Evaluation of land suitability for citrus cultivation in Khana Local Government Area of Rivers State, Southern Nigeria

Soils of Khana Local Government Area of Rivers State, Southern Nigeria were evaluated using semi detailed soil survey for citrus cultivation. The purpose of this research was to evaluate the suitability of soils of the study area for the cultivation of citrus. The total land area covers 49,631.54 ha and was delineated into eight mapping units based on the soil types. One pedon each was dug in each mapping unit and described using the FAO system. The nonparametric method of soil suitability evaluation was used. Four soil orders, majorly Inceptisols/Cambisols, Entosols/Arenosols, Ultisols/Acrisols, and Alfisols/Lixisols, were identified in the area. The results showed that land requirements/characteristics such as climate (mean annual rainfall), wetness (depth to water table) and fertility made the land marginally suitable (S3) to not suitable (N) for citrus cultivation across the eight pedons. The three limitations for citrus cultivation in the area are climate (annual rainfall), wetness (depth to the water table) and fertility (low status of NPK and pH). The land is potentially suitable for citrus cultivation but currently marginally and not suitable due to these three limitations.


INTRODUCTION
Land suitability evaluation is best described as the process of estimating the agricultural land potential for diverse kinds of utilization on a sustainable basis (Peter and Umweni, 2020a). According to Bintang and Tampubolon (2018), land suitability assessment is done by matching land qualities and characteristics to the criteria of the land suitability classes. Land suitability evaluation is also the assessment of agricultural land resources aiming to optimize land productive potentials, adding that land evaluation provides key information on the ability of land for sustainable crop production and soil management (Chukwu et al., 2014;Peter and Umweni, 2020a). It is also the interpretation of soil survey data in order that every hectare of land could be used in accordance with its capability, suitability and limitations (Food and Agriculture Organization, 2006). Soil suitability assessment involves a scientific procedure, which is essential to assess the potential and constraints of a given land for agricultural purposes (Rossiter, 1996). Therefore, to maintain sustainable agriculture, land use planning should be undertaken by investigating the soil through land suitability evaluation studies at both local and regional levels (Sereke, 2002;Essoka and Essoka 2013;Douglas and Peter, 2016;and Peter and Umweni, 2020a). Again, knowledge of the potentials and limitations of agricultural land resources in Khana Local Government Area, will enable crop farmers in the area to make adequate land use initiative to improve and maintain high yield of citrus crops on a sustainable basis and, at the same time, improve their standard of living (Peter and Umweni, 2020b). This requires a proper organization of land and soil data in such a way that they could be interpreted and applied for sustainable agricultural production. Citrus are predominantly produced on large scale within the Middle belt in Nigeria and have enormous market value with very high sales, especially within the dry season. Citrus is an important crop due to its nutritional and medicinal values. There are different citrus cultivars cultivated in Nigeria, such as sweet orange (Citrus sinesis Osbeck), lime (Citrus aurantifolia Swingle), tangerine or tangor (Citrus nobilis) and (Citrus reticulate, Blanco). Therefore, this study aimed to evaluate the suitability of agricultural land resources for citrus cultivation in Khana Local Government Area of Rivers State, Southern Nigeria on a sustainable basis.

MATERIALS AND METHODS
The study was carried out in Khana Local Government Area of Rivers State, Southern Nigeria within the Tropical Rainforest zone. It is located between latitude 4.67172N and longitude 7.34398E (Figure 1) (Peter and Umweni, 2020b). The study location covers 49,631.54 ha of land with a rainfall pattern that is in a bimodal form that usually start effectively from late February to October with a period of low precipitation in August commonly called August break (Peter and Ayolagha, 2012). The period of effective low precipitation occured mainly from late November to early March. Sometimes, it is accompanied by serious dry cold wind commonly called harmathan wind. The average rainfall of the study location was between 2000 mm to 2500 mm with monthly temperature range of 26 °C to 35 °C and relative humidity varying from 81 % to 87 % depending on the season (rainy season and dry season) (Peter and Umweni, 2020b).

Field work
The entire land of Khana LGA was identified and delineated into eight mapping units based on vegetation, topography, soil types, drainage condition, textures and structures. One soil profile pit of 2 m × 2 m × 2 m was dug in each representative soil mapping unit. Each of the soil profile was described in line with soil procedure as recommended by Food and Agriculture Organization (1988). Soil samples were collected from identifiable horizons in each of the profile pit for physical and chemical analysis. Undisturbed core samples were collected from each identified profile pit horizon for bulk density determinations. The coordinates of all profile pits were collected using a hand-held geographical positioning system (GPS). Soil color notation in the field was described using the Munsell color chart (1992).

Laboratory analysis
Soil samples collected were analyzed using routine soil analysis procedures most appropriate at the Soil Science Laboratory, Federal University Technology, Owerri -Imo State, to determine the physical and chemical characteristics of the soils. Soils collected were air-dried, crushed gently and sieved with a 2 mm mesh sieve. Soil particle size distributions were determined using the hydrometer method of Bouyoucus (1962) and Blake (1965), bulk density was determined by oven drying the undisturbed soil samples collected using a cylindrical core samplers, and bulk density was calculated as the weight of the soils divided by the volume of soil sample. Soil reaction (pH) was determined using the glass electrode pH meter (Mclean 1965). Organic carbon was determined using the dichromate wet oxidation method (Walkey and Black, 1934). Organic matter was obtained by multiplying the percentage of organic matter by 1.72. Total nitrogen was determined by the macro-Kjeldahl digestion methods by Jackson (1973) as described by Bremner and Mulvaney (1982). Available phosphorus was determined by Bray and Kurtz No 2 method (1945). Exchangeable cations were determined by extracting them with neutral ammonium acetate  Heald (1965). Base saturation was calculated as the sum of exchangeable bases divided by ECEC multiplied by 100.

Soil classification
Based on the results of the laboratory analysis and field morphological characteristics, the eight pedons were classified according to soil taxonomy United States Department of Agriculture (2014) and correlated with the World Reference Base for Soil Resources (2014).

Land suitability evaluation procedure
Soil characteristics of each soil mapping unit were matched with the requirement of the land qualities (climate, topography, wetness, soil physical characteristics, and fertility) to a suitability class assigned to it, following the guidelines provided by Mohekar (1997), (Table 1 and Table 2). The final suitability class of each mapping unit is the class indicated by the characteristics with the lowest ranking, which is in line with the "Law of Minimum" (Food and Agriculture Organization, 1984). The suitability classes of each mapping unit were ranked, and the rankings were compared using Spearman's ranking correlation coefficient.

RESULTS AND DISCUSSION
Physical and chemical characteristics of the soils Table 3 showed the selected physical and chemical characteristics of soils in the study area. Sand size particle dominated other soil particles in all the eight Pedons. Sand particles varied from (705.5 to 792.4) g.kg -1 in Pedon 1, (833.6 to 863.6) g.kg -1 in Pedon 2, (712.6 to 802.4) g.kg -1 in Pedon 3, (792.4 to 802.4) g.kg -1 in Pedon 4, (702.2 to 802) g.kg -1 in pedon 5, (760 to 781.6) g.kg -1 in Pedon 6, (691.2 to 812.4) g.kg -1 in Pedon 7 and (813.6 to 853.6) g.kg -1 in Pedon 8, accordingly. The high-level sand in surface horizons is also in line with the report of Akamigbo and Asadu (1983), who reported that sand particles were observed more in surface level as a result of the   eluviation and illuviation processes in soils. The high sand fraction in surface horizon was also influenced by the parent material from which the soils are formed (Akpan-Idiok (2012); Peter and Umweni, 2020b). There were some degrees of variability in silt contents in all the eight pedons. It varied from (40 to 120) g.kg -1 in Pedon 1, (23.6 to 103.6) g.kg -1 in Pedon 2, (50 to 70) g.kg -1 in Pedon 3, 60.2 g.kg -1 in Pedon 4, (100.2-140.1) g.kg -1 in Pedon 5, (702.4 to 781.6) g.kg -1 in Pedon 6, (60.2-10) g.kg -1 in Pedon 7 and (53.6 to 63.6) g.kg -1 in Pedon 8. Clay content of soils in the study area also varied between (137.4 to 177.6) g.kg -1 in Pedon 1, (62.8 to 132.8) g.kg -1 in Pedon 2, (147.6 to 227.4) g.kg -1 in Pedon 3, (137.4 to 147.4) g.kg -1 in Pedon 4, (57.9 to 197.6) g.kg -1 in Pedon 5, (64.4 to 206.4) g.kg -1 in Pedon 6, (77.6 to 248.6) g.kg -1 in Pedon 7 and (92.8 to 132.8) g.kg -1 in Pedon 8 accordingly (Peter and Umweni, 2020b). Soil reactions (pH) in water, as shown in Table 4, varied from acidic (4.31) to slightly acidic (6.16). Soil pH increased from 5.43 to 6.13 in pedon 1 and increased from 5.6 to 6.16 in pedon 2. It also increased from 5.4 to 6.11 in pedon 3. It was also observed that soil pH increased from 5.43 to 6.08 in pedon 4 and increased from 4.50 to 5.71 in pedon 5. In pedon 6, there was an increase from 4.31 to 4.81, and in pedon 7, there was a decrease from 5.9 to 4.7 and an increase from 5.59 to 5.83 in pedon 8. There was a decrease in soil pH down the profile depth, which is in line with the finding of Peter and Umweni (2020b). Soil organic carbon was generally low in all pedons (1.40 to 14.15 g.kg -1 ). This is also in line with the findings of Thurow and Smith (1998); Essoka and Essoka (2014); and Peter and Umweni, (2020). Total nitrogen levels ranged from very low (0.24 g.kg -1 ) in pedon 5 to low (1.39 g.kg -1 ) in pedon 1. Total N decreased down the depth the profile across the pedons. The low level of total N in the soils in the study area was as a result of excessive soil planted to leaching due to intensive rainfall experienced in the area, supported by the findings of Udo andOgunwale (1986) &Umweni (2020a). Available phosphorus also varied from 1.82 m.kg -1 (very low) to 57.47 m.kg -1 . There was no decrease in the available P content down the profile depths, but there were differences in the level of available P nonlinear to soil depths in all pedons.

Land suitability of the study area for citrus cultivation
The suitability classification of each pedon in the study area for citrus cultivation showed that Pedons 1 and 5 were currently marginally (S3) suitable for citrus cultivation in the study area due to limitations in climate (rainfall), wetness and fertility (Table 5). This is in line with the findings of Ikhe et al. (2017), who reported that excess rain caused citrus to be waterlogged, which eventually caused molding, 6 Table 4. Chemical characteristics of soils of the study area Remarks: Pedons 1 and 5 (10,700 ha) were marginally suitable (S3) for citrus cultivation with defects in climate (rainfall), wetness (depth to water table) and fertility (Low N, P and K status) in the soils; Pedons 2, 3, 6, 7, and 8 (31,232 ha) were also marginally suitable (S3) for citrus cultivation but with limitations in both climate and fertility; Pedon 4 (7,700) was not suitable (N) for citrus cultivation due to limitation in wetness (soil depth to water table); Source : Mohekar (1997).
adding that under such condition, citrus takes in more water and becomes more diluted. Both pedons (1 and 5) cover an area of land of 10,700 ha, representing 21.55 % of the study area. Pedons 2, 3, 6, 7 and 8 were also marginally (S3) suitable with limitations in climate and soil fertility. The main climatic factor affecting citrus production in the area was rainfall. This is synonymous with the finding of Ali et al. (2017), reporting that rainfall affected the growth and development of citrus in the humid tropical region. They added that excessive rain could lead to the excessive drop of flowers, resulting in fruitless conditions. Pedons 2, 3, 6, 7, and 8 cover a land area of 31,232 ha, representing 62.93 %, of the study area while pedon 4 was permanently not suitable (N) to citrus cultivation due to severity in wetness (depth to the water table) (Figure 2). This confirmed the report of Ridolfi (2006), that tree crops were stressed when the depth to the water table was shallow, leading to poor growth and dead of plants due water logging conditions. Pedon 4 covers a land area of 7,700 ha, representing 15.52 % of the study area. The specific fertility limitation was nutrient availability and status, especially NPK, which was very low in all pedons (1-8), while for pedon 4, it was the soil depth to the water table.

CONCLUSIONS
The land resource (soil) of Khana LGA is marginally suitable (S3), and some were currently not suitable (N) for citrus cultivation due to certain limitations. These limitations ranged from climate (rainfall) to wetness and fertility. A total of 41,931.54 of land representing 84.49 % of the study area were marginally suitable (S3) for citrus cultivation due to constraints in climate (rainfall), wetness and fertility, while 7,700 ha representing 15.51 % of the study area were permanently not (N) suitable for citrus cultivation due to severity in wetness (depth to the water table). However, some of these limitations can be improved through appropriate management practices in the study area for sustainable citrus cultivation.