Status of Selected Physicochemical Properties of Soils under Long Term Sugarcane Cultivation Fields at Wonji-Shoa Sugar Estate

: A declining trend in per hectare yield of sugarcane in Wonji-Shoa Sugar Estate is being observed due to altering of soil physicochemical properties resulted from long term sugarcane cultivation. Cognizant of this fact, a study was conducted in 2017 at Wonji-Shoa Sugar Estate with the objective of determining the effects of more than 62 years of sugarcane cultivation on the physicochemical properties of soil. In order to achieve this objective disturbed and undisturbed soil samples were collected from 0-30 and 30-60 cm layers of selected cultivated and uncultivated light and heavy soils for laboratory analysis. Results of the study indicated that long term cultivation of sugarcane at the same depth and low soil organic matter content of cultivated fields induced soil compaction and consequently highest bulk density was recorded in subsoil layer of cultivated than uncultivated land. The bulk density and total porosity values were out of ranges recommended for optimum sugarcane cultivation and suggest the existence of some degree of compaction. The finding further showed that the pH of study area soils is out of the normal pH range for sugarcane plant growth. The soil organic carbon, total nitrogen content and available phosphorus concentration of soils under both land uses of all soils was found within low range. Therefore, based on the result of the study it can be concluded that under condition of strong base soil pH of study area the availability of essential nutrients are critically affected. This indicates that the strong pH values at Wonji-Shoa Estate require more attention. Moreover, the low levels of organic carbon, total nitrogen and available P contents under both cultivated and uncultivated soils indicated that soil fertility is among the constraints for sustainable sugarcane production in the estate. Based on the findings and conclusions of this study one can recommend that to maintain sustainability of sugarcane production in the estate soil management practices that can protect as well as ameliorate soil compaction, increase soil organic carbon, total nitrogen, soil available P and that can decrease soil pH are important. Nevertheless, in order to give conclusive recommendation further research studies are needed for more soil management units in the estate.


INTRODUCTION
Several researchers suggested that the most serious Long term intensive mechanized tillage operation is soil compaction and the loss of soil organic matter [4].under sugarcane production causes soil degradation Moreover, soil compaction and loss of organic matter are which results in subsequent decline in cane yield [1].
major soil degradation processes leading to deterioration These situations have worsened by the use of heavy of soil physical and chemical properties with concomitant machinery such as tractors for operations like cultivation, decline in cane yield [5,6].Tesfaye et al. [7] reported that planting, fertilizer application, weed control and cane the most serious factor associated with soil compaction extraction is a common practice [2].Research report under sugarcane production is the loss of soil organic indicated that machinery overuse and long term intensive matter due to intensive tillage.Studies in Ethiopian Sugar cultivation have been found to be the main cause for Estates also showed that declining productivity of the major soil degradation processes such as compaction and fields is mainly due to deplete of organic matter along with loss of organic matter [3].
factor associated with soil degradation under sugarcane In Ethiopia sugar cane yield decline is currently Valley system in the Awash River Basin (Figure 1).becoming the major area of attention in the sugarcane The current total area of land covered with cane is about plantations.For instance, Tesfaye [10] clearly indicated 7050 ha for Wonji-Shoa Sugar Estate [15]. the existence of a general decline in cane yield in the Ten years (2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015) climatic data (Figure 2) of the Wonji-Shoa Sugar Estate.Accordingly, the cane yield Wonji-Shoa Estate indicated that the area has a bimodal declined by 48.63% over the last 54 years at Wonji-Shoa rainfall pattern in which small rain is received from between 1954-2008 production years.Research report has February to April, while the main rainy season that shown that long-term monoculture and excessive tillage contributes a significant proportion of the total annual along with practices that deplete organic matter all rainfall is received during June to September.The mean of contribute to yield decline [11].Moreover, Babbu et al. ten years annual rainfall of the study area is 831.47 mm [12] and indicated that long-term sugarcane cultivation [16].The climate at estate is semiarid and average under low soil organic matter condition altered soil maximum and minimum temperature was about 15.19 and properties.These changes in soil properties result in 27.57°C, respectively.increased bulk densities that may consequently reduce The range of altitude of the Wonji-Shoa Estate is nutrition uptake and crop yield [13].
950-1540 meters above sea level (m.a.s.l).While, the slope Identifying and understanding the cause of the yield of the fields was generally very gentle and regular and decline has paramount importance to design and this makes it suitable for gravity irrigation.The estate recommend appropriate management strategies.sugarcane production was undertaken with irrigation [17] Therefore, evidences on the effect of long-term and the sources of water for irrigation were Awash River.mechanized cultivation for sugarcane production on soil The major crop of the estate was sugarcane; while, haricot physicochemical properties are important inputs for bean and crotalaria are also produced.The average length planning soil and land management practices in large of growing period of sugarcane (plant cane) in the study scale mechanized irrigated sugarcane farms in the sugar area is 22 months.estate.
The major geologic materials of Wonji-Shoa Estate Some studies have been done in Ethiopian Sugar was developed under tropical hot condition from Estates on effect of long term sugarcane cultivation on alluvium-colluvium parent materials, which include basic sugarcane yield [14].Nevertheless, a few is known volcanic rocks (such as basalt, limestone) as well as regarding the effects of long term mechanized sugarcane recent and ancient alluvial soils developed from materials production on selected soil physicochemical properties in laid down by river systems [18].Vertisols and Fluvisols the Wonji-Shoa Sugar Estate.Such information is of are major soil types of the estate plantation [19].particular important inputs for sugarcane producing At the study area soils were grouped according to community and for land-use-planners in planning land their moisture content at pF2, i.e. at 10 kPa matrix management practices for sustaining the production and potential.This grouping of soil management approach for productivity of sugarcane in the estate.Keeping all these the estate was adopted from Kuipers [20] though there aspects in consideration, this study was initiated with was no documented information about the exact objective of assessing the effects of long term mechanized methodology, depth of sampling, number of sugarcane production on selected soil physicochemical measurements of samples and types of sampling for pF grouping of soils under different management units of this soils nearby the farms as references.
estate.There are five pF2 classes (namely A , A , BA , B  observation for both soil management units per estate.The experiment was conducted on light and heavy

MATERIALS AND METHODS
The reports of Babbu et al. [22] indicated that the yield soil management unit groups.Three stages stratified reduction was due to soil related constraints.The random soil sampling method was used.In the first stage uncultivated fields were identified per each existing the estate was stratified in to two soil management units.
management unit groups and most of them were located In second stage each soil management unit was between the main drains and access roads.According to categorized into two land use types (cultivated and information from station officers of the estate, these soils uncultivated).In the third stage each land use was have not been cultivated for about forty years.Each represented by three sampling sites so that soil samples cultivated and uncultivated fields were sampled by from each stratum provided good representation of study replicating three times.Accordingly, 18 sampling sites for area soils.Furthermore, qualitative soil compaction each soil management unit was assigned.Global diagnosis at field level was undertaken in order to select Positioning System (GPS) data was taken from each of the the final soil sampling sites.sampling sites.
A representative composite soil samples with three determined soil physicochemical parameters for each replications per each cultivated and uncultivated bare soil management unit group land uses were tested fields was collected from the two depths.Composite and using the general linear model procedure of the SAS undisturbed (for bulk density) samples were collected computer package [33].For statistically different from 0-30 and 30-60 cm soil depths using auger and core parameters (P < 0.05), means were separated using the samplers, respectively.Ten sub-samples were collected Fisher's least significant difference (LSD) comparison.from each sampling site using the X-pattern of sampling Pearson correlation analysis was also executed to reveal technique to make one composite sample per depth.
the magnitudes and directions of relationships between Three undisturbed samples per each cultivated and the selected soil physicochemical properties.uncultivated bare field was taken using core sampler into which 5 cm height and diameter cores were fitted.On the RESULTS AND DISCUSSION basis of this, a total of 72 composite and undisturbed samples were collected from the estate plantation fields.
Effects of Land Use Types on Selected Soil About 500 g of the composite soil samples were Physicochemical Properties properly weighed, labeled and kept in plastic bag and Particle Size Distribution: There were significant transported to Debrezeit Research Center, Wonji Central differences in the soil particle size distribution between Laboratory.In the laboratory, sufficient amount of soil the soil management unit groups of cultivated soils as samples were air dried and ground to pass through 2 mm compared with the adjacent uncultivated soils of each diameter sieve for further laboratory analysis of selected soil management groups with the exception that silt soil physicochemical properties except organic carbon content was not significantly (P 0.05) affected by land and total nitrogen in which case the samples were crushed uses of all soil management unit groups (Tables 1).further to pass through 0.5 mm diameter sieve.
The highest clay content was recorded for cultivated land Laboratory Analysis of Soils: Particle size distribution uncultivated land.The results indicated that the soil of all was determined by the Bouyoucos hydrometer method as soil management units could be categorized as clay described by Okalebo et al. [23].The textural class was textural class except the uncultivated light soils of determined using the USDA soil textural triangle [24].Bulk Wonji-Shoa soils which is sandy clay loam.This indicates density was determined using the core method as that the significant differences in individual separates described by Jamison et al. [25].Particle density (ñp) was between land uses did not cause changes in textural determined using the pycnometer method following classes.procedures described in Rao et al. [26].Total porosity The increase in clay content in cultivated than was calculated from the values of bulk density and uncultivated land might be attributed to the difference in particle density using the method described by Rowell vulnerability of the land uses to eluviation and surface [27].
runoff which is normally highest in the cultivated land The pH of the soils was measured in water (1:2.5 soil: soils.Moreover, occurrence of higher sand fraction in the water ratio) by glass electrode pH meter [28].Soil organic layer of uncultivated land could be ascribed to the carbon was determined by the wet digestion method removal of clay particles through erosion of the area, following the procedure of Walkley and Black [29].
leaving the sand particles behind.In line with this finding, The total nitrogen was determined by the Kjeldal method Wakene [34] also reported the difference in particle size as described by Jackson [30].Relative amount of carbon distribution between cultivated and uncultivated soils due to nitrogen was determined by taking the ratio of soil to eluviation and surface runoff.organic carbon to total nitrogen.Available phosphorus Furthermore, the differences in particle size was extracted according to Olsen's method [31].The P distribution of cultivated from uncultivated soils could extracted with different methods was measured by also be due to mixing of soils of the surface and spectrophotometer following the procedures described by subsurface horizons during tillage activities and Murphy and Riley [32].
subsoiling operations of sugarcane cultivation field soils.

Data Analysis and Interpretation:
A randomized complete distribution due to the removal of soil particles through block design (RCBD) with three replications was used to erosion and mixing of the surface and subsurface soils analysis the variance of soil parameters.Analytically during deep tillage activities.
[40], the optimum soil porosity for sugarcane the transport as well as utilization rate of nutrients in soil growth is 50%.Furthermore, according to Hazelton and directly.Bulk density values were significantly (P < 0.05) Murphy [41], in clay soils total porosity less than 50% can affected by land use for all soil management unit groups be taken as critical value for root restriction.As per these (Tables 1).Soil bulk density of study area soil ranges from ratings, total porosity values for the cultivated lands in 1.26 to 1.36 g.cm .As per dry bulk density ratings this estate was below the optimum value for sustainable 3 suggested by Jones [36] for different textured soils, the sugarcane production and were in the range of root bulk density values of both soils of SMUGs were within growth restriction.the normal range suggested for the respective textural The relatively highest values of total porosity classes except that the bulk density of the cultivated light obtained for uncultivated land use corresponded to the soils of Wonji-Shoa Estate were close to the root higher amount of organic matter contents and lower bulk restriction initiation bulk density values.
density values of uncultivated land uses.In line with this, The lowest value of bulk density was observed for Brady and Weil [42] reported that the low total porosity uncultivated land use.But the highest value of bulk was the reflection of the low organic matter content and density was recorded for cultivated land use (Tables 1).
the high bulk density.This is further supported by The highest bulk density values for cultivated land use Gangwar et al. [43] who reported reduced bulk density might be attributed to soil compaction induced in the soil might be due to increased soil pores as the result of due to long term cultivation of sugarcane and low soil incorporation of higher soil organic matter to soil from organic matter content of cultivated fields.This can be organic fertilizer that ultimately improved soil total evidenced from the strong correlation (r = -0.51*) between porosity.This is also supported by the negative and the soil organic matter and bulk density (Table 5).
significant correlation between bulk density and total Similarly, Negesa and Tesfaye [37] also reported porosity in soils of the estate (Table 5).Furthermore, increasing of bulk density due to soil compaction and the though non-significant, the correlation between total negative correlation between soil organic matter and bulk porosity and organic carbon was also positive (Table 5) density, respectively.
suggesting that maintaining higher soil organic matter in Moreover, the optimum bulk density for sugarcane soils could result in higher total porosity.production is 1.10 to 1.20 g.cm for both clay and loam (Table 2).The clay percentage increased whilst the sand The different land uses (land uses under light soil and silt contents decreased from the surface to the management) had significant (P < 0.05) effect on total subsurface horizons in both cultivated and uncultivated porosity of soil (Table 1).The highest value of total light and heavy soil management unit groups of porosity (42.80%) was obtained from light uncultivated Wonji-Shoa Estate.The general decrease in sand and silt land use and the lowest value (38.19%) was recorded from content and increase in clay content with soil depth, heavy cultivated land use.The total porosity of soils nonetheless, did not result in change in textural class name (Table 2).Texture is an intrinsic soil property, but to soil agricultural additives (filter cake, silt and vinasse) intensive cultivation, leaching and mixing of the surface during cultivation at top soil layer (Table 4).This is in line and subsurface horizons during deep tillage activities with Barzegar et al.
[48] who reported the effectiveness contributed to the variations in particle size distribution of sugarcane residue in reducing soil compactibility.between two depths of the cultivated and uncultivated The subsoil layer data further indicates that the soil bulk lands.
density is mostly close to root restriction initiation.On a relative basis, the clay content at both the The particle density under the two land uses surface and subsurface layers are higher for the cultivated increased slightly with soil depth (Table 2).This could be than the uncultivated soils of almost both the SMUGs in attributed to the relatively higher OM content in the top the estate.The relatively higher clay content at the soils and presence of heavy minerals such as Fe and Mn subsurface layers of the two land uses may indicate the in the subsurface layers.The particle density values selective removal of clay from the surface layers by recorded in this study are less than the average mineral downward movement and its subsequent accumulation in particle density of 2.65 g cm implying that the soils are the subsurface layers.Chemada et al.
[44] also indicated composed of relatively light minerals.Similarly, increase that one of the main processes that could likely lead to in particle density with increasing soil depth was reported increase of clay content with depth in a soil profile is the by Ahmed [49].downward transport of clay suspended in percolating soil Following the variations in bulk and particle water.It might also be due to in situ formation of clay densities, total porosity of the SMUGs showed a within the subsurface layers.In line with these findings, generally decreasing trend with soil depth in the Meyer and Antwepen [45] indicated the existence of cultivated soils and an increasing pattern in the significant variations in particle size distribution in soil uncultivated soils (Table 2).In soils, which have the same profiles due to elluviation and illuviation processes.
particle density, the lower the bulk density the higher is Prasad and Govardhan [46] also reported accumulation of total porosity.Furthermore, total porosity was lower in clay in subsurface layers and attributed this to the in situ cultivated land subsoil layers than uncultivated land formation of clays and weathering of primary minerals in (Table 2).The lower total porosity in the subsoil layer of the B horizon.
the cultivated land is likely attributed to the higher bulk Bulk Density and Particle Densities and Total Porosity: also reported by Smith et al.
[50] that total porosity was In all the soil management unit groups bulk density was lower in the subsoil layer of cultivated land.increased with depth in the soil profile in cultivated soils and decreased with soil depth in the uncultivated soils Effects of Land Use Types on Selected Soil Chemical (Table 2).Numerically the highest mean (1.38 g cm ) Properties 3 value of bulk density was recorded on the subsoil layer of Soil pH: Soil pH is the most important master chemical soil cultivated land and the lowest mean (1.16 g cm ) value parameter and it reflects the overall chemical status of the 3 was under the top layer of cultivated land.Moreover, bulk soil and influences a whole range of chemical and density values of the surface layers of the uncultivated biological processes occurring in the soils [51].Soil pH land were relatively higher than those of the cultivated was significantly (P < 0.05) affected by land uses in heavy lands in the estate, while the reverse was true for the soil management unit group of the estate (Table 3).subsurface layers.The increase in bulk density in subsoil The highest soil pH (8.19) was recorded for uncultivated layer of cultivated than uncultivated soils might be heavy soil management unit group and the lowest soil pH attributed to compaction resulting from intensive (7.57) was obtained from cultivated heavy soil cultivation at the same depth for long time (Table 4).
management unit group (Table 3).According to the This is in line with the findings of Barzegar et al. [47] ratings of soil reaction by Tekalign [51], soil reactions of who reported increase in subsoil bulk density following the study area were moderately alkaline to strongly long term cultivation.

Likewise, the relatively low bulk density in top soil
The lower pH values recorded under the cultivated layers of cultivated land may be attributed to the existence than uncultivated lands in Wonji-Shoa might be due to of high organic matter, tillage and more root extension in the depletion of basic cations as a result of leaching the top layers as a result of cane residues left after during every irrigation of the soils.The higher soil pH harvesting on surface soil layer of cultivated fields or due recorded for uncultivated heavy soil management unit   conditions might have led to higher concentration of groups (Table 3).The organic matter contents were in the sodium carbonate [52].Sodium carbonate in the soil reacts range of 1.21-2.14%for land uses of the estate (Table 3).with water to produce carbon dioxide and sodium
values of soil organic matter contents from all soil This is in line with Pradeep [53] who reported increase in management unit groups of the estate were rated as low.soil pH due to concentration of sodium carbonate in soils.
In all the SMUGs the soil organic carbon content of the The most universal effect of pH on sugarcane growth cultivated soils was significantly higher than the organic is nutritional.As reported by Arain et al. [54], the ideal carbon content of the adjacent uncultivated lands soil pH for sugarcane plant growth is pH 6.5 to 7.
(Table 3).The higher organic matter content of cultivated Moreover, most of the primary nutrients like nitrogen, over the uncultivated land uses is due to the agricultural phosphorus and potassium and secondary nutrients like additives such as filter cake and organic residues calcium and magnesium are best utilized by sugarcane remaining after harvest.Girma [57] also indicated low crop when the soil pH ranges between 5.50 and 7.90 [55].
status of soil organic matter in Wonji-Shoa Estate.Nevertheless, this finding showed that the pH of study The result of the study indicates that the soil organic area soils is out of this normal pH range.Under such carbon content found in the estate was within the range condition the availability of essential nutrients are of minimum quantities required (1.16-1.74%)for sugarcane critically affected.This indicates that in the estate pH production as suggested by Yadava [58].Such low could be one of the major factors affecting sugarcane organic matter content in the soils of the estate could production.Therefore, improving soil pH is clearly presumably be due to the hot climate and intensive valuable in these soils in terms of improving availability of cultivation which increases rate of decomposition.It also nutrients for sugarcane crops.Soil management practices indicates that the current rate of organic matter addition that reduce high pH value of soils at Wonji-Shoa Estate followed by the estate is not adequate to maintain the have positive effect in improving sugarcane production of organic matter content of the soils at the required level.the estate.
If decomposition rate is faster than the rate at which organic matter is added, soil organic matter levels will Narrower ratios permit mineralization to occur.The carbon decrease.As a result, nutrient supplying capacity of soil to nitrogen ratio of the soils was significantly (P < 0.05) declines steadily.
affected by land use in light soil at Wonji-Shoa only The low rating values of soil organic carbon in (Tables 3).In these soil management units, the C:N ratio cultivated and uncultivated lands may increase of the cultivated soils was significantly greater than that susceptibility of soil to compaction during machinery of the uncultivated soils (Table 3).In cultivated operations.Different studies made hitherto have indicated agricultural soils, the C:N ratio ranges from 8:1 to 15:1 [65].that the degree to which soils will compact when a force As mentioned by Tesfaye et al.
[66] when C:N ratio is less is applied by heavy machine on soil is primarily than 20:1 mineral N can be released.In this regard, the C:N dependent on the amount of organic matter content ratio of the estate is in the range where mineral N can be present in the soil [59].The average organic matter released for sugarcane use.As per rating by Newey [67], content was found to be 1.98% for the estate soil the C:N ratio of all the soils was within the medium range management unit groups (Table 3).However, as per the category except in the uncultivated light Wonji-Shoa suggestion by Alvarez et al. [60] soils with organic which was in the low range.However, the amount of N carbon levels above 1.97% (threshold value) are less released by decomposition process may be limited by the vulnerable to soil compaction.This indicates that the amount of organic carbon in the soil.organic carbon level in the estate was even below the threshold value, which can aggravate soil compaction.
Available Phosphorus (P): Soil available phosphorus was Therefore, management of soil organic matter is at the significantly (P< 0.05) affected by land uses of soil heart of sustainable agriculture.One way to reduce management unit groups (Table 3).The content of susceptibility of soil to compaction is to raise organic available P in the cultivated land appeared to be higher matter content of soils.Botta et al. [61] also demonstrated than the uncultivated land use type in all the soil that soil compactibility caused by heavy machinery can management unit groups (Table 3).In the estate, the be reduced by raising soil organic matter content by available P ranged between 3.06 to 5.89 mg kg .incorporating residues.
According to research reported by Arain et al.
[68], the Total soil nitrogen was significantly (P < 0.05) optimum P content for sugarcane growth should range affected by land uses of the estate (Table 3).The highest between 20 and 40 mg kg .Similarly, Tekalign and Haque soil total nitrogen (0.08%) was recorded from cultivated [69] set minimum critical limit (11 mg kg ) for growth of heavy soil management unit groups and the lowest soil crop plants in general.Muhammade et al.
[70] also total nitrogen (0.06%) was obtained from cultivated light suggested that below 6 mg kg , P may cause deficiency soil management unit groups.Nevertheless, based on symptoms in sugarcane plants.The available P content in total nitrogen rating suggested by Berhanu [62], the total all the SMUGs was even below the 6 mg kg , which nitrogen content of soils under both land uses of all the indicates that available P could also be among the limiting soil management unit groups was within the range of low.
nutrient elements for successful growth of sugarcane.This result suggests that nitrogen could be among the This was also evidenced by the red or purple colours major nutrient elements limiting sugarcane production in observed on most sugarcane leaves (P deficiency) during the estate.The higher total nitrogen content in the soil sampling.cultivated soils could be related to the nitrogen fertilizer The available phosphorus concentration in the applied to cultivated land.Application of N fertilizer and soils of the estate was very low according to the available agricultural organic additives in the long and short-term P rating classes suggested by Landon [71] except for cultivation probably increased N content of the heavy cultivated Wonji-Shoa (low).Nevertheless, the cultivated fields as compared with uncultivated ones.low contents of available P observed in these fields is in This finding is in agreement with Bikila [63] who reported agreement with the findings of Tekalign and Haque [72] direct association between total N content of a soil and who reported that the availability of P under most organic carbon (OC) content.soils of Ethiopia is low.The higher available P in the Carbon to nitrogen ratio is an important property of cultivated than uncultivated land could be due to the P soil which controls the rate of decomposition and whether fertilizer added during cultivation.In line with this mineralization or immobilization of N occurs [64].
Birru [73] reported that the concentration of available P Conditions which encourage decomposition of organic was lower in uncultivated lands than in cultivated crop matter results in narrowing of the C:N ratio of the soil.lands.4. The maximum value of soil pH (8.20) was recorded from subsoil layer of light uncultivated and minimum soil pH (7.48) was recorded from subsoil layer heavy cultivated layer.Within each land use type of the studied soil management units, soil pH showed some inconsistent variation with soil depth.
In light cultivated and uncultivated soils at Wonji-Shoa pH increased with soil depth and the reverse was true for heavy soil management unit group (Table 4).Comparing the surface layers of the two land uses, it was observed that the pH of the cultivated soils was relatively lower than the pH of the adjacent uncultivated lands which might be due to the fertilizer and organic inputs applied to cultivated land.

Soil Organic Carbon and Total Nitrogen:
The organic carbon (OC) was affected by soil depth in both land uses.The OC was decreased consistently from the surface to the subsurface horizons in all of the land use systems (Table 4).The organic carbon contents were in the range of 0.7-1.4% for both land uses of the estate (Table 4).
According to the soil organic carbon rating suggested by Murphy [74], the soils of the study area were very low (< 2%) in their organic carbon content.The present study shows that organic carbon content of the soils is even below the minimum quantity of OM required for sugarcane cultivation (2-3%) as suggested by Yadava [75].The relatively higher soil organic carbon content in the top soil layer than the respective subsoil layer of the cultivated soils could be due to addition of the organic agricultural additives to the top soil layer.
Similarly, the relatively higher organic carbon content in the top layer of the uncultivated soils is an indication that most of the organic matter sources are within the upper 0-30 cm layer.Similarly, Angelova et al. [76] also reported high organic carbon content over top surface of the cultivated soils.The low organic carbon content in the study area might be attributed to the low level of organic matter addition and exploitative and continuous tillage activities during seed bed preparation under continuous and intensive cane cropping.Tillage introduces oxygen and break aggregates to expose soil organic carbon that was formerly protected from decomposition.Then, this condition increases the rate of decomposition of soil organic matter and steadily decreases the organic carbon content of soils.In line with this, Wakene [77] also reported decrease in organic matter content as a result of continuous cultivation.
The total nitrogen which is a major nutrient element determining sugarcane yield was in the range of 0.06-0.34%for both land uses of the estate (Table 4).There was a decrease of soil total N down the depth.The total nitrogen content which decreased with soil depth was also in the range of very low (< 0.1%) as per rating suggested by Landon [78].This very low level of total nitrogen is in line with the very low level of organic carbon.The differences of nitrogen contents between soil layers may be attributed to the observed differences in soil organic matter contents between the two layers.
Available Soil Phosphorus: P is the most commonly plant study area the availability of essential nutrients are growth-limiting nutrient in the tropical soils next to water critically affected so that the strong pH values at and N. The rate at which the plant absorbs phosphate Wonji-Shoa Estate require more attention.Moreover, the ions is influenced by their concentration in the soil low levels of organic carbon, total nitrogen and available solutions.The concentration of available p in soils under P contents under cultivated soils showed that soil fertility the two land uses in the estate was ranged from 2.92 to 6.9 is among the constraints for sustainable sugarcane ppm.In the cultivated and uncultivated soils of the production in the estate.Based on the findings and SMUGs in the estate, available P exhibited a decreasing conclusions of this study one can recommend that to trend with soil depth (Table 4).The decrease in available maintain sustainability of sugarcane production in the soil phosphorus with soil depth in both the cultivated and estate soil management practices that can protect as well uncultivated soils might be ascribed to the increment of as ameliorate soil compaction, increase soil organic clay content with depth (Table 2), which can cause carbon, total nitrogen, soil available P and that can fixation of P and higher organic matter content in the top decrease soil pH are important.Nevertheless, in order to layers (Table 4).
give conclusive recommendation further research studies The better accumulation of sugarcane root residues are needed for more soil management units in the estate.and better biological activities in the topsoil layer than that of the subsoil layer can improve available P in the top ACKNOWLEDGMENTS layer soil.Further, the lower concentration of available P in the subsoil layer might also be due to fixation by clay The authors gratefully acknowledge Ministry of which was observed to increase with profile depth.
Education for guaranteeing their salary during the Sugarcane also takes up phosphorus from the subsoil and research work.They would like also to thank Wonji-Shoa in combination with its low mobility of P at the top soil Research Station staff members and field workers layer the values of phosphorus can be found to be very particularly for providing us laboratory facilities, low in the subsoil.These results are in line with the laboratory analysis, labour and technical support during findings by Dang [79] who reported the restriction of soil the implementation of field experiment.P in top soil layer due to its low mobility and decrease of soil P in subsoil due to fixation with clay.Ahmed [80]

Fig. 1 :
Fig. 1: Location maps of Wonji-Shoa Sugar Estate in Ethiopia management unit groups, LSD = least significant difference, ñb = bulk density, ñp = particle density, f = total porosity, San cl lo = sandy clay loam and means with the same letters are not significantly different

3 soils
and 1.30 to 1.40 g.cm for sandy soils[38].Based on Effects of Soil Depth on Selected Soil Physical Properties 3 these critical values, the bulk density values of most of Particle Size Distribution: There was variation in the soil the sampled sites were out of these critical values, which particle size distribution between soil depths of the land indicate presence of soil compaction and sustainability use types of the selected soil management unit groups problem for sugarcane production in the estate.
management unit groups, LSD = least significant difference, pH = soil pH, SOC = soil organic carbon content, TN = total nitrogen, C:N = carbon to nitrogen ratio, P = available soil phosphorus and means with the same letters are not significantly different group soils of Wonji-Shoa Estate could be attributed to Soil Organic Carbon, Total Nitrogen and Carbon to the accumulation of exchangeable sodium and calcium Nitrogen Ratio (C:N Ratio): Soil organic matter content carbonate in the soils of the estate.The reaction of of the soils at this estate was significantly (P < 0.05) exchangeable sodium and CaCO under low CO affected by land use in all the soil management unit 3 2 content, San = sand content, Bd = bulk density, f = total porosity, N = total nitrogen, P = soil available P, SOC = soil organic carbon and ***, ** and * = Significant at P < 0.001, P < 0.01 and P < 0.05, respectively; ns = not significant Variation of Selected Soil Chemical Properties with Soil Depth under Two Land Uses Soil pH: Data pertaining to the soil pH as influenced by depth is given in Table

Table 1 :
Effects of land use on selected physical properties of the soils in Wonji-Shoa Sugar Estate Particle size distribution (%) -

Table 2 :
Variations of selected soil physical properties with soil depth across two land uses of the major soil management units in the Wonji-Shoa Estate

Table 3 :
Effects of land use on selected chemical properties of the soils in the Wonji-Shoa Estate *SMUG = soil management unit group; SCL = sandy clay loam; CL = clay loam; ñb = dry bulk density; ñp = particle density; f = total porosity

Table 4 :
Variation of selected soil chemical properties with soil depth across two land uses of the major soil management unit groups in the Wonji-Shoa Estate management unit groups, pH = soil pH, SOC = soil organic carbon content, TN = total nitrogen, P = available soil phosphorus

Table 5 :
Pearson correlation analysis of some selected soil physicochemical also REFERENCES observed the highest value of available P at the top soil layer in soils of Mount Chilalo. 1. Kodesova, H., 2011.Role of physicochemical The results of the study indicated that clay content E. Teklu, 2018.Long Term Effects of Cultivation on was decreased consistently with depth and higher in Physicochemical Properties of Soils at Metahara cultivated land than uncultivated land.Moreover, long Sugar Estate.American-Eurasian Journal of term cultivation of sugarcane at the same depth and low Agricultural Research, 18: 246-257.soil organic matter content of cultivated fields induced 3. Barzegar, A.R., S.H. Mahmoodi, F. Hamedi and soil compaction and consequently highest bulk density F.Abdolvahabi, 2005.Long term sugarcane was recorded in subsoil layer of cultivated than cultivation effects on physical properties of fine uncultivated land.The bulk density and total porosity textured soils.Journal of Agricultural Science and values were out of ranges recommended for optimum Technology, 7: 59-68.sugarcane cultivation and suggest the existence of some 4. Hamza, M.A. and W.K. Anderson, 2005.Soil degree of compaction.The finding further showed that compaction in cropping systems.A review of the the pH of study area soils is out of the normal pH range nature, causes and possible solutions.Soil and for sugarcane plant growth.The soil organic carbon, total Tillage Research, 82: 121-145.nitrogen content and available phosphorus concentration 5. Albiach, K., 2003.Management practices to of soils under both land uses of all the soil management improve soil health and reduce the effects of unit groups was found within low range.detrimental soil biota associated with yield decline of Therefore, based on the result of this study it can be sugarcane in Queensland, Australia.Soil Tillage concluded that under condition of strong base soil pH of Research, 72: 125-137.properties in soil quality.Chemistry Science Reviews 2. Tesfaye, W., K. Kibebew, B. Bobe, T. Melesse and