Sustainable Soil Management of the Dryland Soils in Northern Nigeria

Table of contents

Dedication

Acknowledgement

Declaration

List of tables

List of figures

List of abbreviations

Abstract

Chapter 1 General introduction
1.1 Introduction
1.2 Area of study
1.3 Key soil types in the study area
1.4 Key crop growing and farming system
1.5 Aims and 0bjectives

Chapter 2 Literature background
2.1 Soil and Agriculture
2.2 Drylands: meaning and characteristic features
2.3 Concept of soil evolution in history
2.4 Soil: properties and components
2.5 Physical property of soil
2.5.1 Soil texture
2.5.2 Soil structure
2.6 Soil organic matter
2.7 Fertile and productive soil in agriculture
2.8 Soil: an essential medium for growing crops
2.9 Environmental degradation and their consequences
2.9.1 Types of soil degradation process
2.9.2 Classes of land degradation severity
2.10 Soil erosion
2.10.1 Types of soil erosion
2.10.2 Erosion by water (rain)
2.10.3 Erosion by wind
2.10.4 Implications of soil erosion by water and wind
2.11 Desertification
2.11.1 Causes of desertification and implications
2.12 Soil sustainability: meaning and importance in agriculture
2.13 Sustainable soil management practices
2.13.1 Use of manure and composting in agriculture
2.13.2 Mulching
2.13.3 Intercropping
2.13.4 Crop rotation
2.13.5 Chemical fertilizers
2.13.6 Cover crops
2.13.7 Afforestation and Shelter belt
2.13.8 Tillage and contour ploughing
2.14 Summary of chapter 2

Chapter 3 General discussions on key soil problems and their possible solutions in northern Nigeria
3.1 Key soil problems in northern Nigeria
3.1.1 Soil Erosion in northern Nigeria
3.1.2 Agents of soil erosion
3.1.3 Desertification in northern Nigeria
3.1.4 Potential impact of soil erosion and desertification in northern Nigeria
3.1.4.1 Impacts and causes of nutrient losses from soil
3.1.4.2 Impact and causes of decreased in crop yield
3.2 Possible solutions to control soil degradation in northern Nigeria
3.2.1 Maintenance of soil fertility in northern Nigeria
3.2.2 Soil management practices in northern Nigeria
3.2.2.1 Use of manure
3.2.2.2 Mulching by spreading straw on topsoil
3.2.2.3 Intercropping
3.2.2.4 Crop rotation system
3.2.2.5 Nutrient and chemical fertilizers
3.3 Alternative measures to control of soil erosion in northern Nigeria
3.3.1 Control of water erosion
3.3.1.1 Protective cover
3.3.1.2 Technical changes
3.3.1.3 Practical/physical changes
3.3.1.4 Political changes
3.3.2 Control of wind erosion
3.3.2.1 Vegetation cover
3.3.2.2 Provision of windbreak and shelterbelt
3.3.2.3 Listing of sandy soils
3.3.3 Control of desertification in northern Nigeria
3.4 Alternative measures to control desertification in northern Nigeria
3.4.1 Afforestation and tree plantation
3.4.2 Agro-pastoral systems
3.4.3 Restoration of rangeland
3.4.4 Regeneration and secondary forest
3.5 Summary of chapter 3

Chapter 4 Conclusions
4.1 Summary and Recommendations

References

Dedication

This thesis is dedicated to my family members Alhaji Yahayya Usman Labbo Allugu, Hajjiyya Aishatu Yhayya, Mr. U. Hashimu, Mr. Y. Nasiru, Mr. A. Abdullahi, Mr. U. Basiru, Mr. U. Idrisu, Mr. U. Samaila, Mr. U. Ishaqa, Miss. U. Aishatu, Miss. U. Maryam and other relatives.

ACKNOWLEDGEMENTS

I wish to express my sincere thanks to All-Mighty Allah Who offered me an opportunity through Kebbi state government scholarship to study this course in the Natural Resource Institute, University of Greenwich, UK.

I also wish to express my thanks to Dr. P. Burt for his kind advice and guidance to finish this project in well arrange manner.

I wish to extend this thanks to my mother Hajjiyya Aishatu, my father Alhaji Yahayya Usman, my Islamic scholars in persons of Sheikh Lawal Argungu and Sheikh Aliyu Bunza and my sister Zalihatu for their kindness to me.

List of tables

1 Estimated vulnerability classes of land 30 degradation in Africa

2 Land areas in risk classes of land degradation 31 in Africa

3 Assessment of human-induced desertification 39 in affected areas of the world

4 Example of the harmful effect of soil degradation 55 in northern Nigeria

List of figures

1 Example of sequestration of carbon and organic matter into the soil and biomass through carbon cycle

2 Map of sub-Saharan Africa indicating Nigeria

3 Map of Nigeria indicating the major stated in northern part

4 Linguistic Groups map of Nigeria indicating Hausa and Fulani as major ethnic group in northern Nigeria

5 Evaporation exceeds rainfall in dry areas of the world

6 Example of dryland farming system in some part of African dryland areas

7 Stages of soil formation of soil

8 USDA textural triangle indicating particle separations: clay, silt sandy and loamy soils

9 The size particles of sand silt and clay. No. means number of particle size

10 The main types of soil structure

11 Global soil regions indicating different types of soil orders

12 (A) Chemical and (B) Biological activities of biochemical breakdown of a plant tissue by soil micro organisms in a process of mineralization and humification respectively

13 Example of high yield in maize farm obtained from fertile soil

14 Chemical transfer in the atmosphere

15 Soil zones where water interacts with other particles such as rocks for plant used

16 Role of soil organisms in nutrient cycling associated with crop production

17 Structure of Earth indicating the crustal zone where geochemical cycles take

18 Causes of soil erosion

19 A simple model of hillside erosion. Source

20 Raindrop is about to hit unprotected mismanaged soil

21 Raindrop hit unprotected soil, detach and erode soil particles

22 Raindrop hitting unprotected mismanaged soil and caused soil compaction

23 Soil erosion by water increasing from sheet leading to gully

24 Rill erosion exceeding to gullies

25 Erosion by wind affects all kinds of soil materials

26 Map of Africa indicating areas at risk of desertification

27 Example of sound land use managements practices for sustainable development

28 Standard Agricultural manure preferred as a soil cover to enhance soil fertility and control soil erosion

29 Example of compost filters berms

30 Practical models on how compost berm works

31 Straws or crop residue covered as mulch spread on top

of soil to maintain soil fertility and reduce soil erosion

32 Field shelterbelt reduced wind erosion and conserve soil moisture

33 Example of an indication of soil erosion starting from sheet and proceeding to rill as in the case of many areas in northern Nigeria

34 An example of how erosion hinders soil productivity

35 Example of soil erosion in Katsina State, northern Nigeria

36 Strip cropping can reduce soil losses by 50%

37 Listing of sandy soil reduces wind erosion

38 Example of using scientific way to control soil erosion in dryland areas

List of abbreviations

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ABSTRACT

Although it is widely recognised that environmental problems such as soil degradation erosion and desertification threaten sustained agricultural production in many States of northern Nigeria including Adamawa, Bauchi, Borno, Gombe, Jigawa, Kaduna Kano, Katsina, Kebbi, Sokoto and Zamfara. Very little information is available about the current status of the potential impact of soil degradation and whether the situation is worsening in all the States of northern Nigeria. However, it is now clear that the major factor explaining the severity and spatial distribution of soil degradation such as erosion and desertification in northern Nigeria is associated with human impact such as deforestation and mismanagement of land resources. Other factors includes overgrazing, use of mechanized equipment, deforestation and lack of government concern to protect the environment in the region, but few studies have been made on applied issues related to the physical and chemical processes including erosion, runoff and leaching. These problems are widely considered as a serious problem to agricultural production and its environmental consequences will remain an important issue during the 21st century.

Possible solutions such as soil management practices will help to minimise/control soil degradations which caused serious hazards to farmer’s land in northern Nigeria. Sustainable soil management practices are vital for enhancing and sustaining the productivity of soil, food, livestock, water quality and other related land resources such as forestry in northern Nigeria. It is more efficient in terms of reduced environmental impact, high risk of soil degradation and soil erosion. These management practices are: (a) applying organic manure regularly, (b) growing cover crops in rotation with millet/sorghum, (c) Intercropping and multiple cropping systems, (d) crop rotation system, (e) planting shelter belt around the farm (f) minimum tillage system, (g) good drainage system and (h) good government policies. However two steps will help to achieve the successful implementaion of those management practices in northern Nigeria. These are: (a) good government policies with quality initiative and (b) general cooperation from individual, community and people in concerned.

Chapter 1
GENERAL INTRODUCTION

1.1 Introduction

The vast importance of soil in the development of various systems of agriculture and types of civilizations has long been recognized (Jenny, 1994). Soil is the basis of production in agriculture and forestry, an important component of the human environment (Zachar, 1982), and is a significant component of arid ecosystems (Russell and Greacen, 1977). Soil provides habitats for organisms (the soil fauna and micro-organisms) (Wild, 1993) and moisture and nutrients for the basic requirements of plant growth (Okigbo, 1991). Therefore, the science of soil has played, and continues to play, an important role in global studies of food production in relation to the Earth’s natural resources (Hartemink, 2003). This work is in progress with the effort of many researchers and organizations through the provision of relevant information on global soil resources e.g. the development of the FAO-UNESCO soil map of the world (Graaff, 1993). However, there is a need to improve the existing information on management and sustainability of soils in areas such as northern Nigeria (e.g. Tor, 2001). This is to help improve the fertility and quality of soils of the region using management practices such as organic matter application, composting, proper irrigation systems, intercropping, and others. These activities can help increase food availability by improving soil quality, sustain soil fertility and maintain yield (Blum, 1994; Zhao, 1995; Lal, 1997; World Bank, 2001; Mortimore and Adams, 2001; Osbahr and Allen, 2002; Pretty et al., 2003).

To achieve these management activities, much more attention from farmers, and general concern from government, should be given to the sustainability of soil (using available and affordable management resources by farmers) and creation of good environmental policies (by government). This is because for many years soils in northern Nigeria, as well as in other parts of sub-Saharan Africa, have been facing many problems ranging from nutrient depletion as a result of land degradation factors such as erosion by wind and water (Lal, 1998), desertification as a result of poor vegetation cover, and human-induced activities in destroying forests (Gad and Abdel, 2000; ICLDD, 2001; FMEN, 2001).

These types of problems might lead to serious damage to farmers’ lands by the loss of organic matter, and the deterioration of soil structure (Bradley and Thompson, 1998; UNEP, 2003) which may also lead to decreases in crop yield year after year (e.g. Gachimbi et al., 2002; GSST, 2006). These problems have been mentioned by researchers studying the dryland area of sub-Saharan Africa (e.g. Stoorvogel and Smaling, 1990), as serious problems occurring in the northern part of Nigeria (FMEN, 2001) and as principal causes of nutrient losses from soil (Mango, 1996), as well as being a major constraint to sustainability in agricultural production (Okigbo, 1991; Gomes et al., 2003; Su et al., 2003).This decline in soil fertility, and an increasing population pressure, has been met with by calls from some international scientists for a soil recapitalisation programme of sustainable development for sub-Saharan Africa (DFID, 2002). This programme could lead to: enhanced and sustained food production, reversal of degradative trends and improvements in soil quality and soil resilience, and enhancement of environmental quality through sequestration of carbon and organic matter into the soil and biomass (Figure 1) (FAO, 2001; Lal, 2004; Lal et al., 2004; Tieszen et al., 2004; Farage et al., 2007) and improvement in water quality (Lal, 1995a).

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Figure 1: Example of sequestration of carbon and organic matter into the soil and biomass through carbon cycle (Ref. 1).

According to previous research by Lal (1995b), these management systems are based on six attributes: (a) soil erosion control; (b) improvement in soil organic matter content; (c) enhancement of soil structure; (e) increase in soil biodiversity; (f) strengthening of nutrient cycling mechanisms and (g) increase in soil resilience. Therefore, the most challenging goal for research and development is to achieve sustainable land/soil management (Syers et al., 2001), particularly in the vulnerable areas as in the case of northern part of Nigeria (FMEN, 2001). These vulnerable areas are usually dry (Kalpage, 1976) and their soils have been characterized with having low moisture content, low organic matter and degrade rapidly under conditions of intensive rainfall (Harris 2000; Aregheore, 2005).

1.2 Area of study

Nigeria is a country which is approximately between latitude 4o and 14o north of the Equator (Salako, 2003) and between longitudes 2o 2’ and 14o 30’ east of the Greenwich Meridian, with a surface area estimated as 91.07 million hectares, in which 57% is believed to be either under crops, or pasture (Cleaver and Schreiber, 1994). Another estimate indicated that 75.9% of the landmass of the country is in northern part of Nigeria (Clara et al., 2003) in which the total area of the dryland is approximately 170,000 km2 (Anderson, 1988).

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Figure 2: Map of sub-Saharan Africa indicating Nigeria (Ref. 2).

The topography of northern Nigeria consists of semiarid plain and lies between the latitude of 11o N and 14o N (Central Intelligence Agency, 2005), collectively covered thirteen states (Figure 3): Adamawa, Bauchi, Borno, Gombe, Jigawa, Kaduna, Kano, Katsina, Kebbi, Niger, Sokoto, Yobe and Zamfara (FMEN, 2001). The region is bordered by the Republics of Niger and Benin (McEwen, 1991; Aregheore, 2005).

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Figure 3: Map of Nigeria indicating the major stated in northern part (Ref. 3).

Northern Nigeria is inhibited primary by the Hausa and nomadic Fulani ethnic groups (see Figure 4) (FMEN, 2001) who together account for 29% of Nigeria’s total population; there are 3 also million Kanuri in and around the north-western state of Borno who make up 4% of the country’s population, and around 300,000 people from the Gwari minority in and around Kaduna state (Gunnemark and Kenrick, 1985). The majority of these people are Muslims, speaking Hausa as their major language (UNICEFF, 2001). They largely depend on farming and cattle rearing (FMEN, 2001) and for that are classified as an agro-pastoralist (Dixon et al., 2001). Indeed, the major binding factors among the people living in northern Nigeria are: the significance of Islamic religious culture, language of communication and agriculture as the mainstay of economic activity, low population density, the communal nature of civil society organization and the preponderance of rural settlements within a wider context of rural poverty (Clara et al., 2003).

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Figure 4: Linguistic Groups map of Nigeria indicating Hausa and Fulani as major ethnic group in northern Nigeria (Ref. 4).

Northern Nigeria covers a wide range of land described as the dryland of Nigeria (FMEN, 2001), which constitutes the main source of fodder and the grazing land for livestock in the north (Gadzama, 1997). It has been characterised as a region where the rainfall is between 400 and 1200 mm per annum (Brendan et al., 2002). The region also has an undulating plain topography, with general elevation from about 450 to 700 m, covered in highly sandy soils, which are usually very low in organic matter, may degrade rapidly under conditions of intensive rainfall (Mortmore, 1989) and have deficiency in elements such as nitrogen (N), potassium (K) and phosphorous (P) (Aregheore, 2005).

Understanding the changing of rainfall pattern in northern Nigeria and its effects on soil processes and the environment is of critical importance in the assessment and management of ecology (Oluwasemire, 2004). Annual rainfall is variable (Adefolalu, 2004) and declining (Ati, 1995), being on average 716 mm during the period 1962-88, against 815 mm over 1962-71, and was only 509 mm in 1993 with significant increase 712 mm in 2004 (Oluwasemire, 2004). Rains normally starts in June and are very regularly distributed from early July onwards and through August, diminishing into September (Onyewotu et al., 2003). In fact, much research carried out in the Sudan and Sahelian zones of West Africa indicated that the mean May to September rainfall is variable (Jenkinson, 1942). Using data on West African dryland’s rain (e.g. Niger republic) from CRU (2005), average rainfall for 1941-1960 was 181 mm; the average for 1961-1980 fell to 161 mm; and the average for 1981-200 was only 148 mm and as such are characterised with having drought condition that contribute to the occurrence of land degradation (Vermani et al., 1994). It is now widely recognised that a search for long term average rainfall can be misleading in sub-Saharan Africa, especially in the Sahel dryland areas (Mortmore and Adams, 1999).

1.3 Key soil types in northern Nigeria

Soils in most parts of northern Nigeria’s dryland are well-drained, sandy, and low in organic matter (Harris, 2000) and are also characterised by having low water-holding capacity (FMEN, 2001). The only exception to this observation, as indicated by the Federal Ministry of Environment in Nigeria (FMEN, 2001), is the fadama soil found in states such as Kebbi Sokoto and Zamfara. Fadama soil in northern Nigeria has been described by many researchers as having a fine texture, high organic matter content and relatively high water-holding capacity (FMEN, 2001; Usman, 2003). For example, research carried out by Usman (2003), found that the rate of infiltration in these soils is generally high compared to that of the fadama flood plains areas of the region. The term fadama refers to seasonally flooded areas named by Hausa people (Iloeje, 2001), described as having flat-floored valleys that are flooded in the wet season only, and recede during the dry season to leave a coating of alluvial soil (Iloeje, 2001).

Generally, soil such as that of dryland of northern Nigeria are named as ‘Aridisols’ by soil taxonomists (USDA, 1975) and are characterised as slowly permeable, most of the water is lost by run-off (Fitzpatrick, 1980). They might have been formed under aridity from wind-stored desert sands that accumulated over long periods of time (Adegbola, 1979). In addition, some soils in states such as Kaduna, Katsina, Kebbi, Sokoto and Zamfara of northern Nigeria have been also attributed to ferruginous tropic soils (D’Hoore, 1964) and characterised as having sandy texture, covering large areas of land with very low water-holding capacity and low organic matter, nitrogen phosphorus content, neutral or moderately acidic in pH and also having a low cation exchange capacity (Jones and Wild, 1975). Fitzpatrick (1980) showed that the vegetation in such desert areas is usually spare and the surface is bare for long periods. This may contribute to soil degradation by wind erosion and, hence, cause soil fertility to decline in the area. Many authors have considered such a problem as one of the major contributing factors to soil and environmental degradation (Hudson, 1981; Mango, 1996). For example, the majority of millet farmers in Kebbi and Sokoto states have suffered from this problem, which results in decreased of food production as noted by Okigbo, (1991). It might also be due to this reason that those soils are unsustained and have less ability to support sustainable agricultural development (e.g. Harris, 2000). It was reported (Fageria, 1992) that loss of topsoil by water erosion caused by poor soil management is by far the largest single factor contributing to the deterioration of physical properties and decline in productivity of most cropland soils. These problems may be attributed to human activities/pressure on the land, either by continuous cultivation with mechanized equipment such as tractors reducing soil quality leading to erosion, leaching, and nutrient depletion (Knapp, 1979; Eswarran et al., 2001). It may also occur as a result of poor management by both government and people including miss management of readily available and affordable resources to be used for sustainable soil management in the area by farmers themselves. Thus, such soils must be maintained in their natural state for the increase of their agricultural production in northern Nigeria.

Generally speaking, there is a need to overcome or minimise these problems in the zone (e.g. Mortimore, 1989) and this can be done by applying soil management strategies as a combination/integration of several readily available and affordable resources, so as to achieve sustainable development (NRC, 1987) and by using agricultural systems such as intercropping (DFID, 2005).

1.4 Key crops grown and farming system

Cereals are the most important stable food crop in northern Nigeria (Muhamman and Gungula, 2006). These include millet, sorghum, maize, cowpea, groundnut, and sometime soybean in parts of Katsina, Kano, northern Kaduna, Sokoto and Zamfara states (Aregheore, 2005). The cropping systems of cereals predominate in the farming systems with one or several other crops in mixture or in rotation (Weber et al., 1996). The mixture mostly found in northern Nigeria is sorghum, millet, cowpea; sorghum, millet; sorghum/groundnut and sorghum/cowpea (Muhamman and Gungula, 2006). However, millet and sorghum are frequently grown on the same plot in areas such as Kano, Kebbi and Sokoto states (Baker, 2000). Millet is sown with the first rains and sorghum is inter-planted later when the rain become more reliable (Mortimore, 1989). According to Asadu et al., (2004) crops that cover the soil, such as cowpea, are integrated in to different cropping system; crop rotation and mixed cropping. These systems include cassava, maize, cowpea; yam, maize, cowpea; millet, cowpea; millet, sorghum, cowpea; and sorghum, millet, cowpea, okro, maize (NAEARLS and PCU, 2004). Also Harris (2000) reported that “In sub-Sahelian northern Nigeria farmers focus on growing millet, sorghum, groundnut, sesame and cowpea and in the Sahelian part they resort to the most drought-tolerant crops: millet and cowpea”. However, the main subsistence crops among them are sorghum and millet (both early and late varieties) (Mortimore and Adams, 1999). In fact, these crops remain the major food for consumption in rural areas and as feed for poultry and livestock for sub-Saharan Africa (see Figure 2) (Reddy, 1989; Sithole, 1990; Kfir, 1998).

As far as farming systems are concerned, research carried out in the dryland areas of northern Nigeria, the region surrounding the major States of Kano, Katsina, Kebbi, Sokoto and Zamfara (Grove, 1962; Mortimore and Wilson, 1965) has noted three categories of farming system in the area: the intensive systems, where permanent, annual or biannual cultivation occurs, less intensive system, where shrubs or short bush fallowing is common, and the extensive systems where long bush fallowing system operates among uncultivated areas (Mortimore, 1989). However, soils which become subjected to permanent intensive cultivation are usually deficient in nitrogen, phosphorus and potassium, requiring high-cost improvement due to due to destruction of organic matter, loss of soil fertility and increase acidity (Stockwel and Fisher, 1996; Harris, 1998), or are situated in regions affected by highly destructive exogenous geomorphologic process attributable to the climate, and therefore at increased danger of erosion (Zachar, 1982). Therefore, the extensive expansion of cultivated agricultural land, which is always increasing in northern Nigeria, increases the danger of erosion and causes hazards to the economic wellbeing of people living there. Sustainable soil management will be the only immediate solution to this problem.

1.5 Aims and Objectives

The aim of this research is to discuss the various aspects of soil problems in relation to soil degradation in northern Nigeria, their causal factors, potential impact and possible solutions for sustainable development.

Chapter 2
BACKGROUND AND LITERATURE REVIEW

2.1 Soil and agriculture

Soil as one of the main resources of the biosphere (Holy, 1980) and a very important factor in production of agricultural crops, forestry and horticulture (Okigbo, 1991), has been defined by many scholars with different views. According to USDA (2005) Soil is a natural substance comprised of solid minerals and organic matter, liquids and gasses that occur on the surface, occupies space, and is characterised by one or both of the following: horizons, or layers, that are distinguishable from the initial materials as a result of additions, losses, transfers and transformations of energy and matter or the ability to support plants in a natural environment. Also GSST (2006) define soil as unconsolidated mineral or organic materials on the immediate surface of the Earth that has been subjected to, and shows effects of, genetic and environmental factors of: climate (including water and temperature effects) and micro-organisms, conditioned by relief, acting on parent materials over a period of time. Soil is a limited and irreplaceable resource and the growing degradation and loss of soil means that the expanding population in many part of the world is pressing this resource to its limits and its absence the biospheric environment of man will collapse with devastating results for humanity (Holy, 1980).

2.2 Drylands: meaning and characteristic features

Drylands, as defined by United Nations Convention to Combat Desertification (UNCCD, 1997), include the arid, semi-arid, and sub-humid zones and cover almost 54 million km2 of the globe. The name dryland was derived from the word arid which implies prolonged dryness (Squires and Tow, 1991). It is used with respect to the climate itself and the land below it in such regions where the ability to produce agricultural crops is restricted (Creswell and Martin, 1998). These regions include the semi-arid areas (the most extensive), followed by arid areas and then dry sub-humid lands, generally spread across all continents, but are found most predominantly in Asia and Africa (WRI, 2006).

Usually on arid lands the potential evaporation (PE) of water from the land exceeds the rainfall (ECHO, 1998; Burt, 2007a). The PE is the amount of moisture that, if it were available, would be removed by evaporation and transpiration, and can be estimated from temperature (i.e. the degree of hotness or coldness of a body or environment) and photoperiod (i.e. the relationship between the length of light and dark in 24-hours period) (WRI, 2006).

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Figure 5: Evaporation exceeds rainfall in dry areas of the world (Ref. 5).

Approximately 75% of rainfall is lost to evapotranspiration in dryland areas (ARIJ, 1994). The land may be characterized according to the degree of aridity as dry forest, chaparral or brush-land, grassland or savannah, or desert (Creswell and Martin, 1998). Farming systems in dryland areas of the world are called ‘dryland farming’, and have been defined as an agricultural technique for cultivating land which received little rainfall (Farage et al., 2007).

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Figure 6: Example of dryland farming in some part of African dryland areas (Ref. 6).

Indeed, dryland agriculture has distinct importance in the sphere of agricultural production (see Laflen and Tian, 2000). It refers to high-yield and high-efficient agricultural production in areas without irrigation capacity and depends on natural rainfall through the adaptation of a set of dryland farming systems (Yu, 2004).

2.3 Concept of soil evolution in history

The process of soil formation and evolution is a process requiring thousands or even millions of years. As such, it is difficult to make categorical statements about the various stages in the development of soil (Fitzpatrick, 1980) and this can only be related within a limited knowledge base to scientific investigations and findings. Many researchers in the 18th and 19th centuries contributed much in this field. According to Dokuchaev (1846 – 1903) (quoted in Krasil‘inikov, 1961), soil may be considered to be a natural body, having its own genesis and its own history of development, a body with complex and multiform processes taking place within it, as different from bedrock, which can become soil under the influence of a series of soil formation factors (climate, vegetation, relief and age), which also include organisms and parent materials that control the degree of soil development (Harden, 1990; Jenny, 1994). These factors govern the geomorphic processes and landscape evolution in soil development (McFadden and Kneupfer, 1990) and result in most soils having a variety of particles such as sand, silt, clay and organic matter, which are also the result of rock weathering and organic materials and are shaped by their transport in water (or rain and hail), wind and ice (Sund et al., 1973). Example of soil formation processes can be seen in Figure 7.

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Figure 7: Stages of soil formation (Ref. 7).

According to early soil scientists, soil should be called the “daily” or outward horizons of rocks regardless of the changes caused naturally by the common effects of water, air and various winds of living and dead organisms (Krasil‘inikov, 1961; Compton and Company, 1914-retrieved in 2006).

Critically, the above views of Dokuchaev are contrary to those of Kostychev (1892) who studied the soil as a botanist not as a geologist. According to Dokuchaev, soil is a formation which is involved with the geography and physiology of plants that perform the decomposition of organic matter, and hence the accumulation of soil humus depends on the intensity and completeness of the decomposition of plant residues, the roots and the plant parts which are above the ground (Krasil‘inikov, 1961). This could be one of the several ways of determining the age and formation of soil because different soil characteristics and horizons have been used in the processes (Semmel, 1989; Ortiz et al., 2006). Although changes in external environmental factors (such as plants, rocks and living organisms) are usually responsible for progressive changes in the soil, vegetation and landscape, there is a number of cases when environmental factors remain relatively constant but continuing soil development might lead to the formation of new features within the soil, in which case some of them can become very prominent and may themselves influence the further course of soil formation (Fitzpatrick, 1980).

2.4 Soil: properties and components

Soil properties

These are a key piece of the picture of how an ecosystem works. For example, Levine (2001) highlighted the following point regarding the importance of understanding the soil properties in agriculture: (a) they can help us to predict how fast water will move through different types of soils; (b) they can tell us whether the soil has the potential to store enough water to keep plant growing through a drought, to withstand a flood, and to provide a right conditions of chemicals to plants (as indicated in measurement of pH, and N, P, K, levels) so that they will grow properly; (c) soil chemical and physical properties also tell us specific information about how well the soil will perform as a filter of wastes, as a home to organisms, as a location for important uses (including garden, forest plantation, horticultures and other agricultural activities).

This means that the more information available on soil properties the more it is possible to evaluate the quality of our natural resources in agriculture all over the globe.

Soil components

These are made up of an extensive variety of substances, minerals and rocks, which can be categorised into four major groups: organic materials, inorganic materials, air and water (Brady, 1991; Koning, 1994). Organic particles consist of chemical molecules that contain atoms of carbon, and these particles are either produced by living organisms, twigs, leaf litter, compost (i.e. a mixture of decomposing plants refuse, manure, etc. for fertilizing and conditioning soil) and humus (i.e. dark organic materials in soil, produced by decomposition of soil) (see Miller, 1963). Inorganic particles consist of chemical molecules that generally do not contain carbon atoms and are produced by the weathering of Earth’s bedrocks, such as pebbles, sand, silt and clay (Koning, 1994). The air (as component of soil) includes the atmospheric gases such as oxygen, nitrogen, hydrogen, argon, carbon dioxide, Neon, hellion, xenon and water (Koning, 1994). Research indicated that in the modern atmosphere the proportion of nitrogen is 78%, Oxygen 21%, and argon 1% and for the rest of the compounds, the proportion is variable (Burt, 2007b). Water is also another critical component of soils for all of the organisms that live in the soil environment, especially plants (Koning, 1994).

In most soils, minerals represent around 45% of the total volume, water 25% and air 25%, and organic matter from 2% to 5% (Sullivan, 2004). The mineral portion consists of three distinct particle sizes classified as sand, silt, and clay (GSST, 2006). Therefore, the space between the solid particles (organic and inorganic) must include a film of water as well as pockets of air exchanging with the atmosphere if this soil is to support life (plants and the micro organisms responsible for the decomposition of organic and inorganic materials) (Koning, 1994).

Organic/inorganic minerals, water and air are essential for the growth and survival of crop plants. Thus, the need for proper management and sustainability of soil is very crucial.

2.5 Physical properties of soil

The physical properties of soil are those characteristics which can be seen with the eye or felt between the thumb and fingers and which are the result of soil parent materials being acted upon by climatic factors (such as rainfall and temperature), affected by topography (slope and direction, or aspect) and life forms (kind and amount, such as forest, grass or soil animals) over a period of time (CECA, 1998). These properties are very important in influencing soil productivity in agriculture. Like its other properties (e.g. chemical properties), physical conditions of soil are dependent upon several internal and external factors, which are constantly changing under natural field conditions, and these are generally built up over an extended period of time (Govinda and Gopala, 1971). For example, the permeability (the rate at which water moves through the soil) and water holding capacity (WHC), the ability of soil micro-pores to hold water for plant use, are affected by (a) the amount, size and arrangement of pores spaces in soil and (b) micro-pores that control the permeability and aeration within the soil (TFRE, 2004). Porosity is, in turn, affected by soil structure, texture, compaction, and organic matter (Millar, 1963). These types of soil can be easily identified through their physical processes of translocation, aggregation, freezing and thawing, wetting and drying, expansion and contraction, exfoliation and unloading (see Fitzpatrick, 1980).

Generally, the most important among all the physical properties of a soil are colour, texture, structure, drainage, depth, and surface features such as slope (CECA, 1998).

2.5.1 Soil texture

Soil texture refers to the proportions by sizes of the smaller minerals particles in the soil (Knapp, 1979) or the fineness/coarseness of the mineral particles in the soil, and depends on the relative amounts of sand, silt, and clay, which are present in a wide range of different soil classes (CECA, 1998). The textural classes are shown on the USDA textural triangle in Figure 8.

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Figure 8: USDA textural triangle indicating particle separations: clay, silt sandy and loamy soils (Ref. 8).

The coarser mineral particles of the soil are called sand, and particles vary in size (e.g. Brady and Weil, 2003) (see Figure 9). Most sand particles can be seen without a magnifying glass and also feel gritty when rubbed between the thumb and fingers (CECA, 1998). Relatively fine soil particles that feel smooth and floury are called silt, and when wet, silt feels smooth but is not slick or sticky (Brady and Weil). When dry, it’s smooth, and if pressed between the thumb and finger, will retain the imprint (Brady and Weil, 2003). Clays are the finest soil particles from muddy waters and rivers (Govinda and Gopala, 1971), and these particles can be feel extremely smooth when dry, and become slick and sticky when wet (Rice, 2002).

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Figure 9: The size particles of sand silt and clay (Ref. 9).

However, clay soils are sometime dominated by cracks formed by swelling clays (Rice, 2002). This pronounced surface cracking, when dried, has been reported to increase the rate of water infiltration (Usman, 2003).

Texture is an important soil characteristic because it will, in part, determine water intake rates (infiltration); water movement through soil (hydraulic conductivity); soil water holding capacity; the ease of tilling the soil; the amount of aeration (which is vital to root growth), and texture will also influence soil fertility (Rice, 2002). According to Rice (2002) a coarse sandy soil is easy to till, has plenty of aeration to stimulate root growth, and is easily irrigated. However, this same sandy soil will rapidly dry out after irrigation due to its low water holding capacity (e.g. Usman, 2003).

2.5.2 Soil structure

Soil structure describes the arrangement of aggregation in soil particles (sand, silt, clay) and the pore spaces (Govinda and Gopala, 1971, RPDC, 2005). The four main types of soil structure are (Figure 9): (a) platy - common with ponding of soil, (b) prismatic - (columnar) - common in subsoil in arid and semi-arid regions, (c) blocky - common in subsoil especially in humid regions, and (d) granular (crumb) common in surface soils with high organic matter (TFRE, 2004). Well-structured soils will appear spongy inside and one should see many pores and fissures clearly (Richardson, 20001). These soils are vital for crop health, as most of their nutrients come from the top 30 cm layer of soil (Richardson, 2001).

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Figure 10: The main types of soil structure (Ref. 10).

These structural components of different types of soils have been grouped and classified by the USDA (1975) (see Fitzpatrick, 1980) into:

Aridisoils: soils of dry areas, mostly water lost by run-off.

Entisols: soils occur under climatic condition characterised with having active erosion or flood plain deposition.

Histosols: soils composed predominantly of organic matter, and non active erosion.

Inceptisols: soils of humid regions, materials are lost by leaching.

Millisols: soils of temperate grasslands also occur at high latitudes, mostly dark in colour.

Oxisols: soils occur on gently slopping sites of great age, they are red, yellow or grey soils, with very low fertility but very productive under modern agricultural methods.

Spodosols: soils occur on late Pleistocene or Holocene surface and in cool humid areas but some are found in inter-tropical areas, most lost by cultivations.

Ultisols: soils of the mid-to low latitudes occurring in the vegetation of the upper centimetres of soil, having low base saturation due to leaching during the wet season.

Vertisols: these are clayey soils that have deep wide cracks at some period of the year and high bulk density between the cracks.

This global distribution of these soil groups (Figure 11) was demonstrated by FAO-UNESCO global soil regions map (USDA, 2005).

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Figure 11: Global soil regions indicating different types of soil orders (USDA, 2005).

2.6 Soil organic matter

Understanding the role that the soil organic matter plays is important in sustainable soil management (Sullivan, 2004), and the influence of organic matter on plant growth may be studied under two main headings: its effects on the physical conditions of soils and the role of organic materials in supplying nutrients to plant (Millar, 1963), which normally occur by converting organic inputs into readily available nutrients and soil organic matter through the process of decomposition (Cherly et al., 1994). Decomposition is controlled by a series of factors: the physiochemical environment, including climate and soil; the resource quality (the chemical and composition of the organic materials); and the nature of the decomposer community (Swift et al., 1979). Hence, the term (soil) organic matter might include the highly decomposed and colloidal soil fraction known as humus, as well as the roots and tops of plants containing much easily decayable carbohydrate and protein materials, and in addition the bodies of micro-organisms, worms, insects, and other animals, and also animal manures and similar materials applied to the soil (Millar, 1963).

Organic material added to the soil will become organic matter after undergoing a process of decomposition, which in turn become humus (Funderburg, 2001). This normally occurs in two main stages: mineralization, which is concerned with biochemical breakdown of dead plant tissue by soil organisms to produce simple structured soluble organic substances, and humification , which changes the simple soluble organic substances into large molecules (polymerisation) to produce humus (Knapp, 1979).

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A B

Figure 12: Examples of (A) Chemical and (B) Biological activities of biochemical breakdown of a plant tissue by soil micro organisms in a process of mineralization and humification respectively (Ref. 11).

However, the physical properties of humus which are most significant in regard to its effects upon the physical nature of soils and their crop-producing ability are its (a) structure, (b) volume weight, (c) adhesive and cohesive qualities, (d) water-holding capacity, (e) shrinkage with loss of water and (f) coagulation by certain electrolytes (Millar, 1963). The amount of materials present in organic matter varies, but Funderburg (2001), reported that an acre of soil measured to a depth of 6 cm weighs approximately 2,000,000 kg, which means that 1% organic matter in the soil would weigh about 20,000 kg/ha. Detwiler, (1986) reported that there is a decrease of 17-27% of soil organic matter (SOM) in on the top 40 cm during the cropping phase of shifting cultivation and estimate it would take 35 years to retain the soil organic matter content of the primary forest. Also, losses of 10 and 30% of SOM were found following two years of maize cultivation, with and without return of residues, respectively, compared with that of bush fallow (Ayanaba et al., 1976).

2.7 Fertile and productive soils in agriculture

A fertile soil may be defined as one which has a good supply of available plant nutrients to be drawn upon by plants throughout their growth (Govinda and Gopala, 1971). This definition describes the quality and ability of soil to provide the essential elements in adequate amounts and in proper balance for the growth of specified crop or plant in agriculture (DFID, 2002; GSST, 2006). However, Miller (1963) have the opinion that a fertile soil must have these nutrients not only in a reasonable amount or in suitable balance, but also in a way plants can take them from mineral and organic soil fractions and must be located in a climatic zone which provides moisture, light and heat sufficient for the need of plant under consideration.

In essence, this idea of fertile soil was opposed to the idea of sterile stone. Rocks differ in their fertility, where some of them are densely overgrown with lichens and micro-organisms, others are sparsely covered and others are more properly parts of one of same rock, which are not overgrown with lichens, but contain only certain microbial forms (bacteria, actinomycetes and fungi) (Glazovskaya, 1950; Krasil‘nikov, 1961).Therefore, continuous and rapid rock and soil decompositions (by micro-organisms), provide a constant supply of minerals for plant growth (Hartemink, 2002), and, hence, the principal factor of soil fertility is determined by biological factors, mainly by micro-organisms (Krsli‘nikov, 1961).

At least 16 elements, called plant nutrients, are considered necessary for optimum growth, development and food value in plants (FPDCC, 1963). Macronutrients are usually required by plant in large quantities (i.e. plants need them through out their growing stages), while micronutrients are normally required in small quantities (i.e. plants need them but not all the time) (Miller, 1963). Macronutrients include elements such as N, P and K, while micronutrients include calcium (Ca), magnesium (Mg), and sulphur (S) (EFMA, 2002). N, P and K can be supplied into soil for plant growth through chemical fertilizer and organic matter, while calcium and magnesium can be added when lime is applied to acid soils (Ramaru et al., 2000). All of these elements are taken by plants in solid form or through recycling of organic matter (Syers and Rimmer, 1994). Elements such as carbon, hydrogen and oxygen (also macronutrients) are taken by plants in the form of water and air (FPDCC, 1963).

In general, improving soil fertility depends greatly upon the crop in question, the weather, the management imposed, and other factors (Maltby and Thomas, 1989). However, Tinker (1984) suggests starting with healthy crops giving the ideal yield and maintaining proper management activities (such as a regular supply of organic and inorganic materials) year after year.

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Figure 13: Example of high yield in maize farm obtained from fertile soil 704 X 570 – 30k gif (Ref. 12).

Thus, a fertile soil may contain all the essential nutrients needed for plant growth, but due to unfavourable soil conditions of say, pH, the plant may not be in a position to utilise them for good growth, hence the fertile soil in combination with environmental factors and management practices can improves soil productivity (Govinda and Gopala, 1971) and also contributes to better sustainability in agriculture. On the other hand, a productive soil has the capacity to produce good crops, and this capacity is derived not only from the fertility of the soil, but also from ancillary factors such as availability of moisture for producing the crops, the facilities for drainage, lack of harmful conditions in the soil (Govinda and Gopala, 1971). These soils have the ability to perform the function of sustaining agriculture even under poor soil management practices (Eswaran et al., 2001). Thus, soil productivity is a function of environmental factors combined with soil fertility (Govinda and Gopala, 1971). For example, soils which are inherently fertile can be made productive through adequate management measures such as the application of inorganic fertilizers and manure (Datta, 1986; Ramaru et al., 2000; Hartemink, 2003).

Such improvement in the productivity of soils through human efforts and management can help to improve the quality and productivity of poor soils in agriculture (Govinda and Gopala, 1971; Lal, 1997; World Bank, 2001; Mortimore and Adams, 2001; Osbahr and Allen, 2002; Pretty et al., 2003).

2.8 Soil: an essential medium for growing crops

As noted earlier, soil plays, and continues to play, an essential role in agriculture. According to Law and Scuderi (1996) soil contains atmospheric gases such as oxygen, hydrogen, and CO2, water, and living organisms, which always support the growth of plants. Normally, the natural chemistry of soil is dominated by these gases, where much of the CO2 in air is introduced by natural processes such as transpiration, and some sulphur compounds, for example, are brought into soil by volcanic processes (Hobish, 2002) (see Figure 14).

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