LITERATURE REVIEW
Our focus in this chapter is to critically examine relevant literature that would assist in explaining the research problem and furthermore recognize the efforts of scholars who had previously contributed immensely to similar research. The chapter intends to deepen the understanding of the study and close the perceived gaps.
Precisely, the chapter will be considered in two sub-headings:
Conceptual Framework
CONCEPTUAL FRAMEWORK
Soil Characterization and Classification
The type of soil formed under a particular set of environment is a function of the parent material and time. Soil physico-chemical properties and micronutrients vary in their contents from soil to soil and from one parent material to the other. The soils of Nsukka in eastern Nigeria have generally been derived from the residua (disintegrated rock materials) of either false-bedded sandstone or upper-coal measures (Asadu, 1990), which give rise to sandy and clayey soils, respectively (Akamigbo and Asadu, 1983). These soils are low in inherent fertility and are subjected to high temperature and rainfall of high intensity (Asadu et al., 2010). Nigerian soils derived from basic rocks have higher content of micronutrients than those derived from acid rocks (Chude et al., 1993). It is not so much abundance or the total content of these micronutrient elements as the availability that is crucial to plant growth, since micronutrients in most soils are ordinarily insoluble and are not easily available to plants.
Soils are the bases for most development projects. In order to ensure that the soil is put to the most appropriate and sustainable use, there is every need for characterization and classification of the soil. Soil characterization, soil classification and soil mapping provide a powerful resource for the benefit of mankind, especially in the area of food security and environmental sustainability (Esu, 2004). According to Ajiboye and Ogunwale (2010), earlier studies conducted on the soils of various regions of Nigeria and subsequent classifications were based majorly on the soil parent materials at the higher category classes. Soil classification study is a major building block for understanding the soil, classifying it and getting the best understanding of the environment. It is categorizing soils on the basis of their characteristics.
The USDA Soil Taxonomy (Soil Survey Staff, 1975 & 1999) and the FAO-UNESCO Soil Classification System (FAO, 2001) are the two most used classification systems in Nigeria (Esu, 1999). For instance, the soils of Ejiba (Ajiboye and Ogunwale, 2010) were classified according to the USDA (Soil Survey Staff, 2003) and World Reference Base (WRB) for Soil Resources (FAO, 2006) systems. The soils were mostly Alfisols and Lixisols, with respect to such criteria as nature of the epipedon, diagnostic master horizon, the cation exchange capacity, percentage base saturation, organic carbon content, soil drainage characteristics, soil temperature, moisture regimes and soil colour. Esu and Akpan-Idiok (2010), characterized the morphological and physico-chemical properties of alluvial soils and classified them according to the USDA Soil Taxonomy System (Soil Survey Staff, 1999) and the FAO/UNESCO/ISRIC World Reference Base for Soil Resources (WRB) Classification System (FAO, 2001). The soils met the requirement as Entisols and Vertisols.
The mineralogical analyses carried out on the clay fractions from the horizons of Nsukka soil series (Akamigbo and Igwe, 1990) showed that the dominant clay minerals are kaolinite and quartz; and the classifications according to Soil Taxonomy and FAO/UNESCO Soil Legend are Ultisols and either Acrisols or Nitisols. The characteristics of Nsukka soils include their sandy loam textural class, considerably high sand and moderate clay fraction formed from false-bedded sandstone (Orajaka, 1975) and acidic reaction, associated with low activity clays, highly degraded and leached profiles; which Akamigbo and Asadu (1983) described as deeply weathered soils mostly derived from the residua of sedimentary materials. The CEC of the soils are generally low (Asadu, 1990) as well as low exchangeable bases in the soils. Akamigbo and Asadu (1983) reported these intrinsic properties as dependent on the parent materials of the soils.
Land Evaluation
In farming, risk is minimized by matching the requirements of land use to land qualities, which is the role of land evaluation. Land evaluation (FAO, 1976) identifies the most limiting land qualities and provides a good basis for advising farmers for optimum production. Dent and Young (1981) stated that land evaluation is a prediction process of land potential for various alternative uses, and it is one important component in the process of land-use planning (FAO, 1976). Land evaluation is a process that matches the characteristics of land resources for certain uses using a scientifically standardized technique. The results can be used as a guide by land users and planners to identify alternative land uses.
The utilization of land resources in accordance to the optimum carrying capacity in the agricultural development can only be done if the information about the suitability of the land is available. Suryana et al. (2005) stated that one of the basic information needed for agricultural development is the spatial data (maps) of land resource potential. According to Wahyunto et al. (1994), to determine potential areas for optimal agricultural development, balanced and sustainable land resource data are required through the evaluation of land suitability. An evaluation of suitable landscape for food crop cultivation based on the value of landscape type is needed for decision making, coordination, and control for researchers and farmers to minimize cost (Azis et al., 2006).
Many systems of land evaluation have been developed. They include the Storie Index (Storie, 1933), Land Capability Classification (Klingebiel and Montgomery, 1961) and Land Suitability Evaluation (FAO, 1976). The basic principle of land evaluation is the identification of the characteristics of the soil in a given landscape, identification of the soil requirement for the land utilization type of interest and matching the two to establish the extent to which they match. Some Land Evaluation Systems use several approaches such as parameters multiplying system, parameters totaling system and matching system between land quality and land characteristics with crop requirements. For instance, the Storie Index uses parameters multiplying system, while Land Suitability Evaluation matches land quality and land characteristics with crop requirements. Obviously, land evaluation has provided the needed solution to the issue of making soil survey information useful to farmers and other land users (Ogunkunle, 2005).
Land quality is the complex attributes of lands and contains one or more land characteristics. Important land qualities in any land evaluation include topography, soil, and climate. These are closely linked to plant requirements (Ritung et al., 2007). The most important soil characteristics in land evaluation include drainage, texture, soil depth, nutrient retention (pH, cation exchange capacity), alkalinity, erosion hazard, and flood/inundation. Soil attribute is important for the overall performance of land and play a preponderant role in checking land quality, and has been used extensively by several authors to monitor land degradation (Senjobi , 2007; Senjobi and Ogunkunle, 2011).
Land Suitability Classification
Hakim et al. (1986) stated that land suitability classification is the process of assessment and classification of land units according to their suitability for a particular use. Land suitability could be assessed for present condition (Actual Land Suitability) or after improvement (Potential Land Suitability). Agricultural land use has benefited significantly from the use of suitability systems in recent years. These systems have jointly showed their capabilities in the evaluation and assessment of suitable sites for a variety of crops.
However, poor knowledge of soil suitability for agricultural production constitutes a major problem to land use. For sustainable crop production, reliable soil data are the most important prerequisite for the design of appropriate land-use systems and soil management practices as well as for a better understanding of the environment (Aderonke and Gbadegesin, 2013). Though soil classification and mapping are necessary and very useful for general land use planning, what is of utmost importance to the farmer is knowing how profitable it is to grow a particular crop or series of crops on a given plot of land, and what amendments are necessary to optimize the productivity of the soil for specific crops (Aderonke and Gbadegesin, 2013).
Aguilar and Ortiz (1992) used the FAO Framework, in combination with the parametric Riquier index to define the suitability classes (S1, S2, S3, N1 and N2) for land capability. In a recent study, (Udoh et al, 2011), the conventional (non-parametric) methods as well as the parametric method were used to evaluate the suitability of the eight pedons for rice and cocoa cultivation in soils developed from alluvial deposit; in which five land quality groups were used for the study and only a member of each of the five land quality groups was used in the calculation, because of the strong correlation among members of the same group (Ogunkunle, 1993). The five land quality groups were climate (c), soil physical characteristic (s), wetness (w), fertility status (f) and toxicity (t).
On the basis of soil parameters provided by Harmonized World Soil Database (HWSD), seven key soil qualities important for crop production have been derived, namely: nutrient availability, nutrient retention capacity, rooting conditions, oxygen availability to roots, excess salts, toxicities, and workability. These soil qualities are related to the agricultural use of the soil and more specifically to specific crop requirements and tolerances (Fischer et al., 2008). If these characteristics fulfill all requirements, the land is classified as “highly suitable (S1);” if one or more characteristics do not meet the requirements the land is classified as “moderately suitable (S2), marginally suitable (S3) or not suitable (N),” with the following implications:
Suitable = The crop can be cultivated without difficulty; no additional land improvement techniques are needed.
Slightly suitable = Yields will be marginal when the soil is not improved. Therefore, additional techniques to improve the soil are needed, such as drainage and irrigation techniques or terrace building techniques.
Not suitable = Implementation causes problems without notable yields.
Crop Growth Requirements
Climate and water requirements for growing oil palms (Elaeis guineensis) restrict it to growing in tropical soil orders such as Ultisols, Oxisols and Inceptisols. Nutrient requirements of oil palms are higher than what can be sustained by any soil for economical yields. Macro-nutrient requirements for nitrogen and potassium are especially high; and a high cation exchange capacity (CEC) is an important requirement for soil where oil palms grow.
Land suitability evaluation for oil palm in heavy rainfall areas of Nigeria (Ogunkunle, 1993) are known. Oil palms require a well-drained soil that also retains water well. A soil that forms a variety of plentiful aggregates is good for growing oil palms. Sustainable plantation of palm oil plants must grow on a naturally level area.
Cassava grows well under a wide range of soils but prefers porous, friable soils with some organic matter content and depth of 30-40 cm. It will not survive extended waterlogged conditions. Cassava prefers soils with pH between 6-7, and clay content less than 18 %. It does not tolerate saline conditions. The limitations found in most soil suitability evaluation for cassava production are poor soil structure and texture. This affects the aggregate and water-holding capacity of the soil. Other constraints include drainage and soil fertility. Ande (2011) in his study noted the properties of Apomu soil suitable for cassava production, essentially for its sandy loamy texture, coupled with its gentle slope of 3-4 %. However, very little attention has been given to the proper cultivation and soil requirement of the crop (Ande et al., 2008). This could be attributed to the ease with which the crop grows and secondly because of its position usually as the last crop in the traditional agricultural system before the land is left to fallow.
Although maize (Zea mays) is found to grow throughout Nigeria under a wide range of agro-climatic conditions, three broad agro-ecological zones can be distinguished for maize production. They are the forest, the moist (guinea) savanna and the forest/savanna transition zone. High insolation, relative high rainfall amount, high radiation, long dry season which limits the incidence of pests and diseases and low night temperature are favorable ecology for maize. The major limitations for maize production are soil texture and structure, which directly affect water-holding capacity, permeability of the soil and other physical properties. Other limiting factors are drainage and soil fertility, measured by CEC, organic matter and total nitrogen content. Sys et al. (1991), stated 16-24 cmol kg-1 CEC and 1.2-2.0 % organic matter as optimal for maize.
Nitrogen, phosphorus and potassium are the primary nutrients most commonly demanded by yam (Dioscorea spp.) Yam can be grown in all tropical countries provided water is not a limiting factor. Deep, fertile, friable, and well-drained soils are ideal for yam cultivation, and optimum textural classification of loam sandy soils (Onyekwere et al., 2009) is required for unhindered anchorage and bulking of roots and tubers and easy harvest. In Nigeria, it is grown in areas where the annual rainfall exceeds 800mm in amount and four months in duration.
Critical level of 0.15 % total Nitrogen is required for sustainable Dioscorea production ; and 2.0 cmol kg-1 exchangeable K is recommended for soils of southeastern Nigeria (FPDD, 1989) for yam production. The cation exchange capacity and base saturation ratings are respectively 16 cmol kg-1 and greater than 35 %. Chukwu et al. (2007) stated that major yam soils of southeastern Nigeria are deficient in total N. However, the northwest of Enugu/Anambra States axes bordered by alluvial parent material are medium in N, having total N ranging from 0.15 – 0.20 %. (Chukwu et al., 2007).