the sites as a manifestation of strongly acidic soil reactions. In all studied soils, the exchangeable potassium (K+) was adequate or even excess in some sites based on the suggested critical thresholds. Some 20.8% of the soils contained low levels of calcium (Ca2+) with values falling below the suggested critical levels. Still some, 12.5% were marginal, leaving 66.7% of the sites to be safe from Ca nutritional problems. Hence, Ca was found to be dominating the soil-colloids, particularly, in the ES zone. Similarly, magnesium (Mg2+) appeared to be deficient in strongly acidic soils (29.2% of the sites). The excess levels of Ca2+, K+ and even Mg2+ were observed in the alkaline soils sampled from ES. In fact, sandy and strongly acidic soils tend to have relative lower levels of the cations, Mg2+ and Ca2+. The overall study revealed that, in addition to the previously reported deficient macronutrients: nitrogen (N), phosphorus (P) and sulfur (S); Mg was also found to be limiting element in some areas followed by Ca. From the micronutrients: boron (B), zinc (Zn), molybdenum (Mo) and iron (Fe) were found to be the most limiting in the studied soils, all in the decreasing order of importance. Therefore, it is suggested that the deficient amounts of nutrients need to be applied and/or formulated for the specific sites, if soil- and plant analytical data are available.