Effects of level soil bunds and stone bunds on soil properties and its implications for crop production: the case of Bokole watershed, Dawuro zone, Southern Ethiopia

doi:10.4236/as.2011.23047

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Vol.2, No.3, 357-363 (2011)

doi:10.4236/as.2011.23047

Agricultural Scienc es

Effects of level soil bunds and stone bunds on soil

properties and its implications for crop production: the

case of Bokole watershed, Dawuro zone, Southern

Ethiopia

Kebede Wolka1, A wdenegest Moges2, Fantaw Yimer1

1Wondo Genet College of Forestry and Natural Resources, Hawassa University, Shashemene, Ethiopia; Corresponding Author:

kebedewolka@gmail.com; fantawyimer2003@yahoo.com

2Biosystems and Environmental Engineering Department, Hawassa University, Shashemene, Ethiopia; Corresponding Author:

awde_moges@yahoo.co.uk

Received 23 March 2011; revised 21 May 2011; accepted 31 July 2011.

ABSTRACT

Level soil bunds (LSB) and stone bunds (SB)

have been widely implemented in the Bokole

watershed since 2000 through support of the

World Food Program (WFP). However, the per-

formance of them against the target of the

structure has not been studied. This study ana-

lyzed the effects of LSB and SB on selected soil

properties, when compared with nonterraced

cropland. The Bokole watershed was divided

into two units. From upper watershed, three

croplands with LSB (aged 4, 6, and 9 years) and

three nonterraced croplands each adjacent to

one of the LSB were selected. Similarly, in lower

watershed, SB aged 4, 6, and 8 years and three

nonterraced croplands each adjacent to one of

the SB were selected. From each cropland with

LSB and SB, three composite soil samples (rep-

licates) were collected systematically in X de-

signed rectangular plot. From each nonterraced

cropland, three composite soil samples (repli-

cates) were collected in X designed square plot.

A total of 36 soil samples were analyzed for Soil

Organic Carbon (SOC), Total Nitrogen (TN),

Available Phosphorus (AP), Available Potas-

sium (AK), pH, and Cation Exchange Capacity

(CEC) following standard laboratory procedures.

Most soil parameters were not significantly dif-

ferent in cropland with LSB and SB comp ared to

nonterraced. However, in LSB aged 4 years and

SB aged 6 years AP and pH were significantly

less than their adjacent-nonterraced cropland.

In SB aged 8 years, SOC, AP, AK, and pH were

also significantly less than adjacent-nonter-

raced cropland. Past erosion, and past land

uses are likely factors contributed to the ob-

served result. It was inferred that the mean con-

tribution of LSB and SB alone for crop produc-

tion with regard to analy zed so il p a rameters was

not significant in the considered sites. Addi-

tional soil fertility management practices should

be incorporated for bett er effect.

Keywords: Crop Yield; Level Soil Bund;

Nonterraced; Soil Fertility; Stone Bund; Water

Erosion; Watershed

1. INTRODUCTION

Human activity, such as conversion of forests to agri-

cultural land, increased cultivation of marginal land,

overgrazing, and low-input or fertility-mining methods

of subsistence agriculture practiced on marginal lands

with steep gradients; accelerate soil erosion [1-3]. The

sorting action of erosion removes large proportions of

the clay and humus from soil, leaving behind the less

productive coarse sand, gravel, and in some case even

stones, impairing the quality of the remaining topsoil [4-

8]. The removal of this organic matter affects soil prop-

erties including texture, structure, nutrient availability

and biological activity [5,6] and makes soil more sus-

ceptible to further erosion as its aggregates becomes less

stable [9] thus, negatively affecting crop production [1,

8-11]. In Ethiopia, measurements from experimental

plots and micro-watersheds showed the annual soil loss

from croplands is about 42 t·ha–1·year–1 [12]. As a con-

sequence, the productive capacity of Ethiopia’s highland

soils is being reduced at an annual rate of 2% - 3%,

which certainly contributes to a higher vulnerability to

K. Wolka et al. / Agricultural Science 2 (2011) 357-363

358

famine [13].

In the agricultural production system of the Ethiopian

highlands, it is not possible to maintain year-round

vegetation cover under given ecological, economic, and

social circumstances [8,14]. According to these re- sear-

chers, structures such as the stone bunds are an indis-

pensable component of soil and water conservation

(SWC) measures for the control of erosion. It was also

reported that plots with stone bunds are more productive

than those without such technologies in semi-arid areas.

This is apparently due to the moisture conserving bene-

fits of this technology being critical in drier areas [15].

Climate change, which alters precipitation patterns and

intensities, is believed to substantially increase the risk

of runoff, soil erosion, drought and other environmental

problems. SWC measures such as soil bunds and stone

bunds are adaptation options to mitigate the problems

caused by climate change [16].

Since 2000 [17], government and nongovernment or-

ganizations such as World Food Programme (WFP) have

been promoting agricultural production through envi-

ronmental rehabilitation in the degraded Bokole water-

shed. Construction of level soil bunds and stone bunds

have been the major program activities on the watershed

croplands [17]. But the long-term effects of those inter-

ventions on production have not been investigated. This

Figure 1. Location of the study site.

K. Wolka et al. / Agricultural Science 2 (2011) 357-363

359359

study assessed the effects of Level Soil Bund (LSB) and

Stone Bund (SB) on selected physical and chemical

properties of soil.

2. MATERIALS AND METHODS

2.1. Description of the Study Area

The study area is situated in Southern Nations, Na-

tionalities and Peoples Regional State (SNNPRS) of

Ethiopia at Dawuro zone, Loma woreda. Geographically,

it lies between 6˚55′N and 7˚01′30″N latitude and 37˚15′

E and 37˚19′E longitude. It is at about 470 km in south

west of Addis Ababa, the capital of Ethiopia. The study

area topography is undulating and rugged (Figure 2).

The watershed drains to the Omo River.

The study area lies between 1160 and 2300 m above

sea level and receives 1400 mm - 1600 mm rainfall an-

nually. The mean temperature ranges from 15.1˚C to

27.5˚C [18]. The soil is grouped as Orthic Acrisols [18].

Mixed agriculture is the major economic activity in this

watershed. The population of Bokole watershed is

11,798 (of which 3832 reside in the upper watershed and

7936 in the lower watershed) [19].

2.2. Methodology

Before soil sampling, two group discussions were

conducted (one in upper watershed and the other in

lower watershed, each comprises 10 farmers) with pur-

posively selected farmers from different part of the wa-

tershed on effect of LSB and SB on soil properties. Indi-

vidual farmers from whose land soil samples collected

were thoroughly interviewed on observed changes as

result of LSB and SB. Soil samples were collected from

purposively selected croplands/plot with LSBs (aged 4, 6

and 9 years), SBs (aged 4, 6, and 8 years) and nonter-

raced (adjacent to each LSB and SB). The specific sam-

pling sites were selected based on criteria: closeness of

treated and nonterraced croplands (not too far from each

other) [11]; age of the structure (4 - 9 years); number of

inter-structure plots per treated cropland (more than or

equal to 5); similarity of treated and adjacent-nonter-

Figure 2. Partial view of the topography of the study site; Up-

per watershed (a) and lower watershed (b).

raced croplands in certain management and natural set

up. Croplands with LSBs were selected from upper wa-

tershed whereas croplands with SBs were selected from

lower watershed because of the structures dominance.

All the selected croplands have been under farmer man-

agement.

The sampling plots in LSB and SB were replicated

three times systematically: the first, third and fifth plots

(inter-structure area) in between two successive LSB and

SB were selected in each treated cropland. From each

sampling plot, a composite sample from the four corners

and one at the center of an X designed rectangular plot

were taken. The X designed rectangular plot has two

sides along the contour with length of 15m each and

adjusted at 1.5 m away from LSBs and SBs [20]. By

repeating those procedures on all the selected croplands

with LSB and SB, a total of 18 composite samples (2

types of structures (LSB and SB) × 3 croplands treated

with each type of structures × 3 inter-structure area or

plot from each LSB and SB) were collected using auger

from depth of 0 - 20 cm.

From nonterraced croplands, three composite sam-

ples—replicates (from upper, middle and lower slope

position within the field) were collected from 15 m × 15

m X designed square plot. By repeating this procedures

18 (2 part of watershed that is upper and lower × 3 crop-

lands in each part of the watershed × 3 sample per crop-

land) were taken using auger at 0 - 20 cm depth.

Each time after sampling (from treated and non-ter-

raced), soil clods in samples was thoroughly broken to

make a uniform mix in clean plastic bucket. The sample

was divided in to four equal parts from which two di-

agonal parts were retained and other two removed. This

was continued until sufficient sample was retained in the

field. Samples were air dried at room temperature, ho-

mogenized and passed through a 2 mm sieve. Soil tex-

ture, Soil Organic Carbon (SOC), Total Nitrogen (TN),

Available Phosphorus (AP), Available Potassium (AK),

Soil pH (pH), and Cation Exchange Capacity (CEC) of

the prepared samples were analyzed following standard

laboratory procedure at City Government of Addis

Ababa Environmental Protection Authority. T-tests were

used to compare soil properties variation [21].

3. RESULTS

3.1. Soil Physical and Chemical Properties

The silt and clay fractions showed significant differ-

ence (P < 0.05) in croplands under LSB aged 6 year

when compared with adjacent-nonterraced croplands

while the LSBs aged 4 and 9 year was showed no sig-

nificant difference in any fractions compared to adja-

cent-nonterraced cropland (Table 1). In the SB aged 4

K. Wolka et al. / Agricultural Science 2 (2011) 357-363

360

Table 1. Mean values (± SEM) of sand, silt and clay fractions of topsoil (0 - 20 cm) from croplands with LSB aged 4, 6, and 9 years

and the respective adjacent-nonterraced (NTU) at the upper watershed (n = 3).

Soil texture LSB-4 year NTU-1 P-value LSB-6 yearNTU-2 P-valueLSB-9 year NTU-3 P-value

Sand (%) 47.17 ± 0.75 43.83 ± 1.89 0.132 47.17 ± 1.7647.83 ± 1.6 0.785 46.00 ± 2.7 47.8 ± 3.000.659

Silt (%) 17.17 ± 0.60 18.67 ± 0.80 0.166 25.67 ± 1.8019.33 + 0.95 0.011* 18.00 ± 1.00 18.17 ± 0.980.908

Clay (%) 35.67 ± 0.56 37.50 ± 1.31 0.227 27.17 ± 1.4532.83 ± 1.72 0.030* 36.00 ± 2.58 34.00 ± 2.130.563

Textural Classes Sandy clay Clay loam Sandy clay

loam Sandy clay loam Sandy clay Sandy

clay loam

*denotes significantly different values from each other at P < 0.05 by 2-tailed t-test

Table 2. Mean values (±SEM) of sand, silt and clay fractions of topsoil (0 - 20 cm) croplands with SB aged 4, 6, and 8 years and the

respective adjacent-nonterraced (NTL) at the lower watershed (n = 3).

Soil texture SB-4 year NTL-1 P-valueSB-6 year NTL- 2 P-valueSB- 8 year NTL- 3 P-value

Sand (%) 55.67 ± 1.31 59.67 ± 1.43 0.066 49.17 ± 1.4053.50 ± 1.630.071 48.67 ± 2.12 47.83 ± 1.64 0.763

Silt (%) 14.00 ± 1.39 16.83 ± 0.60 0.091 20.33 ± 0.6717.33 ± 0.950.028*23.67 ± 1.61 23.33 ± 1.20 0.871

Clay (%) 30.33 ± 1.10 23.50 ± 1.34 0.003*30.5 ± 1.1829.17 ± 1.510.503 27.67 ± 0.95 28.83 ± 1.05 0.429

Textural Classes Sandy clay loam Sandy clay loam Sandy clay

loam

Sandy clay

loam Sandy clay

loam Sandy clay loam

*denotes significantly different values from each other at P < 0.05 by 2-tailed t-test

Table 3. Mean values (± SEM) of SOC, TN, AP, AK, pH, and CEC of topsoil (0 - 20 cm) croplands with LSB aged 4, 6, and 9 years

and the respective adjacent-nonterraced (NTU) at the upper watershed (n = 3).

Soil parameters LSB-4 yearNTU-1 P-valueLSB-6 yearNTU-2 P-valueLSB-9 year NTU-3 P-value

SOC (%) 1.23 ± 0.151.32 ± 0.23 0.736 1.41 ± 0.281.18 ± 0.110.4631.57 ± 0.27 1.68 ± 0.15 0.715

TN (%) 0.11 ± 0.020.07 ± 0.02 0.275 0.21 ± 0.130.12 ± 0.040.5150.08 ± 0.02 0.17 ± 0.06 0.138

AP (ppm) 12.04 ± 0.5616.87 ± 1.94 0.037*5.86 ± 0.953.94 ± 1.660.13110.5 ± 2.70 5.68 ± 0.39 0.105

AK cmol (+)/kg soil) 0.15 ± 0.0030.21 ± 0.04 0.138 0.23 ± 0.060.31 ± 0.130.6000.20 ± 0.06 0.14 ± 0.004 0.326

pH 5.26 ± 0.155.64 ± 0.04 0.034*5.87 ± 0.105.34 ± 0.230.0685.83 ± 0.07 5.93 ± 0.08 0.383

CEC cmol (+)/kg soil) 20.57 ± 2.6329.75 ± 3.48 0.062 17.16 ± 2.1021.27 ± 4.030.38016.75 ± 1.57 18.33 ± 1.53 0.487

*denotes significantly different values from each other at P < 0.05 by 2-tailed t-test

Table 4. Mean values (± SEM) of SOC, TN, AP, AK, pH, and CEC of topsoil (0 - 20 cm) croplands with SB aged 4, 6, and 8 years

and the respective adjacent-nonterraced (NTL) at the lower watershed (n = 3).

Soil parameters SB-4 year NTL- 1 P-valueSB-6 year NTL-2 P-valueSB-8 year NTL-3 P-value

SOC (%) 1.57 ± 0.16 1.22 ± 0 .12 0.101 1.35 ± 0.141.10 ± 0.120.206 0.69 ± 0.11 1.01 ± 0.090.048*

TN (%) 0.28 ± 0.08 0.13 ± 0. 03 0.098 0.09 ± 0.060.06 ± 0.020.619 0.41 ± 0.13 0.26 ± 0.120.409

AP (ppm) 2.92 ± 0.65 4.41 ± 0.46 0.089 1.82 ± 0.137.33 ± 1.740.010*10.62 ± 1.71 27.48 ± 6.200.026*

AK cmol (+)/kg soil) 0.45 ± 0.08 0.44 ± 0.02 0.906 1.39 ± 0.300.83 ± 0.150.123 0.59 ± 0.29 1.61 ± 0.320.039*

pH 7.06 ± 0.0347.13 ± 0.02 0.090 6.34 ± 0.056.63 ± 0.0870.014*6.41 ± 0.08 6.64 ± 0.040.03*

CEC cmol (+)/kg soil) 25.70 ± 5.8521.80 ± 5.19 0.629 30.43 ± 4.4631.43 ± 3.520.864 31.57 ± 2.96 25.93 ± 6.850.468

*denotes significantly different values from each other at P < 0.05 by 2-tailed t-test

year, significantly greater clay content was observed as

compared to its adjacent-nonterraced cropland. Ta bles 3

and 4 shows result for soil chemical properties analysis.

4. DISCUSSIONS

4.1. Soil Properties in Croplands with SWC

Structures and Nonterraced

In the study site, SOC was generally less in soils of

the study area which might be due to the practices such

as intensive tillage, continuous cropping, removal of

crop residues, and low organic carbon input in croplands.

[20] mentioned that SOC values are typically low in the

Ethiopian highlands as a consequence of stubble grazing

and the absence of fallowing.

The soil and water conservation system of LSB and

SB reduce surface runoff and soil loss, retain water that

enhances crop growth and contributes to SOC input.

Despite these all benefits, in most comparison no sig-

nificant difference in SOC observed. Even it was sig-

nificantly low (P < 0.05) in SB aged 8 year compared to

adjacent-nonterraced. This result showed that type and

intensity of other land management practices that indi-

vidual farmers adapted in the past has also role in current

level of SOC. The report by [22] around the Mediterra-

K. Wolka et al. / Agricultural Science 2 (2011) 357-363

361361

nean region indicated SOC significantly differ was not

agree with result of present study in that 50 years old

stone-walled terrace was compared to nonterraced.

AP was significantly high (P < 0.05) in nonterraced

croplands adjacent to LSB aged 4 year, SB aged 6 year

and SB aged 8 year. These were perhaps due to: the dif-

ference in the past land degradation resulting from con-

tinuous cultivation, extractive plant harvest and soil ero-

sion. Similar study by [23] in Anjeni watershed (Ethio-

pia) also showed that AP on nonterraced land was higher

than the terraced.

The relatively higher AK on nonterraced cropland ad-

jacent to SB aged 8 year was most likely because of the

difference in weathering process and past erosion. The

significantly low (P < 0.05) soil pH in LSB aged 4 year,

in SB aged 6 year and SB aged 8 year compared to the

respective adjacent-nonterraced cropland were probably

due to loss of relatively more basic cation resulted from

erosion before the structures built and did not restore yet

after the structures. Under a continuous cropping system

soil acidity increases due to the gradual replacement of

basic cations by aluminum [24]. Even though soil acidi-

fication is natural process involved in soil formation [7],

it is influenced by historic land use practice and subse-

quent soil erosion.

4.2. Relating Soil Analysis and Farmers’

Opinion

The LSB and SB have been accepted and introduced

to reduce soil loss by creating barrier against surface

runoff and reducing slope length and gradient in the

long-term. Farmers in thorough group discussion indi-

cated that structures improved crop yield. Those farmers

whose land the soil sample were taken also perceived

that the structure has improved crop yield when com-

pared to previous situation. However, most of the se-

lected chemical properties in cropland with structures

were not significantly higher than adjacent nonterraced.

Even for AP, pH, SOC, and AK in some sites, they have

been significantly less than nonterraced cropland. This

can be explained by the fact that, on one hand, natural

resource experts recommend the structures when the

croplands become degraded. Farmers also initially ac-

cept the structure when the land gets relatively denuded

and thus it takes time for soil to restore in the pace of

less or no additional soil fertility management. Refer-

ence [25] stated adoption of the SWC technologies is

likely to increase, among other things, with recognition

of the erosion problem and slope gradient. The tradi-

tional diversion ditches that have commonly been con-

structed on nonterraced cropland to expel surface runoff

and to reduce erosion might also have contributed for

reduction of erosion and subsequent effect on nonter-

raced croplands. Past land use and management and past

erosion and deposition also determine the current prop-

erty of soil.

SWC structures are practically used as support for

agronomic and soil management [26] and considered as

the first defense line. Thus, they alone are less likely to

improve soil properties significantly under similar man-

agement to nonterraced. Reference [27] has reported that

combining stone rows barriers to run-off with the appli-

cation of compost was significantly controlled erosion

and reduced organic C and nutrient losses than compost

or stone row alone. Related experimental study in Burk-

ina Faso [24] showed that stone lines, five years after

laying, have a limited effect on soil fertility and a drop in

soil pH, organic C, N, and P concentrations were ob-

served. Similarly, experiment in Maybar research site of

Ethiopia [14] on level soil bund without any agronomic

or biological techniques showed decrease in production

during the first 3 - 5 years, which showed probably no

improvement in soil fertility.

SWC structures such as stone bund traps finer soil

particles that were eroded by rain water and tillage from

inter-structure area. Thus fertile soil remains on the

structure and nearby at upslope side. On hand it might be

partly due to this reason that eight years after construc-

tion of LSB showed significantly less in most analyzed

nutrient concentration for sampled plot (1.5 m away

from structure) when compared with nonterraced.

The in-depth interview and discussion in the present

study watershed revealed that the respondents have posi-

tive attitudes towards introduced SWC structures by ob-

serving mainly crop performance [28]. This could proba-

bly be due to the fact that crop performance is not only

function of soil fertility but also due to water availability,

which most likely improved by water retention ability of

structures as observed by farmers. The explanation by

[29] confirms that evaluating soil fertility by crop yield

alone is not fair because crop performance is a function

of many factors including soil nutrient (fertility) itself,

soil water availability, and weed competition

5. CONCLUSIONS AND

RECOMMENDATION

Even though farmers perceived some improvements

on cropland after construction of SWC structures by

using their own criteria, the standard soil laboratory

analysis from croplands with LSB and SB and nonter-

raced did not show remarkable difference for some pa-

rameters and even less for some sites. Thus, without

withstanding the contribution of structures in reducing

surface runoff and erosion which is partially the cones-

quences of climate change, it can be concluded that the

contribution of LSB and SB alone with regard to im-

K. Wolka et al. / Agricultural Science 2 (2011) 357-363

362

proving soil properties for crop production are not sig-

nificant compared to nonterraced cropland in the con-

sidered site. The perceived improvement in crop produc-

tion was most likely due to water retention ability of the

structures which contributes for adaptation of climate

change.

It can be inferred that soil nutrient restoration for de-

graded land takes long time in the pace of continuous

cultivation and poor soil fertility management. It was

also suspected that most of soil along with nutrient

trapped by the structures deposited on the structure and

adjacent to it at distance range of 1.5 m in upslope area.

To make watershed management particularly LSB and

SB effective for attaining and sustaining food security in

smallholder farming, soil fertility management practices

such as use of crop residue and manuring are recom-

mended beside the structures in the study area. The role

and effectiveness of traditional diversion ditch, which

has commonly been practiced on nonterraced cropland,

in alleviating soil erosion should be studied.

6. ACKNOWLEDGEMENTS

The Center for Environment and Society project (at Hawassa Uni-

versity), Food for Work project (in SNNPRS, Agriculture and Rural

Development Bureau), and Wondo Genet College of Forestry and

Natural Resources, Hawassa University, are gratefully acknowledged

for the financial support.

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