SOIL FERTILITY ASSESSEMENT ALONG SLOPE GRADIENTS
IN GIPO SUB-WATERSHED, METEKEL ZONE OF BENISHANGUL
GUMUZ NATIONAL REGIONAL STATE, NORTH-WESTERN
ETHIOPIA
MSC THESIS
ABDISSA ABEBE
OCTOBER 2015
HARAMAYA UNIVERSITY, HARAMAYA
SOIL FERTILITY ASSESSEMENT ALONG SLOPE GRADIENTS IN
GIPO SUB-WATERSHED, METEKEL ZONE OF BENISHANGUL
GUMUZ NATIONAL REGIONAL STATE, NORTH-WESTERN
ETHIOPIA
A Thesis Submitted to the Postgraduate Program Directorate
School of Natural Resources Management and Environmental Sciences
HARAMAYA UNIVERSITY
In Partial Fulfillment of the Requirements for the Degree of
MASTER OF SCIENCE IN AGRICULTURE
(SOIL SCIENCE)
By
Abdissa Abebe
October 2015
Haramaya University, Haramaya
HARAMAYA UNIVERSITY
School of Postgraduate Program Directorate
We hereby certify that we have read and evaluated the Thesis entitled “Soil Fertility
Assessment along Slope Gradients in Gipo Sub-Watershed, Metekel Zone of Benishangul
Gumuz National Regional State, North-Western Ethiopia”, Prepared under our guidance by
Abidissa Abebe. We recommend that it shall be submitted as fulfilling the Thesis
requirements.
Kibebew Kibret (PhD)
Name of Major Advisor
__________________ __________________
Signature
Date
Muktar Mohammed (PhD) __________________ __________________
Name of Co-Advisor
Signature
Date
As members of the Board of Examiners of the MSc Thesis Open Defense Examination, we
certify that we have read and evaluated the Thesis prepared by ABDISSA ABEBE and
examined the candidate. We recommend that the thesis be accepted as fulfilling the Thesis
requirements for the degree of Master of Science in Agriculture (Soil Science).
Dr. Bobe Badadi
Name of Chairman
Dr. Lemma Wogi
Name of Internal Examiner
Dr.Yihenew G/ selassie
Name of External Examiner
__________________ ________________
Signature
Date
__________________ ________________
Signature
Date
__________________ ________________
Signature
Date
TABLE OF CONTENTS
DEDICATION
Error! Bookmark not defined.
STATEMENT OF THE AUTHOR
Error! Bookmark not defined.
BIOGRAPHICAL SKETCH
Error! Bookmark not defined.
ACKNOWLEDGMENTS
Error! Bookmark not defined.
LIST OF ACRONYMS AND ABBREVIATIONS
Error! Bookmark not defined.
LIST OF TABLES
Error! Bookmark not defined.
LIST OF FIGURE
ix
LIST OF TABLES IN THE APPENDIX
Error! Bookmark not defined.
ABSTRACT
Error! Bookmark not defined.
1. INTRODUCTION
Error! Bookmark not defined.
2. LITERATURE REVIEW
4
2.1. General Soil Fertility Concept
4
2.2. Soil Physical Properties
4
2.2.1. Soil Texture
5
2.2.2. Bulk Densities
5
2.2.3. Total Porosity
6
2.3. Soil Chemical Properties
7
2.3.1. Soil Reaction (pH)
7
2.3.2. Soil Organic Matter, Nitrogen and C: N Ratio
8
2.3.3. Available Phosphorus
10
2.3.4. Available Potassium
Error! Bookmark not defined.
2.3.5. Cation Exchange Capacity
Error! Bookmark not defined.
iii
2.3.6. Exchangeable Acidity
Error! Bookmark not defined.
2.3.7. Exchangeable Basic Cations ofPotassium and Sodium
defined.
Error! Bookmark not
2.3.8. Exchangeable Basic Cations ofCalcium and Magnesium
defined.
Error! Bookmark not
2.3.9. Percent Base Saturation
Error! Bookmark not defined.
2.3.10. Micronutrients (Fe, Mn, Zn and Cu)
Error! Bookmark not defined.
3. MATERIALS AND METHODS
Error! Bookmark not defined.
3.1. Site Description
Bookmark not defined.
Error!
3.1.1. Location
Error! Bookmark not defined.
3.1.2. Climate
20
3.1.3. Land use, Farming System, Soils and Vegetation of the Study Area
Bookmark not defined.
3.2. Site Selection, Soil Sampling and Preparation
Bookmark not defined.
Error!
Error!
3.2.1 Site Selection
Error! Bookmark not defined.
3.2.2 Soil Sampling and Preparation
Error! Bookmark not defined.
3.3. Analysis of Soil Samples of Physico-Chemical Peroperties
Bookmark not defined.
Error!
3.3.1. Soil Physical Properties
Error! Bookmark not defined.
3.3.2. Soil Chemical Properties
Error! Bookmark not defined.
3.4. Data Analysis
Bookmark not defined.
Error!
4. RESULTS AND DISCUSSION
Error! Bookmark not defined.
4.1. Soil Physical Properties
Bookmark not defined.
Error!
4.1.1. Soil Texture
Error! Bookmark not defined.
4.1.2. Bulk Density and Total Porosity
Error! Bookmark not defined.
4.2. Soil Chemical Properties
26
4.2.1. Soil Reaction (pH)
26
4.2.2. Soil Organic Carbon, Total Nitrogen and Carbon to Nitrogen Ratio (C: N)
27
iv
4.2.3. Available Phosphors and Potassium
29
4.2.5. Exchangeable Basic Cations and Exchangeable Acidity
29
4.2.4. Cation Exchange Capacity and Percentage Base Saturation Error! Bookmark not
defined.
4.2.6. Micronutrients (Fe, Mn, Zn and Cu)
Error! Bookmark not defined.
5. SUMMARY AND CONCLUSIONS
35
6. REFERENCES
37
7. APPENDIXES
47
DEDICATION
This Thesis manuscript is dedicated to his lovely father, Abebe Gerbi, and his mother Gode
Seyum, his beloved wife, Chaltu Melaku, his brothers Daba Abebe and Mitiku Nikues and all
his sisters.
v
vi
STATEMENT OF THE AUTHOR
I declare that this Thesis is my genuine work and that all sources of materials used for
preparation of this Thesis have been suitably acknowledged through citations. This Thesis has
been submitted in partial fulfillment of the requirements for M.Sc. degree at Haramaya
University. I strictly declare that this Thesis is not submitted to any other institution anywhere
for the award of any academic degree, diploma or certificate. This Thesis shall be deposited at
the University Library to be made available to borrowers under rules of the Library.
Brief quotations from this Thesis are allowable without special permission provided that
accurate acknowledgement of source is made. Requests for permission for extended quotation
from or reproduction of this manuscript in whole or in part may be granted by the head of the
major department or the Director of the post Graduate Directorate, when in his or her
judgment the proposed use of the material is in the interests of academic scholar.
Name: AbdisaAbebe
Place: Haramaya University, Haramaya
Date of Submission: __________
Signature: ______________
iv
BIOGRAPHICAL SKETCH
The author, Abdisa Abebe, was born in West Metekel Zone of the Benshangul Gumuz
Regional State, in January, 1988. He began his education at Gallessa Elementary and Junior
School and completed his Secondary School at Bullen Secondary and preparatory School in
2007. Then he joined the science faculty of Addis Ababa University in 2007 and graduated
with Bachelor of education Degree in chemistry in 2009.
The author has served at Dibati distinct of Metekel zone, Gallesa preparatory school for two
years and then, he joined the Postgraduate program at Haramaya University in 2011 to pursue
his study for M.Sc. degree in soil science.
v
ACKNOWLEDGMENTS
The author is indebted to and gratefully acknowledges Dr. Kibebew Kibret his major advisor,
for shaping his thinking and keeping momentum going throughout the course of this study.
Without his intellectual stimulation, professional guidance, encouragement and inspiring
discussions during the research work and valuable criticisms of the thesis manuscript, the
completion of this research work would not have been possible. He would also like to extend
his special thanks and gratitude to Dr. Muktar Mohammed his co-advisor, for commenting the
thesis manuscript and technical advice on matters pertinent to the thesis.
The staff member of Pawi national agriculture research center and Central Soil Laboratory of
Haramaya University are highly accredited for allowing him to use their laboratory facilities
and highly appreciated for providing information important for his work. Finally, his special
heartfelt gratitude and deepest appreciation goes to Mr. Abebe Gerbi, Mrs. Gode Seyum, Mr.
Daba Abebe, Mr. Azo Jibat, Mr. Abdisa Jegora, Mr. Mitiku Nikues, Mr. Lemessa Abebe, Mr.
Takele Abebe Mr. Haile Derje, Mr. Asefa Derje and Mr. Tekele Derje and to all his sisters for
their unreserved financial, moral and material support throughout his educational career. His
deepest and unreserved thanks also go to Dibati educational and agricultural site for their
material support. If he forget someone unmentioned, it is his mind not his heart to be blamed.
vi
LIST OF ACRONYMS AND ABBREVIATIONS
BD
AGU-R
CEC
CSA
CGS
DGC
DTPA
ECEC
ESSS
Exch.A
Exch. Al
FAO
GIS
GPS
IAR
IBSRAM
IFA
IITA
ISRIC
LU
LSD
masl
OC
OM
PD
ppm
PBS
RSA
SAS
TN
TP
Bulk density
Benishangule gumuz region
Cation exchange capacity
Central Statistical Authority
Council of Graduate Studies
Department Graduate Committee
Di-ethylene tri-amine penta-acetic acid
Effective cation exchange capacity
Ethiopian Society of Soil Science
Exchangeable acidity
Exchangeable aluminum
Food and Agriculture Organization
Geographic information system
Geographic positioning system
Institute of Agricultural Research
International Board for Soil Research and Management
International Fertilizer Industry Association
International Institute of Tropical Agriculture
International Soil Reference and Information Center
Land unit
Least significant difference
Meters above sea level
Organic carbon
Organic matter
Particle density
Parts Per Million
Percent base saturation
Republic of South Africa
Statistical analysis system
Total nitrogen
Total porosity
vii
LIST OF TABLES
Table
Page
1. Slope class, slope, surface soil color and soil textural classes of the study area
22
2. Variations of selected soil physical properties along slope gradients
26
3. Variation of soils pH, TN, OC, C: N, available phosphors and available potassium
29
along slope gradients
4. Variation of Soils Exchangeable Basic Cations, Exchangeable Acidity, CEC in
33
(cmol (+)/kg) and PBS (%) along slope gradient
5. Variation of selected soil micro nutrients along slope gradient in (mg/kg)
viii
34
LIST OF FIGURE
Figure
Page
1. Location map of the study area
2. Mean annual rainfall (mm) and Temperature (0C) from (2007-2014)
ix
22
21
LIST OF TABLES IN THE APPENDIX
Appendix Table
Page
1. Ratings of soil pH, organic carbon and CEC
49
2. Ratings of soil av.P, av.K, percentage base saturation and total nitrogen
49
3. Ratings of exchangeable cations in the soil
49
4. Mean square of soil physical and chemical parameters from the different land units
50
5. Geographical coordinates (grids) of the sampling sites
50
x
SOIL FERTILITY ASSESSEMENT ALONG SLOPE GRADINT IN GIPO
SUB-WATERSHED, METEKEL ZONE OF BENISHANGUL GUMUZ
NATIONAL REGIONAL STATE, NORTH-WESTERN ETHIOPIA
ABSTRACT
Soil fertility assessment is key for designing sit-specific management intervention. A study was
conducted in Dibati distirict of Benishangul Gumuz National Regional State to assess
variation of soil fertility status along slope gradients. Composite surface soil samples were
collected from three slope classes’ categories (2-10%, 10-15%, 15-30) and three slope classes
for the analysis of physico-chemical properties. Analysis of variance revealed that soil texture
were not significantly (P > 0.05) affected by changes in slope gradients. Relatively, clay
content decreased with increase in slope gradient. On the other hand, silt and sand contents
followed the opposite trend. Bulk density and total porosity were not significantly (P > 0.05)
affected due to variations in slope gradients. Relatively higher bulk density value was
recorded on upper slope, followed by middle slope gradient, while the lower bulk density was
recorded on lower slope gradients. On the other hand, total porosity followed the opposite
trend. Soil pH did not vary significantly (P > 0.05) with variation in slope gradients.
Nevertheless, it slightly decreases with increase in slope gradients. Soils organic carbon was
significantly (P ≤ 0.05) different due to differences in slope gradients. Accordingly, the mean
soils organic carbon contents of lower slope gradients and middle slope gradients were
slightly higher than that of upper slope classes. Soils total nitrogen was significantly (P ≤
0.05) different due to differences in slope gradients. Furthermore, the trend of total nitrogen
was similar to that of organic carbon. Soil C; N did not vary significantly (P > 0.05) with
variation in slope gradients. Nevertheless, it slightly decreases with increase, in slope classes.
Cation exchange capacity of all the slope classes are significantly (P ≤ 0.05) influenced by
changes in slope classes. The analysis of variance showed that the percentage base saturation
did not vary significantly (P > 0.05) with variation in slope classes. The mean values of
available K and P were statistically significant (P ≤ 0.05) different. The highest mean value of
available K and available P were observed on the lower slope classes, while the lowest
available K and available P were found under the upper slope classes. The analysis of
variance show that except exchangeable Na+, all the other exchangeable basic caions were
significantly (P ≤ 0.05) different under the three slope classes. Accordingly, except for
exchangeable Mg2+, Ca2+and k+ increased significantly with decrease in the slope classes.
There was no significant (P > 0.05) difference among soils under the three slope classes due
to variations in slope classes. Nevertheless, it slightly increases with increases in slope
classes. There was a significant (P ≤ 0.05) variation in mean values of micro nutrient in the
soils under the different slope classes. The soils of the study area showed relatively potentially
desirable physical fertility, deficient of some chemical parameters, such as low total nitrogen,
low organic carbon, very low available P and low Na+ cation. Therefore, to solve the problem
and restore the loss of nutrients and maintain sustainable agricultural production, there
should be restoring, maintaining and increasing the fertility status of the soils by integrated
soil conservation measures to avoid removal of soil and soil nutrients through incorporation
of crop residues, application of compost and animal dung for in improving soil fertility as
well as other physicochemical characteristics of the soils.
xi
1. INTRODUCTION
Providing food for the ever-growing population is one of the critical challenges of today. Soil
fertility assessment and mapping is key for designing sit-specific management intervention.
According to Beets (1982), production can be increased by expanding the areas cultivate for
crops, raising the yield per unit area of individual crops or by growing more crops per year. In
the future, most of additional food the world needs must come from high yields on the lands
already under cultivation and/or from lands now considered marginal (Chatterjee and Maiti,
1994). A major share of this increase will likely come from the use of irrigation, commercial
fertilizers, pesticides, improved crops culture, mechanization and improved soil and water
management (FAO, 1984). The demand for fertilization is evident, as growers around the
world have already recognized the return, which can be realized from added plant nutrients.
Quinenes et al. (1992) stated that unless something is done to restore soil fertility first, other
efforts to increase crop production could end up with little success. Moreover, using chemical
fertilizers that bring more than100% extra yield is inevitable in most cases (Kelsa et al.,
1992).
The quest for increased and sustainable productivity to match with population growth has
been a central issue in agriculture for as long as crops have been grown. Its stark significance
is seen today in areas of Africa that are suffering from frequent drought and the consequent
famine. In countries with a capacity for excess food production, maintenance of soil fertility is
a requirement for both economic and environmental viability of their farming system, with
production matched to national needs and export demands (Rowell, 1994). Soil fertility
decline has been described as the single most important constraint to food security in SubSaharan Africa (SSA) Teferi (2008). It has been also described by the same author; soil
fertility is not just a problem of nutrient deficiency. It is a problem of soil physicochemical
and biological degradation. The problem relates the linkage between poverty and land
degradation, often perverse national and global policies with respect to incentives and
institutional failures (Verchot et al., 2007).
Soil fertility maintenance is a major concern in tropical Africa, particularly with the rapid
population increase, which has occurred in the past few decades (Gebeyaw, 2007). It has been
also described by the same author; in traditional farming systems, farmers use bush fallow,
1
plant residues, household refuse, animal manures and other organic nutrient sources to
maintain soil fertility and soil organic matter. Although this reliance on organic nutrient
sources for soil fertility regeneration is adequate with low cropping intensity, it becomes
unsustainable with more intensive cropping unless fertilizers are applied (Mulongey and
Merck, 1993). In Ethiopia, low soil fertility is one of the factors limiting agricultural
productivity. It may be caused as a result of removal of surface soil by erosion, crop removal
of nutrients from the soil, total removal of plant residue from farmland, and lack of proper
cropping systems (Tamirie, 1982). The results of several studies conducted on the status of P
in Ethiopian soils (Eylachew, 1987; Tekalign and Haque, 1987) indicated that most of the
soils studied require addition of P fertilizer for profitable crop production.
Eyasu (2002) indicated that under increasing demographic pressure, cultivation becomes
permanent. It has been also described by the same author; the conventional hypothesis is that
the traditional farming systems in SSA lead to the mining of plant nutrients when cultivation
becomes more permanent due to increasing population pressure. In many cases, removal of
vegetation cover, depletion of soil nutrients and organic matter (OM), and accelerated soil
erosion have all led to the drastic decline in soil fertility. Soil OM not only plays a major role
in affecting physical and chemical soil properties, but also controls soil microbial activities by
serving as a source of carbon and nitrogen (Gebeyaw 2007). In many management options
such as keeping grasses in the crop rotation, returning all crop residues to the fields and
conservation tillage, controlling erosion, using cover crops whenever possible, manures
application to the soil and adding organic materials are considered as important sources of
plant nutrients and improvement of soil physical and chemical properties (Campbell et al.,
1996).
Research results have shown that the success in soil management to maintain soil quality
depends on understanding of the properties of a given soil. This is a requisite for designing
appropriate management strategies and thereby solving many challenges that the Ethiopians
are facing in the crop and livestock production sectors and in their efforts towards natural
resource management for sustainable development (Wakene, 2001). The removal of
vegetative cover (such as straw or stubble) or burning plant residues as practiced under the
traditional system of crop production or the annual burning of vegetation on grazing lands are
2
major contributors to the loss of nutrients (Mesfin, 1998), while the use of chemical fertilizer
is also minimal.
To maintain where soil fertility is high and to improve where it is low, assessment is a
prerequisite to rate soils on the basis of their fertility status. In Ethiopia, the information
presently available on soil fertility status is not adequate to meet the requirement of
agricultural development programs, and rational fertilizer promotions and recommendations
based on actual limiting nutrients for a given crop (Teferi, 2008).
Information on soil fertility is important in enhancing production and productivity of the
agricultural sector on sustainable basis. However, little information is available in Northwestern Ethiopia, Benishangul Gumuz Regional State, Metekel Zone in general and in Dibati
distinct in particular. The regional agricultural bureau assumes as most of the area in the
region are fertile and productive before few years. However, due to intensive cultivation and
over grazing, the fertility status of soils of the study area has been declining. In addition, the
loss of soil fertility in the study area is related with burning of plant residues under traditional
crop production systems which may contribute to the loss of nutrients. Therefore, this study
was proposed with the following objective:
 To assess the fertility status of soils of the study area in terms of selected soil physical
and chemical properties along slope gradients
3
2. LITERATURE REVIEW
2.1. General Soil Fertility Concept
The study of soil fertility involves examining the forms in which plant nutrients occur in the
soil, how these become available to the plant, and factors that influence their uptake (Martin
1993).This in turn leads to a study of the measures that can be taken to improve soil fertility
and crop yield by applying nutrient to the soil- plant system. This is usually done by adding
manure, fertilizers, and amendments to the soil. A mineral element is to be essential to plant
growth and development if the deficiency of that element is not corrected by other elements, if
the element directly involved in plant metabolic functions and plant cannot complete its life
cycle without the element Teferi (2008). There for, caution should be taken when diagnosing
deficiency symptoms and determination of soil fertility through soil analysis.
Soil chemical properties determine the nutrient supplying power of the soil to the plants. The
chemical processes that occur in the soil affect its pedological development and soil fertility.
Minerals inherited from the soil parent materials overtime release chemical elements that can
undergo various transfers and transformations within the soil. The more important chemical
characteristics, which influence soil fertility and hence plant growth and yield, are soil
reaction (pH), cation exchange capacity (CEC), available nutrients, OM content and salt
concentrations Damtew (2007). The land qualities such as moisture and oxygen availability,
and availability of foothold for root development depend for a great deal on soil physical
characteristics such as texture, coarse fragments, stoniness, depth of the soil and structure
(Ranst, 1991).
2.2. Soil Physical Properties
The physical properties of soils determine their 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. Many soil physical properties
change with changes in land use system and its management such as intensity of cultivation,
4
the instrument used and the nature of the land under cultivation, rendering the soil less
permeable and more susceptible to runoff and erosion losses (Sanchez, 1976).
2.2.1. Soil Texture
Soil texture is perhaps the most fundamental and most permanent soil property, not readily
subject to change by normal soil management practices in the field (Brady and Weil, 2002).
Texture is an important soil characteristic because it, in part, determines water intake rates,
water storage in the soil, the ease of tilling the soil, the amount of aeration and influences soil
fertility (Sharma. 2002; Gupta, 2000). It is also a guide to the value of land and as a result
land use capability and soil management practices are largely affected by the texture (Gupta,
2000). Moreover, the rate of many important chemical reactions in soil is governed by soil
texture because it determines the amount of surface area available for reaction (Youndeowiei
et al., 1990). It also critically influences the response of crop to fertilization. It affects the
infiltration and retention of water, soil aeration, absorption of nutrients, microbial activities,
tillage and irrigation practices (Foth, 1990; Gupta, 2004). It is also an indicator of some other
related soil features such as type of parent material, homogeneity and heterogeneity within the
profile, migration of clay and intensity of weathering of soil material or age of soil (Miller and
Donahue, 1995; Lilienfein et al., 2000).
Soil texture is one of the inherent soil physical properties less affected by management. The
rate of increase in stickiness or ability to mould as the moisture content increases depend on
the content of silt and clay, the degree to which the clay particles are bound together into
stable granules and the OM content of the soil (White, 1997). Over a very long period of time,
pedogenic processes such as erosion, deposition, eluviations and weathering can change the
textures of various soil horizons (Forth, 1990; Brady and Weil, 2002).
2.2.2. Bulk Densities
Measurement of soil bulk density is required for the determination of compactness, as a
measure of soil structure, for calculating soil pore space (Barauah and Barthakulh, 1997).
Bulk density also provides information on the environment available to soil microorganisms.
5
White (1997) stated that values of bulk density ranges from < 1 g/cm3 for soils high in OM,
1.0 to 1.46 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 normally decreases as mineral soils become
finer in texture. Soils having low and high bulk density exhibit favorable and poor physical
conditions, respectively. Bulk densities of soil horizons are inversely related to the amount of
pore space and soil OM (Brady and Weil, 2002; Gupta, 2004). Any factor that influences soil
pore space will also affect the bulk density.
Woldeamlak and Stroosnijder (2003) and Mulugeta (2004) revealed that the bulk density of
cultivated soils was higher than the bulk density of forest soils. Soil bulk density increased in
the 0-10 and 10-20 cm layers relative to the length of time the soils were subjected to
cultivation (Mulugeta, 2004). Similarly, Ahmed (2002) reported that soil bulk density under
both cultivated and grazing lands increased with increasing soil depth. On the other hand,
Wakene (2001) reported that bulk density was higher at the surface than the subsurface
horizons in the abandoned and lands left fallow for twelve years. The changes in the physical
soil attributes on the farm fields can be attributed to the impacts of frequent tillage and the
decline in OM content of the soils. Particle density is the mass or weight of a unit volume of
soil solids. It affects soil porosity, aeration and rate of sedimentation of particles. The mean
particle density of most mineral soils is about 2.60 to 2.75 g/cm3, but the presence of iron
oxide and heavy minerals increases the average value of particle density and the presence of
OM lowers it (Hillel, 1980).
2.2.3. Total Porosity
As soil particles vary in size and shape, pore spaces also vary in size, shape and direction
(Foth, 1990). Coarse textured soils tend to be less porous than fine texture soils, although the
mean size of individual pores is larger in the former than in the latter. There is close
relationship between relative compaction and the larger (macro pores) of soils (Ike and
Aremu, 1992). According to the same authors, tillage reduces the macro pore spaces and
produces a discontinuity in pore space between the cultivated surface and the subsurface soils.
Generally, intensive cultivation causes soil compaction and degradation of soil properties
6
including porosity Gebeyaw (2007). According to the same author, macro pores can occur as
the spaces between individual sand grains in coarse textural soils.
The decreasing OM and increasing in clay that occur with depth in many soil profiles are
associated with a shift from macro-pores to micro-pores (Brady and Weil, 2002). Fine
textured soils, especially those without a stable granular structure may have a dominance of
micro pores, thus allowing relatively slow gas and water movement, despite the relative large
volume of total pore space (Landon, 1991). Considering the surface soils, Wakene (2001)
stated that the lowest total porosity 36.2% was observed on the abandoned research field,
followed by 41.6% under the land left fallow for twelve years and the highest 56.7% was
recorded on the farmer’s field. Along with the increase in soil bulk density, soil total porosity
showed marked decline in both soil layers (0-10 and 10-20 cm) with increasing period under
cultivation (Mulugeta, 2004). The lowest total porosity was the reflections of the low OM
content.
2.3. Soil Chemical Properties
2.3.1. Soil Reaction (pH)
Soil reaction (pH) is the degree of soil acidity or alkalinity, which is caused by particular
chemical, mineralogical and/or biological environment. It is the simplest and the most
important chemical parameter measured in soils Teferi (2008). It has vital role in determining
several chemical reactions and in influencing plant growth by affecting the activity of soil
microorganisms and altering the solubility and availability of most of the essential plant
nutrients and particularly the micronutrients such as Fe, Zn, Cu and Mn (Miller and Donahue,
1995; Rao, 1995). Soil acidity is common in area where precipitation is high enough to leach
appreciable quantities of cations (Ca2+, Mg2+, K+ and Na+) from the surface layers of soils
(Brady and Weil, 2002). Leaching processes, decomposition of organic materials, continuous
cultivation, acid rains and application of acidic inorganic fertilizers usually bring about
changes in soil reactions where the increasing intensities of these factors could significantly
bring about the reduction of the soil pH by increasing the concentrations of the H+ ions in soil
solutions that eventually produce acidic soils (Tisdale et al., 1993). Smith et al. (1995)
7
disclosed that soil pH is sensitive to changes in the natural environment and soil management
processes due to human activity.
Solomon et al. (2002) pointed out that a gradual reduction of surface soil pH with increasing
time was observed in land units that have undergone conversion from forest to cultivated land
uses in Ethiopia. Wakene (2001) also reported very low pH (3.45) for fields that were under
intensive Cultivation compared to fields that were abandoned (3.80) and left fallow (4.07) for
over 12 years. Wakene and Heluf (2003) attributed the lowest pH values recorded in the soils
of the Bako research farmlands to the continuous removal of basic cations by high yielding
crop varieties, use of acidifying inorganic N and P fertilizers and intensive cultivatition.
2.3.2. Soil Organic Matter, Nitrogen and C: N Ratio
Soils are characterized as mineral or organic, on the basis of OM content. Mineral soils form
most of our cultivated land and may contain from a mere trace to 20 to 30% OM, but organic
soils contain 80% or more OM (Prasad and Power, 1997). Soil OM controls many soil
physical and chemical properties. It improves aggregate stability, structure of soils and is a
source of several essential plant nutrients, especially N, S and P (Prasad and Power, 1997).
In all forms of agricultural systems, whether traditional or modern, soil organic matter (OM)
plays an essential role in sustaining crop production and preventing land degradation
(Ouedraogo,2004). Due to its positive influence on several soil processes, crop productivity
and environmental quality, soil OM is often considered to be the single most important
indicator of soil quality and sustainable land management (Roming et al., 1995; Vance, 2000;
Doran, 2002). The positive effect of OM on structural stability is more pronounced on sandy
than on more finely textured soils (FAO, 1998). According to (Assefa et al. 2003), crop
management practices such as stubble management play an important role in maintaining an
optimal soil environment. Since carbon makes up a large and definite proportion of soil OM,
it is not surprising that the C:N ratio of soils tends to be fairly constant, generally between 8:1
to 15:1, the median being between 10:1 to 12:1 (Teferi, 2008). Temperature and precipitation
influence the C: N ratio of soil OM. When rainfall is constant, the C: N ratio is lower in
8
warmer than in cooler regions. When annual temperatures are about the same, the C: N ratio
tends to be lower in drier regions (Prasad and Power, 1997).
According to (Saikh et al., 1998a), cultivation of land results in reduction of OM and total N,
increase C: N of soils. Tesfu and Laktionov (1996) reported that intensive cultivation of
Nitosols at Holetta decreased their OM content as compared to the uncultivated soils. The
wider C: N ratios in the surface soils than in the corresponding subsoil layers suggested that N
was limiting in agricultural productivity (Tamirat et al., 1996). Nitrogen occurs in soils in
both organic and inorganic compounds of which nitrate and ammonium ions contribute the
plant available forms Teferi (2008). Apart from application of N fertilizers, the main source of
N in soils is the breakdown and mummification of OM; slow decomposition of humus
releases NH4 ions which are greatly influenced by microbial activity (Landon, 1991).
Formation and decomposition of OM determines the parallel gains and losses of soil N.
Addition of organic N occurs annually through immobilization as crop residues. Conversely,
organic N in the soil is continuously converted to the inorganic form through mineralization.
Under any cropping system and soil management, immobilization and mineralization
processes tend to balance each other in magnitude to render the system in equilibrium
(Mesfin, 1996).
Despite the above considerations, N is the most deficient nutrient element that frequently
limits yield in the tropics as well as in the temperate region (Sanchez, 1976). The amount of N
loss from Ethiopian soils is tied up with cultural practices or cultivation (Mesfin, 1998). The
removal of vegetative cover such as straw or stubble or burning plant residue, be it under the
traditional system of land preparation or the annual burning of vegetation on grazing land, are
major contributors to the loss. For soils cultivated for 30 to 39 years, there was a substantial
decrease in total N to about 0.05%. Whereas soil OM levels are usually expected to decrease
exponentially with cultivation, eventually reaching a steady state reflecting soil type, climate
and land use and/or management practices (Syers et al., 2001).
According to (Buresh and Tian, 1998), Some agro-forestry trees have potential to provide N
in quantities sufficient to support moderate crop yields through N inputs from biological N2
fixation and retrieval of nitrate deep soil layers and cycling of nitrogen from plant residues
9
and manures .Biomass transfer, alone or integrated with the use of inorganic fertilizers, is one
of the promising land management options for improving soil fertility and hence increasing
crop yields (Shehu et al., 1997; Rao et al., 1998). For instance, findings from India indicate
that pigeon pea can contribute about 40 kg N ha-1 through N fixation, leaf fall and roots
(Sheldrake and Narayanan, 1978; Rao and Willey, 1981). It was also suggested that Cajanus
biomass application seems a promising alternative for maize production in the Bako areas and
it can also be mineralized and the nutrients contained therein made available to crops during
the year of application (Abebe and Diriba, 2003).
2.3.3. Available Phosphorus
Available phosphorus (P) possibly has the most complicated chemistry in the soil, at least as
far as assessment of P levels and the P fertilizer requirements are concerned Teferi (2008).
According to the same author, it occurs in soils in both organic and inorganic forms, the latter
usually being the more important for crop nutrition. Plants absorb P in the forms of HPO4 and
H2PO4. The physical and chemical properties of soils were reported to influence the
availability of P and its adsorption reactions in the soil. These include the nature and amount
of soil minerals; soil pH, cation effects, anion effects, reaction time and temperature, flooding
and fertilizer management practices (Landon, 1991; Tisdale et al., 1993). Mitiku (1987)
reported that the available P in Ethiopian Vertisols of the central highlands as determined by
the Olsen method showed a wide range of values (8 to 28 gm kg-1), which is low to
moderately high in relation to the fertility status of the soils. Mohammed (2003) also reported
the existence of better P levels in some of the surface horizons of soils in the Chercher,
eastern Ethiopia and attributed this to the presence of relatively high soil OM contents.
Consequently, the decreasing characteristic in P with depth of profiles is partly due to the
corresponding decline of soil OM levels and the increasing levels of Ca phosphate which is
unavailable to be extracted by plant roots.
According to research conducted at Adet Agricultural Research Canter, the available P
(Olsen) content ranged from 1.5 ppm at Bichena to 9.5 at Injibara while most sites had less
than 5 mg kg- 1, which was below the critical limit for the growth of most crops (Yihenew,
2002). The dynamics of soil P could be affected by land use changes, which often involves
10
changes in vegetation cover, biomass production and nutrient cycling in the ecosystem
(Solomon et al., 2002; Wakene and Heluf, 2003). According to research on some soils of
Ethiopia, Acrisols from Bebeka and Masha (with low PH values) had considerably higher
available and total P than the rest of the soils studied. The soils of these areas were being
converted from forest to agricultural use and still hold their fertility reserves as compared to
the other soils, which have been cultivated for a long time (Berhane and Sahilemedhin, 2003).
Sustainable crop production in many soils of SSA requires P inputs because the soils are
either derived from parent materials with low levels of P or have been depleted of plant
available P through continuous cropping with insufficient P inputs (Sanchez et al., 1997).
Nega (2006) reported that available P content of soils drastically declined due to conversion
of natural ecosystem into managed agro-ecosystems. According to same, the statistical
analysis indicated that available P content was significantly affected by land uses (P < 0.01) in
which the average P content of various land uses were found to be significantly lower than P
content of adjacent forest land soils. Besides depletion of P from the soil, its availability varies
from soil to soil. Available soil P was relatively lower at Bekoji than at Eteya-Gonde on
account of P fixation being higher in Nitisols (Yesuf, 2006). Most of the soils in Ethiopia
show deficiency for inherent total and available P (Murphy, 1968) and this P deficiency is
directly related to food security issues, especially in the tropics where severe soil degradation
is responsible for series deterioration in soil quality (Stocking, 2003; Zhang and Zhang, 2005).
The average total P content for soils of the eastern highlands of the country was 0.05% as
reported by Eylachew (1987). Different researchers (Yesuf and Duga, 2001; Ahmed, 2002)
reported that P and N plant nutrients to be the most deficient mineral components in the soils
of Arsi causing strong challenges to the farming communities particularly in the upper
highlands. This can be ascribed to the high rainfall in the highlands, erosion hazards and
intensive cultivation.
The form of P that occurs in soil parent materials is generally of low availability to plants. The
concentration of P in soil solution provides useful information about P nutrition since
concentration gradients are the driving force for flow of P to the roots and its uptake by roots
also is concentration dependent (Birru and Heluf, 2003). Soil tests for plant available P is used
worldwide to determine the current P status of soils so as to estimate fertilizer P requirements
11
for specific yield goals. The current P status is due to indigenous (native) P present in the soil
and P from previous fertilizer P application (residual P) (Indiati, 2000).
2.3.4. Available Potassium
The total K content of soil is usually many times greater than the amount taken up by a crop
during a growing season, but in most cases only a small fraction of it is available to plants
(Tisdale et al., 1993). The exchangeable K maintains an equilibrium with the K in solution,
and together the exchangeable and solution K make up the available K (Foth and Ellis, 1997).
According to FAO (2006) K is absorbed as the monovalent cation K+ and it is mobile in the
phloem tissue of the plants. According to same author, K is involved in the working of more
than 60 enzymes, in photosynthesis and the movement of its products (photosynthates) to
storage organs (seeds, tubers, roots and fruits), water economy and provide resistance against
a number of pests, diseases and stresses (frost and drought). It plays a role in regulating
stomata opening and, therefore, in the internal water relations of plants. Potassium in soils is
found in minerals that weather and release K ions. The ions are adsorbed on the cation
exchange site and are readily available for plant uptake. Leaching in humid regions removes
available K and creates a need for K fertilizer application when moderate or high crop yields
are desired (Foth, 1990). In tropical soils, the total K content may be quite low because of the
origin of the soils, high rainfall and continued high temperatures.
In acidic soils under virgin conditions, K rarely limits crop production in spite of its low
availability to plants in most acidic soils. However, under intensive crop production where
these acid soils are improved by liming and application of P, availability of K becomes an
important factor (Somani, 1996). Unlike N and P nutrients, which become immediately
deficient in most tropical soils due to leaching and/or fixation particularly for P, the need for
K frequently arises only after a few years of cropping a virgin soil (Tisdale et al., 1993).
2.3.5. Cation Exchange Capacity
Cation exchange capacity (CEC) of the soil is defined as the sum of the positive charges of the
cations that a soil can absorb at a specific pH value. In other words, it is the sum of the
12
positive charges of all of the adsorbed cations (Miller and Donahue, 1995; Rai, 1995). The
Cation exchange capacity (CEC) of soils is stated as, the capacity of soils to adsorb and
exchange cations (Brady and Weil, 2002).
The CEC of a soil is strongly affected by the amount and type of clay, and amount of OM
present in the soil (Curtis and Courson, 1981). Both clay and colloidal OM are negatively
charged and therefore can act as anions (Kimmins, 1997). As a result, these two materials,
either individually or combined as a clay-humus complex, have the ability to absorb and hold
positively charged ions (cations). Soils with large amounts of clay and OM have higher CEC
than sandy soils low in OM. In surface horizons of mineral soils, higher OM and clay contents
significantly contribute to the CEC, while in the subsoil particularly where Bt horizon exist,
more CEC is contributed by the clay fractions than by OM due to the decline of OM with
profile depth (Foth, 1990; Brady and Weil, 2002). Soil solutions contain dissolved chemicals,
and many of these chemicals carry positive charges (cations) or negative charges (anions)
(Fisher and Binkley, 2000). Cation exchange is considered to be of greater importance to soil
fertility than anion exchange, because the majority of essential minerals are absorbed by
plants as cations (Poritchett and Fisher, 1987).
The nutrients required for plant growth is present in the soil in a variety of forms (Kimmins,
1997).They may be dissolved in the soil solution, from where they can be utilized directly.
They may be absorbed onto exchange sites, from where they inter soil solution or be directly
exploited by tree roots or microorganisms that come in contact with the exchange site.
Alternatively, they may be firmly fixed in clay lattices, immobilized in decomposition
resistant OM, or present in insoluble inorganic compounds. An exchangeable cation is one
that is held on a negatively charged surface and displaced by another cation. The
exchangeable cation is a desirable form of a nutrient being quickly brought into solution and
made accessible to roots by the exchange with proton. Although the cation nutrients held on
the exchange sites form a readily available pool, they do not represent the cation supplying
ability of the soil (Binkley and Sollins, 1990; Binkley et al., 1992). Cations removed from the
exchange sites often are replenished rapidly from other sources, such as OM decomposition,
mineral weathering, or release of ions fixed within the layers of clay minerals (Gebeyaw,
2007).
13
2.3.6. Exchangeable Acidity
Exchangeable acidity refers to the sum of the concentrations of hydrogen (H) and aluminium
(Al) ions in the soil exchange complex, and is inversely related to base saturation and pH of
the soil. For instance, exchangeable Al does not commonly occur in an appreciable quantity in
soils with pH values above 5.5 Teferi (2008). Soil acidity occurs when acidic H+ ion occurs in
the soil solution to a greater extent and when an acid soluble Al3+ reacts with water
(hydrolysis) and results in the release of H+ and hydroxyl Al ions into the soil solution
(Rowell, 1994; Brady and Weil, 2002). When the soils become strongly acidic, they may
develop sufficient Al in the root zone and the amount of exchangeable basic cations decrease,
solubility and availability of some toxic plant nutrient increase and the activities of many soil
microorganisms are reduced, resulting in accumulation of OM, reduced mineralization and
lower availability of some macronutrients like N, S and P and limitation of growth of most
crop plants (Rowell, 1994) and ultimately decline in crop yields and productivity (Miller and
Donahue, 1995; Tisdale et al., 1995; Foth and Ellis,1997; Brady and Weil, 2002). Foth and
Ellis (1997) stated that during soil acidification, protonation increases the mobilization of Al
and Al forms serve as a sink for the accumulation of H+.
The concentration of the H+ in soils to cause acidity is pronounced at pH values below 4 while
excess concentration of Al3+ is observed at pH below 5.5 (Nair and Chamuah, 1993). In
strongly acidic conditions of humid regions, where rainfall is sufficient to leach exchangeable
basic cations, exchangeable Al occupies more than approximately 60% of the effective cation
exchange capacity, resulting in a toxic level of aluminium in the soil solution (Buol et al.,
1989). Generally, the presence of more than 1 parts per million of Al3+ in the soil solution can
significantly bring toxicity to plants. Hence, the management of exchangeable Al is a primary
concern in acid soils.
2.3.7. Exchangeable Basic Cations of Potassium and Sodium
Soil parent materials contain potassium (K) mainly in feldspars and micas. As these minerals
weather, and the K ions released become either exchangeable or exist as adsorbed or as
soluble in the solution (Foth and Ellis, 1997). Potassium is the third most important essential
14
element next to N and P that limit plant productivity. Its behavior in the soil is influenced
primarily by soil cation exchange properties and mineral weathering rather than by
microbiological processes. Unlike N and P, K causes no off-site environmental problems
when it leaves the soil system. It is not toxic and does not cause eutrophication in aquatic
systems (Brady and Weil, 2002). Wakene (2001) reported that the variation in the distribution
of K depends on the mineral present, particles size distribution, degree of weathering, soil
management practices, climatic conditions, degree of soil development, the intensity of
cultivation and the parent material from which the soil is formed.
The greater the proportion of clay mineral high in K, the greater will be the potential K
availability in soils (Tisdale et al., 1995). Soil K is mostly a mineral form and the daily K
needs of plants are little affected by organic associated K, except for exchangeable K
adsorbed on OM. Mesfin (1996) described low presence of exchangeable K under acidic soils
while Alemayehu (1990) observed low K under intensive cultivation. Normally, losses of K
by leaching appear to be more serious on soils with low activity clays than soils with highactivity clays, and K from fertilizer application move deeply (Foth and Ellis, 1997).
Exchangeable sodium (Na) alters soil physical and chemical properties mainly by inducing
swelling and dispersion of clay and organic particles resulting in restricting water
permeability and air movement and crust formation and nutritional disorders (decrease
solubility and availability of calcium (Ca) and magnesium (Mg) ions) (Szabolcs, 1969;
Sposito, 1989). Moreover, it also adversely affects the population, composition and activity of
beneficial soil microorganisms directly through its toxicity effects and indirectly by adversely
affecting soil physical and as well as chemical properties. In general, high exchangeable Na in
soils causes soil sodcity which affects soil fertility and productivity.
2.3.8. Exchangeable Basic Cations of Calcium and Magnesium
Soils under continuous cultivation, application of acid forming inorganic fertilizers, high
exchangeable and extractable Al and low pH are characterized by low contents of Ca and Mg
mineral nutrients resulting in Ca and Mg deficiency due to excessive leaching (Dudal and
Decaers, 1993). Soils in areas of moisture scarcity (such as in arid and semi arid regions) have
15
less potential to be affected by leaching of cations than do soils of humid and humid regions
(Jordan, 1993).
According to (Brady and Weil, 2002), exchangeable Mg commonly saturates only 5 to 20% of
the effective CEC, as compared to the 60 to 90% typical for Ca in neutral to somewhat acid
soils. 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; Eyelachew, 2001). Different crops
have different optimum ranges of nutrient requirements. The response to calcium fertilizer is
expected from most crops when the exchangeable Ca is less than 0.2 cmol (+)/kg of soils,
while 0.5 cmol (+)/kg soil is reported to be the deficiency threshold level for Mg in the tropics
(Landon, 1991).
2.3.9. Percent Base Saturation
The percent base saturation (PBS) is as much a measure of the actual percentage of cation
exchange sites occupied by exchangeable bases Teferi (2008). It is influenced by the pH of
the CEC determination. The denominator includes oxide-mineral complexes between the
initial soil pH and the reference pH (7.0 or 8.2) (Bohn et al., 2001). According to same author,
since neither the content of exchangeable Al nor exchangeable H is appreciable above pH 5.5,
the effective CEC of the soil above this pH should be essentially 100% base saturated.
However, soils in the pH range of 5.5 to 7.0 or 8.2 generally still have measured base
saturations well below 100%. Such base saturation values are particularly low for minerals
that have a high proportion of PH dependent charge, such as kaolinite clays.
A soil with PBS less than 20% is considered to be of low, 20–60% medium and greater than
60% high fertility (Landon, 1991). The PBS of the lower B and upper C-horizons is especially
diagnostic of the extent to which exchangeable basic cations have been removed from the soil
and replaced by exchangeable acidity. Therefore, this characteristic is extensively used in soil
classification, soil fertility evaluation, and mineral nutrition studies. Two of the soil orders are
separated from each other by differences in PBS of the subsoil. Those soils in regions of
higher rainfall, warmer temperatures, and an older landscape surface have been observed to
16
have PBS less than 35 in their B-horizons (soils with argillic horizons only), or PBS
decreasing from B to C horizons (Buol et al., 1980).
Experiment conducted at vegetable growing area of Kolfe showed that the PBS of soils is
more than 50%. However, the soils of the new farmland indicate higher values of base
saturation as compared to the soils of old farms (Dereje, 2004). In tropical regions under
rainforest, the base saturation in humus-rich top-soils is usually in the order of 50 to 80%,
while below the humus rich layer, it drops very sharply to the levels of less than 20% (FAO,
1998).
2.3.10. Micronutrients (Fe, Mn, Zn and Cu)
The micronutrients in soils are of diverse groups of elements that are slowly released into the
soil solution from the pedochemical and geochemical weathering of rocks and minerals.
While the four cationic micronutrient elements namely iron (Fe), manganese (Mn), zinc (Zn)
and cupper (Cu) occur mainly in the divalent form in soils; differences in the ionic characters
of their chemical bonding are great enough so that only Fe2+ and Mn2+ can substitute
extensively for each other Damtew (2007). According to same author, Micronutrient supply in
plants and soils occur in much smaller concentrations, and their supply depends not only on
the amount of nutrient available in the soil but also on various soil intrinsic factors like parent
materials, soil texture, OM content, soil pH, moisture content and so on. Fine texture soils are
likely to have been derived from more easily weather able minerals, which are also the main
sources of micronutrients, whereas coarse texture soils are derived from mineral such as
quartz, which are resistance to weathering and have low content of micronutrients (Hodgson,
1963, Tisdale et al., 2002) .The four essential micronutrients that exist as cations in soils
unlike to boron and molybdenum are zinc (Zn), copper (Cu), iron (Fe) and manganese (Mn)
(Foth and Ellis, 1997).
Tisdale et al. (1995) stated that micronutrients have positive relation 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. According to
17
Hodgson (1963), the presence of OM may promote the availability of certain elements by
supplying soluble complexing agents that interfere with their fixation. Krauskopf (1972)
stated that the main source of micronutrient elements in most soils is the parent material, from
which the soil is formed. Iron, Zn, Mn and Cu are somewhat more abundant in basalt. 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). Adsorption
of micronutrients, either by soil OM or by clay-size 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 salts (Foth and Ellis, 1997). Factors
affecting the availability of micronutrients are parent material, soil reaction, soil texture, and
soil OM (Brady and Weil, 2002).
The Cu content in soils ranges nearly from 5 to 60 mg kg-1, although both lower (< 2 mg kg-1)
and higher values are not uncommon Steveson, (1986) (cited by Prasad and Power, 1997).
The average value of Cu in soils is about 9 to 10 mg kg-1. Total Zn content in soils ranges
from 10 to 300 mg kg-1 with an average of 50 mg kg-1. The concentration of both Cu and Zn
in soil solutions remain very low, approximately 0.5 to 70 mg kg-1. At any time the
concentration of Zn2+ in the soil solution is generally greater than that of Cu2+ (Prasad and
Power, 1997).
The information available about the status and limitation of micronutrients particularly in soils
of SSA is not adequate and is currently difficult for potential users to access (Haque, 1988).
The first attempt of diagnosing the micronutrients status in Ethiopian soils showed that Fe and
Mn were adequate, Zn varied between low to high and Cu was deficient throughout the
investigation sites (Sillanpää, 1990). The concentrations of Fe, Mn and Zn are negatively
correlated with soil pH (He et al., 1999). Differences in soil temperature, wetting and drying,
tillage practices, liming, OM maintenance and reduction and oxidation processes also affect
micronutrient availability in soils (Fisseha, 1996).
FAO (1983) reported that increased yields through intensive cropping and use of high yielding
varieties, losses of micronutrients through leaching and liming decreasing proportion of
18
farmyard manure compared with chemical fertilizers and several other factors are contributing
towards accelerated exhaustion of the available micronutrients. According to the same author,
hidden micronutrient deficiencies are far more widespread than is generally estimated and the
problems which today may be considered local may well become more serious in the
relatively near future, occurring extensively over new areas and creating widespread and
complicated production restrictions if they are not properly studied and diagnosed in time.
19
3. MATERIALS AND METHODS
3.1. Site Description
3.1.1. Location
The study was conducted at Gipo, which is located in Dibati District of Metekel Zone,
Benishangul Gumuz National Regional State North-Western Ethiopia. Gipo is found at the
South-Western part of Dibati District in North-East Assosa, Benishangul Gumuz Regional
State. It is specifically located at about 968 km from Addis Ababa in the North-West direction
and about 300 km in the North direction of Assosa, the capital city of the Benishangul Gumuz
Regional State (Figure 1). Dibati District shares its border with Bullen in the South, Guba in
the West, Ganga in the East and Mandura in the North. Geographically, the study site lies
between 100 28’ 220’-100 27’ 984’’ N latitude and 360 07’ 425’’-360 72’ 203’’ E longitudes and
at an altitude ranging from 1645 to 1685 masl.
3.1.2. Climate
Gipo area is characterized by monomodal rainfall distribution pattern. The rainy season
(Kermit) extends from May to November, with higher rain fall in August (figure 2). While the
remaining months (December to April) are dry season or season with no rain. Mean annual
rainfall was 1370 mm for the years (2007-2014) and the total rainfall is sufficient for rain fed
agriculture. The annual mean minimum and mean maximum temperatures of the study area
for the periods from 2007 to 2014 were 25 and 29 0C, respectively. Due to monomodal
rainfall distribution pattern, crop production in the Gipo area is mono-modal rain-fed that is
only one production season per year. It is a representative of the moist Kola agro-climatic
zone.
20
Figure 1. Location map of the study area
35
450
30
400
350
25
300
20
250
15
200
150
10
100
5
50
0
0
JAN
FEB
MAR APR MAY JUN
JUL
AUG
SEP
OCT NOV DEC
Figure 2. Mean annual rainfall (mm) and Temperature (0C) from (2007-2014)
21
RAIN
Temp
3.1.3. Land use, Farming System, Soils and Vegetation of the Study Area
Cultivated and grazing lands are the major land use types of the watershed. The major crops
produced are maize (Zea mays L.), and sorghum followed by teff and noug. The land use of
the study area is dominated by traditional rain-fed subsistence farming with individual holding
traditional grazing on communal lands. There are few types of natural vegetation in the area in
addition to different grass species covering the ground on the grazing lands. The dominant
vegetation in the study area includes bisana, wanza and shola. Soils of the study area were
black, dark brown and dark radish brown. The farmers locally call the black soils “boondi”
and the dark radish brown red soils “biyyee diimaa” in afaan Oromo. The dominant soil on
slopes ranging from 2-10 % of the study area is black (5YR2.5/1), while dominant soil colour
on slopes ranging from 10-15 % and 15-30 is dark brown (7.5YR3/2 and dark radish brown
(5YR3/3) colour respectively. The textural class for soils of the study area was clay.
3.2. Site Selection, Soil Sampling and Preparation
3.2.1 Site Selection
A preliminary site observation and appropriate sampling to accommodate spatial variation at
the study area were important considerations when attempting to measure changes in surface
soil chemical and physical properties. At the beginning, a general visual field observation of
the study area was carried out to have a general view of the variations in land feature of the
site. Following the general site selection, three representative slope classes were selected
based on slope gradient. Slope classes 1 was, a slope classes with slope gradient of 2-10%,
slope classes 2 and slope classes 3 were slope classes with slope gradients of 10-15 and 15-30
%, respectively. Representative soil sampling sites were selected based on slope gradient.
Table 1. Slope class, slope (%), surface soil color and soil textural classes of the
study area
Slope classes
1
2
3
Slope (%)
2-10
10-15
15-30
Moist soil color
Black (5YR2.5/1)
Dark brown (7.5YR3/2
Dark radish brown (5YR3/3)
22
Textural class
Clay
Clay
Clay
3.2.2 Soil Sampling and Preparation
Following the general site selection, three representative slope classes were selected based on
slope gradient. Slope class 1 is a land with slope gradient of 2-10%, while slope class 2 and
slope class 3 are with slope gradient of 10-15 % and 15-30 %, respectively. Based on this
classification 1 composite sample was collected from each slope classes which were repeated
three times, from the depths of 0-20cm using an auger (Wilding, 1985). During collection of
samples, dead plants, furrow, manures, wet spots, areas near trees and compost pits were
excluded. This was done to minimize differences, which may arise because of the dilution of
soil OM due to mixing through cultivation and other factors. The soil samples collected from
representative field was then air-dried, mixed well, grinded and passed through a 2 mm sieve
for the analysis of selected soil physical and chemical properties and 0.5mm sieve for
determination of TN and OC analysis.
3.3. Analysis of Soil Physico-Chemical Properties
3.3.1. Soil Physical Properties
The soil physical properties, which were determined in the laboratory, include soil texture,
bulk density, and for calculation purposes, a particle density of 2.65 g/cm3 was used. Soil
texture was analyzed by the Bouyucous (1965) hydrometer method. Bulk density of the soil
samples were analyzed on undisturbed soil samples collected using the core sampling method
(Sahlemedhin and Taye, 2000). Finally, total soil porosity was estimated from the values of
bulk density and particle density as;Total porocity 
1  Bulck density
 100
Particle density
3.3.2. Soil Chemical Properties
The pH of the soils was measured in water suspension in a 1:2.5 (soil: liquid ratio) potentio
metrically using a glass-calomel combination electrode (Van Reeuwijk, 1992). The Walkley
and Black (1934) wet oxidation method was used to determine soil organic carbon content.
23
Total N was analyzed using the Kjeldahl digestion, distillation and titration method as
described by Black (1965) through oxidizing the OM in concentrated sulfuric acid solution.
extractable soil P was analyzed according to the standard procedure of Olsen et al. (1954)
extraction method, since the Olsen method is the most widely used for P extraction under
wide range of pH both in Ethiopia and elsewhere in the world (Landon, 1991; Tekalign and
Haque, 1991). Exchangeable soil potassium was measured by flame photometer, following
extraction of the soil sample by sodium acetate (CH3COONa.3H2O). Basic exchangeable
cations (Ca, Mg, K and Na) were extracted by saturating the soil samples with neutral
ammonium acetate (1N NH4OAc) solution. Following this extraction, exchangeable Ca and
Mg were measured using atomic absorption spectrophotometer (RDP, 1986) and
exchangeable Na and K were determined by flame photometer (FAO, 1984a).
Cation exchange capacity (CEC) was determined by extracting the ammonium acetate
saturated soil samples used for the determination of exchangeable cations by sodium chloride
solution. Then, the ammonium ion displaced from the soil exchange site was distilled and the
evolved ammonia was determined by the macro-Kjeldhal method (Jackson, 1958) and its
concentration was reported as CEC. Percent base saturation (PBS) was calculated from the
sum of exchangeable bases as percent of the sum of bases retained by the CEC sites.
Exchangeable acidity (Al and H) was determined by saturating the soil sample with potassium
chloride solution and titrating with sodium hydroxide (Rowell, 1994). Micronutrients (Fe, Mn,
Zn and Cu) were extracted with DTPA as described by Lindsay and Norvell (1978). Atomic
absorption spectrophotometer was used to determine the amounts of the micro-nutrients in the
extracts at 248.3, 279.5, 324.7 and 213.9 nm wave lengths for Fe, Mn, Cu, and Zn,
respectively.
3.4. Data Analysis
The data recorded was subjected to analysis of variance using the general linear model
procedure of the statistical analysis system (SAS Institute, 1999). The least significance
difference (LSD) test was used to separate significantly different parameters after ANOVA
was found significant at P ≤ 0.05.
24
4. RESULTS AND DISCUSSION
4.1. Soil Physical Properties
4.1.1. Soil Texture
The analysis of variance revealed that soil texture were not significantly (P > 0.05) affected by
changes in slope gradients (Table 2). Relatively, the highest clay content was recorded for
soils of lower slope position, while the lowest was recorded in soils of upper slope position
followed by middle slope position. On the other hand, silt and sand contents followed the
opposite trend in that both separates increased significantly with increase in slope gradient
(Table 2). The probable reason for decrease in clay content and increase in silt and sand
content with increases in slope gradient could be related to the balance between increase of
erosion and depositions. On the upper and middle slopes, erosion exceeds deposition and,
thus, the clay particles are washed down to the lower slopes where the runoff water velocity is
decreased.
In line with this, Regina et al. (2004) reported that on the steep cultivated hill slope, the most
noticeable changes were a decrease in clay and a corresponding increase in sand and silt
fractions as the slope gradient increases. In addition to this, Belay (1998) also reported, as
soils on the uplands commonly are well drained, whereas those in depressions are poorly
drained and hence are rich in clay and organic matter, with signs of various degrees of
gleying. The soil textural class varied with position of slopes in the landscape. It ranged from
silt clay loam in the upper slope to clayey in the lower slope positions, suggesting that amount
of clay increase down slope (Mohammed et al, 2005).
4.1.2. Bulk Density and Total Porosity
Bulk density and total porosity were not significantly (P > 0.05) affected by variations in
slope gradients (Appendix Table 4). Relatively higher bulk density value was recorded for
soils of upper slope gradient, followed by middle slope gradient, while the lowest bulk density
was recorded for soils of lower slope gradient. On the other hand, total porosity followed the
opposite trend in that total porosity decreased significantly with increase in slope gradient
25
(Table 2). The probable reason for the increase in bulk density and decrease in total porosity
with increases in slope gradient could be related to wash down of clay particles from the
upper slopes and deposited on lower slopes where the runoff water velocity is the lowest. In
addition to this coarser particles like sand soils have higher bulk density due to lower total
pore space.
In line with this, Foth (1990) reported that the greater development of structure in the finetextured surface soils and relatively higher organic matter content accounts for their lower
bulk density as compared to the more sandy soils with less structural differentiation. Soil bulk
density increased in the 0-10 and 10-20 cm layers relative to the length of time the soils were
subjected to cultivation (Mulugeta, 2004). Low bulk density values (generally below 1.3 gm
cm-3) indicate a porous soil condition (FAO, 2006b). Increase in OM content lowers bulk
density while compaction increases bulk density.
Table 2.Variations of Selected Soil Physical Properties across Slope Gradients
Slope
gradient
Particle size (%)
Textural class
BD (g cm3)
TP (%)
Sand (%) Silt (%) Clay (%)
1
7.98b
14.01b
78.01a
Clay
1.16
56.48
b
a
b
2
10.02
24.02
65.98
Clay
1.23
56.48
3
16.01a
25.99a
58.00c
Clay
1.27
52.20
LSD (5 %)
3.5
8.1
7.8
NS
NS
CV (%)
13.8
16.8
5.1
22.0
18.7
*Means in the column represented by different letters are significantly different at P ≤ 0.05;
NS = non significant P > 0.05; BD=bulk density; TP= total porosity
4.2. Soil Chemical Properties
4.2.1. Soil Reaction (pH)
Soil pH did not vary significantly (P > 0.05) with variation in slope gradient (Table 3).
Nevertheless, it slightly decreased with increase in slope gradient. The absence of significant
difference among the slope classes, which represents different slope gradients, could be
related to the generally high clay content, which gives high buffering capacity. Belay, (1996)
and Abayneh, (2001) also reported that the soils in high altitude and those with higher slopes
26
had low pH values, probably suggesting the washing out of basic cations from upper slopes.
Gupta (2000) also suggested that soils on the upper slope are usually lower in pH, less clayey;
low in soluble salts and organic matter contents; lighter in color and well drained, whereas
those at lower slope are more clayey, higher in pH, CaCO3 and soluble salt contents, darker in
color and have impeded drainage.
According to Tekalign (1991), soil pH rating, soil pH (H2O), the study area represented by
upper slope gradient were strongly acidic while soils of lower slope gradient and middle slope
gradient were moderately acidic (Appendix Table 1). In agreement with this study, Yerima
(1993) also reported that, lower pH values may occur in very acidic, humus-rich top-soils,
sulfuric horizons, and extremely weathered (or highly leached) soils. From this, it could be
concluded that the soils of the study area have acidic nature. The acidic nature of the soil
might be related to leaching of basic cations due to the high rainfall and intensive weathering
expected in such high temperature and humid environment. Although the soils under the study
area have acidic nature, the pH ranges are within the range where the mobility and availability
of a number of essential plant nutrients is not affected.
4.2.2. Soil Organic Carbon, Total Nitrogen and Carbon to Nitrogen Ratio (C: N)
Soils organic carbon was significantly (P ≤ 0.05) different due to differences in slope
gradients (Table 3). Accordingly the mean soils organic carbon content of the slope gradients
1 and slope gradients 2 was slightly higher than that of slope gradients 3 (Table 3).
Nevertheless, soils organic carbon under the study area slightly decreases with increase in
slope gradient, which means inversely related with slope gradient. This may be due to the
steepness of the slope which attributes organic matter transportation from the upper slope to
the lower slope position. Gregorich (1998) also reported the dependence of soil organic
carbon content with landscape position where the increasing soil water content and fertile soil
deposition at lower slope positions would favor high crop biomass production and the higher
soil organic carbon content. According to the rating of Landon (1991), organic carbon content
of the soils in the study area was categorized under low organic carbon content. This could be
due to the soil fertility management practices of the respective farmers.
27
Soils total nitrogen was significantly (P ≤ 0.05) different due to differences in slope gradients
(Table 3). Furthermore, the trend of total nitrogen was similar to that of organic carbon,
clearly indicating the strong relationship between these two soils attributes. The relatively
higher total nitrogen values was recorded in soils of slope gradients 1 (the lower slope
gradient) and slope gradients 2 (middle slope gradient) while the relatively lower soils total
nitrogen was recorded in soils of slope gradients 3 (upper slope gradient) (Table 3). The
probable reasons for the decreased of total nitrogen with increased in slope gradient could be
related to removal (transportation) of organic matter from the higher or steep slope as a result
of high erosion on the upper slopes.
Tisdale et al., (2002) observed that the total nitrogen content of soils ranges from less than
0.02% in subsoil to greater than 2.5% in peat soils which is attributed to the generally low
biomass productions and fast oxidations of organic matter. Similar finding was reported by
Regina, et al. (2004) for low soil organic carbon and total nitrogen content on steeper slopes
from southern American. According to the study of Havlin et al. (1999), soil total N contents
of less than 0.15, 0.15-0.25 and greater than 0.25% are categorized as low, medium and high,
respectively. According to this study soil total N content of slope gradients 3 which is 0.12%
is low, while soil total N content of slope gradients 1 and 2 which were 0.16% and 0.15%
categorized under medium total N content (Appendix Table 2). Mesfin, (1998) also reported
that nitrogen is one of the essential nutrient elements that is taken up by plants in greatest
quantity after carbon, oxygen and hydrogen, but is one of the most deficient macronutrients in
crop production.
Soil C: N did not vary significantly (P > 0.05) with variation in slope gradient (Table 3).
Nevertheless, it slightly decreases with increase in slope gradient. The results recorded for all
land units showed optimum range for active microbial activities of humification and
mineralization of organic residues. Foth and Ellis (1997) reported that soils with C: N ratios in
the range of 10-12 provide nitrogen in excess of microbial needs.
28
4.2.3. Available Phosphors and Potassium
The mean values of available K and P were statistically significant (P ≤ 0.05) difference as
indicated in Table 3. The highest mean value of available K (700.10 mg kg-1) and available P
(1.21 mg kg-1) were recorded on the lower slope gradients (slope gradient 1) while the lowest
available K (150.2 mg kg-1) and available P (0.08 mg kg-1) were found under the upper slope
gradients (slope gradients 3) (Table 3). According to the study results of Foth (1990), leaching
in humid regions removes available K and creates a need for K fertilizer application when
moderate or high crop yields are desired. IFA (1992) categorized extractable potassium (K) in
mg kg-1 of < 50 as very low, 50 to 100 low, 100 to 175 medium, 175 to 300 high and > 300 as
very high. According to these ratings, the mean available K content of the lower slope
gradient (700.1 mg kg-1) and the middle slope gradient (310.1 mg kg-1) were very high, while
the mean available K content of the upper slope gradient (150.2 mg kg-1) was medium
(Appendix Table 2). Havlin et al. (1999) rated available P (Olsen extracted P) of < 3, 4-7, 811 and > 12 mg kg-1 as very low, low, medium and as high, respectively. According to these
rating, the mean available P content of the entire slope class was very low (Appendix Table
2).
Table 3. Variation of Soil Chemical Properties of pH, TN, OC, C: N, Available Phosphors and
Available Potassium across Slope Gradients
pH
TN (%) OC (%)
C: N (%) Available P
Available K
Slope
gradients
1
5.67
0.16a
1.38a
8.47
1.21a
700.10a
2
5.56
0.15a
1.28a
8.46
0.59b
310.10b
b
b
c
3
5.20
0.12
0.95
7.88
0.08
150.20c
LSD (5 %)
NS
0.02
0.2
NS
0.5
13.09
CV (%)
13.4
5.6
7.9
11.7
35.30
1.49
Where means in the column represented by different letters are significantly different at
P ≤ 0.05; NS= non significant P > 0.05; TN= total nitrogen; OC= organic carbon;
C: N= carbon to nitrogen ratio.
4.2.4. Exchangeable Basic Cations and Exchangeable Acidity
The analysis of variance show that except exchangeable Na+, all the other exchangeable basic
caions were significantly (P ≤ 0.05) different under the three slope gradients (Table 4).
29
Accordingly, except Na+ other exchangeable cations (Mg2+, Ca2+and k+) increased
significantly with decrease in the slope gradient (Table 4). On the other hand, the
exchangeable Mg2+ level of soils under the lower and middle slope gradients (slope gradient
2) show non-significant difference. The possible reason for the decrease in basic cations with
increases in slope gradient might be related to their removal from the upper slopes and
subsequent accumulation in soils of lower slopes. The exchange complexes of the soils of all
slope classes were dominated by calcium followed by magnesium, potassium, and sodium
(Table 4).
Exchangeable K+ was highest (0.81 cmol (+)/kg) on slope gradient 1 (lower slope gradient),
followed by slope gradient 2, while the lowest exchangeable K+ content (0.206 cmol (+)/kg)
was recorded under slope gradient 3 (upper slope gradient). The highest content of K+ in the
lower slope gradient could be related to its slightly high pH value. In line with this study,
Mesfin (1996) also reported that high K+ was recorded under high pH tropical soils. The
amount of mean exchangeable K+ values recorded in this study show that K+ content of slope
gradient 1 and slope gradient 2 was above the critical levels (0.38 cmol (+)/kg) for the
production of most crop plants as indicated by Barber (1984), while that of slope gradient 3 is
relatively lower than the critical levels.
Statistically the difference in exchangeable Na+ was not significant (P > 0.05 cmol kg-1)
(Table 4). High exchangeable Na+ (0.21 cmol kg-1) was observed on the lower slope gradient
(slope gradient 1), whiles the exchangeable Na+ for slope gradient 2 and 3 was equal (0.10).
The slightly decrease in exchangeable Na+ from slope gradient 1 to slope gradient 2 might be
due to high erosion and leaching processes on slope gradient 2, which removes soluble salts
from upper slope and accumulate at the down slope positions. According to the rating of FAO
(2006), exchangeable Na+ content was low for slope gradient 1 and very low for slope
gradient 2 and 3 (Table 4). Statistically significant differences (P ≤ 0.05) in the values of Ca2+
were observed for different slope gradient (Table 4). Accordingly, the highest exchangeable
Ca2+ (7.54 cmol kg-1) was recorded on soils of slope gradient 1, and the lowest exchangeable
Ca2+ (6.75 cmol kg-1) value was recorded on soils of slope gradient 3, followed by slope
gradient 2 (middle slope gradient) .The possible reason for the decrease value for
30
exchangeable Ca2+ with increases in slope gradient could be related to the removal of soluble
salts from upper slope and accumulate at the down slope positions by erosion.
Statistically, the difference in exchangeable Mg2+ was not significantly (P > 0.05) affected by
difference in slope gradient (Table 4). Nevertheless, it slightly increased with decrease in
slope gradient (Table 4). Slightly higher exchangeable Mg2+ value was recorded under slope
gradient 1 and 2. While exchangeable Mg2+ value of slope gradient 3 was lower. Similar to
exchangeable Na+ and Ca2+, the possible reason for the decrease of exchangeable Mg2+ with
increase in slope gradient could be related to the removal of soluble salts from upper slope
and accumulate at the down slope positions by erosion. According to rating of FAO (2006),
exchangeable Mg2+ content of all the slope gradient was medium (in the interval of 1-3 cmol
(+)/kg) (Table 4). According to Havlin et al.(1999), the prevalence of Ca2+ followed by Mg2+,
K+, and Na+, in exchange site of soil is favorable for crop production.
There is no significant (P > 0.05) difference of exchangeable acidity among soils under the
three slope classes due to variations in slope gradient (Table 4). Nevertheless, it slightly
increases with increases in slope gradient. Accordingly, the relatively higher exchangeable
acidity value was recorded in soils of slope gradient 3 (upper slope gradient), while the
relatively lower exchangeable acidity value was recorded in soils of slope gradient 1 followed
by slope gradient 2 (Table 4). The possible sources of the acidity of the soils in the study area
might be related to release of organic acids during decomposition of organic matter, intensive
cultivation, and steepness of the topography, which causes removal of basic cations by erosion
and leaching. The exchangeable Al3+ of the study area is zero. Therefore, the exchangeable
acidity is due to proton (H+). Nair and Chamuah (1993) reported that the concentration of the
H+ to cause acidity is pronounced at pH value below 4, while excess concentration of Al3+ is
observed at pH below 5.5. However, the results of this study indicate that the pH of the study
area were above 5.5 except for slope gradient 3 (upper slope gradient). Therefore, the
concentration of exchangeable Al3+ was trace and Al toxicity is not expected in the area.
31
4.2.5. Cation Exchange Capacity and Percentage Base Saturation
The analysis of variance show that Cation exchange capacity was significantly (P ≤ 0.05)
influenced by changes in slope gradient (Table 4). Accordingly, the highest CEC value of
28.27 cmol (+)/kg was recorded for soils in the lower slope gradient, followed by 24.94 cmol
(+)/kg of the soils in the middle slope gradient. On the other hand, the lowest CEC (22.48
cmol (+)/kg) was recorded for soils of under the slope gradient 3 (upper slope position) (Table
4). The probable reason for the decreased of CEC with increases of slope gradient would be
related to variability in amount of clay and OM contents of the soils. Curtis and Courson
(1981) suggested that the CEC of a soil is strongly affected by the amount and type of clay
and amount of OM present in the soil. It is a general truth that both clay and colloidal OM
have the ability to absorb and hold positively charged ions. Thus, soils containing high clay
and organic matter contents have high cation exchange capacity. According to Landon (1991),
the surface soils of the study area can be classified as high CEC soils for slope class 1 and
medium CEC soils for slope gradient 2 and 3 (Appendix Table 1).
The mean separation result in Table 4 showed that the percentage base saturation did not vary
significantly (P > 0.05) with variation in slope gradient. Furthermore, it did not show any
consistent trend with slope gradient, although relatively higher percentage base saturation
value was recorded in soils of the lower slope gradient, followed by middle slope position.
While relatively lower percentage base saturation value was recorded in soils of the upper
slope gradient. The relative mean values of PBS for all the three slope classes were found to
be far below 50%. The probable reason for this could be related to intensive cultivation and
continuous fertilization uses in the study area that enhanced loss of basic cations through
leaching, erosion and crop harvest. Singh et al., 1995; He et al. 1999, Getachew and Heluf,
(2007) reported that, intensive cultivation, continuous fertilization and grazing of land that
enhanced loss of basic cations through leaching, erosion and crop harvest could be affect PBS.
Furthermore, the relatively high precipitation of study area might be leached the basic cations
from surface of the soils that resulted into low PBS.
32
Table 4. Variation of Soils Exchangeable Basic Cations, Exchangeable Acidity, CEC in (cmol
(+)/kg) and PBS (%) across Slope Gradients
Slope gradient
Exchangeable bases
Exch H
CEC
PBS
+
2+
2+
Na
K
Ca
Mg
a
a
1
0.21
0.81
7.54
1.72a
0.02
28.27a
38.43
2
0.10
0.45b
7.17b
1.63a
0.03
24.94b
36.30
3
0.10
0.21c
6.75c
1.49b
0.03
2.48c
35.66
LSD (5%)
NS
0.08
0.32
0.13
NS
1.7
NS
CV (%)
48.7
6.8
1.9
3.5
20.87
2.9
17.95
*Means in the column represented by different letters are significantly different at
+
P ≤ 0.05; NS =non significant P > 0.05; Exch H+ = Exchangeable acidity CEC= cation exchange
capacity; PBS=percentage base saturation
4.2.6. Micronutrients (Fe, Mn, Zn and Cu)
There was a significant (P ≤ 0.05) variation in mean values of micro nutrients in the soils of
different slope gradients (Table 5). The highest Fe (26.88 mg/kg) and Mn (336.78 mg/kg) was
recorded in the lower slope gradient followed by Fe (21.18 mg/kg) and Mn (192.70 mg/kg) of
the soils in the middle slope gradient. On the other hand, the lowest value of Fe (16.13 mg/kg)
and Mn (143.27 mg/kg) were for the slope gradient 3 (upper slope gradient) (Table 5). This
could be due to the difference in soil organic matter, in which much of the potentially
available micronutrients are held by erosion of topsoil. The availability of micronutrients in
the soils of the study area inversely related with slope gradient. The possible reason might be
related to the influences of soil pH, organic matter, CEC, phosphorus level in the soil which
are variable a long slope gradients. Fisseha (1992) suggested that the solubility and
availability of micronutrients is largely influenced by clay content, pH, organic matter, CEC,
phosphorus level in the soil and tillage practices.
The highest Cu (94.20 mg/kg) and Zn (4.94 mg/kg) contents were registered at the lower slope
gradient (slope class 1) of the study area and the lowest Cu (58.87 mg/kg) and Zn (1.64 mg/kg)
contents were registered at the upper slope gradient (slope class 3) of the study area. In
general, DTPA extractable Cu and Zn in the surface decreased almost consistently along the
slope gradient from the upper slope to lower of slope gradient (Table 5).
33
Table 5. Variation of Selected Soil Micro Nutrients across Slope Gradients in (mg/kg)
Slope gradients
Fe
Mn
Zn
Cu
1
26.82a
336.78a
4.94a
94.19a
2
21.18b
192.70b
2.47b
93.81a
c
c
c
3
16.13
143.27
1.65
58.87c
LSD (5%)
0.23
11.22
0.27
7.13
CV (%)
0.47
2.20
3.98
3.82
*Means in the column represented by different letters are significantly
Different at P ≤ 0.05
34
5. SUMMARY AND CONCLUSIONS
Soil fertility assessment is key for designing sit-specific management intervention. This study
was conducted in Dibati distinct of Benishangul Gumuz National Regional State to assess the
variation of soil fertility status with slope gradient. A preliminary field observation was
undertaken to select the study area. Following the general site selection, three representative
slope classes were selected based on slope gradient. The slope classes were slope classes 1, a
land with slope gradient of 2-10%, slope classes 2 and slope classes 3 with slope gradients of
10-15 and 15-30 %, respectively. From each slope class soil samples collected, from the
depths of 0-20cm using an auger. Result showed that, the textural class of the soils of all
slope classes was clay. The bulk densities of the soils were below 1.34 gm cm-3. Hence, in
terms of soil fertility, the bulk density value indicates that the soils were not too compact to be
penetrated by plant roots and thus they are potentially good for agricultural activities.
Some soils chemical properties were significantly (P ≤ 0.05) influenced by variation in slope
gradient. The soil pH (H2O), in the study area represented by slope classes 3 was strongly
acidic while slope classes 1 and 2 were moderately acidic. Soils organic carbon was
significantly (P ≤ 0.05) different due to differences in slope classes. Accordingly soils organic
carbon under the study area slightly decreases with increase in slope gradient, which is
inversely related with slope gradient. Soils total nitrogen was significantly (P ≤ 0.05) different
due to differences in slope classes. Relatively higher total nitrogen values was recorded in
soils of slope classes 1 (the lower slope gradient) and slope classes 2 (middle slope gradient)
while the relatively lower soils total nitrogen was recorded in soils of slope classes 3 (upper
slope gradient). Soil C: N did not vary significantly (P > 0.05) with variation in slope
gradient. Cation exchange capacity of all the soil separates are significantly (P ≤ 0.05)
influenced by changes in slope classes. Accordingly the highest CEC value of 28.27 cmol
(+)/kg was recorded in the lower slope gradient, followed by 24.94 cmol (+)/kg of the soils in
the middle slope gradient. The analysis of variance showed that the percentage base saturation
did not vary significantly (P > 0.05) with variation in slope classes.
The mean values of available K and P were statistically significant (P ≤ 0.05) difference. The
highest mean value of available K (700.10 mg kg-1) and available P (1.21 mg kg-1) were
35
observed on the lower slope gradient (slope class 1) while the lowest available K (150.2 mg
kg-1) and available P (0.08 mg kg-1) were found under the upper slope gradient of (slope class
3). The analysis of variance show that except exchangeable Na+, all the other exchangeable
basic caions were significantly (P ≤ 0.05) different under the three slope classes. According to
rating of FAO (2006), exchangeable Na+ content was low for slope class 1 and very low for
slope class 2 and 3, while exchangeable Mg2+ content of all the slope classes was medium (in
the interval of 1-3 cmol (+)/kg). There was a significant (P ≤ 0.05) variation in mean values of
micro nutrient in the soils under the different slope gradient. The availability of micronutrients
in the soils of the study area inversely related with slope gradient.
In conclusion, the physical and chemical properties of soils may vary from place to place due
to slope gradient. Even if some physical and chemical parameters exist in adequate and
desirable amount in one of the three slope classes, they may be deficient for the other. The
soils of the study area showed relatively potentially desirable physical fertility, but soils of the
study area showed deficiency of some chemical parameters, such as low total nitrogen, low
organic carbon very low available P and low Na+ cation. Therefore, to solve the problem and
restore the loss of nutrients and maintain sustainable agricultural production on the study area,
there should be restoring, maintaining and increasing the fertility status of the soils by
integrated soil conservation measures to avoid removal of soil and soil nutrients and
incorporation of crop residues, compost and animal dung would be important in improving
soil fertility as well as other physicochemical characteristics of the soils. Soil analysis alone
did not fairly indicate the toxicity, sufficiency or deficiency level. In addition, the nutrient
supplying powers of the soils and demanding levels of the plants need should be known from
soil-plant analysis for specific fertilizer recommendation. The very low existing available P
problems should be addressed sooner by introducing resistant crop varieties and applying
organic fertilizers.
36
6. REFERENCES
Abayneh Esayas, 2001. Some physico-chemical characteristics of the Raya valley soils. Ethiopian
Journal of Natural Resources, 3(2): 179-193.
Abebe, Y and Diriba, B. 2003. Effect of biomass transfer of Cajanus cajan with or without inorganic
fertilizer on BH-660 hybrid maize variety at Bako, Western Oromia. In: Tilahun, A. and
Eylachew, Z. (eds.), Proceedings of the 6th ESSS conference, February 28- March 1, 2002.
Ethiopian Society of Soil Science. Addis Ababa, Ethiopia.
Ahmed Hussen. 2002. Assessment of Spatial Variability of Some Physcio-chemical Properties of
Soils under Different Elevations and Land Use Systems in the Western Slopes of Mount
Chilalo, Arsi. MSc Thesis, Alemaya University, Alemaya, Ethiopian.
Asefa Taa . 2003. Effects of Straw Management, Tillage and Cropping Sequence on Soil Chemical
Properties in the South –Eastern Highlands of Ethiopia. In: Tilahun Amede and Eylachew
Zewdie (eds.), 2003. Challenges of land Degradation to Agriculture in Ethiopia. Proceedings
of the 6th ESSS conference, Feb 28- March 1, 2002. Ethiopian Society of Soil Science.
Barauah, T.C. and Barthakulh, H.P. 1997. A textbook of soil analysis. Viskas Publishing House, New
Delhi, India.
Barber, S. 1984. Soil nutrition bioavailability: A Mechanistic Approach. J. Wiley and Sons, Inc.,
New York.
Beets, W.C. 1982. Multiple Cropping and Tropical Farming System. Gower, London, Britain and
West Views Press Colorado, USA. 256p.
Belay Simane. 2003. Integrated watershed management approach to sustainable land management
(Experience of SARDP in East Gojjam and South Welo). pp.127-136. In: Tilahun Amede.
(ed.), In: Proceedings of the Conference on the Natural Resource Degradation and
Environmental Concerns: Impact of Food Security, 24-26 July, 2002.The Ethiopian Society of
Soil Science (ESSS). Addis Ababa, Ethiopia.
Berhane Fisseha and Sahilemedhin Sertsu. 2003. Assessment of the different phosphorus forms in
some of agricultural soils of Ethiopia. Ethiopian Journal of Natural Resources, 5(2):193-213.
Binkley, D., Dunkin, K.A. DeBell, D. and Ryan, M.G. 1992. Production and nutrient cycling in
mixed plantation of Eucalyptus and Albizia in Hawaii. Forest Science, 38(1): 393-408.
37
Birru Yitaferu and Heluf Gebrekidan. 2003. Kinetics of phosphorus for some selected soils of the
north-western highlands of Ethiopia. In: Tilahun, A and Eylachew, Z (eds.), Challenges of
land Degradation to Agriculture in Ethiopia Proceedings of the 6th ESSS conference,
February 28- March 1, 2002. Ethiopian Society of Soil Science. Addise Ababa, Ethiopia.
Bohn, H.L., McNeal, B.L. and O'Connor, G.A. 2001. Soil chemistry, 3rd Edition. John Wiley and
Sons, Inc., USA.
Bouyoucos, G.J. 1962. Hydrometer method improvement for making particle size analysis of soils.
Agronomy Journal, 54(3): 179-186.
Brady, N.C. and Weil, R.R. 2002. The Nature and Properties of Soils, 13th Edition. Macmillan
Publishing Company, Inc, USA
Buol, S.W., Hole, F.D. and McCracken, R.J. 1980. Soil Genesis and Classification. 2nd Edition. Iowa
State University Press.
Buresh, R.J. and Tian, G. 1998. Soil improvement by trees in Sub-Sharan Africa. Agro forestry
Systems, 38: 51-76.
Campbell, C.A., McConkey, B.G., Zentner, R.P., Sells, F. and Curtin, D. 1996. Long-term effects of
tillage and crop rotations on soil organic carbon and total nitrogen in a clay soil in south
western Saskatchewan. Canadian Soil Science Journal, 76(2): 395-340.
Chatterjee, B.N. and S. Maiti, 1994. Cropping System; Theory and Practice. Oxford and BBH
Publishing Co, New Delhi, Bombay, Calcutta. 323p.
Curtis, P.E. and Courson, R.L. 1981. Outline of soil fertility and fertilizers, 2nd Edition. Stipes
Publishing Company, Champaign.
Damtew Bekele. 2007. Dynamics of some physicochemical properties of soils under different land
use systems at Ambomesk water shed in Mecha distinict, Northwest Ethiopia. MSc Thesis,
Haramaya University, Haramaya, Ethiopia.
Dereje Tilahun. 2004. Soil Fertility Status with Emphasis on Some Micronutrients in Vegetable
Growing Area of Kolfe, Addis Ababa, Ethiopia. MSc Thesis, Alemaya University, Alemaya,
Ethiopia.
Doran, J.W. 2002. Soil health and global sustainability: translating science into practice. Envi Agri.,
Ecos. 88: 119-127.
Dudal, R. and Deckers, J. 1993. Soil organic matter in relation to soil productivity. pp. 377-380. In:
Mulongoy, J. and Marckx, R. (eds.), Soil Organic Matter Dynamics and Sustainability of
38
Tropical Agriculture. Proceedings of an International Symposium Organised by the
Laboratory of Soil Fertility and Soil Biology, Katholieke Universities, Leuven and the
International Institute of Tropical Agriculture (IITA) in Leuven, Belgium, 4-6 November 1991.
John Wiley and Sons. Ltd. UK.
Eyasu Elias. 2002. Farmers Perception of Soil Fertility Changes and Management. Institute for
Sustainable Development, Addis Ababa, Ethiopia.
Eylachew Zewdie. 1987. Study on Phosphorus Status of Different Soil Types of Chercher Highlands,
South Eastern Ethiopia. PhD. Thesis, Jestus Liebig University, Germany.
Eylachew Zewdie. 2001. Study on the physical, chemical and mineralogical characteristics of some
Vertisols of Ethiopia. pp.87-102. In: Wondimagegn Chekol and Engida Mersha (eds.),
Proceedings of the 5th Conference of the Ethiopian Society of Soil Science/ESSS, March 3031, 2000. The ESSS. Addis Ababa.
FAO (Food and Agriculture Organization). 2006b. Guidelines for soil description. FAO, Rome.
FAO. 1983. Micronutrients. FAO, Fertilizer and Plant Nutrition Bulletin 7. Rome, Italy.
FAO. 1998. Top Soil Characterization for Sustainable Land Management. Soil Resources,
Management and Solutions Service. FAO, Rome.
FAO. 2006a. Plant nutrition for food security: A guide for integrated nutrient management. FAO,
Fertilizer and Plant Nutrition Bulletin 16.
Fisseha Itana. 1992. Macro and micronutrients distributions in Ethiopian Vertisols landscapes. PhD.
Dissertation Submitted to the Institute fur Bondenkunde und Standorslehre, University of
Hohenheim, Germany.
Fisseha Itanna.1996. Micronutrient status of three Ethiopian vertisol landscapes at different
agroecological zones. In: Teshome Yizengaw, Eyasu Mekonnen,and Mintesinot Behailu
(eds.), 1996. Soils Information and Database: The Basis for Improved Agricultural
Productivity and Sustainable Land Resource Management. Proceedings of the third
conference of Ethiopian Society of Soil Science (ESSS), February 28-29, 1996, Addis Ababa,
Ethiopia. 194p.
Foth, H.D. 1990. Fundamentals of Soil Science, 8th Edition. John Wiley and Sons, Inc., New York,
USA.
Foth, H.D. and Ellis, B. G. 1996. Soil fertility, 2nd Edition. Department of Crop and Soil Science,
Michigan, Lewis Publisher, New York.
39
Gebeyaw Tilahun. 2007. Soil fertility status as influenced by different land use in Mayber area of
South Welo zone, North Ethiopia. MSc Thesis, Haramaya University, Haramaya, Ethiopia.
Getachew Fisseha and Heluf Gebrekidan. 2007. Characterization and fertility status of the soils of
Ayehu Research Substation, Northwestern Highlands of Ethiopia. East African Journal of
Sciences 1(2): 160-169.
Gregorich, E.G. Carter, M.R. Anger, D.A. Monereal, C.M. and Ellert, B.H. 1994. Towards
aminimum data set to assess soil organic matter quality in agricultural soils. Can. J. Soil Sci,
74: 367-385.
Gupta, P.K. 2000. Handbook of soil fertilizer and manure. Anis offset press, Paryapuni, New Delhi,
India.
Gupta, P.K. 2004. Soil, plant, water and fertilizer analysis. Shyam Printing Press, Agrobios, India.
Haque, I. 1988. Bibliography on micronutrients in soils, plants and animals in Sub Saharan Africa
Plant Science Division Working Document Number B8. International Livestock Center for
Africa(ILCA), Addis Ababa.
Havlin, J.L., Beaton, J.D., Tisdale, S.L. and Nelson, W.L. 1999. Soil fertility and Fertilizers. Prentice
Hall, New Jersey.
He, Z.L., Alva, A.K., Calvert, D.V., Li, Y.C. and Banks, D.J. 1999. Effects of nitrogen fertilization of
grapefruit trees on soil acidification and nutrient availability in Riviera fine sand. Plant and
Soil, 206: 11-19
Hillel, D. 1980. Fundamentals of Soil Physics. Academic Press, Inc., London.
Hodgson, J.F. 1963. Chemistry of the micronutrients in soils. Advances in Agronomy, 15: 119-149.
IFA (International Fertilizer Industry Association). 1992. World Fertilizer Use Manual. Paris, France.
Ike, I.F. and Aremu, J.A. 1992. Soil physical properties as influenced by tillage practice. In:
International Board for Soil Research and Management (IBSRAM). Soil management
abstract, 4(2): 15
Indiati, R. 2000. Addition of phosphorus to soils with low to medium phosphorus retention capacities.
II. Effect on soil phosphorus extractability. Commun. Soil Sci. Plant Anal, 31: 2591-2606.
Jackson, M.L. 1958. Soil chemical analysis. Prentice Hall, Inc., Englewood cliffs, New Jersy. 183204 pp.
Jordan, C.F. 1993. Ecology of tropical forests. pp.165-195. In: Panxel, L. (ed.), Tropical forestry,
Hand book.
40
Kelsa Kena. 1992. Influence of fertilizer and its related management practices on maize grain yield in
major producing areas of Ethiopia. pp. 15-104. Proceedings of the First National Maize
Workshop of Ethiopia 5-7 may, 1992 Addis Ababa, Ethiopia.
Kimmins, J.P. 1997. Forest ecology: A foundation for sustainable management, 2nd Edition. New
Jersey.
Krauskof, K.B. 1972. Geochemistry of micronutrients. pp. 7-35. In: Mortvedt, J.J., Giordano, P.M.
and Lindsay, W.L. (eds.), Micronutrients in Agriculture. Soil Science Society of America.
Washington, USA
Landon, J.R. 1991. Booker Tropical Soil Manual: a handbook for soil survey and agricultural land
evaluation in the tropics and subtropics. John Wiley & Sons Inc., New York.
Lilienfein, J., W. Wilcke, M.A. Ayarza, L. Vilela, S.D.C. Lima and W. Zech, 2000. Chemical
fractionation of phosphorus, sulphur, and molybdenum in Brazilian savannah Oxisols under
different land uses. Geoderma. 96: 31-46.
Lindsay, W.L. and Norvell, W.A. 1978. Development of a DTPA soil test for zinc, iron, manganese
and copper. Soil Sci. Soc. Am. J, 42: 421-428.
Martin. A.1993. Estimate of organic matter turnover rate in a savanna soil by 13C natural abundance
measurements. Soil Biology and Biochemistry.
Mesfin Abebe. 1996. The challenges and future prospects of soil chemistry in Ethiopia. pp. 78-96. In:
Teshome, Y., Eyasu, M. and Mintesinot, B. (eds.), Proceedings of the 3rd Conference of the
Ethiopian Society of Soil Science, 28-29 February 1996. Ethiopia science and Technology
Commission. Addis Ababa, Ethiopia.
Mesfin Abebe. 1998. Nature and Management of Ethiopian Soils. Alemaya University of
Agriculture.
Miller, R. W. and Donahue, R. L. 1995. Soils in Our Environment, 7th Edition. A Simon and Schuster
Company Englewood Cliffs, New Jersey.
Mitiku Haile. 1987. Genesis, Characteristics, and Classification of Soils of the Central Highlands of
Ethiopia. Thesis submitted to the State University of Ghent, Belgium, in fulfillment of the
requirements of the Degree of Doctor in Soil Science.
Mohammed Assen. 2003a. Land suitability evaluation in the Jelo Catchment, Chercher Highlands,
Ethiopia. PhD Thesis University of Free State, Bloemfontein, South Africa.
41
Mohammed Assen. 2005. Soils of Jelo Micro-catchment in the Chercher highlands of Eastern
Ethiopia: Morphological and Physicochemical Properties. Ethiopian Journal of Natural
Resources, 7(1): 55-81.
Mulongey, K. and R. Merck (Eds.), 1993, Soil organic matter dynamics and sustainability of tropical
agriculture. John Wiley and Sons, Inc., New York. 392p
Mulugeta Lemenih, 2004. Effects of land use changes on soil quality and native flora degradation and
restoration in the highlands of Ethiopia: Implication for sustainable land management. PhD
Thesis presented to Swedish University of Agricultural Sciences, Uppsala.
Murphy, H.F. 1968. A report on fertility status and other data on some soils of Ethiopia. Addis
Ababa, Ethiopia.
Nair, K.M. and Chamuah, G.S. 1993. Exchangeable aluminium in soils of Meghlaya and
Management of Al3+ related productive constraints. Indian Soil Science Society Journal,
4(1/2): 331-334.
Nega Emiru. 2006. Land Use Changes and Their Effects on Soil Physical and Chemical Properties in
Senbat Sub-Watershed, Western Ethiopia. MSc Thesis, Alemaya University, Alemaya,
Ethiopia.
Olsen, S.R., Cole, C.V., Watanabe, F.S. and Dean, L.A. 1954. Estimation of available phosphorus in
soil by extraction with sodium bicarbonate. USDA circular, 939: 1-19.
Ouedraogo, E. 2004. Soil quality improvement for crop production in semi-arid West Africa. Tropical
Resource Management Paper No. 51.Wageningen University.
Poritchett, W.L. and R.F. Fisher, 1987. Properties and management of forest soils, 2nd Ed., John
Wiley and Sons, Inc., New York. 494p.
Prasad, R. and James F. Power, 1997. Soil Fertility management for sustainable Agriculture. Lewis
Publisher, New York Printed-Hall Inc. U.S.A.
Quinenes, M., A. 1992. Methodology used by SG 2000 Project in Africa for transfer of improved
production technologies to small scale farmer. pp. 149-153. Proceeding of the First National
Maize Workshop of Ethiopia, Addis Ababa.
Rai, M.M. 1995. Principles of Soil Science. 3rd ed. Rajiv Beri for Macmillan India Limited, New
Delhi.
42
Ranst, V. 1991. Land Evaluation: Principles in Land Evaluation and Crop Production Calculations
(Part I). International Training Center for Post Graduate Soil Scientists. State University
Gent, Belgium.
Rao, M.R., Niang, A., Hwesiga, F., Duguma, B., Franzel, S., Jama, B. And Buresh, R. 1998. Soil
fertility replenishment in Sub-Saharan Africa: New techniques and the spread of their use on
farms. Agroforestry Today, 10(2): 3-8.
RDP (Rural Development Program), 1986. Analytical methods of the laboratory for soil, plants and
water analysis. Royal Tropical Institute, the Netherlands. 110p.
Regina.k .2004. Introduction to Environmental Soil Physics. Elsevier Academic Press
Roming, D.E., Garlynd, M.J., Harris, R.F. and McSweeney, K. 1995. How farmers assess soil health
and quality. Soil and Water Solutions Journal, 50: 229-236.
Rowell, D.L. 1994. Soil Science: Methods and Applications. Longman group UK Limited.
Sahlemedhin Sertsu and Taye Bekele. 2000. Procedures for Soil and Plant Analysis. National Soil
Research center, Ethiopian Agricultural Research organization.
Saikh, H., Varadachari, C. and Ghosh, K. 1998a. Changes in carbon, nitrogen and phosphorus levels
due to deforestation and cultivation. A case study in Simplipal National Park, India. Plant
Soil, 198: 137-145.36.
Sanchez, P.A. 1976. Properties and management of soils in the tropics. John Wiley and Sons, Inc.
USA.
Sanchez, P.A. 1997. Soil fertility replenishment in Africa. An Investment in Natural Resource
Capital. pp. 1-46. In: Buresh, R.J. Sanchez, P.A. and Calnoun, F. (eds.), SSSA special
publication No.51. SSSA. Am. Soc. Argon. Madison, Wisconsin, USA.
SAS (Statistical Analysis System) Institute, 1999. The SAS system for windows, version 8.1, Volume
1. SAS Institute Inc. Cary NC., USA.
Sharma, A.K, 2002. Handbook of organic farming. Agrobios, India. 627p.
Shehu, Y., Alhassan, W.S., Mensah, G.W.K., Aliyu, A. and Philips, C.J.C. 1997. The effects of green
manuring and chemical fertilizer application on maize yield, quality and soil composition.
Tropical Grasslands, 32: 139-142.
Sheldrake, A.R. and Narayanan, A. 1978. Growth, development and nutrient uptake of pigeon pea.
Agricultural Science Journal, 92:513-526
43
Sillanpääa, M. 1990. Micronutrient assessment at country level: an international study. FAO Soils
Bull. Rome.
Singh, H., Sharma, K.N. and Aroa, B.S. 1995. Influence of continuous fertilization to a maize
system on the changes in soil fertility. Fert. Res 40: 7-19.
Smith, C.J., Goh, G.M., Bond, W.J. and Freney, J.R. 1995. Effects of organic and inorganic calcium
compounds on soil solution pH and Al concentration. European Soil Science Journal, 46:5363.
Solomon Derje .2002. Soil organic matter dynamics in the sub-humid Ethiopian highlands: Evidence
from natural 13C abundance and particle-size fractionation. Soil Sci. Soc. Amr, J. 66: 969-978.
Somani, L.L. 1996. Crop Production in Acid Soils. Agrotech Publishing Academy: Udaipur,
Daryagani, New Delhi.
Sposito, G. (Ed.), 1989. The Chemistry of Soils. pp. 226-245. Oxford Uni. Press, New York.
Stevenson, F.J. 1986. Cycles of Soil. John Wiley & Sons, Inc., New York.
Stocking, M.A. 2003. Tropical soils and food security: the next 50 years. Science, 302:1356- 1359.
Syers, J.K. 2001. Sustainable Nutrient Management of Vetisols, pp 43-55. In: Syers, K.J., Devaries,
P. and Nyamudeza (eds.) (2001). CABI publishing , New York.
Szabolcs, I., 1969. The influence of sodium carbonate on soil forming processes and on soil
properties. Symposium on the Reclamation of Sodic and Soda-Saline Soils, Yerevan 1969.
Tom. 18: 37-52.FAO (Food and Agriculture Organization), 1984a. Physical and Chemical
Methods of Soil and Plant Analysis. FAO Soils Bull. No. 10, FAO of the United Nations,
Rome 1984.
Tamirat Tsegaye. 1992. Vertisols of the Central Highlands of Ethiopia – Characterization,
Classification, and Evaluation, of the Phosphorus status. MSc Thesis, Alemaya University,
Alemaya, Ethiopia.
Tamirat Tsegaye. 1996. Vertisols of the central highland of Ethiopia, physical and chemical
characterization. pp.46-77. In: Tekalign Mamo and Mitiku Haile (eds.), Soil- The Resource
Base for survival. Proceedings of the Second Conference of Ethiopian Society of Soil Science
(ESSS), 23-24 September 1993, Addis Ababa, Ethiopia.
Tamirie Hawando, 1982. Problems of soils and its implications on crop improvement program in
Ethiopia context. Department of plant science, college of Agriculture, Addis Ababa
University. pp 5-12
44
Teferi Tadesa. 2008. Soil fertility assessment and mapping in selected Highland areas of Kuyu
district in north Shoa Zone of Oromia Region. MSc Thesis, Haramaya University, Haramaya,
Ethiopia.
Tekalign Mamo and Haque, I. 1991. Phosphorus Status of Some Ethiopian Soils. III. Evaluation of
soil test methods for available phosphorus. Tropical Agriculture (Trinidad), 68: 51-56.
Tesfu Kebede and Laktionov, I. 1996. Quantitative changes of humus on some soils around Ginchi
and Holeta. pp. 231-241. In: Tekalign Mamo and Mitiku Haile (eds.), Soil the Resource Base
for Survival. Proceeding of the second conference of Ethiopian Society of Soil Science (ESSS),
23-24 September 1993, Addis Ababa, Ethiopia.
Tisdale, S.L., Nelson, W.L., Beaton, J.D. and Havlin, J.L. 2002. Soil fertility and fertilizer, 7th
Edition. Prentice-Hall of India, Newdelhi.
Van Reeuwijk, L.P., 1992. Procedures for soil analysis, 3rd Edition. International Soil Reference and
Information Center (ISRIC), Wageningen, Netherland.
Vance, D.E. 2000. Agricultural site productivity: principles derived from long-term experiments and
their implications for intensively managed forests. Forest Ecology and Management 138: 369
396.
Verchot, L.V. 2007. Science and Technology Innovations for Improving Soil Fertility and
Management in Africa: A report for the NEPAD science and technology Forum. World Agro
forestry Center United Nations Avenue, Nairobi Kenya.
Wakene Negasa and Heluf Gebrekidan. 2003. Influence of land management on morphological,
physical and chemical properties of some soils of Bako, Western Ethiopia. Agropedology,
13(2): 1-9.
Wakene Negasa. 2001. Assessment of Important Physico-Chemical Properties of District Udalf
(District Nitosols) Under Different Management Systems in Bako Area, Western Ethiopia.
MSc. Thesis. Alemaya University, Alemaya, Ethiopia.
Walkley, A. and Black, I.A. 1934. An examination of the Degtjareff method for determining soil
organic matter and a proposed modification of the chromic acid titration method. Soil Science
journal, 37: 29-38.
White, R.E. 1997. Principles and practices of soils science: The soil is the natural resource.
Cambridge University Press, UK.
45
Wilding, L.G., 1985. Soil spatial variability: Its documentation, accommodation and implication to
soil surveys. pp. 166-187. In: D.R. Nielsen and J. Bouma (Eds.), Soil Spatial Variability
Proceedings of a Workshop of the ISSS and the SSA, Las Vegas PUDOC, Wageningen.
Woldeamlak Bewket and Stroosnijder, L. 2003. Effects of agro-ecological land use succession on soil
properties in the Chemoga watershed, Blue Nile basin, Ethiopia. Geoderma, 111: 85-98.
Yerima, B., 1993. Manual on good laboratory practice. Technical Report. NDP / FAO / ETH / 87 /
10. Addis Ababa, Ethiopia. 180p. In review.
Yesuf Assen and Duga Debele. 2001. The effect, rate and time of nitrogen application on the uptake
by bread wheat varieties and soil characteristics on farmers’ fields. Ethiopian Society of Soil
Sciences. Ethiopian Journal of Natural Resources, 2(2): 165-187.
Yesuf Assen. 2006. Nitrogen fertilizer requriment of bread wheat (Triticum aestivum L.) after legume
pre-cursor crops in Arsi zone of Ethiopia. Ethiopian Society of Soil Sciences. Ethiopian
Journal of Natural Resources, 8 (2): 217-228.
Yihenew Gebreselassie. 2002. Selected chemical and physical characteristics of soils of Adet
Research Center and its testing sites in north-western Ethiopia. Ethiopian Journal of Natural
Resources, 4(2): 199-215.
Youndeowei, A. 1990. Introduction to tropical agriculture. English Language Book, Longman. 344p.
Zhang, H. and Zhang, G. 2005. Landscape-scale soil quality change under different farming systems
of a tropical farm in Hainan, China. Soil Use and Management, 21: 58-64.
46
7. Appendixes
47
Appendix Table 1. Ratings of soil pH (Tekalign, 1991), organic carbon (Berhanu, 1980) and
CEC (Landon, 1991)
pH (1:2.5, H2O)
PH
Rating
< 4.5
Very strongly acid
4.5-5.2
Strongly acid
5.3-5.9
Moderately acid
6.0-6.6
Slightly acid
6.7-7.3
Neutral
7.4-8.0
Moderately alkaline
> 8.0
Strongly alkaline
Organic carbon
OC (%) Rating
>10
High
4-10
Medium
<4
Low
Cation exchange capacity (CEC)
CEC (cmolc kg-1) Rating
> 40
Very high
25 – 40
High
15 - 25
Medium
<5
Low
Very low
Appendix Table 2. Ratings of soil available phosphors (mgkg-1), available potassium (mgkg-1),
percentage base saturation (%), and total nitrogen (%) (Haviln, 1999)
AVP
<3
4-7
8-11
>12
Rating
Very low
Low
Medium
High
AVK
<50
50-100
100-175
175-300
>300
Rating
Very low
Low
medium
High
Very high
PBS
<20
20-60
>60
Rating
Low
Medium
High
TN
<0.15
0.15-0-25
>0.25
Appendix Table 3. Ratings of exchangeable basic cations in the soil (FAO, 2006)
Rating
Very high
High
Medium
Low
Very low
Exchangeable cations Rating (cmolckg-1)
Na
K
Ca
>2
>1.2
> 20
0.7-2
0.6-1.2
10-20
0.3-0.7
3-8
5-10
0.1-0.3
0.2-0.3
2-5
< 0.1
< 0.2
<2
48
Mg
>8
3-8
0.3-1
0.3-0.7
< 0.3
Rating
Low
Medium
High
Appendix Table 4. Mean square of soil physical and chemical parameters from the
different slope gradients
Slope gradients
Mean squares for source variation
Slope gradients (2)
Error (4)
Sand
52.1942111*
0.00024440
Silt
123.800700*
0.00128330
Clay
304.293911*
0.00119440
Bulk density
0.01040678NS
0.00000728
Total porosity
14.5976920NS
0.01022200
PH (H2O)
0.18111078NS
0.00005428
Organic Carbon
0.07412228*
0.00000092
Total nitrogen
0.00141062*
0.00000003
C; N
0.16716947NS
0.00009430
Available P
0.96649733*
0.00000433
Available K
0.07968500NS
0.13005749
Exchangeable acidity
0.07968500NS
0.00054632
Exchangeable Na
0.00547994NS
0.00000394
Exchangeable K
0.13679111*
0.00000011
Exchangeable Ca
0.12984711*
0.10881244
Exchangeable Mg
0.00741667*
0.00013333
CEC
25.8777221*
0.00005494
Available Fe
256.560233*
0.05745220
Available Mn
1769.50034*
0.00338775
Available Zn
58.3456023*
0.13865470
Available Cu
789.284675*
0.53411739
Figures in parentheses are values of degrees of freedom for respective source
of variation; NS = Non significant;* = Significant at P ≤ 0.05
Appendix Table 5. Geographical coordinates (grids) of the sampling sites
Points Slope gradients 1
1
2
3
4
5
6
7
8
9
10
11
Slope gradients 2
Slope gradients 3
X (East)
Y (North)
X (East)
Y (North)
X (East)
Y (North)
184625.62
184708.60
184601.21
184780.61
184599.98
184619.51
184711.04
184718.37
184778.16
184692.34
184829.42
1158605.80
1158594.83
1158256.48
1158509.39
1158458.14
1158386.14
1158484.99
115838652
1158300.71
1158275.08
1158369.05
184831.86
184920.95
185022.24
185002.78
184957.56
185000.28
184895.32
184848.95
184770.84
184844.06
184913.63
1158721.75
1158709.54
1158442.28
1158705.88
1158509.40
1158602.15
1158619.23
1158516.72
1158641.20
1158450.81
1158388.58
185018.70
185187.12
185274.99
185334.79
185223.73
185360.42
185157.83
185069.96
185261.56
185138.30
185271.05
1158765.68
1158794.97
1158793.75
1158766.90
1158610.69
1158615.57
1158679.03
1158621.67
1158504.51
1158472.78
1158689.55
49