This study was initiated to assess soil fertility, quantify nutrients content in maize tissue, and mapping selective soil parameters of Migna Kura Kebele, Wayu Tuka District, east Wollega, Ethiopia in 2019. A total of 32 surface soils and leaf samples were randomly collected for nutrients concentration quantify. Soil physicochemical properties and maize leaf nutrients of 32 samples were analyzed using standard laboratory procedures.
Declined soil fertility is one of the main constraints to improve food production in Ethiopia and inadequate information about soil fertility in the study area. To replenish impoverished soils, site and crop-specific studies of soil fertility parameters are required to devise appropriate suggestions for site-specific balanced fertilizer recommendation and soil fertility management in the study area.
TABLE OF CONTENTS
BIOGRAPHICAL SKETCH
ACKNOWLEDGMENTS
ACRONYMS AND ABBREVIATIONS
LIST OF TABLES
LIST OF FIGURES
LIST OF TABLES IN APPENDIX
ABSTRACT
1. INTRODUCTION
2. LITERATURE REVIEW
2.1. Concept of Soil Fertility and Productivity
2.2. Soil Fertility Indicators
2.2.1. Soil physical properties as indicators of soil fertility
2.2.1.1. Soil moisture content
2.2.1.2. Soil texture
2.2.1.3. Bulk density
2.2.1.4. Total porosity
2.2.2. Soil chemical properties as indicators of soil fertility
2.2.2.1. Soil pH and electrical conductivity
2.2.2.2. Exchangeable acidity
2.2.2.3. Soil organic matter and C: N
2.2.2.4. Total nitrogen
2.2.2.5. Available phosphorous
2.2.2.6. Exchangeable Bases
2.2.2.7. Cation exchange capacity
2.2.2.8. Micronutrients
2.3. Soil Fertility Mapping
2.4. Factors Affect Available Macro and Micronutrients in Soil
2.5. Soil and Plant Relationship
3. MATERIALS AND METHODS
3.1. Description of the Study Area
3.1.1. Location
3.1.2. Climate
3.1.3. Topography and Soils
3.1.4. Land use and Vegetations
3.1.5. Farming system
3.2. Site Selection and Soil Sampling
3.3. Soil Samples Preparation and Handling
3.4. Laboratory Analysis of Soil Samples
3.4.1. Soil physical analysis
3.4.2. Soil Chemical Properties Analysis
3.5. Plant Tissue Sampling and Preparation
3.6. Plant Tissue Analysis
3.7. Data Analysis and Interpretation
3.8. Spatial Interpolation and Soil Fertility Mapping
4. RESULTS AND DISCUSSION
4.1. Soil Management Practices in the Study area
4.2. Selected Soil Physical Properties
4.2.1. Moisture content
4.2.2. Particle size distribution
4.2.3. Bulk density and total porosity
4.3. Selected Soil Chemical Properties
4.3.1. Soil pH and available phosphorous, potassium and Sulfur
4.3.2. Exchangeable acidity and percent acid and aluminum saturation
4.3.3. Soil organic carbon, total nitrogen, and carbon to nitrogen ratio
4.3.4. Exchangeable bases, cation exchange capacity, and percent base saturation
4.3.5. Status of micronutrients in soil of the study area
4.4. Concentration of Nutrients in Maize Leaf Tissue
4.4.1. Macronutrients
4.4.2. Micronutrients
4.5. Relationship of Nutrients Concentration in Soil and Maize Leaf Tissue
4.6. Soil Fertility Status Maps of the Study Area
5. SUMMARY AND CONCLUSION
6. REFERENCES
7. APPENDIXES
DEDICATION
This Thesis manuscript is dedicated to my beloved mother Mrs. Bizu Bayana for her kindness, devotion, and endless support to bring me to this level and the memory of my late sister Zewdie Desalegn (1987 -2008 E.C). May God rest her soul in heaven in his domicile forever.
BIOGRAPHICAL SKETCH
The author, Mintesinot Desalegn Eshetu was born on 21 December 1991 at Balakasa Gedo Kebele, Arsi Robe district, Arsi Zone of Oromia Regional State, Ethiopia. He attended his primary education at Habe and Robe Elementary (1-8) school and Secondary and Preparatory school at Robe Didea high school from 1997 to 2009.
After he successfully passed the Ethiopian School Leaving Certificate Examination (ESLCE), he joined Ambo University in 2009 academic year and graduated with a Bachelor of Science Degree in applied Chemistry in 2012 June. After his graduation, he was employed by Oromia Agricultural Research Institute and has been serving as Junior Researcher I and II, and Assistant Researcher II (Soil Chemistry Researcher) in Soil Fertility Improvement Team at Nekemte Soil Research Center, Oromia Region state since June 2013. Then, he joined Postgraduate Program at Haramaya University,School ofNatural Resources Management and Environmental Science in October 2018 to pursue his MSc studies in Agriculture (Soil Science).
ACKNOWLEDGMENTS
First and foremost, I have no words to express my God's endless love, safety, and for supporting me to register success through many ups and downs. This all happened according to his perfect plan. This is only to say thanks to the Almighty God and His Mother Saint Merry. I express my sincere gratitude and respect to my major advisor, Dr. Lemma Wogi for his consistent guidance, encouragement, critical remark, suggestion and valuable comments throughout the development of this Thesis starting from the inception of the proposal. I shared with him his accumulated professional experiences and was cooperative from the beginning of proposal writing to the completion of the Thesis work. I also extend my sincere appreciation to my co-advisor Dr. Samuel Feyissa for his precious time invested in reviewing the proposal and the Thesis thoroughly and giving critical comments and suggestions which became an important part of this Thesis. Both advisers have agreat contribution of the important part of this Thesis with encouragement, endless support, friendly constructive and detailed examination and reviewed the planning stage of the research work.
I would like to extend my special thanks to Oromia Agricultural Research Institute for granting me the study leave and Agricultural Growth Program II for providing financial support for this study at the Haramaya University. My special thanks go to Migna Kura Kebele Rural Development for providing me all information important for the Thesis work and the farmers willingness to allowme their own maize fieldsfor the study.
My heartfelt thanks go to Nekemte Soil Laboratory Center and Oromia Agricultural Research Institute of Food Science Laboratory technician for offering me all possible help in soil and plant tissue laboratory analysis. The contribution of all my friends, particularly that of Mr. Shure Saboka, is highly recognized for the moral and unreserved knowledge shared with me. I would like to express my very profound gratitude to my Mother Bizu Beyana, my Father Desalegn Eshetu, my sisters Nigata, Mestawot and Mezengash, brother Endashew and my aunt sons Engineer Feleke Amde and Birtukan Asrat and Yigazu Sime for their supports and continuous encouragement throughout my life. Words fail to convey my deepest thanks to them. Finally, I express my thanks to all individuals and institutions for their support, encouraged and discouraged in the entire work of the research.
ACRONYMS AND ABBREVIATIONS
Abbildung in dieser Leseprobe nicht enthalten
LIST OF TABLES
Table Page
1. Slope gradient, elevation, altitude classes, and soil fertility management practices of the study area.
2. Soil physical properties of maize fields in Migna Kura Kebele
3. Soil pH, available phosphorous, potassium, and sulfur of maize fields in Migna Kura Kebele
4. Soil exchangeable acidity, percent acid, and aluminum saturation of maize fields
5. Organic carbon, total nitrogen and C: N in soils of maize fields
6. Exchangeable bases, CEC, ECEC and PBS in soils of maize fields of the study area
7. Micronutrients in soil of maize growing fields in Migna Kura Kebele
8. Mean values of macronutrient Concentration of maize leaf at Migna Kura Kebele
9. Mean values of micronutrient concentration of maize leaves in Migna Kura Kebele
LIST OF FIGURES
1. Location map of the study area and sample points
2. Mean monthly rainfall, max and min temperatures of the study area (2006 – 2018)
3. Soil pH (A) and organic carbon (%) (B) status map of Migna Kura Kebele
4. Soil total nitrogen (%) (C) and available phosphorous (mg/kg soil) (D) status map of Migna Kura Kebele
5. Soil available potassium (mg/kg) (E) and sulfur (mg/kg) (F) status map of Migna Kura Kebele
6. Soil calcium (G) and magnesium (cmol(+)/kg) (H) status map of Migna Kura Kebele
7. Soil CEC (cmol(+)/kg) (I) and PBS (%) (J) status map of Migna Kura Kebele
8. Soil hot water extractable boron (K) status map of Migna Kura Kebele
9. Soil DTPA extractable iron (L) and manganese (mg/kg) (M)Status map of Migna Kura Kebele
10. Soil DTPA extractable copper (N) and (mg/kg zinc (mg/kg) (O) status map of Migna Kura Kebele
LIST OF TABLES IN APPENDIX
Appendix Table Page
1. Soil samples points and soil fertility management practices of the study area
2. Mean values of monthly rainfall, minimum and maximum temperatures of Migna Kura Kebele
3. The general scale of bulk density and classification of total porosity and slope
4. Rating of soil pH (H2O) and soil OC, OM, and TN Values of soil
5. Rating of soil available phosphorous and sulfur values of soil
6. Rating of soil exchangeable base, Cation exchangeable capacity, and percent of base saturation 79 Appendix Table 7. Interpretation values DTPA extractable Fe, Mn, Cu, and Zn and hot water extractable boron
8. Pearson Correlation matrix coefficients (r) among selectively soil physicochemical properties
9. Correlation matrix coefficients (r) for relationship of nutrients concentration in soil and maize leaf tissue
10. Soil parameters with the validated test results of selected models
11. Selective soil parameters and rate, class and area (ha, %) of soil fertility status map in study area
12. Macro and micronutrients concentration of maize leaf critical values at tasselling
13. Macro and micronutrient concentration of maize leaf sufficient range values at tasselling
Soil Fertility Status and Nutrients Content in Maize (Zea Mays L.) Tissue at Migna Kura in Wayu Tuka District, East Wollega, Ethiopia
ABSTRACT
Declined soil fertility is one of the main constraints to improve food production in Ethiopia and inadequate information about soil fertility in the study area. To replenish impoverished soils, site and crop-specific studies of soil fertility parameters are required to devise appropriate suggestions for site-specific balanced fertilizer recommendation and soil fertility management in the study area. With this milieu, this study was initiated to assess soil fertility, quantify nutrients content in maize tissue, and mapping selective soil parameters of Migna Kura Kebele, Wayu Tuka District, east Wollega, Ethiopia in 2019. A total of 32 surface soils and leaf samples were randomly collected for nutrients concentration quantify. Soil physicochemical properties and maize leaf nutrients of 32 samples were analyzed using standard laboratory procedures. The data obtained were analyzed by Microsoft Excel and SPSS software version 20. Soil fertility status maps were prepared using the ordinary kriging interpolation technique and employed with ArcGIS10.4.1 and rated as very low to very high following the criteria nutrient rating guidelines for pH, OC, TN, P, K, S, Ca, Mg, CEC, PBS, B, Fe, Mn, Cu,and Zn.The Laboratory analysis results revealed that textural classes of soils are clay loam and clay. Soil bulk density and total porosity varied from 1.13 to 1.46 g cm-3 and 42.57 to 55.15 %, respectively. Soil reaction varies from slightly acidic (pH=6.7) to strongly acidic (pH=4.91). Soil exchangeable acidity values ranged from 0 to 2.46 cmol (+) kg-l. Soil OC values ranged from the medium (1.79 %) to the high (3.51 %) range. Soil total N and available P values were between very low to low (0.19 to 1.11 % and 6.71 to 13.44 mgkg-l, respectively). Available K and S values ranged from very low to medium (27.57 to 290.78 mg/kg) and (1.34 to 13.76 mg/kg), respectively. Exchangeable Ca, Mg, K, and Na of the soil values varied from 6.93 to 51.15, 2.85 to 23.63, 0.21 to 1.76, and 0.05 to 0.71 cmol (+) kg-1, respectively, while medium to avery high level of CEC (22.19 to 77.42 cmol (+) kg-1 was registered. Soil PBS varied from 42.13 to 98.30 % within a rating of moderate to very high. Soil DTPA extractable Fe, Mn, Cu, Zn and B values varied from 12.07-33.51, 16.02-48.26, 0.18-0.62, 1.09-3.68 and 0.19-0.62 mg kg-1, respectively. Based on the Maize leaf macronutrients concentration analysis of N, P, K, S, Ca and Mg concentration values varied between 1.90-3.20, 0.07-0.21, 0.69-2.89, 0.05-0.48, 0.2-0.91, and 0.38-0.90 %, respectively. Ca, Mg, S, and K nutrients concentrations were at sufficient levels whereas 81.25% and 100 % of the total leaf samples were deficient in Nand P nutrients content, respectively. The levels of micronutrients concentration in maize leaves tissue values ranged from 2.43-8.02 mg/kg for B, 96.36-190.38 mg/kg for Fe, 32.43-226.01 mg/kg for Mn, 18.7044.38 mg/kg for Cu, and 15.87-45.40 mg/kg for Zn. Fe,Mn, and Cu nutrients concentrations were sufficient, while in 37.5 and 56.25 % of the total leaf samples, Zn and B were below the critical level. Soil micro and macronutrient results revealed highly significant at (P< 0.01) and positive correlations with maize leaf macro and micronutrient concentrations. These results of the present study indicated that the soils affected by soil acidity and deficiencies of six yieldlimiting nutrients N, P, K, S, B, and Zn were identified in soils of the study area. Organic matter, N, P, S, K, B, and Zn containing fertilizers and lime should be applied to soils for sustainable crop production in the study area.
Keywords: Soil physicochemical, Macronutrients, Micronutrients, maize leaf, and Mapping
1. INTRODUCTION
Agriculture is the backbone of the national economy, but soil fertility decline is a big issue in the Agriculture of Ethiopia. Soil fertility depletion is one of the main causes of low soil productivity and Agricultural in Ethiopia. Declining soil fertility is one of the most significant constraints to increased food production in Ethiopia (Gete et al., 2010). Ethiopia is facing a wider set of issues in soil fertility beyond chemical fertilizer use, which has historically been a major focus for extension workers, researchers, policymakers, and donors. If left unchecked, this wider set of issues will limit future agricultural productivity across the country, and in some areas; they already limit the effectiveness of chemical fertilizer in crop production. Farming without adequate fertilizer and manure has stripped the soil of vital nutrients needed to support plant growth (IFPRI, 2010). Soil fertility decline has been recognized as one of the most challenging and limiting factors for food security in the country (MoARD, 2010). The major one for the decline of soil fertility is land degradation because of deforestation, human and livestock population pressure, inadequate use of crop residues and animal dung, and little or no use of modern technologies to restore soil fertility (Taye and Yifru, 2010).
The soil fertility decline causes include loss of organic matter (OM), macro and micronutrient depletion, soil acidity, topsoil erosion and deterioration of physical soil properties, nutrients depletion includes farming without replenishing nutrients over time (loss through continuous crop harvest), removal of crop residue, and low level of fertilizer use and unbalanced application of nutrients (IFPRI, 2010; Sommer et al., 2013). The Ethiopian highlands have been experiencing declining soil fertility and severe soil erosion due to intensive farming on steep land, complete crop residue removal, and high nutrient depletion in the country have been cited as pressing challenges that aggravate soil degradation and thereby low crop productivity (Haileselassie et al., 2005; Tadele et al., 2013; Fanuel, 2015). Even if the government of Ethiopia has made several interventions, the country still loses a tremendous amount of fertile topsoil, and the threat of land degradation is broadening alarmingly (Teklu and Gezahegn, 2003). Replenishing soil fertility is the primary biophysical requirement for increasing crop production in sub-Saharan African countries (Sanchez, 2010).
To counterbalance the soil nutrient problems, the intervention in the country has been based on the research efforts made during the 1950s and the 1960s. Accordingly, N and P were applied to the soil in the form of di-ammonium phosphate (DAP) and Urea fertilizers, whereas other macro and micronutrients were neglected from being used for crop production. In most nutrient studies in Ethiopia, more emphasis was given to macronutrients, especially N and P, and micronutrient investigations received little attention. Studies related to the micronutrient's status of Ethiopian soils, on the other hand, are scarce, although the role of micronutrients in plant nutrition is very important as macronutrients (Yifru and Mesfin, 2013).
Thus, to increase crop yields, the government of Ethiopia has launched an extension of a full package, which gives more attention to high external inputs and high-yielding crop varieties (Elias, 2002). Accordingly, the national recommendation for applying N and P rates was set at 100 kg DAP and 50 Kg of Urea per hectare, respectively. However, the real existence shows that farmers are applying only a smaller amount of mineral fertilizers between 7 and 10 kg ha-1 annually and a low rate of fertilizer use suggests the limited capacity of the farmers to purchase and fear of debt (Elias, 2002). By 1995 only two-thirds of rural households in Ethiopia had been using mineral fertilizers at these lower rates (Pender et al., 1999). The unreliable rainfall also other problems (World Bank, 2007) and the increased cost of mineral fertilizers leads farmers to use lower rates of mineral fertilizer, which is the cause of yield decline.
Maize (Zea mays L.) is the most widely cultivated cereal crop in terms of area coverage (16.79 %) and production (27.43 %) with about 8.39 million tons of production in Ethiopia (CSA, 2018). It is a major grain crop used for consumption and the market in Ethiopia. Soil, being the natural medium for plant growth, has a direct impact on the yield and quality of crops growing on it. Maize is a crop with a high demand for soil nutrients. To produce one ton of maize grains, the plant removes 24 kg N, 3 kg P, 23 kg K, 5 kg calcium (Ca), and 4 kg magnesium (Mg) from the soil (Fageria et al., 2011). Jensen and Cavalieri (1983) reported that crop growth and development characteristics, especially for a crop such as corn (Zea mays L.) are very sensitive to changes in soil fertility. Cultivation of improved varieties without balanced nutrient management further aggravated the problem of nutrient depletion and was reported to be yieldlimiting factors (Mesfin et al., 1998).
At the country level, a higher depletion rate of macronutrients and their deficiencies has been reported by Haileselassie et al. (2005). In addition to these, several investigators (Wassie and Shiferaw, 2011; Abdenna et al., 2013; Alemayehu and Shelem, 2013), sulfur (S), (Itanna, 2005; EthioSIS, 2014; EthioSIS 2015) reported K deficiencies and micronutrients such as boron (B), copper (Cu) and zinc (Zn) (Wakene and heluf, 2003; Teklu, 2004; Wondwosen and sheleme, 2011; EthioSIS, 2015) and iron (Fe) (EthioSIS, 2014). In east Wollega Zone, Western Ethiopia farmers are reporting yield decline despite the application of nitrogen and phosphorus fertilizers in the form of di-ammonium phosphate and Urea and there are also reports that show soil acidity is increasing and basic cation such as calcium, magnesium, and potassium are deficient while acidic cations are at toxic level (Abdenna et al., 2006; Abdenna et al., 2007). This has led to the causes of nutrient imbalance. Thus, soil fertility needs to be maintained, agricultural systems need to be transformed to increase the productive capacity and stability of smallholder crop production (FAO, 2011). This demands the need to investigate the soil nutrient status and the responses of crops growing on it.
The farmers of the study area have been practicing a mono-cropping system and do not use rotation crops every year. This farming system accelerates the depletion of soil fertility status. The landscapes associated with high rainfall also contribute to the loss of nutrients. This highlights the need for regular soil fertility status and maintenance in the study area. The sitespecific soil fertility status, particularly for the soil of maize growing fields, has not been studied. In addition to this, periodic assessment of important soil physical, chemical, and biological properties and their responses to changes in land management is necessary to apply appropriate agricultural technologies and effective design of soil fertility management techniques and to improve and maintain fertility and productivity of soil (Wakene and Heluf, 2003). The best use of plant analysis is to monitor nutrient status and diagnose existing nutrient problems (Flynn et al., 2004; Cleveland et al., 2008). Plant tissue analysis, on the other hand, reveals the nutritional status of the plant directly and when combined with the soil tests can be used to evaluate the nutritional sufficiency of the soil-plant system.
Soil tests are designed to help farmers predict the available nutrient status of their soils. Once the existing nutrient levels are established, producers can use the data to best manage what nutrients are applied, decide the application rate and make decisions concerning the profitability of their operations (Getachew and Berhane, 2013; Getachew et al.,2015). Assessing soil physicochemical properties is used to understand the potential status of nutrients in the soil.
Soil fertility assessment is a key for the sustainable planning of a particular area. According to EthioSIS (2015) at the national level assessments of soil fertility and soil fertility status maps were initiated, but from different land used soil samples and suggested fertilizer types for cultivation land to the study area. The soil fertility map needed to manage soil fertility status spatial variability should follow the most suitable prediction method by implementing geospatial analyst tools. The ordinary kriging technique is widely used to map spatial variation of soil fertility, because it provides a higher level of prediction accuracy (Song et al., 2013). The lack of site-specific fertilizer recommendations to replenish declining soil fertility has been the major challenge to boost crop production in Ethiopia (Alemu et al., 2016).
Inadequate information about soil fertility is one of the main constraints in the study area as a result of the lack of area-specific information on soil fertility status. Lack of area-specific information on soil fertility status is one of the major challenges for site-specific balanced fertilizer recommendation and sustainable natural resource management in the study area. There was a gap between soil-plant tissue analysis and site-specific soil samples survey to know the soil fertility status in the study area. Site-specific estimates of soil nutrient status and subsequent maize tissue analysis are very important for rational fertilizer use in the study area. Combined use of soil and maize tissue analysis is believed to evaluate the complex interaction, get an accurate picture of limiting nutrients. Unfortunately, in Migna Kura Kebel e, extensive surveys dealing with the assessment of soil fertility status, soil fertility map, and maize tissue nutrients content relations have not been studied and the information was still very scarce. Therefore, the general objective of this study was to assess soil fertility status, quantify maize tissue nutrients content, and mapping selective soil fertility parameters at Migna Kura Kebele, Wayu Tuka district, east Wollega, Ethiopia. Moreover, the specific objectives were:
- To assess selected soil fertility parameters in maize growing fields at Migna Kura Kebele
- To quantify nutrients content of maize tissue growing at Migna Kura Kebele
- To evaluate the relationship between soil and maize tissue nutrients content
- To map selective soil fertility status of Migna Kura kebele
2. LITERATURE REVIEW
2.1. Concept of Soil Fertility and Productivity
Soil fertility is a quality of soil to supply nutrients in proper amounts without causing toxicity, whereas soil productivity is the capacity of a soil to produce a specific crop or sequences of crops at a specific management system (Foth and Ellis, 1997). The optimum productivity of any cropping system depends on an adequate supply of plant nutrients. Soil fertility and productivity is more than just plant nutrients and can be defined as “the physical, biological, and chemical characteristics of soil, for example, its organic matter content, acidity, texture, depth, and water retention capacity all influence fertility” (Gruhn et al., 2000). The whole world in general and developing the world in particular, need reliable information and knowledge on soil fertility and agriculture productivity which are the most challenging issues of rural livelihoods. To attain sustainable crop production, improving crop nutrition through appropriate soil fertility management is highly essential.
Soil productivity in Africa is declining as a result of soil erosion, nutrient and organic matter (OM) depletion (Abreha, 2013). Declining soil fertility is one of the most significant constraints to increased food production in Ethiopia (Gete et al.,2010). In the Ethiopian cultivated fields, about 42 t ha-1of fertile soils have been lost every year (Akamigbo and Asadu, 2001) together with essential plant nutrients mainly due to poor soil management. Soil nutrient availability changes over time. Ethiopian soils indicated that elements like K, S, Ca, Mg and micro-nutrients particularly Cu, Mn, B, Mo, and Zn are becoming depleted and deficiency symptoms are being observed on major crops in different areas of the country (Asgelil et al., 2007). It is necessary to assess the capacity of soil to supply nutrients to supply the remaining amounts of needed plant nutrients.
2.2. Soil Fertility Indicators
Sustaining soil and environmental qualities is the most effective method for ensuring a sufficient food supply to support life (Soares et al., 2005). Most of the important soil fertility indicators are significantly influenced by different land-use systems, particularly at the surface horizon. The bulk density, soil structure, OC, soil pH, CEC, total N, different forms of P, exchangeable bases, and available micronutrients are affected due to intensive cultivation, soil erosion, and use of acid-forming inorganic fertilizers for the past three decades. Maintaining soil fertility mainly depends on the knowledge of the physicochemical properties of a given soil.
2.2.1. Soil physical properties as indicators of soil fertility
The soil's physical properties determine its adaptability to cultivation and the level of biological activity that can be supported by the soil. Soil physical properties also largely determine the soil's water and air supplying capacity to plants. Any soil physical properties change with changes in its soil fertility management such as the intensity of cultivation, crop rotation, crop residue management, farmyard manure application, and the nature of the land under cultivation, rendering the soil less permeable and more susceptible to runoff and erosion losses. Soil physical properties change with changes in the land-use system and its management such as the intensity of cultivation, the instrument used, and the nature of the land under cultivation, rendering the soil less permeable and more susceptible to runoff and erosion losses.
The soil's physical properties strongly influence the soil fertility and productivity of soils through their parameters such as structure, texture, bulk density, moisture-holding characteristics, and soil aeration (Mulugeta, 2004).
2.2.1.1. Soil moisture content
Water is the most limiting factor in the arid to the semi-arid area. Soil moisture is a major significant indicator of indirectly affecting soil fertility. Soil moisture influences crop growth not only by affecting nutrient availability, but also nutrient transformations and soil biological behavior.
2.2.1.2. Soil texture
Soil texture (or particle size distribution) is a stable soil characteristic that influences the physical and chemical properties of the soil. The sizes of the soil particles have a direct relationship with the surface area of the particles. Soil particles remain aggregated owing to various types of binding forces and factors. These include the content of OM, other colloidal substances present in the soil, oxides of iron (Fe) and aluminum (Al), and the hydration of clay particles. Soil texture is an essential aspect of the soil and the one most often used to characterize its physical make-up, having a bearing on such soil behaviors as nutrient and water holding capacity, organic matter (OM) level and decomposition, aeration, infiltration rate, drainage and/or permeability and workability (Sys et al., 1991). The distribution of clay in profile increased with depth and the total sand fraction highest in topsoil.
2.2.1.3. Bulk density
Bulk density is an indicator of soil pore space and compaction and it is a measure of the density of a porous material that takes into account the density of the solid material and the amount of porosity (Hazelton and Murphy, 2007). Textural differences between soils influence the value of bulk density (for example, clay, silt clay, and clay loam surface soils show low bulk density as compared to sands and sandy loam soils that show high bulk density values) (Gupta, 2000). Soil Bulk density is increased with the increase in soil profile depth because of variations in organic matter content, porosity, and compaction (Pravin et al., 2013). White (1997) stated that values of soil bulk density vary from < 1 g/cm3 for soils high in organic matter content, 1.0-1.40 g/cm3 for well- aggregated loamy soils, and 1.2 to 1.8 g/cm3 for sands and compacted horizons in clay soils. Bulk density commonly decreases as mineral soils become finer in texture. Soils having low and high bulk density show favorable and unfavorable physical conditions respectively (Mitiku et al., 2006). Low bulk density values (generally below 1.3 gm cm-3) indicate a porous condition of the soil (FAO, 2006b).
Soil particle density refers to the average density of the soil particles not including fluid or pore space and is commonly stated in grams per cubic centimeters (g cm-3). Hussein (2002) reported that surface soil layers possessed lower particle density values than the subsurface soil horizons. Soil porosity is also part of the soil volume, which is not occupied by solid particles, but occupied with water and air. Soils having high bulk density exhibit poor physical conditions and can limit root growth, air circulation (Assefa, 2009), and availability of less mobile nutrients such as P and K.
2.2.1.4. Total porosity
Soil porosity refers to the total volume of pores in the soil. Pores can be filled either with air or water. Large pores allow the rapid exchange of air and drainage of excess water. Small pores retain water against drainage and the availability of this water for crops depends on the diameter of the pores. For instance, the percentage volume occupied by small pores in sandy soils is low, which accounts for their low water holding capacity but good aeration due to the high percentage of the macro-pores. Clay soils, in contrast, are characterized by having many small pores with a large water holding capacity, but not all of this water is available to plants. The total porosity of soils usually lies between 30 to 70% and may also be used as a very general indication of the degree of compaction where management exerts a decisive influence on it as in the case with bulk density.
2.2.2. Soil chemical properties as indicators of soil fertility
Soil chemical properties are those that are mainly influenced by the chemical components of soil, which are responsible for the chemical reactions and processes of soil. The result of soil mineral component weathering, decomposition of OM in the soil, and the activity of plants and animals of plant and animal growth and human development. Determinants of soil chemical quality include soil organic matter content, total nitrogen (N), soil reaction (pH), the salt content of soil (EC), cation exchange capacity (CEC), exchangeable bases, and plant available nutrient reserves. The indicators that showed the greatest change from pristine conditions were organic C, N, P, Mg, K, B, Ca, and Zn contents and cation exchange capacity (Allotey et al., 2008; Noredin et al., 2013). Rapid nutrient depletion caused by intensive cropping may result in drastic yield reduction (Subbian et al., 2000). Maintaining a favorable plant nutrient balance is critical to soil chemical fertility. Attributes include soil reaction, salinity, aeration status, organic matter content, cation exchange capacity, the status of plant nutrients, concentrations of potentially toxic elements, and possibly the most important attribute, the capacity of the soil to buffer against change (Swarp, 2004 ). The chemical reactions that arise in the soil highly affect processes leading to soil improvement and soil fertility makeup.
2.2.2.1. Soil pH and electrical conductivity
The significance of soil pH is one of the most commonly measured soil properties because it indicates processes that are taking place in soils and their effect on plant growth. The soil pH is a measure of soil acidity or alkalinity that gives an indication of the activity of the hydrogen ion (H+) and hydroxyl ion (OH-) in a water solution. Both these ions have high chemical activity. Soil pH has an appreciable influence on soil fertility, activity of organisms and nutrient availability, and plant growth (Prasad and Power, 1997; Daji et al., 1996; McBride, 1994; Chesworth, 2008). The principal adverse effects of acidity on soil fertility occur at soil pH values below 5.5 due to the acid dissolution of aluminum (Al3+ and Fe2+ phytotoxicity to susceptible plants (Lal, 2006).
Their chemical activity is lowest when the solution or soil is close to a neutral pH of 7.0. The pH characterizes the chemical environment of the soil and may be used as a guide to the suitability of soils for various pasture and crop species. Soil pH is also an indicator of the chemical processes that occur in the soil and is a guide to likely deficiencies and/or toxicities (Slattery et al., 1999). In strongly acid soils (soil pH less than 5.2) the availability of the essential elements such as calcium (Ca), magnesium (Mg), nitrogen (N), and phosphorus (P) as well as the micronutrients molybdenum (Mo) and boron (B) is reduced. In contrast, the elements of zinc (Zn), copper (Cu), iron (Fe), and manganese (Mn) are highly soluble in strongly acid soils and may even approach concentrations that are toxic to some plants. Aluminum (Al) toxicity to plants can also occur strongly to very strongly acid soils, (pH 5.1) and below. Conversely, elements such as Zn, Fe, Cu, and Mn are quite insoluble in slightly to moderately alkaline (pH 7.3 to 8.2) soils, which can lead to plant deficiencies for these elements. Soil pH is probably the most important master chemical soil parameter. It reflects the overall chemical status of the soil and influences a whole range of chemical and biological processes occurring in the soils. Most plants and soil organisms prefer a pH range between 6.0 and 7.5 (Hazelton and Murphy, 2007).
Electrical conductivity is the common measurement of soil salinity and is indicative of the ability of an aqueous solution to carry an electric current. By agricultural standard, soil with an EC greater than 4 dS/m is considered saline. Salt-sensitive plants may be affected by the increasing electrical conductivity is less than 4 dS/m range. The electrical conductivity of soil is heavily dependent on the climatic conditions of the area. In soils of sub-humid tropics where there is sufficient rainfall to flush out base-forming cautions from the root zone, EC is found to be too low, usually, <4 dS/m (Landon, 1991). Soil's EC is related to soil pH, nutrient availability, water holding capacity, cation exchange capacity which affect crop yield (Aimrun et al., 2007).
2.2.2.2. Exchangeable acidity
Soil acidity is one of the most important soil factors which affect plant growth and ultimately limit crop production and profitability (Fageria, 2009), and its problems are common in all regions where precipitation is high enough to leach appreciable amounts of exchangeable bases from the soil surface (Achalu et al., 2012). Soil acidity mainly at soil pH < 5.5 affects the growth of crops due to the high concentration of aluminum (Al) and manganese (Mn), and deficiency of P, nitrogen (N), sulfur (S), and other nutrients (Abreha, 2013). The declining fertility status of most of the Ethiopian soils continues to decline to pose a serious threat to crop production and food security. In Ethiopia, large areas of highlands with an altitude of >1500 mals located in almost all regional states of the country are affected by soil acidity. According to Ethiosis (2014) study, about 43% of the Ethiopian arable land is affected by soil acidity. The main factors giving rise to increased soil acidity in Ethiopia include climatic factors such as the high amount of precipitation, temperature, and severe soil erosion, morphological and anthropological factors. Most cultivated lands of the Ethiopian highlands are prone to soil acidity due to the removal of several nutrients by leaching, crop mining, and runoff as compared with grazing and forest lands. A recent study also showed that soil acidity increased on cultivated land as compared with forest and grazing lands in the western part of Ethiopia (Achalu et al., 2012).
The largest areas of the Western Oromia highlands are dominated by Nitisols with high acidity (Mesfin, 1998; Temesgen et al., 2011). In the Oromia regional state most huge surface areas of highlands are affected by soil acidity. Also, Mesfin (2007) reported that moderately acidic soils with pH less than 5.5 considerably influence crop growth and require intervention, and the Western part of the Oromia region, most of the Wollega zone, is affected by soil acidity with pH less than 5.5. Aluminum concentration is one of the indicators of soil acidity (Rengel, 2003). Soil aluminum concentration of 2-5 ppm is toxic to the roots of sensitive plant species and above 5 ppm is toxic to tolerant species. A high concentration of Al in acidic soils means the soil is toxic to plant growth. The strongly acid soils are found in ecologies that receive or have historically received a high incidence of rainfall and have warm temperatures much of the year. They are often found in Oxisols, Nitisols and Ferralsols. Thus, the most strongly acidic soils are found in the western and southwestern parts of the country (Taye, 2008).
Soil acidity limits or reduces crop production primarily by impairing root growth thereby reducing nutrient and water uptake (Marschner, 1995). Moreover, low pH or soil acidity converts available soil nutrients into unavailable form and also acidic soils are poor in their basic cautions such as Ca, K, Mg, and some micronutrients which are essential to crop growth and development (Wang et al., 2006). The extent of damage posed by soil acidity varies from place to place depending on several factors. But there are also occasions where total loss of crops occurs due to soil acidity. Activities of exchangeable basic (Ca2+, Mg2+, and K+) cautions; orthophosphate (H2PO4-), nitrate (NO3-), and sulfate (SO42- ) anions with soil organic matter content and their availability to plant roots might be hampered by acidifying ions (Thomas and Hargrove, 1984). Soils become acidic during geological evolution, especially in areas of high rainfall, because bases are relatively easy to leach from soils, leaving them acidic. Also, soil acidification is frequently inevitable in agriculture that relies on either N2 fixation or cheaper ammonium-containing N fertilizers (Rengel, 2003). When the soil is becoming acidic, the concentration of acidic cautions such as H, Al, Fe, Mn, ammonium (NH4+) increases and becomes toxic to plant roots and inhibits microorganisms' activity which influences nutrient uptake and crop growth. Soil acidity is becoming a serious threat to crop production mainly in the areas of the western, southern, and central highlands of Ethiopia (Wassie and Shiferaw, 2009).
2.2.2.3. Soil organic matter and C: N
Organic matter is the material in soil that is directly derived from plants and animals. Organic carbon is the main element in organic matter (58%) (Sarkar and Halder, 2005). Clay-humus complex, a mixture of organic matter and clay, is a storehouse of nitrogen, phosphate, and sulfur in soil (Biswas and Mukherjee, 1994). In general, soil having a cover of grass or forest contains more organic matter than arable soils (Daji et al., 1996). Soil is regularly plowed or harrowed, containing less organic matter than those not so cultivated. With greater aeration, decomposition of organic matter and depletion of humus has been increased with cultivation (Daji et al., 1996; Wolf & Synder, 2003). Through its breakdown and interaction with other soil constituents, it is largely responsible for much of the physical and chemical fertility of the soil (Charman and Roper 2007). Organic matter is an important source of some plant nutrients.
Soil organic matter (SOM) influences many soils' biological, chemical, and physical properties that favorably influence nutrient availability (Tisdale et al., 1993). Soil organic matter acts as a conditioner by improving soil structure, moisture content, and ion retention, besides being an important source of some nutrient elements such as N, P, K, S, Zn (Uriyo et al., 1979). The only important natural source of N in the soil is organic matter (Davies et al., 1993). The organic fraction of P generally constitutes 20-80% of total P in surface horizons (Brady and Weil, 2000). Stevenson and Ardakani (1972) reported that generally, Zn associated with the soluble fraction of (SOM) such as organic acids and amino acids is readily available, whereas that associated with humic acids is less available. An organic matter depletion of the soils is a widespread problem in Ethiopia investigation and soils of the different areas can be rated as low concerning their organic matter and total nitrogen content (Gete et al., 2010). Soil to be productive it needs to have an organic carbon content in a range of 1.8-3.0% to achieve a good soil structural condition and structural stability (Charman and Roper, 2007). In the same document, that soil with organic carbon of 1% and less is considered as low to extremely low. The nitrogen rating of soil is also indicated as high when its soil concentration is in the range of 0.25-0.5% but can be considered as low below these values (Bruce and Rayment, 1982). Therefore, organic matter content is also affecting soil fertility status.
2.2.2.4. Total nitrogen
Nitrogen is one of the macronutrients for plant growth. Nitrogen is available to plant in the form of ammonium (NH4+) and nitrate (NO3-). In a natural environment, nitrogen is synthesized from organic protein and organic compounds. In a geo-pedological system, nitrogen is incorporated through chemical fertilizers, such as di-ammonium phosphate, (NH4)2HPO4, urea, CO (NH2)2. Nitrifying bacteria normally convert ammonium rapidly to nitrate (Addiscott, 2005). As suggestion by Wakene et al. (2007) elucidated that integrated nutrient management is an option to alleviate soil fertility problems as it utilizes available organic and inorganic nutrients for sustainable agricultural production and productivity. Aspasia et al. (2010) also revealed that combined use of NPK and FYM increased soil OC, total N, P, and exchangeable K by 47, 31, 13, and 73%, respectively compared to the sole application of NPK fertilizers. From this finding generalizing applying compost along with chemical fertilizers and consequently, integrated use of inorganic fertilizers and compost improved the efficiency of chemical fertilizers and crop productivity as well as sustain soil health and fertility.
2.2.2.5. Available phosphorous
Phosphorus (P) is an essential plant nutrient, and its deficiency in soils severely restricts crop yields. Tropical and subtropical soils are predominantly acidic and often extremely deficient in phosphorus. Moreover, most of these soils possess a high phosphate sorption capacity. Strongly high P sorption capacity or fixed phosphate is unavailable for plant uptake. Therefore, substantial P inputs are required for optimum plant growth and adequate food and fiber production. Phosphate is another macronutrient in the growth of the plant. Phosphorus is not fixed in the soil by natural processes but its losses from agricultural land through leaching, crop removal, and insoluble forms of phosphorous. Phosphorous availability in soil is higher in the pH range of 6.0 to 6.5 (Prasad and Power, 1997). In an acidic solution, applied phosphate reacts with Fe-2 and Al+3 and produces Al-phosphate. In the solution of pH 7.0, both H2PO4- and HPO42- ions are found because Ca-clay plays a dominant role in phosphate retention (Prasad and Power, 1997).
Although P is known as the master key to agriculture, the lack of available P in the soil limits the growth of both cultivated and uncultivated plants (Foth and Ellis, 1997). Phosphorus also forms insoluble compounds in strongly alkaline soils (pH >8.3) rendering it unavailable to plants. Tropical and subtropical soils are predominantly acidic with high P fixation capacities and often are extremely P deficient (Mamo, 2011). In acidic soils, there are very high contents of iron (Fe) and Al bonded P fractions compared to the other fractions which could be due to the high content of Fe/Al-oxides, low pH, and advanced stage of weathering of the soils that control the plant available P, and the high content of Al and Fe oxides and hydroxides are the main factors for the strong P fixation in acidic soils (Asmare, 2014). Soil acidity and low availability of P are among the major problems that limit crop production in the highlands of Ethiopia (Melese, 2014).
2.2.2.6. Exchangeable Bases
Potassium is the third macronutrient for plant growth. Potassium is available as K+ and K2O ions in soil. Potassium is also removed from soil through crop removal and leaching. Due to gab on the potassium dynamics in Ethiopian soils and the absence of a remarkable response to potassium application in the central and northern part of the country, as a result, there has not been an adequate focus on potassium in the national fertilizer scheme (Mesfin et al., 2007). This is happening because of soil fertility status from time to time, not inventory.
Research works conducted on Ethiopian soils indicated that exchangeable Ca and Mg cations dominate the exchange sites of most soils and contributed higher to the total percent base saturation particularly in Vertisols (Mesfin, 1998 and Achalu et al., 2012). Heluf and Wakene (2006) revealed that variations in the distribution of exchangeable bases depend on the mineral present, particles size distribution, degree of weathering, soil management practices, climatic conditions, degree of soil development, the intensity of cultivation, external inputs (ash and manure) and the parent material from which the soil is formed.
2.2.2.7. Cation exchange capacity
The cation exchange capacity (CEC) of soils is defined as the capacity of soils to absorb and exchange cautions (Brady and Weil, 2002). Cation exchange capacity is an important parameter of soil because it indicates the type of clay minerals present in the soil, its capacity to retain nutrients against leaching, and assessing their fertility and environmental behavior. Highly reduced CEC of the cultivated lands is due to OM depletion and soil erosion. Other literature also justifies that soil exchangeable cautions are highly dependent upon soil texture and OM content (Fantaw and Abdu, 2011). Since organic matter and soil clay particles have negatively charged sites and can attract positive charges, soil with high OM and clay fractions have higher CEC than soil with low OM and clay content. Achalu et al. (2012) stated that intensive weathering and the presence of more 1:1 (kaolinite) clay minerals lower the value of CEC and percent base saturation (PBS) in cultivated lands of Guto Gida District, Oromia Regional State.
2.2.2.8. Micronutrients
The term micronutrients refer to several elements that are required by plants in very small quantities. The four essential micronutrients that exist as cautions in soils unlike boron (B) and molybdenum (Mo) are zinc (Zn), copper (Cu), iron (Fe), and manganese (Mn). Absorption of micronutrients, either by soil OM or by clay-sized inorganic soil components is an important mechanism of removing micronutrients from the soil solution (Foth and Ellis, 1997). Thus, each may be added to the soil's pool of soluble micronutrients by weathering of minerals, by mineralization of OM, or by addition as a soluble salt (Foth and Ellis, 1997).
Asgelil et al. (2008) documented the status of some micronutrients in agriculturally important soils of the country. In their work, Fe and Mn are above the critical limit and in some cases, Mn surpasses the sufficiency level. On the other hand, Zn and Cu are deficient in most zones studied. The frequency of Zn deficiency is highest in Vertisols and Cambisols 78% and the lowest in Nitisols, whereas Cu deficiency is the highest in Fluvisols and Nitisols with the value of 75 and 69% respectively. In the same study, wheat tissue analysis revealed that no deficiency of Fe and Mn, whereas the deficiency of Zn and Cu are severe, ranging from 43 to 87 % of the total samples analyzed (Teklu et al., 2007) also reported the status of Mn, Zn, and B is an insufficient range on Andosols in the rift valley of Ethiopia. Besides, the wheat flag leaves micronutrient analysis from ten sites in central highlands Vertisols of Ethiopia showed that Cu, Fe, Mn, and Cl Concentrations are within the sufficiency range while Zn is deficient in all of the samples (Hillette et al., 2015). Also, a recent nutrient survey conducted by EthioSIS exhibited widespread B and Zn deficiency in the country.
The Factors affecting the availability of micronutrients are parent material, soil reaction, soil texture, and soil OM (Brady and Weil, 2002). Tisdale et al. (1995) stated that micronutrients have a positive relationship with the fine mineral fractions like clay and silt while negative relations with coarser sand particles. This is because their high retention of moisture induces the diffusion of these elements (Tisdale et al., 1995). Soil OM content also significantly affects the availability of micronutrients. The presence of OM may promote the availability of certain elements by supplying soluble chelating agents that interfere with their fixation. Brady and Weil (2002) indicated that the solubility, availability, and plant uptake of micronutrient cations (Cu, Fe, Mn, and Zn) are more under acidic conditions (pH of 5.0 to 6.5).
The availability of boron is related to soil pH. This element is the most available in acid soils. Boron is also easily leached from acid sandy soils. Therefore, deficiency of boron is relatively common in acid sandy soils. This occurs due to the low supply of total boron rather than due to the low available boron present. Boron deficiencies can be found most often in humid region or sandy soils. Boron is subject to lose by leaching, particularly in sandy soils, and thus responses to boron are common for sandy soils as summarized by Gupta (1993). Boron in soils is primarily in the 3+oxidation state taking the form of the borate anion: anion (B (OH) 4- ). The two most common solution species of B are neutral boric acid (H3BO3) and borate anion (B(OH)4-). Below pH 7, (H3BO3) predominates in solution, resulting in only a small amount of B adsorbed onto soil minerals. Boron deficiency is more common in calcareous arid soils and in neutral to alkaline soils. Boron is absorbed by humus and bound more firmly than inorganic colloids, only the B in solution is important for plants.
2.3. Soil Fertility Mapping
Soil fertility status maps are used to determine the soil fertility status and provide information for farmers. Soil fertility maps are the process of characterizing soil fertility status in a given area and geo-encoding such information. It involves different operations such as identification of soils to be mapped, development of map unit descriptions, fieldwork, and compilation. According to FAO (2006a), nutrient supply maps can be drawn for farms, larger regions, and countries based on soil testing. Such maps provide a useful generalized picture of the soil fertility of an area. The soil fertility status of a given area can be delineated on fertility maps so that it is easy to detect whether the soil fertility is low, medium, or high. The information obtained from soil fertility maps is very useful for planning nutrient management programs. The extent to which soil fertility maps can be used for planning nutrient management strategies depends upon how thorough, recent, and representative the soil sampling has been on which such maps are based FAO (2006a).
2.4. Factors Affect Available Macro and Micronutrients in Soil
Calcium, potassium, and nitrogen concentrations in both the soil and plant can affect boron availability and plant function, the calcium: boron (Ca: B) ratio relationship being the most important. Therefore, soils high in calcium will require more boron than soils low in calcium. The chance for boron toxicity is greater on low calcium-content soils.
The availability of micronutrients is sensitive to environmental change, organic matter, soil pH, lime application, and soil texture in the soil system (Nazif et al., 2006 and Vijayakumar et al. 2011), also stated that soil properties like soil texture, CEC, OM, pH, EC, and CO32- are the main characteristics playing a major role in controlling the availability of micronutrients. Factors affecting the availability of micronutrients are parent material, soil reaction, soil texture, and soil OM (Brady and Weil, 2002: Tisdale et al., 1995) stated that micronutrients have a positive relationship with the fine mineral fractions like clay and silt while negative relations with coarser sand particles.
2.5. Soil and Plant Relationship
Soil is the main source of nutrients for crops. Soil also provides support for plant growth in various ways. Knowledge about soil health and its maintenance is critical to sustaining crop productivity. The health of soils can be assessed by the quality and stand of the crops grown on them. However, this is a general assessment made by farmers. A scientific assessment is possible through detailed physical, chemical, and biological analysis of the soils. Soil fertility and plant nutrition are two closely related subjects that emphasize the forms and availability of nutrients in soils, their movement to and their uptake by roots, and the utilization of nutrients within plants (Foth and Ellis, 1997).
Soil tests generally overestimate the nutrient content and analysis of associative plant analysis can be considered as a quick and reliable technique for assessing the available status of nutrients according to (Mari et al., 2006). Different factors affect the soil and plant relationships. Thus, the combined use of soil and plant tissue analysis is believed to evaluate the complex interaction, get an accurate picture of limiting nutrients, and design corrective actions (Aref, 2011; H0gh- Jensen et al., 2009). Ideally, soil and plant nutrient concentrations are expected to be positively correlated for most nutrients (Melgar et al., 2001). Nevertheless, factors such as soil nutrient level, soil physical conditions, genotype or climate influence the required nutrient concentrations in plant tissue as a consequence; sufficient nutrient uptake from deficient soils (Akhter, 2012) was reported. This could be related to the modification of the rhizosphere environment (Fageria and Baligar, 2005; Diatta and Grzebisz, 2006; Akhter, 2012) and nutrient interaction (H0gh-Jensen et al.,2009; Loide, 2004). This would also call for the need to know more about soil-plant interaction before fertilizer interventions.
Nitrogen (N) is one of the most important plant nutrients and the most frequently deficient of all nutrients (Tisdale et al., 1993). Low nitrogen-supplying power by the soils calls for large additions of nitrogen to soils as fertilizers to meet the N needs of high-yielding non-leguminous crops such as maize, rice, sorghum, and finger millet (Foth, 1990). Plants normally contain between 1 and 5% N by weight (Tisdale et al., 1993). Phosphorus (P) is second only to nitrogen in the frequency of use as a fertilizer nutrient (Trohel and Thompson, 1993). Potassium (K) in the soil occurs as potassium ions in the mineral structure and as hydrated K ions either in solution or adsorbed on cation exchange sites (Trohel and Thompson, 1993). The content of K in maize plants ranges between 1.7 and 2.8% (Robert,1998). Total zinc (Zn) in soils is very variable depending on the nature of parent materials, clay content, and soil type. The content of Zn in maize plants ranges between 20 and 70 mg kg-1 (Campbell and Plank, 2000).
Maize requires at least 16 nutrients for normal growth and the completion of its life cycle. Those used in the largest amounts include carbon, hydrogen, and oxygen and are supplied by air and water. The other 14 nutrients are taken up by plants only in mineral forms from the soil or must be added to the soils as fertilizers. Maize needs relatively large amounts of nitrogen (N), phosphorus (P), and potassium (K). These nutrients are referred to as primary nutrients because they usually are lacking from the soil first and plants use large amounts for their growth and survival. Most frequently supplied to plants in fertilizers. Secondary nutrients are usually enough nutrients in the soil so fertilization is not always needed; these are calcium (Ca), magnesium (Mg), and sulfur (S). Other nutrients essential for maize plant growth that are needed in only very small quantities are called micronutrients include iron (Fe), manganese (Mn), zinc (Zn), copper (Cu), boron (Bo), molybdenum (Mo), and chlorine (Cl) (Johnson et al., 2000).
Interpretation of soil test results and plant analysis data is essential for further development of nutrient management (Hillette et al., 2015). The deficiency or sufficiency of a nutrient in the plant is measured and interpreted based on critical nutrient concentration (CNC). The CNC is located in that portion of a growth curve of the plant where the nutrient concentrations change from deficiency to adequate level. The critical nutrient concentration of maize is about 3% for N, 0.3 for P, and 2% for K in the leaf opposite and below the uppermost ear at the time of silking.
3. MATERIALS AND METHODS
3.1. Description of the Study Area
3.1.1. Location
The study was conducted at Migna Kura Kebele in Wayu Tuka District of Oromia Regional State, Western highlands of Ethiopia. Gute is the administrative center of the district, it is located at a distance of 12 km from Nekemte Zonal capital town. The district is located 320 km from the capital city, Addis Ababa toward the west of the country and 10 km away from Nekemte. It is bounded by Sibu Sire in the north and east, Leka Dulecha in the south, and Guto Gida in the west. Geographically, the district is located in the Western highlands of Ethiopia (Figure 1) lying between 8°56'56"N and 9°7'49"N and 36°32'38"E and 36°49'3"E. According to WTWAO (2007), the altitude of the district ranges from 1300-3140 m.a.s.l. The study area is located 5 km from the Gute administration town toward the east of the district and 1 km from Gaba Jimata town toward the south. It is bounded by Gaba Jimata town and Kebeles of Wara Babo Migna in the North, Gara Hudha in the south, Wali Galte in the east, and Gute Badiya in the west.
Abbildung in dieser Leseprobe nicht enthalten
Figure 1. Location map of the study area and sample points
3.1.2. Climate
The climate of the district is classified traditionally into three main agro-climatic zones, low land, midland, and highland. The total landmass of the district, about 49.23% (14,249 hectares) is Midland ‘ Bada daree' while 37.665% (10,704.73 hectares) is categorized as high land ‘ Baddaa ' and 13.16% (10,704.73 hectares) is low land ‘ Gammojjii ' as reported by WTWAO (2007). The thirteen years (2006-2018) climatic data from Nekemte Meteorological Station was recorded and the area has a unimodal rainfall pattern that extends from April to October with average annual precipitation of 2166.43 mm. Maximum rain is received in June, July, and °
August, with a mean monthly temperature varying from 11.93-28.21 C (Figure 2 and Appendix
A climate Graph of the Migna Kura Kebele
Abbildung in dieser Leseprobe nicht enthalten
Figure 2. Mean monthly rainfall, max and min temperatures of the study area (2006 - 2018)
3.1.3. Topography and Soils
The topography of the study area's district is a mountainous and slopping landscape. Topographically, the study area is characterized by very gentle slopping to strongly slopping according to the rating FAO (2006b) (Table1). Nitisols are the major soils that cover the western part of Ethiopia (Mesfin, 2007). The study site soils were classified as Nitisol covering large production areas, according to the FAO (1990) classification legend and the main soil group of most of the east Wollega zone is Nitisols cited by (Achalu et al., 2012). Also, the Wayu Tuka District is among the 17 districts of the East Wollega zone and the soil is acidic in reaction. The district has a various topographic features, 62% (17,950.8445 ha) of the land area is plain, 17% (4,922.00575 ha) hilly areas, and mountain and Clifts account for 13% (3,763.88675 ha) and 8% (2,316.238 ha) respectively. Clay loam soil coverage of the district is 17371.68 ha (60%). On the other hand, Sandy soil covers an area of 10133.49 ha (35%) and clayey soil 1447.64 ha (5%) the latter two soil textural classes are suitable for agriculture such as cereal crops, maize, sorghum, and teff production in the district as reported by WTWAO (2007).
3.1.4. Land use and Vegetations
Cultivated land and grazing land are the major lands used in the area. However, cultivated land is the dominant land use of the area because crop production is widely practiced through traditional subsistence farming on individuals held under rain-fed conditions. The second land use is the grazing land which is individually held by the farmers. The Forestland that is found in the limited area is the third land use observed in the study area. The major annual crops grown in the study area under rain-fed conditions are maize (Zea mays L.), coffee (Coffee Arabica L.), pepper, and potato (Solanum tuber soum L.) and produced usually once a year.
3.1.5. Farming system
The economic activities of the local society of the study area are primarily a mixed farming system that involves animal husbandry and crop production. The farming system is a subsistence involving a mixing crop-livestock production agricultural system. The nutrients usually applied as fertilizer for crop production in Ethiopia are nitrogen and phosphorus in the form of Urea and DAP (Hillette et al., 2015). Farmers have been using fertilizer applications based on the blanket recommendation methods of DAP and Urea applied to all major crop types for about 60 years. Recently, EthioSIS suggested blended chemical fertilizer for the study area. The new blended fertilizers NPSZnB (17.8N-35.7P-7.7S+0.1B+2.2Zn) and 200 kg blend fertilizer are currently being used by the farmers with urea in the study area (Appendix Table 1). However, the rate of this fertilizer was not determined by researchers particularly for maize production in the study area. Farmers commonly practice the traditional way of crop production like continuous maize growing or mono-cropping and complete removal of residue from the farm field as a source of fuel and livestock feed (Table 1 and Appendix Table 1).
3.2. Site Selection and Soil Sampling
For the present study, Migna Kura Kebele was purposely selected from the Wayu Tuka district as it is among the kebeles in the districts where maize crop has been predominantly produced. A preliminary survey and field observation was carried out to have general information about the landforms, land uses, and topography of the study area. Based on the in-situ survey, thirty- two maize growing fields were selected from Migna Kura Kebele s' farmer fields. To select 32 representative maize-growing fields across the study area, the agricultural lands of the Migna Kura kebele were divided into three major strata, based on its geographical location for stratified sampling techniques where maize crops have largely been produced in the study area.
The local names of major divisions are Gergo, Kilil, and Migna Kura Zones are selected based on purposively stratifying sample technique. Consequently, a total of 32 maize growing fields: 12 from Gergo, 8 from Kilil, and 12 from Migna Kura Zone were selected with a simple random sampling technique. Thirty-two maize growing fields relevant information regarding topography, cropping history, fertilizer type and application method, soil fertility management practices were recorded. Soil and maize ear leaf sampling points were geo-referenced using German 60 (GPS) as shown in the study area map (Figure 1). Location of soil and plant sampling sites (x, y coordinate(s) of the study area were recorded with the help of GPS during the collection of soil samples. However, crop residue management and the amount of fertilizers used for selected maize growing fields were recorded (Table 1 and Appendix Table 1).
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