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Assessment of inland valley soils for sugarcane (Saccharum officinarum L.) production in some floodplains in central Nigeria

Abstract Sugarcane is an energy crop with great economic potentials. Information on soil evaluation for sugarcane in central Nigeria is very scanty. This study was carried out to evaluate the suitability of some soils of the floodplains in central Nigeria for sugarcane cultivation. A semi-detailed soil survey was carried out on 18,500 hectares of land straddling the floodplains of Rivers Niger and Benue in Korton-Karfe Local Government Area of Kogi State. Land resource survey was carried out using a 500 m×500 m grid pattern. Identified soil types were further examined with standard soil profiles. Samples were collected from the soil genetic horizons as well as surface (0–30 cm) soils at selected sampling points for fertility analysis. Parametric and non-parametric suitability evaluation methods were used to relate land qualities with land use requirements for commercial sugarcane cultivation. Relationships between evaluation methods were assessed using Spearman’s rank correlation coefficients. Nine soil mapping units were identified in the study areas which were mainly Inceptisols (55.4%) and Alfisols (22.5%) which correlates with Fluvisols (40.4%), Cambisols (15.1%) and Lixisols (22.5%) in the World Reference Base (WRB) classification system. The evaluation methods used revealed that 0%, 4.95%, 49.48%, 23.55% of the soils were highly suitable (S1), moderately suitable (S2), marginally suitable (S3) and not suitable (N) for sugarcane cultivation, respectively. Potentially, 0%, 9.52%, 44.91% and 23.55% of the soils were found to be highly suitable (S1), moderately suitable (S2), marginally suitable (S3) and not suitable (N) for sugarcane cultivation, respectively. The soils were strongly limited by low soil nutrient availability, soil acidity and flood hazard. Soil management practices such as application of organic manures, fertiliser and liming could be adopted to ameliorate the soil acidity and supply deficient nutrients while land development strategies such as drainage, flood control and possibly sub-soiling would mitigate other major limitations to sugarcane cultivation.


INTRODUCTION
The floodplains defined as a strip of relatively smooth land bordering a stream and overflows at a time of high water (Leopold et al., 1964) is referred to as Akuro in South Western Nigeria and Fadama in Northern Nigeria. The floodplains cover an estimated area of four million hectares in Nigeria and is the most exploited for agricultural production in Africa (Fasina, 2005). This 'strip' could be several kilometers wide in many parts of the Northern Nigeria holding high potentials for commercial agricultural development (Uzu et al., 2003). The major soils in Nigerian floodplains have been classified as Alfisols, Inceptisols, Entisols, Histosols and Vertisols (Olaleye, 1998;Akamigbo, 2009;Ogbaji, 2010). The floodplains usually adjoining major rivers consist mostly of soils formed under conditions of poor drainage with active or passive erosional and depositional features (Schmudde, 1968;Fasina, 2005). They are characterised by seasonal superficial flooding or shallow, seasonally fluctuating groundwater table (Okusami, 1985).
Sugarcane cultivation is a key provider of income and employment in many countries, particularly in developing economies with a high proportion of poor and unemployed groups thereby enhancing the economic growth of developing economies (Gopinathan, 2010). With the increase in electricity demand in recent years, and the imminent risk of blackouts as a result of insufficient capacity, bioelectricity generation through sugarcane processing has been advocated for (Olivério and Ferreira, 2010). With crisis and environmental concerns associated with the use of fossil fuels, the trend towards biofuels and renewable energy has further positioned sugarcane to play an increasing role in producing bioethanol. The generation of surplus electricity from bagasse (a byproduct of sugarcane processing) is already well established while rapidly advancing technologies based on biotechnology and bioprocess engineering has converted bagasse into a wide range of other products (Rogers et al. 2001;Olivério and Ribeiro, 2006). Sugarcane production if properly managed is expected to lead to a shift in the demandsupply situation of sugar and associated products in favour of the Asian, South East and African countries within a foreseeable future (Gopinathan, 2010). With the inauguration of the sugar policy for Nigeria, it is expected that Nigeria will key into the wide range of opportunities available in sugarcane agriculture.
Appropriate protection and judicious utilisation of the floodplains is essential to enable these ecosystems survive and continue to provide sustainable sugarcane productivity. It is therefore necessary that appropriate strategies are developed for the sustainable use and management of floodplains for sugarcane cultivation and other agricultural purposes. Development of such policies and strategies can only be possible where information, on the characteristics and functioning of floodplains is carefully collected, assembled and interpreted. Land evaluation is thus necessary to assess the potentials for sugarcane in the floodplains as the basis for proper land use planning (Fasina and Adeyanju, 2007).
This work was carried out in the floodplain soils in the Dangerri area in Kogi State to contribute to the information database required for the understanding of floodplain resources in Nigeria with a view to develop management strategies for the exploitation of this vast resource for agricultural productivity. Therefore the objectives of this study are to characterise the soils of the floodplains of Rivers Niger and Benue around Dangerri area in Central Nigeria, evaluate the suitability of the floodplain soils of the area for sugarcane cultivation and suggest management practices to ameliorate possible limitations to the sustainable use of the soils for sugarcane production.

Study Location
This study was carried out on a parcel of land covering about 18,500 hectares at Dangerri (Near Lokoja) in Kotton-Karfe Local Government Area, Kogi State from November 2013 to April 2014. The specific study area is located within latitudes 7°52'02.51"N -7°58'01.28"N and longitudes 6°48'11.15"E-6°56'57.96"E. The land area lies on the banks of River Niger and River Benue.
The study area as shown in Fig. 1, lies within the Zone F (Sub-Humid Niger-Benue Trough) (Ojanuga, 2006) with mean annual rainfall of 1120 mm at Lokoja (NiMet, 2015). A distinct dry season of about 5months duration occurs from November to March while the rains occur substantially from April to October. Mean annual temperature ranges from 25 °C-29 °C. The study area was very close to the confluence of Rivers Niger and Benue near Lokoja. Therefore drainage conditions of the soils are influenced by the proximity to the rivers which characteristically overflow their banks yearly at the peak of raining seasons. Geologically, the area is an extension of the Middle Niger Basin and primarily characterised by an overlay of laterite capped Nupe sandstones (Ojanuga, 2006). Parent material of the soils is sandstone materials in the uplands and alluvial deposits of the rivers Niger and Benue.

Field Survey
A rigid grid method of soil survey was adopted for the land resource survey. Transects were laid out 500 m apart and auger observations were taken at 500 m interval along transects. Examination points were pre-determined in a GIS environment and the co-ordinates were pre-loaded into a Global Positioning System (GPS) devise with which the points were located on the field. Soil morphological properties such as texture, colour, consistency, mottles, etc. were examined at 0-15 cm, 15-30 cm, 30-60 cm, 60-90 cm and 90-120 cm depths at each examination point. Similar (points) sites having same morphological characteristics were grouped together to form mapping units. These were further examined with modal soil profiles measuring 200 × 150 cm dug up to 180 cm depth where possible. A total number of eight soil mapping units were identified and eight soil profiles were described and sampled according to standard (FAO) guidelines. In addition, composite soil samples from 0-30 cm depth from the examination points were also collected randomly and bulked for fertility studies therefore; a total of about 180 soil samples were collected for laboratory analysis.

Laboratory Analysis
Soil samples collected from the field were air dried, crushed and passed through 2 mm sieve for physical and chemical analysis while a part of each sample was passed through 0.5 mm mesh sieve to achieve maximum surface area for organic carbon determination (Udoh and Ogunwale, 1986). The soils were analysed for particle size using the modified hydrometer method. The pH and electrical conductivity were determined potentiometrically using a glass electrode pH meter, total nitrogen was determined using the micro-Kjeldahl method, available P was determined colourimetrically, exchangeable bases and exchangeable acidity were extracted with neutral normal acetic acid.
Wetness ( The exchangeable bases were determined using Atomic Absorption Spectrometry whereas exchangeable bases were determined titrometrically. All parameters were determined by standard procedures as contained in IITA (1982).

Soil Classification
The soil types on the study site were identified, characterised and classified using the internationally accepted USDA Soil Taxonomy (Soil Survey Staff, 2014) and the classifications were correlated with World Reference Base (WRB) system (FAO/ISSS, 2006).

Suitability Evaluation
The suitability evaluation according to the modified FAO framework (FAO, 2007) was adopted for land rating using modified suitability criteria for sugarcane from Sys et al. (1993) and Charuppat (2002) (Table 1).
In the non-parametric approach, land requirements are simply matched with the land qualities and characteristics to place soil units or pedons in classes designated as S1 (highly suitable), S2 (moderately suitable), S3 (marginally suitable), and N (not suitable) according to the suitability indices indicated in Table 2. For parametric evaluation, each limiting characteristics was rated by scoring using the criteria presented in Table 1. The index of suitability (actual and potential) was calculated using the equation: 100 100 100 Where IP = Index of Productivity, A = overall fertility limitation and B, C …F is the lowest characteristics ratings of each land quality group. Five land quality groups climate (c), topography (t), soil physical properties (s), wetness (w), and fertility (f) were used in this method of evaluation. Only one member in each group was used for calculation purpose because there are usually strong correlations among members of the same group (e.g. texture and structure). For actual suitability index, all the lowest characteristics ratings for each land qualities group were substituted into the index of suitability equation above. However, in the case of potential suitability index, it is assumed that the corrective limitation observed will no longer have such constraints.
The overall suitability index (SI) calculated for each pedon is assigned to suitability class as shown in Table 2.

RESULTS
Nine soil mapping units were discovered in the surveyed area and the Soil Map of Dangerri Study Area is presented as Fig. 2. The taxonomic classification of the different soil mapping units identified are denoted with alphabets A, B, C, D, E, F, G, H, I, and their area coverage is as shown in Table 3. The matching of the land qualities of the various mapping units with suitability evaluation for Non-Parametric Suitability Evaluation (actual and potential) and Parametric Suitability Evaluation (actual and potential) for sugarcane in the Dangerri study area are presented in Tables 4 and 5, respectively.
The Suitability map of the Dangerri study area is given in Figures 3, 4, 5, and 6, which present the Non-Parametric (actual) Suitability map, Parametric (actual) Suitability map, Non-Parametric (potential) Suitability map, and Parametric (potential) Suitability map of the Dangerri study area, respectively. Table 6 shows the summary of the Non-Parametric suitability classification and land area of the Dangerri study area which shows that a greater proportion (49.48%) of the soils are marginally suitable for sugarcane cultivation following the current status of the soil. If some land developments are made, the potential marginally suitable area will be 44.91% while the potential moderately suitable increased from 4.95% to 9.52%. Table 7 shows the summary of the Parametric suitability classification and land area of the Dangerri study area and this reveals that carrying out some land development measures on the soil will make the soil have a potential highly suitable area of about 4.57%, and a potential moderately suitable area increases from 9.52% to 41.87% thereby giving a potential marginally suitable area of 16.52% from the actual marginally suitable area of 44.91%, and further improve currently not suitable area potentially from 23.55% to 15.02%. Table 8 gives the ranking of the various mapping units by the suitability classification for sugarcane in the Dangerri study area, while Table 9 presents the Spearman's rank correlation coefficient among land suitability evaluation procedures used for both actual and potential evaluation. This table shows that there is a perfect positive and significant correlation in the ranking of the mapping units by the evaluation procedures used.
In terms of colour, the soils of the study area are characterised by dark brown to yellowish brown, very dark greyish brown to greyish brown and brownish yellow colour. The soils are very acidic to slightly acidic soils with pH range of 4.25-5.9. This characterisation

Wetness (w) Drainage
(S3) 60 (S1) 100 (S1) 100 --(S1) 100 (S1) 100 (S1) 100 (N) 40 (S3) 60 agrees with Ogbaji (2010) and Ukabiala (2012) who reported that the river Benue floodplain soils are characterised by dark greyish brown, greyish brown, yellow surface overlying pinkish, grey olive yellow subsurface; medium or strong medium / coarse sub-angular blocky structure with very sticky consistence in the subsurface and strongly acid to slightly acid/neutral when dried. It was also observed that the limiting factors for sugarcane cultivation in the study area are available P, exchangeable K, total N, pH, wetness (prolonged water logging conditions), soil texture, topography, and climate which is a common limitation to all the mapping units. This observation is also in line with Isitekhale et al. (2014) who reported available P, exchangeable K, and total N as the limiting factors for sugarcane in lowland soils of Anegbette, Edo State. By Non-Parametric (actual) evaluation, mapping unit C is moderately suitable (S2) for sugarcane cultivation with soil characteristics (s), topography (t) and fertility (f) identified as the limitations apart from climate which is the only constraint common to the entire mapping units. If some of the identified constraints in mapping unit C are ameliorated, the potential suitability evaluation for this mapping unit remains (S2) with topography (t) left as the only limitations which may not be easily ameliorated. The S2 class covers about 4.95% of the total land area of the Dangerri study site.
Mapping units A, B, E, F G and I, have a common suitability class which is marginally suitable (S3) for sugarcane cultivation. Mapping units A and I are (S3) with fertility (f) and wetness (w) being the common constraints. The potential suitability evaluation of both mapping units therefore remains (S3) with wetness (w) as the only limitation; Mapping unit B is (S3) with fertility being the only limitation, the potential suitability evaluation becomes moderately suitable (S2) with climate being the only constraint; Mapping units E, F, and G are marginally suitable (S3) with soil characteristics (s), topography (t) and fertility (f) being the major constraints identified. If the possible Legend: NPa = Non-Parametric (actual); NPp = Non-Parametric (potential); Pa = Parametric (actual); Pp = Parametric (potential).  constraints are eliminated, the potential suitability evaluation of these mapping units remains marginally suitable (S3) with topography (t) as the only limitations which are somewhat difficult to correct due to its cost implication. The (S3) class covers about 49.48% of the total land area of the Dangerri study site.
Mapping units D and H are permanently not suitable (N2) and currently not suitable (N1) for sugarcane cultivation, respectively, owing to the following limitations: soil characteristics (s), topography (t), and wetness (w) for mapping unit D, while mapping unit H have fertility (f) and wetness (w) as its limitations. The potential suitability evaluation of mapping unit D remains permanently not suitable (N2) because this particular mapping unit is a mountainous area and therefore would be difficult to correct any constraints or limitations on this mapping unit. So also for mapping unit H, the potential suitability evaluation remains marginally not suitable (N1) with wetness (w) being the only constraint after fertility constraints has been ameliorated. The (N) class covers about 23.55% of the total land area of the Dangerri study site.
In the case of Parametric suitability evaluation (actual and potential) for sugarcane, Actual suitability evaluation for mapping units A, E, F, G, and I are marginally suitable (S3) with Suitability Index of 41.58 (S3), 32.22 (S3), 32.22 (S3), 32.22 (S3), and 37.20 (S3), respectively, and a corresponding potential suitability index (SIp) of 53.64 (S2), 53.64 (S2), 53.64 (S2), 53.64 (S2), and 48.00 (S30), respectively. Mapping units B and C are moderately suitable (S2) with an actual suitability index of 53.64 (S2) and 57.28 (S2), respectively, and a corresponding potential suitability index (SIp) of 80.00 (S1) and 71.52 (S2), respectively. Mapping unit H is marginally not suitable (N1) with an actual suitability index value of 22.64 (N1), and a corresponding potential suitability index (SIp) of 35.76 (S3). Mapping unit D is permanently not suitable (N2) with an actual suitability index of 7.16 (N2) and a corresponding potential suitability index (SIp) value of 8.00 (N2). The major limiting factors for sugarcane cultivation in the study area are available P, exchangeable K, total N, pH, wetness (drainage), soil texture, topography, and climate which is a common limitation to all the mapping units. However, following the suitability classes of the various mapping units of the Dangerri study area, most of the soils are marginally suitable for sugarcane cultivation unless serious land development measures such as flood control, drainage and liming are effected.

RECOMMENDATIONS
1) Soil management practices and corrective measures which are cost effective like application of organic manures, compound fertilizers and liming should be applied to the soils of the study area to supply deficient nutrients and correct soil acidity. 2) Physical land development like drainage, flood control, and sub-soiling could be done in the area to mitigate some of the limitations in the area. 3) Acid tolerant crops, fruits and vegetables like low land rice, radishes, sweet potatoes, peppers, potatoes, beans, cabbage, carrots, cucumbers, onions, squash, sweet corn, tomatoes, other than sugarcane could be considered as alternative to sugarcane following the current status of the soils of the study area. 4) Further studies should be carried out in this area to ascertain the effects and the cost/ benefits of the various soil management practices to be employed for successful sugarcane cultivation in the area.