RJOAS April 2025
by Nigussie Ashenafi (Department of Soil and Water Management, Wondo Genet Agriculture Research Center, Shashemene, Ethiopia)
Providing up-to-date soil information to the beneficiaries is indispensable in tackling specific and local problems of agriculture. However, the soil resources studied so far in Hawassa Zuria, and Meskan districts were shallow and could not be applicable for site-specific soil management practices. Therefore, this study was conducted to characterize and identify the soils in terms of their morphological, physical, and chemical properties. Purposively the field of maize was selected from two districts. One pit from each location was opened and the profiles were described in situ. The studied soils had considerable heterogeneity in morphological, physicochemical properties within and among the profiles. The pH of the surface soil was moderately acidic (6.18) and neutral (6.94) in Dore Bafeno and Bati Dubeno pedons, respectively. The organic carbon contents in pedons ranged from low to high. The total nitrogen and available phosphorus contents of the surface soils of the two pedons ranged from 0.24 to 0.36% and 4.5-23.6 mg kg-1, respectively. In both pedons, PBS was greater than 50 and had an increasing tendency with depth. The Pedon of Dore Bafeno had a Mollic epipedon and a Cambic subsurface horizon, and this soil was classified as Cambisols (Loamic, Aric, Humic). However, Bati Dubano had a Mollic surface horizon and a PBS (> 50) throughout the profile and classified as Someric Phaeozems (Pantoclayic, Aric). The significant variations in the physicochemical properties of the studied soils, indicating site-specific soil management is required for each soil to maintain and improve soil organic matter and essential nutrients.
Agriculture is the mainstay of Ethiopia’s economic system, which accounts for 35.5% of the gross domestic product (GDP) and over 80% of employment (CSA, 2020). However, the sector has been characterized by low productivity, specifically due to low soil fertility and the absence of site-specific balanced fertilization and applicable soil management practices, among others. Agricultural intensification requires information on the soil resources in terms of their potential and constraints. Former and existing soil management practices tended to reinforce the physical, chemical, and biological degradation of the soils, causing reduced soil productivity (Zingore et al., 2015). As indicated by Benton (2003), sound management of cropping systems requests information on the physicochemical and biological properties of soils in the course of the growing season. However, the acquirement of this information is difficult due to the limited data on characteristics of soils and the excessive stage of variants in soil properties.
Soil characterization is a scientific way of collecting soil facts that allow recognizing the physical, chemical, and mineralogical properties of the soils that alleviate the unfavorable outcomes of soil variety and resource precision agriculture (Verma and Jayakumar, 2012; Sharu et al., 2013). According to Esu (2005) and Giessen et al. (2009), soil characterization is critical for describing and understanding the status and quality of soils. Characterizing the soil can function as an indicator of the soil potential to provide secure and nutritious food, enhance human and animal health, and overcome degradation (Schoenholtza et al., 2000).
Moreover, soil characterization records allow the ideal classification of the soil to serve as a basis for a more detailed assessment of the soil in addition to accumulating preliminary facts on nutrient, physical or other limitations for crop production (Sharu et al., 2013). However, in the absence of reliable soil fertility information, agricultural land use, and in particular crop production has negatively led to crop yields, with the next land degradation. Therefore, for decision-making, knowledge of the kinds and properties of soils throught distinctive measurement is fundamental in aspects of crop production and different land-use types.
Providing up-to-date and location-specific soil information to the beneficiaries based on precise soil studies at the farm level is indispensable in tackling specific and local problems of the agriculture sector (Hailu et al., 2015). Furthermore, Wang et al. (2001) showed the importance of location specific soil analysis in the evaluation of the nutrient status of a given semi-natural and cultivated ecosystems. Different soil types have varying traits in micro-and macro-morphological, physical, chemical, and mineralogical properties (Ukut et al., 2014). Even though the knowledge of the kinds and properties of soils is important, soil resources studies so far have not been detailed and could not be applicable for site-specific soil management practices. Moreover, Hawassa Zuria and Meskan areas are intensive maize-producing districts in the central rift valley of Ethiopia, but site-specific soil characterization and classifications were not done. Hence, this study was executed to characterize the soils in terms of their morphological, physical, and chemical properties, and classify them following the World Reference Base for Soil Resources (IUSS, 2015).
Soil characterization and identification of the soil types were conducted at Dore Bafeno in Hawassa Zuria district, and Bati Dubeno of Meskan district, in the Central Rift Valley of Ethiopia (Table 1). Prior to the study, an investigation survey was carried out within the selected study sites to identify the major information including soils, topography, vegetation, land use and land management and socioeconomic condition. Detailed descriptions of the study sites are indicated under section.
One representative profile was selected at each study site in the main maize growing areas. Before digging the pits, an auger inspection was performed to determine typical profiles at each location. A pedon of 2 x 1.5 x 2 m was opened to study the soil type at each study site. The representative soil profile of each site was described in situ following the Guidelines for Field Soil Descriptions (FAO, 2006). For each pedon, a recently excavated profile was used for the morphological description and soil sampling. The soil profiles were geo-referenced with the use of the Global Positioning System (GPS).
Soil color (dry and wet) notation was determined using the Munsell color chart (Munsell Color Company, 2009). The soil structure was described in terms of the sequence: grade, size, and type (shape) of aggregates, whereas consistency was identified at dry, moist, and wet moisture conditions. The horizon boundaries were described in terms of depth, distinctness, and topography. Both disturbed and undisturbed soil samples were collected from the entire areas of every identified horizon for analyses of the selected physicochemical properties. The soil samples collected from every recognized horizon were air-dried and passed through 2 mm sieve for the analysis of soil pH, texture, avail-P, exchangeable bases, and CEC; however, a 0.5 mm sieve size was used for determinations of organic carbon and total nitrogen.
Undisturbed soil samples were collected by core samplers and dried at 105°C for 24 h for the determination of bulk density, which was estimated by dividing the oven-dried soil sample taken with a core sampler by the volume of the core sampler (Blake and Hartge, 1986). Particle size distribution (percentage of sand, silt, and clay) was determined using the Hydrometer method following the procedure of FAO (1974) and the soil texture was determined using the USDA textural triangle.
The pH (pH-H2O) and electrical conductivity of the soils were determined in 1: 2.5 soils to water solution ratio using a pH and conductivity meter as outlined by Sahilemedihin and Taye (2000), respectively. Organic carbon was determined using the wet oxidation method (Walkley and Black, 1934) whereby carbon was oxidized with potassium dichromate in sulfuric acid solution. Total nitrogen content was determined according to the Kjeldahl procedure as described by Van Reeuwijk (1992), while available P was extracted using the sodium bicarbonate (pH 8.5) following the procedure described by Olsen et al. (1954) and the amount was measured by a spectrophotometer.
The cation exchange capacity of the soil (CEC) was determined by saturating the soil with neutral 1M NH4OAc (ammonium acetate) at pH 7 and the adsorbed NH4+ were displaced by using 1M KCl and then determined by Kjeldahl distillation method for estimation of CEC of the soil (Sahilemedihin and Taye, 2000). Exchangeable bases were determined after leaching the soil with ammonium acetate; Ca2+ and Mg2+ in the leachate were analyzed by atomic absorption spectrophotometer (AAS), whereas exchangeable K+ and Na+ were measured using a flame photometer. Percent base saturation was estimated from the sum of exchangeable bases as a percent of the CEC of the soil. Available micronutrients (Fe, Mn, Zn, and Cu) contents of the soils were determined by extracting with diethylene triamine pentaacetic acid (DTPA) method (Lindsay and Norvell, 1978) and the contents in the extract were determined by AAS.
Based on the field morphological characteristics and laboratory analytical data, the soil types of the study areas were identified according to the World Reference Base for Soil Resources (IUSS Working Group, 2015)
Pearson’s simple correlation matrix was generated using Statistical Analysis Software 9.3 (SAS, 2014) to examine the relationship between different soil parameters.
Soil morphological properties are vital tools for soil classification purposes than those of physical and chemical properties in view that they are located under natural and undisturbed conditions (Sharma, 2006). According to USDA (2010) soil depth class, the soil pedons were very deep (> 190 cm) and are not restricted by the parent materials and/or groundwater (Table 2). Four genetic horizons: Ap, Bw1, Bw2, and BC horizon sequences were identified at Dore Bafeno site (Table 2). Whereas five: Ap, A1, Bk1, Bk2, and Btkg horizon sequence, were identified at Bati Dubano site.
The soil color (moist) in the pedon at Dore Bafeno site varied from very dark grey (2.5Y3/2), black (2.5Y2.5/1), dark olive-brown (2.5Y3/3), to olive-brown (2.5Y4/4). The color of the horizons had the same hue (2.5Y) with varying value and chroma, which ranged from 2.5/1 to 4/4 (Table 2). According to Sharma et al. (2006), values greater than three indicate low organic matter while chroma greater than two suggests a deeper water table in the soil. On the other hand, the profile at Bati Dubano indicated soil color (moist) changing from black 10YR/2/1 to glayic Glay1/2.5/N (Table 2). The soils of the horizons had similar hue, values and chroma, except for the Btkg horizon. The last horizon (Btkg) showed a strong gleying color indicating either iron has been reduced under stagnant saturated conditions and is preserved in the soil (reduced matrix) or removed (redox depletion). The soil color variations observed among and within the pedons suggest the differences in OM contents, parent material, and drainage conditions (Nahusenay et al., 2014; Alem et al., 2015). In both pedons, there was no mottling (red, yellow, or brown splotches) in all horizons suggesting that the soil pores have not been filled with water for prolonged periods (Sharma et al., 2006).
In both pedons, the surface horizon had a weak, fine to coarse granular structure that changed from weak to moderate, fine to strong, and subangular blocky down the profile (Table 2). This result is in agreement with Yitbarek et al. (2016) and Kebede et al. (2017) who found granular soil structure in the surface horizons that changed to angular and subangular blocky structures in the subsurface horizons. In the pedon at Dore Bafeno site, except the bottom horizon (BC), the others possessed slightly sticky and slightly plastic consistency when wet and very friable, loosely, and very firm when moist (Table 2). The surface horizon of Dore Bafeno pedon had a very friable and slightly sticky/slightly plastic consistency, which might be due to the higher OM contents of the surface than those of subsurface layers. Mulugeta and Sheleme (2010) indicated that the surface layers had a friable consistency compared to the subsurface. In pedon of Bati Dubano, however, both the surface and subsurface layers had a friable and slightly sticky/slightly plastic consistency when moist and wet, respectively. These indicate that the soils have a high OM content and biological activities exist throughout the designated strata. Moreover, the friable consistency proves the workability of the soil at the appropriate moisture content (Ashenafi et al., 2010).
Horizon boundaries provide facts on the dominant soil-forming processes that have made the soil. The distinctness of the boundaries of the soil horizons at Dore Bafeno was clear and gradual with smooth topography, whereas at Bati Dubeno the boundaries were abrupt, clear, and gradual distinctness with smooth topography (Table 2). The variations in horizon boundaries among the soil pedons indicate that the soils formed in different soil-forming processes. The root distribution at Dore Bafeno pedon varied from few in the surface layer to very few in Bw1 horizon, no root was observed in the third and fourth horizons. Whereas at Bati Dubano, the root distribution varied among horizons as common, few, and very few at Ap, A1, and Bk1 and Bk2 horizons. In Dore Bafeno, no effervescence was observed by application of 10% HCl, indicating the absence of calcium carbonate. However, in Bati Dubano, effervescence was observed at Bk1, Bk2, and Btkg horizons. This result was also supported via soil analytical results of calcium carbonate equivalency through the titration technique (Table 2).
The description and abbreviations in the table are the following guidelines for field soil description (FAO, 2006). WE, week; MO, moderate; ST, strong; F, fine; C, coarse; M, medium; GR, granular; SAB, subangular blocky; CR, FR, friable; Lo, loose; VFR, very friable; VF, very firm; SST, slightly sticky; NS, non-sticky; SPL, slightly plastic; NPL, non-plastic; BOU, boundary; C, clear; S, smooth; G, gradual; D, diffuse; N, non; SL, slight; MO, moderate; ST, strong (for CaCO3 by HCL).
Soils with a high bulk density exhibit poor physical soil condition, whereas low soil bulk density shows favorable condition for agriculture (Patil and Prasad, 2004). The results of the present study showed inconsistent bulk density values across soil depth in both pedons (Table 3). The surface horizon at Dore Bafeno pedon had a bulk density of 0.94 gm cm-3 (Table 3), while the values in the subsurface horizons ranged from 0.98 to 1.09 gm cm-3. In general, low bulk density values were recorded across the identified horizons, which is below the normal range for mineral soils (1.3-1.4 gm cm-3) as indicated by Bohn et al. (2001). This low bulk density could be attributed to the parent material of the soil at this site, which is pumice and rich organic matter content (Hazelton and Murphy, 2007). Similarly, Akiro et al. (2014) reported that most Andisols show a low bulk density of 0.9 gm cm-3 or less than that of mineral soils. The surface soil had a lower bulk density value than the subsoils, owing to the presence of higher organic matter on the surface soil. This result was fully supported by different scholars, Wakene (2001); Achalu et al. (2012); Mesfin (2020) who reported that the surface soils had lower bulk density compared with their subsurface counterparts.
Likewise, the soils of the Bati Dubano pedon generally showed low bulk density; whereby the lowest (0.99 gm cm-3) and the highest (1.12 gm cm-3) values were recorded on the surface horizons Ap and A1 horizons, respectively (Table 3). Lower bulk density in the surface (Ap) horizon could be due to the higher OM content at the surface (Celik, 2005). In natural soils, bulk density has an increasing tendency with soil depth, due to changes in OM content, porosity, and compaction (Chaudhari et al., 2013). However, in the present study, the soil bulk density decreased with profile depth from A1 to Btkg horizons, possibly due to clay separate increment along with soil depth.
Soil texture is the most stable physical property, which influences other soil properties like soil structure, soil moisture regime, permeability, consistency, workability, root penetrability, and the fertility of the soil (Landon, 2014). In the soils of Dore Bafeno pedon, sand, silt, and clay particles varied from 34 to 42, 32 to 40, and 24 to 30%, respectively. According to Hazelton and Murphy (2007), who rated all sand, silt, and clay contents of the soil into high (>40%), moderate (25-40%), and low (10-25%), respectively. Thus, the sand, silt, and clay contents of the soils ranged between moderate to high, moderate, and low to moderate, respectively (Table 3). Whereas, the soils of the Bati Dubano pedon contained 16 to 28% sand, 24 to 34% silt, and 46 to 48% clay fractions, which could be rated as low to moderate, moderate, and high, respectively (Table 3).
Loam was the main textural class throughout the genetic horizon of the Dore Bafeno pedon. Clay increment by 4% was found just below the overlying horizon (Ap), which is not adequate to characterize it as an argic horizon since argic horizon has an 8% clay increment within 2.5 cm soil (Soil Survey Staff, 2003). At Bati Dubano pedon, clay was the dominant textural class throughout the genetic horizons, and clay separates showed an increasing trend with profile depth (Table 3). The increase in clay content of the subsoil horizons could be due to the weathering of primary minerals and secondary clay synthesis (Buol et al., 2011). The soils in Bati Dubano pedon had higher clay content as compared with that of Dore Bafeno, which might be due to the fine-grained basaltic nature parent materials at Bati Dubano site.
Silt to clay ratio is an index used to evaluate the rate of weathering potential and determine the relative degree of soil development. In the soils of Dore Bafeno Pedon, the silt to clay ratio varied from 1.07 to 1.58, which is more than one, indicating the soils are comparatively young with a high degree of weathering potential. This might be due to the parent materials, which are mainly composed of volcanic rocks, with alkaline lavas, ashes, and ignimbrites (Meron, 2007). This result agrees with the findings of Girma and Endalkachew (2013), Alemayehu et al. (2016), and Mesfin (2020), which indicated silt to clay ratio of 1.1 to 2.0 in the Ziway and Hawassa area soils. Moreover, the surface soil had moderately higher silt to clay ratio than that of the subsurface layers, which is probably because of tillage practices in the topsoil and illuviation of clay minerals into the subsoils.
In Bati Dubano Pedon, however, the silt to clay ratio was below one, which varied from 0.45 to 0.71; this could be due to the presence of high clay content. Previous research report (Buol et al., 2011) indicated that higher clay content confirms a complete alteration of weatherable minerals into secondary clays and oxides. Additionally, the variation in particle-size distribution among the horizons of the profiles shows the distinction in the material from which the horizons have been formed (FAO, 2006).
The analytical results revealed that the pH (H2O) of the soils in Dore Bafeno pedon varied from slightly acid (6.18) in the surface horizon to moderately alkaline (7.91) in the subsurface horizon (Table 4). Whereas in Bati Dubano pedon the soil pH ranged from 6.94 in the surface horizon to 8.38 in the lower horizons, which is neutral to moderately alkaline according to Benton (2003), and Tekalign (1991). In both pedons, the soil pH increased gradually with soil depth, the lowest value being in the surface layers, and this might be due to the removal of basic cations from the overlying horizon by leaching and crop uptake.
The EC data on the surface horizons of the two pedons revealed very low electrical conductivity in general, ranging between 0.03 to 0.06 dS m-1, indicating that the soils are nonsaline (FAO, 2006). However, the sub-surface horizons of the Bati Dubano pedon had higher EC, ranging between 0.06 to 0.09 dS m-1, which might be due to the results of basic cation leaching and parent material. Similarly, the calcium carbonate content in the subsurface horizons of the Bati Dubano pedon ranged from 1 to 5% and exhibited an increasing trend with soil depth (Table 4). This may be either the impact of the parent material or the result of neo-formation (secondary carbonate). This result is in covenant with Ozsoy and Aksoy's (2007), who reported that calcium carbonate content increased with depth. The soil pH values also confirmed the accumulation of a basic (calcic) character (pH 8–8.7) due to the dominance of CaCO3 (FAO, 2014).
According to the rating of Tekalign (1991), the OC contents in Dore Bafeno pedon ranged from low to medium, whereas in Bati Dubano pedon the values varied from low to high (Table 4). The OC contents in the surface soils of Dore Bafeno and Bati Dubano pedons were 2.61 and 4.10%, respectively. Medium and high OC contents at the surface horizons could be attributed to the accumulation of more organic materials. Considering the depth of the pedon, OC showed a decreasing pattern (Table 4). Various findings showed that the surface soils had higher OC than the subsoils (Abay et al., 2015; Alemayehu et al., 2016). In this study, the experimental soils had variable organic carbon content, suggesting that the need for site-specific soil management options. In agreement with this result, OC content in soils had a variable distribution in different areas of the country (Ashenafi et al., 2010; Daniel and Tefera, 2016).
The total nitrogen (TN) concentration in the surface soils of the two pedons ranged from 0.24 to 0.36% (Table 4). According to London (2014) rating, TN content of the surface soils is in the medium range, which might be due to nitrogen removal through crop harvest and continuous cultivation. Soils having low to medium TN require external N containing inputs for sustainable production, as N is dynamic and prone to leaching, volatilization, and crop uptake. Moreover, the TN distribution within the studied pedons accompanied a comparable pattern to that of OC, implying that OM was the major source for TN. This results in agreement with the findings of Mulugeta and Sheleme (2010); Alemayehu et al. (2016) which reported that the TN concentration decreased with increasing profile depth. Therefore, enhancing and/or keeping OM contents of the soil through addition of organic residues is essential for sustainable soil productivity including ecosystem services.
The C: N ratio varied from 10.89 to 13.5:1 in Dore Bafeno pedon and from 10.75 to 13.37:1 in Bati Dubano pedon, and categorized in a medium range, indicating good quality for mineral soils (Msanya et al., 2000) (Table 4). As indicated by Assen and Yilma (2010), the C: N ratio of about 10:1 is ideal and comparatively better for OM decomposition and serves as an index of improved N availability to plants. The surface horizon in Bati Dubano pedon had a relatively wider C: N ratio than that of Dore Bafeno pedon, which might be due to the variation in soil texture, since clay textures have a wider C: N ratio. Organic matter decomposition rate was low in clay-textured soils, which seems to be a major mechanism of soil organic matter preservation (Christensen, 2001).
The available phosphorus content of the soils in pedons ranged from 0.16 to 23.16 mg kg−1 (Table 4), which could be categorized from very low to very high (Benton, 2003). According to Havlin et al. (1999), P-Olsen between 12 to 18 mg kg-1 is considered sufficient for crop production. Thus, the available P contents in the surface horizons of Dore Bafeno and Bati Dubano pedons were low (4.36) and very high (23.16 mg kg-1), respectively. The low and insufficient available P in Dore Bafeno pedon could be due to the high phosphorus fixation power of Andisols. The surface soil in Bati Dubano pedon, however, had very high available P, which could be due to the neutral pH of the soil, as neutral pH has low P fixation power. Moreover, the carryover effects of continuous inorganic P fertilizer application for vegetable production (onion and tomato) might have also contributed to the high available phosphorus as was also reported by Dinku et al. (2014). Alemayehu et al. (2016) also reported a similar finding. Additionally, available P values declined with increasing profile depth, which might be attributed to decrease in soil OM and an increase in clay content. Mulugeta and Shelem (2010) also stated that available P decreased with depth in soils of Kindo Koye site. In general, the available phosphorus content varied in most Ethiopian soils, mainly due to the difference in soil properties, the intensity of soil disturbance, abundant crop harvest, and erosion (Dawit et al., (2002).
The cation exchange capacity (CEC) can help to decide the suitable fertilizer packages and the amount of nutrients needed to correct imbalances. Hazelton and Murphy's (2007) rated CEC of the soils < 6, 6-12, 12-25, 25-40, and >40 cmolc kg-1 soil as very low, low, medium, high, and very high, respectively. Thus, the overall CEC values of the soils in the present study ranged from 22 to 62 cmolc kg-1, which could be rated as medium to very high. High and very high CEC values indicate the soils have a greater ability to hold cations and require higher rates of fertilizer as compared to those with lower CEC (Hamza, 2008). The CEC of soil in the Dore Bafeno pedon ranged from 22 to 32 cmolc kg-1and had an increasing trend with soil depth: from the surface horizon (Ap) to the second horizon (Bw1), due to increase in clay content. However, the CEC values decreased with further profile depth suggesting a decrease in OM content and andic properties with depth. On the other hand, the CEC values of the Bati Dubano pedon ranged between 56 to 66 cmolc kg-1 without a regular trend with soil depth (Table 5). The high CEC values might be due to the high clay and OM contents. A similar result was reported by Benton (2003) indicating the CEC value is closely related to the soil texture, i.e., sandy soils have CECs ranging from 1 to 8; loamy sands, 9 to 12; sandy or silty loam, 13 to 20; loam, 21 to 28; clay loam, 29 to 40; and clay soils >40 cmolckg-1, respectively.
Exchangeable Ca2+ levels vary among inside pedons with a general tendency of increasing with soil depth (Table 5). Its content in the soils of Dore Bafeno and Bati Dubano pedons varied from 10.22 to 49.9 cmolc kg-1 and could be rated as medium to very high (FAO, 2006). In both pedons, the subsurface horizons contained higher exchangeable Ca2+ than the surface horizon, which could be due to leaching from the overlying horizons. Other authors, Ashenafi et al. (2010), Nahusenay et al. (2014) also indicated that the exchangeable Ca2+ accumulation increased with profile depth. However, the exchangeable Mg2+ ranged from low to medium and very high in Dore Bafeno and Bati Dubano pedons, respectively (Table 5). Relatively the highest exchangeable Mg2+ contents were recorded in Bw2 horizon of Dore Bafeno pedon and Bk2gt horizon of Bati Dubano pedon, but Mg2+ content had an irregular trend with soil depth.
In accordance with the EthioSIS (2014) rating, the potassium contents of the soils >2.3, 1.5-2.3, 0.51-1.5, 0.2-0.5, and <0.2 are classified as very high, high, optimum, low, and very low, respectively. Thus, the soils of the two pedons contained optimum to high exchangeable K+ that could be sufficient for crop production. This result supports the common belief that Ethiopian soils contain sufficient K for crop production. The exchangeable Na+ in the soils of Dore Bafeno and Bati Dubano pedons were high and medium, respectively (FAO, 2006), and showed an increasing pattern with profile depth. Generally, the contents of exchangeable bases increased with increasing soil depth, possibly due to leaching.
The order of exchangeable cations in fertile agricultural soils is Ca2+ > Mg2+ > K+ > Na+ as described by Bohn et al. (2001) and any deviations from this order may create an ion-imbalance problem for plants. Thus, the result of the present study revealed that the exchangeable Ca2+ dominates the exchange complex, but there were different orders of exchangeable cations. In Dore Bafeno pedon, the order was K+, Na+, and Mg2+ followed Ca2+. In Bati Dubano pedon however the order was Ca2+ followed by Mg2+, K+, and Na+, which could be considered as suitable cation distribution for plant growth in accordance with FAO (2006b).
Agricultural soils having a percent base saturation (PBS) > 50 are considered as more fertile and favorable for crop production, however, soils with less than 50 are reflected as less promising (FAO, 2006). In both pedons, PBS was greater than 50 and had an increasing tendency with depth, which is probably due to leaching bases from the overlying layers and subsequent accumulation in the subsurface horizons (Table 5). It varied from 55.89 to 69.5% in Dore Bafeno pedon and 73.66 to 105.5% in Bati Dubano pedon and rated as moderate to very high (Hazelton and Murphy, 2007). The Bk2 horizon in Bati Dubano pedon had a PBS greater than 100 (Table 5), which might be due to solubility of calcium carbonate that contributes calcium during ammonium acetate leachate at pH 7. Similar research finding reported by Sekhar et al. (2014) who described that larger PBS observed due to high exchangeable Ca2+ ions occupying the soil exchange sites.
It is critical to know a soil's Ca: Mg ratio to assess the potential impact of calcium on Magnesium (Mg) and Phosphorus (P) uptake by plant roots. The analytical result of the soils in the surface horizon indicated that the Ca: Mg ratios were 27.79 and 5.89 for pedon 1 and 2, respectively, which are greater than 5:1. Thus, calcium may reduce the availability of both Mg and P (Landon, 2014). On the contrary, Engdawork (2015) suggested that the Ca: Mg ratio in most cases was above 3:1, representing the balance between the two mineral elements.
Potassium (K) availability depends on not only the amount of exchangeable K+ contents of the soil but also influenced by the relative amounts of other cations (Ca and Mg). The analytical result of the soils in the surface horizon indicated that the K: Mg ratios were 5.19 and 0.12 for pedon 1 and 2, respectively (Table 5). The surface horizon of the second pedon had K: Mg ratio of 0.12: 1, which is lower than the treshold value of 0.7:1 (Loide, 2004). Thus, Mg may reduce the availability of K (Laekemariam et al., 2016).
Currently, micronutrient deficiencies emerged as a new tricky to crop production in Ethiopia. Micronutrient deficiencies mainly Zn and Cu were widespread in different parts of the country (Yifru and Sofia, 2017). The results of this study showed that the available micronutrients such as Fe, Cu, and Zn were very low in the surface layers of both pedons, which are insufficient for crop production (Benton, 2003). Conversely, the surface layer of the two pedons contained an adequate amount available Mn and considered as enough for crop production. Generally, in Dore Bafeno pedon, the available Fe and Zn distribution decreased consistently with profile depth, the maximum values were found in the surface than the subsurface layers. While Cu and Mn showed inconsistent trends along with profile depth. Therefore, the two experimental soils require application of fertilizers containing Fe, Cu, and Zn for better crop production.
The soils of the study sites were identified according to World Reference Base Legend (IUSS Working Group, 2015) classification system based on the morphological, physical and chemical characteristics of the soils observed in the field and the results obtained from laboratory analyses. Soils of Dore Bafeno pedon had a texture of loam or finer, a clay increment up to 4% observed just below the overlying horizon, which is not enough to represent it as an argic horizon. The profile had granular and subangular blocky structures at the surface and subsurface horizons, respectively. Moreover, the surface horizon is particularly characterized by a quite thick profile depth, very dark grey colored with a Munsell color value of ≤ 3 moist, and chroma of ≤ 3 moist, a high percent base saturation (> 50), and a medium content of organic carbon. These characteristics demonstrate a Mollic diagnostic epipedon. However, most of the subsurface horizons showed evidence of pedogenetic alteration, structural development and removal of carbonates or gypsum, absence of rock structure in over 50 % of the volume of the fine earth fraction and have a thickness > 15 cm. These properties qualify for Cambic horizon. Therefore, the soils is classified as Cambisols according to WRB soil classification system (IUSS Working Group, 2015). Moreover, the profile had loamy and clay loam textural classes in a layers ≥ 30 cm thick, with the surface soils ploughed to a depth of ≥ 20 and OM content greater than 1% throughout the profile. These properties were suitable for clayic, aric and humic supplemental qualifiers, respectively. Therefore, the soil is classified as Cambisol (Loamic, Aric, Humic).
The surface horizon in Bati Dubano pedon is characterized by black colored with a Munsell color value of ≤ 3 moist, and chroma of ≤ 3 moist, weak, coarse, granular structure; friable consistency; clay textural class; low bulk density; neutral soil reaction; and ≥ 0.6% soil OC content, very high CEC, thickness ≥ 20 cm and base saturation (> 50%) throughout the profile. These properties qualify for Mollic horizon. Thus, considering the morphological and analytical properties of the surface and subsurface horizons, the soils of the Bati Dubano site are classified as Phaeozems according WRB soil classification system (IUSS Working Group, 2015). The profile had mollic horizon < 20 cm thick to be classified as Someric principal qualifier. Moreover, the profile had a textural clay class throughout the profile and the surface soils ploughed to a depth of ≥ 20, these properties suitable for Pantoclayic and aric supplemental qualifier, respectively. Therefore, the soil is classified as Someric Phaeozems (Pantoclayic, Aric).
Pearson`s correlation coefficient (r) was conducted to explore the magnitude and direction of the relationships among selected chemical properties of the soils in the pedons along its profile depths. In both pedons, soil reaction (pH) positively and significantly correlated to calcium carbonate concentration, suggesting that dawn ward movement of basic cations partcicularly Ca. Similarly, the soil OC positively and significantly correlated with soil TN in both peofiles, suggesting that the OM was the major source for TN. In line with this result, Meysner et al. (2006) showed that as much as 93 to 97% of the total N in soils is closely associated with OC. Therefore, it may be imperative to maintain and increase the level of organic soil matter in the management of total nitrogen.
In conclusion, two soil types were identified based on their morphological, physical and chemical characteristics observed in the field and the results obtained from laboratory analyses. The soil of Dore Bafeno pedon had a Mollic epipedon and a Cambic subsurface horizon, and this soil was classified as Cambisol (Loamic, Aric, Humic). However, Bati Dubano pedon had a Mollic surface horizon and a PBS (> 50) throughout the profile and classified as Someric Phaeozems (Pantoclayic, Aric). The significant variations in the physicochemical properties of the studied soils, indicating site-specific soil management needs of each soil type to maintain soil organic matter and essential plant nutrients.
The author expresses his gratitude to the Ethiopian Institute of Agricultural Research and the Agricultural Growth Program II for funding the experiment. He also appreciates the technical and field support provided by the staff members of the Soil and Water Management Research Process at the Wondo Genet Agricultural Research Center.
Original paper, i.e. Figures, Tables, References, and Authors' Contacts available at http://rjoas.com/issue-2025-04/article_06.pdf