Surface soil factors and soil characteristics in geophysical milieu of Kebbi State Nigeria

Received : 15.12.2015 Accepted : 08.02.2016 Soil factors and surface soil characteristics are important components of agricultural environment. They support surface and subsurface soils to perform many functions to agriculture and economic human developments. Understanding these factors would aid to the recognition of the values that our soil and land offered to humanity. It is therefore, aim of this study to visualise and examine the soil factors and surface soil characteristics in Kebbi State Nigeria. An Integrated Surface Soil Approach (ISSA) was used in the classification and description of soil environment in the study region. The factors constituted in the ISSA are important components of soil science that theories and practice(s) noted to provide ideas on how soil environment functioned. The results indicate that the surface soil environments around Arewa, Argungu, Augie, Birnin Kebbi and Dandi are physically familiar with the following surface soil characteristics: bad-lands, blown-out-lands, cirque-lands, fertile-lands, gullied-lands, miscellaneous and rock-outcrops.The major soil factors observed hat played an important role in surface soil manipulations and soil formation are alluvial, colluvial, fluvial and lacustrine; ant, earthworms and termite; and various forms of surface relief supported by temperature, rainfall, relative humidity and wind. Overall, the surface soil environment of the region was describe according to their physical appearance into fadama clay soils, fadama clay-loam soils, dryland sandy soils, dryland sandy-loam soils, dryland stony soils and organic-mineral soils.


Introduction
Soil science has been considered as an applied science that focussed on improving the understanding of soils and how they function (Thompson and Rimmer, 2008).It is one of the objectives of this science to generate soil information that is useful in the understanding of the true nature and properties of soil (FAO, 2006).This information has been considered as part of sustainable agriculture and human development in the 21 st Century (DESA, 2013).Therefore, the future of soil science emphasise the need to integrate ideas so as to obtain necessary soil information which could be use in many aspects of agriculture and sustainable economic development (Hartemink, 2006).Integrated idea through field assessment has been reported to provide adequate knowledge, detail information and overall insight tothe understanding of many important components of soil environment (Olson, 1981).The question here is 'what factors will be considered to achieve the need of soil science?'.Sometimes, answer to this type of question could be difficult primarily due to the fact that many issues and research interests are involved, although weights are always given to the objectives outline in a particular study.And for this, this study lays its emphasis on soil forming factors and surface soil characteristics.Information on these factors is very limited and scarce in the study region (KARDA, 1997;Usman, 2007).
The potential benefits of gathering information on surface soil characteristics and soil factors in the region is largely related to the need for informed priority to improve soil quality and environmental surroundings for diverse crop productions (Hartemink, 2006).Unfortunately, the ways of collection and management of the soil information in the study regionare not able to gather the information required for the proper management of surface soil quality.Accurate soil information about the current status of surface soil characteristic and soil factors is critical in achieving the goals of sustainable agriculture in the 21 st Century (DESA, 2013).Without proper assessments of the important soil components such as surface characteristics, soil factors and land use activities, knowledge of managing soils is almost unfeasible.Without clear and sound approach, adequate and reliable soil information is unpractical.And, without an international field guides such as FAO guidelines for soil description (FAO, 2006), understanding the classes of each soil characteristics and soil factor is impossible.Therefore, it can be justified that the need for soil assessment in the study region is vital to efficient soil management and environmental conservation (Usman, 2007).Thus, the objective of the study was to assess and discuss the major classes and characteristics of surface soil features and soil factors around Arewa, Argungu, Augie, Birnin Kebbi and Dandi areas of Kebbi State, Nigeria.

Study region
Kebbi State is located geographically in the north-western part of Nigeria in sub-Saharan West African region (Figure 1).The State has a total land area of 36,229 km 2 of which 12,600 km 2 is under cultivated within the latitude 10 o 30 o 5'N and longitude 3.5 o and 6 o E (KARDA, 1997).Socially and culturally, the State has been dominated by Hausa-Fulani who depend fully on crop production, cattle rearing, fishing and mining.Their population is believed to bearound 3, 630, 9313 people according to National Population Commission of Nigeria Kebbi State chapter (NPCN-KB, 2007).The land is quite important resource to million people in the State and is been utilized for many purposes -cropping, irrigation, grazing among others.

Field visit exercise
Field visit exercise was aimlessly carried out on 74 different sites between 2009 and 2011 around Arewa, Argungu, Augie, Birnin Kebbi and Dandi local government areas of Kebbi State (Figure 2).These sites were considered because of their relevant to crop production in the State.During the visit, soil factors (parent materials, organisms, topography) and surface soil characteristics were assessed and classified, accordingly.
It is important to mention that priority was only given to those soil classes, which occur more often withinthe 74 areas visited.Guidelines of soil assessment by the FAO (2006), USDA-NRCS (2002) and FAO-SWALIM (2007) were employed as complete guides in the field.

Soils and their geographical distributions around the 74 sites visited
The major surface soil types around the 74 sites visited in the study region are characterised as Aridisols (USDA soil order), Calcisols (FAO soil order) and Vertisols (FAO-USDA soil order) (Usman, 2013a).Aridisols and Calcisols are dryland soils described as well-drained, low organic matter, low water holding capacity and up-and-down undulating landform (Yalwa, 2008).These soils are believed to have been formed many years ago under aridity and sand dunes of sub-Saharan desert (Ahn, 1970).Geographically, they are distributed uniformly as sands, mineral stones and limestone particles covering large areas of land (Usman, 2007).The vertisols on the other hand, is a fadama soil that has been attributed with cracks during the dry season, shrink and swelling due to high clay content (Usman, 2013a).

Approach used
In order to facilitate acquisition of relevant primary information needed for this study, an Integrated Surface Soil Approach (ISSA) was designed and used from 2009 to 2011.The factors constituted in this approach were in line with the information presented in FAO Guideline for soil descriptions (FAO, 2006), Somalia Field Survey Manual (FAO-SWALIM, 2007) as well as Soils and the environment: A guide to soil surveys and their applications (Olson, 1981).

Definition of the approach
Integrated Surface Soil Approach (ISSA) is considered as direct method of visual and digital assessments of geo-physical surface soil conditions within a scope of various ideas, skills and knowledge on factors that could potentially explain the natural surface conditions of soil environment.The factors constituted in the ISSA are important components of soil science that theories and practice(s) noted to develop trust in the understanding of the overall image of soil environment (Olson, 1981;Jenney, 1994;Usman, 2013b).The concept of the approach is depicted in Figure 3. Figure 3 describes the conceptual framework of the ISSA.The approach has two independent components: VSA and DSA.However, the two arrows between VSA and DSA provide linkages that help to understand the current status of the surface soil components intended to assess and visualise in the study area.These linkages served as meansto record the results of the assessment together with some examples of the images of surface soil characteristics (Information I) and soil formation factors (Information II).Finally, the overall results were discussed according to the information obtained under VSA and DSA tallied with the surface soil characteristics and factors of soil formation of the study region.

Digital Soil Assessment (DSA)
The DSA was adapted to provide an overview of the true picture of surface soil characteristics and soil factors in the study region, and the concept is depicted in Figure 4. Simple procedure of photograph using Cyber-shot DSC-W510 (12.1 mega pixels) SONY camera was used in the field to take records of cloud as related to rainfall and temperature, parent materials, presence of soil organisms, characteristics of topography and land use as related to landform 's formation of the study region.

Visual Soil Assessment (VSA)
The VSA was used to evaluate parent materials, organisms, topography and those surface soil characteristics, which are visible and can be assessed directly by the naked eye primarily to explain the current status of surface soil in the study region.This type of assessment was considered important in assessing the surface soil properties, which are capable of changing as a result ofdiverse land use practices (FAO, 2008;Soil Survey Staff, 2010).In addition, previous climate data for rainfall, temperature, wind speed and relative humidityas well as the dynamic formation of the atmosphere were considered as preliminary information to further understand the physical milieu of the region.

Soil factors
Soil factors are: climate + parent materials + topography + biota + time (Jenney, 1994).The survey on these factors in the study region is discussed below.

Climate
In order to understand the climate condition of the study region, four factors were considered namelyrainfall, temperature, relative humidity and wind speed.Physically, these factors are influence by aridity, poor vegetation cover and temperatures of annual and daily fluctuations as a result of periodic energy input of the sun (Yalwa, 2008).According to local meteorological data record in the region, the annual rainfall is variable and declining, it is 600 mm to 875 mm, and on average 650 mm during the period 1995 to 2010.Monthly temperature throughout the year falls between 25 O C and 45 O C.The relative humidity also varies slightly, being around 50% to 80% during the rainy season and decreased to low as 14% in dry period.The major winds occur around Decembers and January with an average wind speed of up to 18 m/s.This clearly indicates that the region has a tropical weather conditions and can be characterised into three seasons, namely: rainy, dry and hot.Rainy (wet) season starts normally from May/June to September/October, and dry season starts from December to early April whereas hot season begins in April and lasted in May as noted between 2009 and 2011.Thus, the atmospheric sky condition in the study region can be explained according to the physical nature of the sky in rainy season (Figure 5).
Generally, this physical nature of the atmospheric sky condition in the region can be grouped into four dynamic formations: the clear sky, the bright sky, the dust sky, and the cloudy sky.The clear sky normally occurs from mid-October and March to early April usually brings stable atmospheric conditions with no cloud, no dust.During the months of December and early January, the sky experiences dust condition making atmosphere unstable due to accumulation of million dust particles overlooking the atmospheric surface environment.The sky become bright mostly in the mid of April through May and June causing rapid temperature increases as a result of direct solar heat radiation to the surface soils (temperature is from 30-45 O C).Cloudsky happens during the rainy season when the atmosphere saturated by air causing different forms of clouds with different shapes, sizes and colours (Figure 5).

Parent materials
Table 1 gives the summary of the major classes of parent materials in the study region.These parent materials are grouped according to origin of their formations, noted from igneous to metamorphic and finally to consolidated and unconsolidated classes.Lithologically, the surface soil parent materials in the study region vary significantly probably due to landform variations and differences in land use activities.In dryland sites, large portion of the land has been covered by sedimentary sand and silt particles of which are clastic and carbonated anthropogenic sediments by their physical appearances.And, in fadama sites, the land is covered by clay and clay-silt/loam particles, which is believed to have been originated from alluvial, colluvial, fluvial and lacustrine deposited particles (Table 1).Typical examples of these parent particles are depicted in Figure 6 and 7.
Physically, it is believe that the result of diverse agricultural activities has caused many surface soil changes, and probably led to the continuous formation of other soil particles (Brady and Weil, 2007).These particles are classified into: fluvial such as pure sand, gravel and clay; alluvial such as clay, clay loam; lacustrine such as sand-loam, silt and clay; estuarine such as fine sand and clay particles and many others (Table 1; Figure

Soil biota and biodiversity
The major types of soil organisms observed are termites, earthworms, ants and nematodes.These organisms were physically assessed according to the nature of their presence, biodiversity or activities and appearance on/within surface soil voids.Remarkably, these organisms play important role in the breakdown and decomposition of many organic materials in soil (Ritz et al., 2010).They contribute widely in the improvement and transformation of soil quality, soil organic matter and soil nutrients availability (Brussaardet al., 2007;Li et al., 2014;Castro-Huerta et al., 2015).Typical examples of biota and their biodiversity are depicted in Figure 9.

Surface soil characteristics
Table 3 gives the major surface characteristics in the study region.Physically, the most risk and poor surface characteristic in this classification are gullied land areas, which most of them are bad-lands.These areas require an engineering and well planned sustainable land management.By contrast, the rock-outcrop areas could be considered as support to surface soil, they bind soil particles and thus, minimise the risk of erosion (FAO, 2006).Water areas are important sites for irrigation and other agricultural activities in the region.Typical examples of these surface characteristics are depicted in Figure 10.

Time
The period of time in the formation of soil, is a factor that can be related to the historical background of human development and agricultural activities in the region.It is believe that the surface soil of the region received changes long time ago related to the human settlements in the region (e.g.Ahn, 1970).

Land use activities
Table 4 presents the major land use activities in the study region.The results conformed to other previous studies in the region (Mortimore, 1989;Baker, 2000;Usman, 2007).The typical examples are depicted in Figure 11.Important land areas for rice and wide range of horticultural crops (tomato, chilli, leaves, okra, etc).
Cattle, sheep's, goats, are the common animals under trans-human system.
Local hens, layers and broilers.
Mainly grasses and shrubs of various types.

Conclusion
The study captured some values of surface soil environment in the study region.Soil factors, surface soil characteristics and land use activities are covered.It has been understood that a very close and interdependent relationship had always existed between all phases of soil components in the study region.Soil parent materials and biota were found interacted with one another, adding values to soil quality and soil fertility.Also, climate factors such as rainfall, temperature, relative humidity and wind were also noted to have independently interconnected with soil particles and added values tosoil for different agricultural land use activities.
An alternative approach, as adapted in this study, would give necessary information to the local people and government in general usefully, understanding both the values of their land resources and current status of its condition related to future management and transformation.The difficulties of this approach however, are the fact that it requires time, careful observation and field visit all over the region as well as additional guidelines for environmental soil assessment.Thus, further assessment is required to determine if improvement can be made to the approach for wide use to other local environments.

Figure 1 .
Figure 1.Location map of the study region from sub-Saharan Africa.The concentric ovals at the two parts of the map depicted the position of Nigeria and Kebbi State on the sub-Saharan African's map and Nigeria's map, accordingly.The yellow area at the map of Nigeria shows the position of the study area and the blue spots show the sites of the bordering states of Sokoto, Zamfara and Niger, consequently.

Figure 2 .
Figure 2. Map of the study sites

Figure 4 .
Figure 4. Digital Soil Assessment (DSA).Arrows at the top indicate the relationship between the three components of atmosphere (i.e.temperature, rainfall and cloud) considered in this assessment.Similarly, arrows at the middle indicate the relationship between soil factors, land and land use at the surface soil environment.

Figure 5 .
Figure 5. Physical condition of atmospheric sky in the study region

Figure 7 .
Figure 7. Examples of some major groups and types of consolidated and unconsolidated particles in the study region: (a) clastic sediments sandstone, (b) lacstrine clay soil, (c) organic limestone (carbonate), (d) lacstrine sand, (e)clastic sandstone, and (f) estuarine marine clay

Figure 9 .
Figure 9.Some major activities of biota and their biodiversity in study region: (a), (b) and (d) indications of biological activities and (c) mobility of termite population

Figure 10 .
Figure 10.Examples of some major surface characteristics in the study region: (a) miscellaneous area, (b) rockoutcrop area, (c) cirque land, (d) bad land, (e) water body area, and (f) Slickness area

Table 1 .
Major classes of parent materials in the study region 1 According toFAO (2006)guidelines for soil description; 2 Based on VSA in the field

Table 3 .
Classification of major surface characteristics in the study region

Table 4 .
Major land use practices in study region Land