Soil physicochemical properties and their significance for sustainable sugarcane production in Kesem Allaideghe plains irrigation project area, Eastern Ethiopia

A feasibility study was carried out to assess the suitability of Allaideghe plains, located in middle awash valley, for the cultivation of sugarcane plantation. The project area, covering 38000 ha, lies between 9o 07’ to 9o 26’ N latitude and 40o 30’ to 40o 50’ E longitude. A detailed soil survey was carried out on 500 m x 500 m grid consisting 1520 auger holes and 76 soil profile observations. The soils of the entire project area were very deep (>200 cm). The textural classes included silty clay loam, clay and heavy clay which revealed that texture varied from fine to very fine with high clay content. Soil reaction (pH), electrical conductivity of saturated extract (ECe), exchangeable sodium percentage (ESP), cation exchange capacity (CEC), organic carbon (OC), CaCO3 ranged between 7.7 to 8.2, 0.9 to 8.0 dS m-1, 9.9 to 42.7%, 40.6 to 61.7 cmol (+) kg -1, 0.3 to 1.2 g kg-1, and 8.3 to 18.3 %, respectively. Soluble cation contents of Na, Mg, Ca and K varied from 8.5 to 20.2, 0.2 to 2.6, 4.7 to 12.6, and 0.1 to 0.2 meq l-1, respectively. Major hydraulic properties influencing water availability and irrigation scheduling for sugarcane included infiltration rate and hydraulic conductivity. Basic infiltration rate varied from 0.2 to 6.2 cm h-1 and in-situ hydraulic conductivity varied from 0.07 to 0.60 m d-1. Low hydraulic conductivity may cause waterlogging in the project area. In order to improve soil structure and water availability, addition of gypsum, plant residues and organic matter are recommended.


INTRODUCTION
Ethiopia has 12 river basins with an annual runoff volume of 122 billion m 3 of water and an estimated 2.6-6.5 billion m 3 of groundwater potential, which makes an average of 1575 m 3 of physically available water per person year -1 (Seleshi Bekele et al., 2007).However, the total irrigated area is only about 250,000 ha (Seleshi Bekele et al., 2005).The expansion of irrigated agriculture for achieving food self sufficiency is currently the most promising option (Mekuria Tafesse, 2003;Dessale Kidane et al., 2014;Gebremedhin Gebrehawarya, 2015).The study was conducted in one of the basins of Awash River that starts in the central highlands of Ethiopia and ends at Lake Abe in Afar Region.
The Food and Agriculture Organization of the United Nations (1965) surveyed the Awash River Basin as a whole on reconnaissance scale and classified the soils as marginally irrigable land.Halcrow (1989) also reported that soils of the Awash Basin are classified as fine textured and concluded that there are good prospects for irrigation of Awash River Basin.The latter study was also carried out on limited scale.The above studies indicated the potential to utilize Awash *Corresponding author: yihenewgs@gmail.complains for commercial cultivation.However, there is paucity of data on detailed scale.In order to classify the soils for their suitability for sugarcane cultivation in Allaideghe plains, a detailed soil survey on 1:50,000 scale was carried out studying 1520 augur holes and 76 soil profiles, with detailed analysis in the laboratory.
Climatically, the Allaideghe plain is well suited for cultivating sugarcane.The upper Awash valley already has two sugarcane plantations at Wonji Shoa and Metehara.Tendaho, located at downstream, has also started sugar production.The middle Awash valley where these plains are located is still unexploited for commercial cultivation.
The government of Ethiopia, keeping in view the infrastructure that has already been developed in surrounding areas for sugarcane plantation, is planning to establish a sugar factory at Kesem Allaideghe plain area too.In order to become selfsufficient in sugar production and to be able to export as well, there is immediate need to expand sugar production in the country.
Thus, the principal objective of investigating the soils of Allaideghe plains located in the middle Awash Valley was to assess the possibility of introducing sugarcane plantation.The study area, in general, can be characterized by its almost homogenous topography.The plain is predominantly flat.Presently, this area is subjected to intensive grazing throughout the year, especially, during the dry seasons when grass cover is scanty.
The water requirement to irrigate Allaideghe plain is intended to be met by abstracting water mainly from Kesem River and supplementing from Awash River.Frequent dry spells and droughts exacerbate the incidence of crop failure and hence food insecurity and poverty.To meet the ever increasing demand for food, sugar, fiber and other agricultural products, there is a need for increasing the land area under production and for intensifying agricultural productivity mainly through expanding new irrigation schemes.The sugarcane plantation will be utilized as input for the new sugar factories to be established in middle Awash plains by the Ethiopian Sugar Corporation (WIC, 2012).
In view of the above premises, the present feasibility study was carried out at middle Awash.
The study was intensive and multidisciplinary covering wide ranges of natural and socioeconomic issues of the project area.In this paper, it is attempted to briefly present characteristics of soils of Kesem Allaideghe plains with respect to its potentiality for sugarcane cultivation.

Description of the Study Area
The project area is located at the right bank of Awash River in the middle of Awash basin in Afar National Regional State, between 9 o 07' to

Field Investigations
In order to characterize the project area with respect to physicochemical and hydro-dynamic soil properties, a detailed soil survey (FAO, 1979;FAO, 1985;Landon, 1991) on 500m x 500m grid was carried out.Prior to field investigation, land satellite images (EMA, 2005) on 1:50000 scale were collected and interpreted.Topographical maps at a scale of 1:50,000 depicting slopes and other relevant features were also studied.Auger-hole studies were made to distinguish soil variation in lateral directions using a grid of 500m x 500m; i.e. one auger-hole observation for every 25 ha area with a total number of 1520 auger holes covering 38,000 ha area.
Each auger-hole observation point was recorded by hand-held GPS.The salient soil characteristics including horizon-wise color using Munsell color chart, texture, consistence, structure (feel method), presence of CaCO 3 through HCl reaction; EC (1:2.5) and pH (1:2.5)soil-water using portable digital EC and pH meter, respectively, were observed at each auger-hole site.
In order to characterize the soil, particularly for irrigation purpose, soil profile pits were excavated

Infiltration Rate
The infiltration rate is exceedingly important for selection of suitable irrigation methods, design of irrigation system and water management techniques.In the present study, infiltration rate was measured in situ using double ring infiltrometer at 28 representative sites each in triplicate.The UTM coordinates, north and east Figure 1.Location map of the study area for each location, was recorded using hand-held GPS.The infiltration rates were computed for three quantities as cumulative intake (cm), average infiltration rate (cm h -1 ) and basic infiltration rate (cm h -1 ).

In-situ Hydraulic Conductivity
In the absence of groundwater table, inverse augerhole method (Kessler and Oosterbaan, 1974) was used to determine hydraulic conductivity in field at twenty one representative locations replicated three times for active rooting zone depth of 1m.
The procedure in brief included: drilling of hole of 8 cm diameter using post-hole auger to a depth of 1 meter.Care was taken to drill the hole as straight as possible.The hole was saturated over night.Water was poured in wetted hole and about 2/3 of depth of hole was filled with water.The rate of falling of water in hole was measured using a specially designed water level indicator assembly consisting of two electrode terminals, flexible wire, and an attached multimeter.The readings of fall of water level against time were recorded till steady state rate was achieved.Generally, the last three consecutive readings showed stable rate.Hydraulic conductivity (k) was computed using following relation: k=1.15r tan α, cm sec -1 (Kessler and Oosterbaan, 1974) where k = hydraulic conductivity, cm sec -1 r = radius of hole i .e 4 cm α =slope of straight line by plotting water falling depth against time on a semilogarithmic paper.
The units of hydraulic conductivity were changed to m d -1 by multiplying with a suitable factor.UTM reading was recorded for each location of hydraulic conductivity test in the field.

Laboratory Studies
For mechanical and chemical analysis, the soil samples were dried, ground and passed through a 2 mm standard sieve.Mechanical analysis (particle size distribution) was carried out by the modified hydrometer method (Sahlemedihin Sertsu and Taye Bekele, 2000).Soil pH and electrical conductivity (EC) of saturated paste extract were determined by pH and EC meter, respectively (Schoffied and Taylor, 1955); organic carbon by wet digestion method (Nelson and Sommers (1982); cation exchange capacity using ammonium saturation method; exchangeable sodium percentage (ESP) using analytical procedure as described in diagnosis and improvement of saline and alkali soils (Richards, 1969;FAO, 1988).

Infiltration Rate
Infiltration is one of the most important parameters which is considered in designing the optimum stream-size of furrow irrigation for sugarcane crop.In order to characterize the intake behavior of project lands, infiltration was measured in triplicates at twenty eight representative locations using double ring infiltrometer.Infiltration characteristics are expressed in three quantities as cumulative infiltration, average infiltration rate and basic infiltration rate (Table 1).The cumulative and average infiltration rates plotted on log-log scale are expressed by equations as Y cum = ax n and Y ave = ax -n , respectively.The basic infiltration rate is a stable rate arrived after 4 to 5 hours, the wetting front reaching a depth of 35 to 50 cm.Thus, infiltration rate has practical significance in recharging soil root zone depth.
The basic infiltration rate of soils of the project area generally varied from 0.2 to 6.2 cm h -1 , i.e. slow to moderate (Landon, 1991).This could be due to high clay content and relatively high organic matter attributed to the no tillage operations, which restricted the oxidation of organic matter.
Owing to the low hydraulic conductivity, soils are less permeable, and such soils are thus classified as poorly to imperfectly drained.This is one of the most important parameters, which downgrades the suitability of these soils for irrigated sugarcane cultivation.(2004) for soils of the Rift Valley.
In most of the soils of the study area, ECe and ESP indicate the occurrence of higher level of sodicity as they ranged between 0.9 to 8.0 dS m -1 and 9.9 to 42.7%, respectively.Results indicated that ESP values above 15% cover large area of the project that revealed the occurrence of high sodicity in soil.As a matter of fact sugarcane is not tolerant to sodicity where there is considerable risk of yield reduction and crop cultivation is not commercially viable.In addition to the effect of the salt on yield, soils having high sodicity have poor soil structure that affect crop performance (Mass and Gratton, 1999;FAO-UNESCO, 1973;FAO, 1979a) 9 o 26' N latitude and 40 o 30' to 40 o 50' E longitude and bordered by the main road from Awash Arba to Tendaho (Figure 1).The area is subjected to intensive grazing and comprised of old alluvium and colluvium deposits.The slope of the command area is gentle ranging from 0.01 to 0.05 m km - 1 ; i.e., almost flat topography.The maximum and minimum mean monthly temperatures of the project area are 38 o C in June and about 15 o C in December.The annual pan evaporation rate is 2400 mm (NMA, 2005).The land use of the entire area is virgin grazing land which has never been cultivated.
to a depth of 2 m with 1 m width and 2 m length.The number of soil profile pits observations at representative sites covering entire command area were 76 (one profile per 500 ha of area).The chemical analysis using standard techniques was carried out for pertinent soil properties in soil laboratories of the Ministry of Water Resources, Addis Ababa, and of Wonji Sugar Factory.The major physical and chemical properties analysis of model profiles included dominant textural classes, soil reaction (pH), ECe, ESP, CEC, organic carbon, soluble cations and CaCO 3 .

Table1:
Infiltration Characteristics (respective average of three replicates) at representative sites of project

Table 2 :
Results of hydraulic inductivity tests (respective average of three replicates) conducted at representative locations in the project area.

Table 3 :
Major physicochemical properties of soils of the project area Note: HC-heavy clay; C-clay; SiCL-silty clay loam; CL-Clay loom; Si-Silt