Characterization and classification of greenbelt soils in Yambio and Nzara counties, Western Equatoria State, South Sudan

The objectives of this study were to characterize, classify and evaluate the potential and constraints of the soils of Sakure and Nginda Payams in Nzara and Yambio counties in the Greenbelt zone of Western Equatoria State, South Sudan. Ten soil pits were dug, described and sampled based on FAO soil profile description guideline and samples were analysed using standard routine lab analyses for physical and chemical properties. Data generated were analysed statistically using the coefficient of variation (CV) and correlation. Results showed that top and subsoil were dominated by sandy clay loams. The soil reactions were strongly to slightly acidic (pH = 5.4 - 6.7). The most limiting nutrients were P and N. SOC was highest in the top soil and consistently decreased with depth, the CEC was low (4 - 14.4 Cmol kg -1 ). The soils were classified into six major soil types: Ferralsol, retisols, acrisols, umbrisols, fluvisols, and chernozems. The soils have poor inherent soil fertility. It is recommended that further soil survey be carried out in the Greenbelt zone and to conduct more research to determine the type of soil fertility management feasible.


INTRODUCTION
Soil characterization provides the basic information necessary to create functional soil classification patterns and assess soil fertility to provide insight of some unique soil properties which are key to sustainable use of the soil resources (Adegbite et al., 2019;Yacob and Nigussie, 2022;Gomes et al., 2023).It provides information for understanding of the physical, chemical, and mineralogical and uses soil horizons and factors of soil formation as basis of classification on which crop/forest growth depend (Umare, 2018;Kafle, 2022).The characterization also analyses and quantifies the morphology of the surface earth in terms of landform characteristics to better understand the physical, chemical, and biological processes that take place within the landscape (Adegbite et al., 2019).It is also important for soil researchers to promote the importance of soil in supporting human life and wellbeing to better support environmental, agricultural and climate policies to policy makers and society/farmers (Bouman et al., 2019;Panagos et al., 2022;Gomes et al., 2023).
The soils of the Greenbelt deteriorate quickly in fertility under cultivation.Because of this, farmers tend to adopt shifting cultivation as a natural way to improve soil productivity in the absence of the use of fertilizer (ASPF, 2012).Shifting cultivation is destructive to the natural environment (Wineman et al., 2021;Kadoya et al., 2022;Gomes et al., 2023), but Nath et al. (2022) and Martin et al. (2023) urgue that there is a need for a careful diagnosis of this system and a rethink before claiming that the system is unsustainable.However, increasing agricultural production without land expansion requires increased fertilizer inputs as well as improved water management (Leitner et al., 2020).In addition, crop production through intensification, also requires efficient soil-plant-nutrient management, where the amount of mineral fertilizer to be applied depends on how much of each nutrient is already in the soil and readily available to plants (Yacob and Nigussie, 2022), Denmark has succeeded to reduce crop land areas by 6.5% in the last 30 years (Gomes et al., 2023) using fertilizer.To determine nutrient credit balance, area specific soil classification and suitability assessment for different crops is required (Lal, 2015;Gomes et al., 2023).According to Lal (2015), it is logical to find site specific farming systems to meet the site specific needs in terms of nutrient managements.Currently, South Sudan lacks or has limited data that can be used for soil fertility management (Odra, 2004;ASPF, 2012;WOSSAC, 2017).Tothil (1948) and Lebon (1956) had reported that there were no land use nor soil maps for any part of South Sudan and it has remained so to date.This type of information is required.According to Gomes et al. (2023), when the Danish government realized that the information on soil in agricultural land was insufficient for effective land use planning, they embarked on soil assessment.
Although, the Greenbelt could be the breadbasket of the whole country, yet the production per unit area is low and the farmers' practice of shifting cultivation is on the rise.This study aimed at characterizing the soils of Sakure and Nginda Payams in Nzara and Yambio counties of the Greenbelt zone to evaluate their potential and constraints for maize.
The study area is located in the Greenbelt zone, one of the six agro ecological zones that covers approximately 14% of the total land (648 000 km 2 ) of South Sudan (AfDB, 2013).The zone is characterized by tall broad leaf trees and thick forest.It runs along the boarders of South Sudan, DR Congo and Uganda.Maize can be planted and harvested twice a year and it has the greatest potential to produce a variety of annual and perennial crops (ASPF, 2012).The rainfall and temperature of the study area is presented graphically (Figure 2).The data were extracted from the University of East Anglia website in 2021 (https://crudata.uea.ac.uk/cru/data/hrg/cru_ts_4.04), using ArcMap version 10.5.

Study design
Stratified preliminary map units were established by unsupervised reclassification according to elevation using the 30 m (1 arc) Shuttle Radar Topography Mission (SRTM) terrain model (USGS, 2014) and conditioned Hypercube Latin Sampling (cLHS) was used in Rsoftware environment to generate thirty points for pit/auger observations on the study area.For soil description the FAO ( 2006) guideline was used.

Data collection and analysis
Geographic Positioning System (GPS) was used to navigate to the identified profile sites in preliminary mapping unit (MU).Site characteristics including land use, elevation, vegetation and slope characteristics were recorded in a Soil Profile Description Form, adapted from the National Soil Services, Tanzania.Soil profiles were dug in identified representative locations (Musell Colour Charts, 2009).
The soil samples obtained from the ten profiles were prepared and analysed for physical and chemical soil properties according to the descriptions in Okalebo et al. (2002).Particle size distribution was determined by Bouyoucos hydrometer method after dispersing soil with calgon.The pH was measured in water at the ratio of 1:2.5 soil-water.Organic carbon was determined by the Walkley and Black method.Total Nitrogen by Kjeldahl method and available phosphorus was extracted by Bray I method.The Cation Exchange Capacity (CEC) and exchangeable bases were extracted by saturating soils with neutral 1 M NH 4 OAc and the absorbed NH 4 + displaced by K + using 1 M KCl and then determined by Kjeldahl distillation method for the estimation of CEC of the soil (Summer and Miller, 1996).The bases Ca 2+ , Mg 2+ , K + , Na + displaced by ammonium were measured by atomic absorption spectrophotometer (Ca 2+ and Mg 2+ ) and flame photometry (K + and Na + ) (Reeuwijk, 2002).One percent EDTA was used to extract micronutrients (Cu, Mn, Zn and Fe).Booker Tropical Manual (Landon, 1991) was used for soil results interpretations unless specified.Statistical analysis was done using MS Excel version 2013 to calculate descriptive statistic and minitab for pearson corelation of all the selected physical and chemical soil properties in eight pedons excluding two pedons, 5 and 8 where only top soil horizon in each was taken.

Soil texture and silt/clay ratio
Thirty seven genetic horizons were observed and sampled, and the results are indicated in Table 1.
Figure 3B shows the trend of the clay, silt and sand fractions.Sixteen horizons were sandy clay loam, 10 clay, 7 sandy clay, 2 loam sand, and sand and sandy loam one each.The top soil and subsoil up to about 40 cm down the profile had more sand in most of the profiles ranging from 50 to 85% sand in all profiles.The dominant texture of sandy clay loam is consistent with the report of Ombina (2008) in Nzara County.The soils being well drained imply that the topsoils and subsoil have high proportions of sand and silt to clay; therefore, rainwater infiltration rate into the soil is fast and it carries with it the nutrients.In all profiles the clay contents consistently increased with depth, suggesting illuviation and formation of argillic horizon while the sand fraction decreased except for pedon 7 where clay content decreased downward while the sand fraction increased.Pedon 7 was obstracted by hard rock at 80 cm deep and it was classified as fluvisol.Probably, the parent material here conforms to the description of Morison et al. (1948), 'the geology of the Nile Congo divide are all composed of Basement Complex of Schists and Gneissess with intrusions; except it is overlaid by ironstone or more recent deposits.''The silt content did not have any particular trend throughout the ten profiles.
The silt-clay ratio is recorded in Table 1.The values ranging between 0.05 and 0.67 in pedon 1 and 7, respectively.The low silt-clay ratio suggests that the soils of the study area are highly weathered and leached (Adegbite et al., 2019;Yacob and Nigussie, 2022).The low silt-clay ratio also suggests that the soils have moderate resistance to erosion.According to Adegbite et al. (2019), soils with a threshold of less than 1 SCR are susceptibe to erosion.
The results of correlations have been run for 8 pedons excluding the pedon 5 and 10 that had only one horizon sampled.The results showed that SCR was positively and strongly correlated to sand (r = 0.758, p < 0.000), and weakly correlated to silt (r = 0.333, p < 0.05) but negatively and strongly correlated to clay (r = -0.825,p < 0.000).In the analysis of individual pedons, SCR was strongly and negatively correlated to silt (r = -0.979,p < 0.05) in pedon 1, and clay (r = -0.811,p < 0.05) in pedon 2, to clay (r = -0.886,p < 0.05) in pedon 4 but in pedon 8 SCR was strongly and positively correlated to silt (r = 0.946, p < 0.05).

Soil chemical properties
The selected soil chemical properties of the investigated pedons have been presented in Figure 4 and the means and the coefficient of variations of the individual pedons are discussed but no table is provided because the tables are too long.In the statistical analysis, pedon 5 and 10 have been excluded because in both only one horizon each was sampled, however, the results are captured in the graphs (Figure 4).Throughout the discussions, positive correlation coefficient (r) value indicates both soil parameters (x) and (y) show positive relationship and negative R-values indicate that one parameter increases and the other decreases and vice versa.Only significant values have been recorded.

Soil pH
The observed pH in the studied soils is strongly to slightly acidic (5.4 -6.7) in all profiles (Figure 4A) that is good for nutrient uptake by most crops (Landon, 1991;Adegbite et al., 2019).This range   (1985).
Agroecological Zone in Eastern Equatoria State, South Sudan.Another study by Ombina (2008) in Nzara where he considered soils around and inside the existing teak plantations revealed that the soils were acidic.However, below < pH 5.5 aluminium toxicity may exist (Neenu and Karthika, 2019).
Pearson correlation coefficienct for 8 profiles revealed that pH was positively but weakly correlated to OC (r = 0.376, p < 0.05), available P (r = 0.330, p < 0.05) and Ca (r = 0.345, p < 0.05).However, for the individual pedons pH was positively and strongly correlated to percentage base saturation (r = 0.962, p <0.05) in pedon 8, Na (r = 0.978, p < 0.05) in pedon 4, and in pedon 2, pH was positively correlated to OC (r = 0.914, p < 0.05), available P (r = 0.928, p < 0.05), Mg (r = 0.909, p < 0.05), CEC (r = 0.847, p < 0.05) and ESP (r = 0.922, p < 0.05).Positive correlation means that for every increase in pH the other soil properties will also increase and vice versa.Strong correlation means pH controls the base saturation of the soil, CEC and other plant nutrients such as P and Mg and also vice versa.

Soil organic carbon (SOC)
Soil organic carbon in the investigated pedons is presented in Figure 4G.In the topsoil, OC in nine out of ten pedons ranges from 0.6 to 2.7% and pedon 10 is 4.1%; these results indicate very low to high and very high OC content in the topsoil and the results conform to the description of Kimaro et al. (2001).According to Lal (2015), the concentration of SOC in the root zone must be maintained above the critical threshold level of 1.5% to enable farming systems to thrive.SOC performs a crucial role in ecosystem functioning and the global C cycle, and its decline can affect important soil processes, such as regulating water dynamics, stabilizing the soil structure, and releasing and holding nutrients for plants (Gomes et al., 2023).Based on Lal's critical level, the current study indicates that only P2, P5, P9 and P10 have good quantity of SOC in the topsoil, the other six pedons are marginally susceptible to SOC depletion.According to Prout et al. (2021), SOC is depleted under arable land use compared to natural system.Cultivation has caused losses of SOC in many parts of the world (Prout et al., 2022).The implication is that the probability of losing SOC is very high in the study area unless proper SOC management practice is undertaken.All pedons exhibited a consistent decrease of OC with depth (Figure 4G) except for pedon 1 at B2 about 100 cm deep, where there was more OC compared to the horizon above it.The high content of SOC on the surface than the subsurface layers can be attributed to the presence of plant materials as well as root and biological activity (Adegbite et al., 2019;Yacob and Nugissie, 2022).Zhong et al. (2018) reported similar results, OC decreases with depth and the report further added that clay content may also lead to more organic carbon molecules being adsorbed by clay surfaces and the presence of polyvalent cations forming organo mineral complex to control the protection of SOC from microbial and enzymatic decay, in turn increasing SOC storage.Several studies have confirmed the results that total amount of SOC increase with silt and clay sized fraction (Matus, 2021); and in addition to clay, precipitation increases the accumulation of SOC while it is decreased by high temperature (Prout et al., 2021).The means of individual pedons range from 0.3 to 1.1% with CV varying from 48.7 to 119%.The CV indicated moderate to high variations of OC within the pedons (Table 2).However, 90% of the soils indicated that the organic carbon content was in the low level category (Landon, 1991).
The ratio of carbon to nitrogen was lower than the critical level 24:1 (Schultheis et al., 2020); this means there is high level of carbon mineralization by the microbes and rapid release of nitrogen into the soil for immediate crop use.According to Schultheis et al. (2020), soils with a carbon-to-nitrogen (C:N) ratio of 24:1 have the optimum ratio for soil microbes to stimulate release of nutrients like N, P and Zn to crops.Statistically, SOC is strongly correlated to clay, sand, SCR and pH (r = -0.873;r = 0.847; r = 0.867; r = 0.914; p < 0.05).The negative correlation means as SOC increases the clay content will decrease.

Total nitrogen (TN)
Total nitrogen in the topsoil of the investigated pedons varied from 0.1 to 0.3% and decreased with depth in all pedons and follows a similar trend as exhibited by SOC (Figure 4G).According to Kimaro et al. (2001), TN is very low to medium.About 90% of the soil samples analyzed showed low levels of total nitrogen and only 10% showed medium levels (Landon, 1991).The CV ranged from 28.3 to 83%, this indicates moderate to high variability among the pedons.However, correlation analysis showed that TN was strongly correlated to SOC (r = 995; p < 0.000); this implies that SOC was the primary source of TN.This is consistent with other studies (Yacob and Nugissie, 2022).This means maintain organic matter on the soil will control sheet erosion, supply SOC and some of the essential plant nutrients such as N, P, S, etc., that in turn contribute to good crop performance.

Available phosphorus (P)
The results of the P in the topsoils of the investigated pedons ranged from 1.2 to 2.7 mg/kg and all pedons exhibited a consistent decrease of P with depth (Figure 4E).In this study, P has measured very low in all the soil samples, far below the low level < 7 mg/kg by Bray-Kurtz 1 method (Kimaro et al., 2001).In another study in Torit county, Eastern Equatoria State by Deng and Marchelo-d Ragga (2020) also found all sixteen top soil samples taken randomly from farmers fields exhibited low phosphorus.In Nzara County, Ombina (2008) found that phosphorus was very low in absolute terms.
This study indicates that P is negatively correlated to clay and positively and strongly correlated to SOC; this implies that as the clay increases with depth, available P decreases and vice versa for the SOC.The decrease of P in the lower layers within the profile could be attributed to P-fixation.This is consitent with the study of Yacob and Nigussie (2022).

Base saturation percentage (BS %)
In the topsoil BS% ranged from 51 to 88% and all pedons did not exhibit any trend (Figure 4D).The means of individual pedons ranged from 55.6 to 82.0% and the CV ranged from 5.8 to 13.3, this result means low variability Bazugba et al. 497 (Table 3).Generally, base saturation ranges are low < 20; medium 20 -60; high > 60 (Landon, 1991).The studied soils base saturation ranges are within medium to high base saturation percentage in acid soils.According to Landon (1991), some soils are base saturated at pH 5 and this explains why acid-sensitive crops can be grown in the tropics and liming does not increase their yield.According to Kimaro et al. (2001), since the studied soils are dominated by kaolinitic and sandy soils the Ca (2.1 -7.1) cmol (+) kg -1 (Figure 4C) is moderate to very high, Mg (0.51-4.2 cmol (+) kg -1 ) (Figure 4C) is moderate to very high, K (0.1 -1.25 cmol (+) kg -1 ) (Figure 4H) is low, 9 out of ten pedons exhibit very low and Na (0.2 -0.43 cmol (+) kg -1 ) (Figure 4C) is very low too.This implies that Ca and Mg are optimum, but potassium is in short supply.

Cation exchange capacity (CEC)
All ten topsoils of the investigated soils exhibited CEC range between 4 and 14.4 cmol (+) kg -1 and the means ranged from 3.3 to 10.9 cmol (+) kg -1 and coefficience of variation varied from 14.7 to 81.8%.In general, CEC decreased with depth in each pedon but it was not consistent (Figure 4H).According to Kimaro et al. (2001), CEC is very low to medium levels, seven pedons are in the low level, this implies that the soils easily lose their fertility and have low water holding capacity as well (Brown and Lemon, 2021).Brown and Lemon (2021) remarked that CEC is the inherent soil characteristic and is difficult to alter significantly, however, the addition of organic matter will increase the CEC of a soil but requires many years to take effect.

Exchange sodium percentage (ESP)
In the top-soil, ESP ranged from 0.5 to 3% and there was a general decrease in all pedons downward but with no

Figure 3 .
Figure 3. Topographic map of elevation, slope aspect, slope and streams.Source: Author.

Figure 4 .
Figure 4. Relationship of selected soil physical and chemical properties with with depth.Source: Author.

Table 1 .
Soil morphological and physical properties of the studied sites in Sakure and Nginda Payams.

Table 2 .
Coefficient of variation ranked to Wilding