Effects of Sustainable Land management (SLM) interventions on Soil Physical Properties (texture, bulk density and moisture content)
Soil Texture
Soil textural fractions such as Sand, silt and clay; soil bulk density and moisture content showed no significant difference with SWC treatments. The non- significant difference in texture may be due to the age of the implementation of watershed practice which was Eight years that can’t make significant change on weathering. The higher mean value of the sand content of the soil was 36.6% and 34.5% that recorded from Koriso and Jamjam respectively in untreated farm plots while the lower mean value was 26.4% and 26% respectively from Koriso and Jamjam in areas with SWC measures. The overall mean value of sand recorded from conserved farm plots was 29.65 ± 3.44 while that of unconserved farm plots was 31.76 ± 4.2. The mean value of sand contents were relatively greater in unconserved farms than conserved cultivation lands this, might due to soil aggregation greater in untreated for less contents of organic matter that minimize the sandy aggregates to contain moisture (Table 1).
Similar results were reported elsewhere, for example Terefe et al., (2020), Husen et al., (2017), Mulugeta and Karl (2010), Anshebo (2009), who reported higher mean value of BD in non-conserved plots than in the plot treated with SWC measures.
Table 1: Mean values of selected soil physical properties
Land use type
|
Variables
|
Sand
|
Silt
|
Clay
|
SMC
|
BD
|
Conserved land
|
29.65 ± 3.44
|
18.54 ± 0.88
|
51.79 ± 3.36
|
10.64 ± 1.73
|
1.23
|
Non-conserved land
|
31.76 ± 4.2
|
19.88 ± 1.74
|
47.6 ± 1.36
|
5.529±1.435
|
1.37
|
Overall mean
|
30.0 ± 3.66
|
18.2 ± 1.67
|
51.8 ± 3.26
|
8.096±3.14
|
1.3
|
LSD (0.05)
|
0.641
|
0.642
|
0.175
|
0.22
|
|
SEM
|
1.49
|
0.685
|
1.326
|
1.284
|
|
Slope Gradient
|
|
|
|
|
Upper (>30%)
|
34.27 ± 1.889
|
18.2 ± 1.621
|
47.525 ± 1.51
|
5.25 ± 0.884
|
|
Middle (15-30%)
|
31.125 ± 2.09
|
19.5 ± 2.415
|
49.875 ± 2.26
|
5.679±0.862
|
|
Lower (8-15%)
|
26.725 ± 1.56
|
19.95 ± 1.865
|
51.725 ±4.38
|
6.047±0.732
|
|
LSD (0.05)
|
0.001
|
0.467
|
0.189
|
0.371
|
|
SEM
|
1.053
|
0.567
|
0.932
|
0.871
|
|
The result of ANOVA revealed highly significant differences (P = 0.001) for sand at landscape positions. The recorded mean value of sand for upper (>30 %), for the middle (15-30%) and lower is (8-15%) which were 34.27 ± 1.889, 31.125 ± 2.09 and 26.725 ± 1.56 respectively. This result is in agreement with (Tiki, et al., 2015, Ademe et al., 2017). Soils of the conserved farm land with SWC measures had the highest percent clay compared to the soils of the untreated/unconserved with SWC measures. However, sand and silt percent were lower in land treated with SWC measures. The sand contents of the soil of cultivated farm plots decreased from upper to lower landscape positions. This might be due to the top fertile soil is removed and deposited on the lower slope mixed with the former sand present on gentle slope change parts of sandy to clay loam and silt. High moisture content is rather present in the lower 6.047±0.73 landscape position compared to the upper and middle one, this enhance better condition for decomposition.
The Silt contents of the recorded results from the two micro-watersheds was not significantly varied (p>0.05) among conserved and unconserved with SWC measures farm plots as well as across landscape position. The mean value of silt contents of soil recorded from conserved farms of both micro watersheds was 18.54 ± 0.88, while that of untreated cultivation field was of 19.88 ± 1.74. The silt contents of cultivated field untreated with SWC practices relatively smaller than that of untreated farmlands which might be due to farm plots treated with conservation structures contain greater organic matter contents there by decomposer breakdown residue (litters) then sandy and silt contents changed to clay loam and also bulk density reduction indicated soil compaction reduced thereby micro-organisms freely moves in soil horizon to decompose immobilized nutrients and improve soil structures and texture.
There was no significant difference of soil texture in slope position except sandy soil. The mean value of silt contents of both micro watersheds along upper, middle and lower was 18.2 ± 1.621, 19.5 ± 2.415 and 19.95 ± 1.865 respectively (Table 1). The recorded results indicated that the mean value of the silt contents from both micro watersheds of similar slope position from upper streams to lower streams relatively decreased, because the silt is very fine particle size that formed from sediments deposited in the lower sides and large mass of grass cover and residue present on the lower side that increase fine particles so that silt contents increased.
The result depicted that the farm plots mainly dominated by clay contents (Table 1). The mean value of clay contents of the farm plots was relatively lower in farm plots with SWC measures than that of without SWC measures. The mean value of clay contents of soil in conserved cultivation farm plots from both watersheds was 51.79 ± 3.36 while that of non-conserved cultivation field was 47.6 ± 1.36. The farms mainly dominated by clay contents in conserved plots than that of non-conserved due to large mass of sand and silt mixed with organic matter and became finer soil particles and also change normal clay to larger clay loam textural class by disintegrate clay colloids to finer soil available to plant growths.
The clay contents of the soil from both micro watersheds relatively increase through slope gradients (from upper to lower). The mean value of clay contents across slope position from upper (>25%), Middle (20-25%) and lower side (15-20%) was 47.525 ± 1.51, 49.875 ± 2.26 and 51.725 ± 4.38 respectively (Table 1). The presence of higher clay fraction in the lower slope might be due to larger deposition of silt and sand mixed with organic matter then large mass of clay and clay loam that mainly used for crops growth.
Soil Bulk Density
The soil bulk density of the study areas were significant (p<0.05) between conserved and unconserved farm plots with SWC measures (Table 1). Similarly, Aşkin and Özdemir (2010); Chaudhari et al. (2013), indicated that soil bulk density is significantly influenced by sand content more than other soil properties. The overall mean of soil bulk density of the study areas covered with SWC practices at effective soil depth (0-20cm) was lower than that of the areas not treated (non-conserved) with structures. The untreated plots were found to exhibit significantly higher mean value of BD than treated plots at both sites. Lower soil bulk density of 1.23 gcm-3 was observed in treated farm plots as compared to untreated farms plot which was 1.37 gcm-3 (Table 1), which might be due to soil bulk density increase with subsurface compaction and also due to the presence of significantly higher organic matter and moisture availability differences in conserved farms.
The finding is in agreement with Husen et al., (2017), Challa et al.(2016); Bezabin et al. (2016); Demelash & Karl (2010), who reported that the mean value of bulk density in conserved areas with SWC practice was lower than that of unconserved areas mainly due to the decomposition of plant biomasses on the conserved field increase organic matter contents which reduces soil bulk density. Alemayehu and Fisseha (2018), also, found higher bulk density in untreated farm land than the conserved farm land in Ethiopia. Heuscher et al. (2005) described soil bulk density, has inversely proportional relationship with soil organic matter. Land management practices like SWC can accumulate soil organic matter and modify soil properties such as bulk density and this innovation have agreement with present study (Amare et al., 2013). Low bulk density was observed for conserved crop lands than non-conserved one (Haweni, 2015).
Soil Moisture Content (SMC)
Soil moisture contents of the study areas has shown significant (p<0.05) variation between Conserved and unconseved land with SWC measures (Table 1). The highest SMC recorded from conserved farm plots with SWC was (10.94) and with non-conserved farm plots (5.87%) both from Jamjam micro watershed. which may be a result of water conservation structures which reduces runoff and evaporation and increases infiltration and soil moisture content (Tiki, et al., 2015, Stroosnijder and Hoogmoed, 2004).
The overall mean of SMC recorded on conserved areas was 10.64 ± 1.73 while 5.53 ± 1.43 from non-conserved farm plots might due to slope length shorten by structures that makes barrier to run off and enhance soil water holding capacity thereby fill soil pores with moisture within the conserved areas (Table 1).The finding is in line with Challa et al. (2016) who stated moisture contents of farms land with SWC practices was higher than that of cultivation farms without any conservation structures. The area covered with improved soil bunds have higher infiltration capacity than cultivation fields without bunds due to runoff reduction for decreased slope length and allow longer time for infiltration on conserved areas with bunds in Melka watershed (Anania, 2015). Therefore, improving infiltration to make rain-water available for plant uptake, erosion control and fertility management practices are necessary (Vancampenhout et al., 2006).
The variation of SMC was not significantly different (p>0.05) in relation to slope. The result showed that SMC is higher in the lower slope (8-15%), 6.047± 0.732 followed by middle slope (15-30% and upper slope position (>30%) with value of 5.679 ± 0.862 and 5.25 ± 0.884 respectively (Table 2) because of organic matter contents of the study areas increase from upper steep slopes to lower parts of the watersheds. The area having larger organic matter contents has ability to capture moisture. Similar to Haweni, (2015) moisture availability in lower slope indicated greater in the upper and middle slopes of untreated farms land that might be related to accumulation of moisture in the lower which eroded from upper slope position. Soil moisture is necessary for the absorption of nutrients and increase yield (Abdzad et al., 2014).
Soil chemical Properties
There were significant differences for selected soil chemical properties at micro watersheds for conserved and unconserved and landscape positions at p<0.05.
Soil pH
The mean value of soil pH recorded was significantly different (p<0.05) among conserved and non-conserved. The maximum and minimum pH value recorded in the study area was 6.735 and 4.265. The mean pH value recorded in conserved areas were 6.33 ± 0.36 while that non-conserved were 4.97 ± 0.45 (Table 2) which might due to more cation ion (Hydrogen ions (H+) release from non-conserved areas as a result of leaching than that of conserved farm plots. It might also be due to more residue and grass left on conserved areas that used to maintain organic matter. In general, the mean value of soil pH recorded in the cultivation fields was 5.65 ± 0.84 which is followed by soil pH rating by Hazelton, and Murphy, (2007) and (Tekalign and Haque, 1991) that the pH value fallen in moderately acidic which is favorable for growth of crops.
Similar to Worku, (2017), the mean value of soil pH were lower in non-conserved farm land as compared to conserved farms due to leaching of cations in controlled farm plots for the absence of SWC practiced used to trap soil as well as lower ground cover in the farms as compared to the conserved farm plots.
The relatively higher mean pH on soil bunds than the control (non-conserved plot) may be explained by the difference in the extent of soil loss between cropland treated with conservation measures and those merely cultivated without any means of protection at least to keep the soil in place (Bezabih, 2015). Similar finding was reported by Bezabih et al., (2016), in which the lowest value of soil pH in cultivated land in untreated with conservation structures, which can be due to result of high microbial oxidation which produce organic acid, soil erosion processes as well as basic cations depletion.
The statistical analyses revealed that there is no significant difference in pH levels between slope positions at p<0.05. The mean pH value was lower in the upper slope (<30%) which was pH 5.25 ± 0.88, in middle (15-30%), pH 5.67 ± 0.86 and higher in the lower slope (8-15%) which was pH 6.05 ± 0.73 (Table 2) that might be attributed to some organic matter removal from steep slope and deposited on the lower side. Similar to such as Bekele et al. (2016) found pH value was lower in steep slopes and higher in gentle slopes due to the fact that the high rainfall coupled with steeper slope might have increased leaching, soil erosion and a reduction of soluble base cations leading to higher H+ activity.
Total Nitrogen (TN)
The result has shown that TN contents of the soil in both (Jamjam laga batu and Koriso odo guba) selected areas were significantly different (p<0.01) with conserved and non- conserved watershed as well as along slope gradients. The overall mean contents of TN under the conserved land 0.228 ± 0.091% and non- conserved land 0.154 ± 0.012 %. This is because the area covered with structures treated with biological measures that are used to conserve soil such as Acacia spices and Sasbania sesban that is used as fodder and have nodule on their roots that are used in fixation of nitrogen. The higher TN recorded on conserved area was 0.247% and the lower content of soil nitrogen identified was 0.137% mainly in the upper parts of the watershed areas (Table 2).
This study is in line with other finding (Mulugeta & Karl, 2010; Anania, 2015; Keberku, 2017), who reported that farmland with physical SWC measures have high TN as compared to the non conserved land. In general, TN content of a soil is directly associated with its Organic Carbon (OC) content and become lower in continuously and intensively cultivated and highly weathered soils of the humid and sub humid tropics due to leaching and then low OM content (Tisdale et al., 1995; Haweni, 2015). The result of this study is also in line with the report of Haweni, (2015) who stated total nitrogen in conserved lands of Dimma watershed was higher than the total nitrogen content in the corresponding sites without conservation measures and Shafi et al., (2019) who reported an increment of total nitrogen in conserved soil of Ezha District.
There was also significant difference in TN (p<0.01) in relation of slope. The mean value of TN higher in the lower slope (8-15%) was 0.205 ± 0.044 % followed by middle slope (15-30%) and upper slope position (>30%) with value of 0.192 ± 0.046 % and 0.177 ± 0.039 % respectively (Table 2) which might be because of the removal of top fertile soil which contain organic matter from upper stream and deposited at lower parts of the watersheds. Following Landon, (2014) the overall mean contents of the study areas was low (0.191 ± 0.043%) which need nitrogen recommendation for the areas.
Soil Organic Matter (SOM) and Soil Organic Carbon (SOC)
Results of the study indicated that there was significant difference in SOM contents between conserved and non-conserved areas in the watersheds. The higher OM contents recorded from conserved areas were 5.983% and 4.584% while the lower were 2.930% and 3.204% in Jamjam and Koriso micro watersheds respectively. The overall mean recorded in conserved areas was 4.915 ± .47 % and 3.404 ±.473 % in non-conserved areas might be due to loss larger mass of effective soil depth by erosion from non-conserved farm plots (Table 2).
The mean value of SOM was not significantly different across slope position. The recorded SOM in the upper (>30%) was 3.8 ± 0.86 % , in the middle (15-30%) was 4.06 ± 0.99% and at the lower part (8-15%) was 4.16 ± 0.97% that indicated SOM increased from upper to lower might due to greater available soil condition to convert litter and other cover crops to soil. This study is in agreement with Kediro, (2015), who stated organic matter content of the soil increased down the slope both conserved and no-conserved suggesting the accumulation of humus-rich fine particles eroded from upper slopes and levels of increasing in OM contents down the slope were higher in the treated fields suggesting the accumulating of sediments behind the conservation structures.
The depletion of SOC as a result of soil degradation within intensified agricultural systems can lead to loss of nutrients and soil structure, loss of soil resilience, a loss of soil biodiversity, and disruption of key biotic and abiotic processes necessary for productivity (Lal, 2015). The mean value of organic carbon (OC) obtained was significantly affected (p<0.05) between conserved and non-conserved cultivation plots. The overall mean of SOC recorded in conserved farms was 2.789 ± 0.2263 while that of non-conserved areas was 1.974 ± 0.275 % (Table 2). The mean value of carbon contents of soil in conserved areas relatively greater than that of non-conserved might due to greater land cover by residues as mulching thereby greater carbon sequestration (carbon stock) than that of non-conserved where severity of erosion case land left bare and soil carbon contaminate with air and react and released to environment. . The study agree with that of Gebreselassie et al. (2009), Wolka et al. (2011) and Shafi et al., (2019) who reported the presence of higher SOC in the field with different conservation measures
The mean value of SOC was not significantly different among upper, middle and lower parts of selected watersheds. The mean value of SOC recorded from upper (>30%), middle (15-30%), and lower (8-15%) were 2.272 ±.579 %, 2.356 ±.574 % and 2.675± 0.564 % respectively (Table 2). The value recorded relatively decreased down slope might be due to degree of residue and grass cover of soil surface is higher on the lower parts.
Soil Electric Conductivity (EC)
Fertile soil with high amount of mineral compounds will have high conductivity while depleted soil with less minerals will have lower conductivity and soil conductivity also depends on types of mineral salts present. The result of the study showed that there was no significant variation (p<0.05) between mean value EC of soil in the conserved and non-conserved as well as across slope position on the farmers’ farm plots. The mean value of soil electric conductivity recorded on conserved cultivation plots was 0.0485 ± 0.015 dS/m and 0.0402 ± 0.005dS/m in non-conserved (Table 2). The mean value of EC recorded from conserved farm is relatively greater than the mean recorded on non-conserved areas might due to soil acidity minimized for the leaching of cations (H+) in the conserved farm plots.
The finding is similar to the finding of Anania, (2015) who reported that the higher electrical conductivity in soil obtained from control farm plot might be due to higher clay content than that of a farm with soil conservation. Gankiso, (2017) also reported the mean value of EC recorded from treated with SWC measures was greater than that of the EC recorded from non-conserved farm plots. The electric conductivity measurement detects the amount of ions in the solution; the greater the amount of cations, the greater conductivity reading.
The overall mean value of EC recorded from the upper stream, Middle and lower of both watersheds were 0.0384 ± 0.0055dS/m, 0.0449 ± 0.006 dS/m and 0.0499 ± 0.006 dS/m respectively (Table 2). The mean value of EC increased from upper to lower slope position since soil pH has positive correlation with soil EC.The increases were due to erosion and leaching of soluble salts from the upper slope and accumulation at the down-slope land positions (Olarieta et al., 2008). The overall mean value of EC recorded in the study areas was 0.045 ± 0.0073dS/m so that soil of the selected farm plots salt free following Scherer (1996) of rated electric conductivity.
Available Phosphorous (Av.P)
The results indicated that Av.P was significantly different (p<0.05) with the conserved and no-conserved farm plots. The relative higher value of Av.P that recorded from conserved farms (Table 2) was 7.481 mg/Kg and the lowest value from non-conserved cultivation plots was 4.358 mg/Kg. The mean value of Av. P in soil under plots with conservation structures (from both Jamjam and Koriso) was 6.627 ± .77 mg/Kg while the mean value of Av. P in non-conserved farm plots was 4.13 ±0.3 mg/Kg might due to soil organic matter contents of conserved farms with SWC structures have greater than that of non- conserved plots. The overall mean value of available phosphorous recorded from the farm plots was 5.37 ±1.47 mg/Kg in that the available phosphorous in soil of the study areas was low following Barber, (1984) available phosphorous.
The finding is similar to the finding of Worku, 2017, that the mean Av. P in soil under conserved plots was relatively better than in the no-conserved plots might be due to higher organic matter contents of the conserved plots than the non-conserved ones. The level of Av.PinSebata central Ethiopia is significantly higher on treated field (11.87 ppm) compared to the untreated fields (6.84 ppm) and its level decreased down the slope (Kediro, 2015).
There was no significant variation shown in soil Av.Pacross slope position. The recorded result indicated that the mean value of Av. P increased down the slope from steep slope (>30%), Middle (15-30%) and Lower slope 8-15%) in both watersheds that were 4.798 ± 1.13 mg/Kg, 5.49 ± 1.61 mg/Kg and 5.84 ± 1.63 mg/Kg respectively (Table 2) might due to limited organic matter that make better condition for soil micro microorganism used to breakdown other fresh organic matter so that phosphorous present in the form of immobility rather than changes to plants available forms. It might be also due to fertilizers and animal manure added to the cultivation field removed by rainfall and run off and laid on the lower side so that the Av. P increased gentle slope.
Available Potassium (Av. K)
The result of soil Av. K of the study areas were significantly affected (p<0.05) by land use type (conserved with SWCstructures and no-conserved farm plots). The mean value of the available potassium was relatively higher in both conserved farm plots 0.874 ± 0.009 Cmol (+)/Kg (341.756 mg/Kg) than the mean value of soil Av. K of non-conserved of both areas of the cultivation farms was 0.835 ± 0.013 Cmol (+)/Kg (326.828 mg/Kg) (Table 2) might due to excessive rainfall can cause potassium to leach out soils in no-conserved areas with the structures and less surface cover (barriers) that hinders the run-off velocity of rainfall.
The finding is similar to the finding of Bekeleet al. (2016), who reported that the mean value of the Av. P in soil of conserved areas with structures was relatively greater than that of non-conserved farm plots due to the fact that soil conservation practices which were applied on the land have created conducive environment for the progress of the nutrients available in the soil.
The result obtained from laboratory indicated the mean value of Av. K was not significantly affected (p>0.05) by slope position. The mean value of Av. K recorded with in both watershed of similar slope position from upper (>30%), middle (15-30%) and lower (8-15%) was 0.84 ± 0.025Cmol (+)/Kg, 0.858 ± 0.23Cmol (+)/Kg, and 0.864 ± 0.21Cmol (+)/Kg respectively (Table 2).
Table 2: Mean values of selected soil chemical properties
Land use type
|
Variables
|
pH
|
OM
|
TN
|
Av. P
|
Av. K
|
EC
|
OC
|
Conserved land
|
6.33 ± .36
|
4.915 ± .47
|
0.228 ± .091
|
6.627± .77
|
0.874±.009
|
0.0485 ± .015
|
2.789 ±.2263
|
Non-conserved land
|
4.97± .45
|
3.404 ±.473
|
0.154 ± .012
|
4.13 ±0.3
|
0.835±.013
|
0.0402 ± .005
|
1.974 ±.275
|
Overall mean
|
5.65 ± .84
|
4.159 ± .94
|
0.191± .043
|
5.37 ±1.47
|
0.85 ± .023
|
0.045 ± .003
|
2.435 ±.5566
|
LSD (0.05)
|
0.0205
|
0.0205
|
0.003
|
0.0085
|
0.0195
|
0.175
|
0.014
|
SEM
|
0.343
|
0.384
|
0.087
|
0.601
|
0.0095
|
0.003
|
0.227
|
Slope Gradient
|
|
|
|
|
|
|
Upper (>30%)
|
5.25 ± .88
|
3.8 ± 0.86
|
0.177 ± .039
|
4.798 ± 1.13
|
0.84 ±.025
|
0.0384 ± .0055
|
2.272 ±.579
|
Middle (15-30%)
|
5.67 ± .86
|
4.06 ± 0.99
|
0.192 ±.046
|
5.49 ±1.61
|
0.858 ±.23
|
0.0449±.006
|
2.356 ±.574
|
Lower (8-15%)
|
6.05 ± .73
|
4.61 ± 0.97
|
0.205 ±.044
|
5.84±1.63
|
0.864 ±.21
|
0.0499±.006
|
2.675 ±0.564
|
Overall mean
|
5.66 ± .23
|
4.16 ±0.92
|
0.1913 ±.04
|
5.37±1.41
|
0.855 ±.23
|
0.045±.007
|
2.435 ±.548
|
LSD (0.05)
|
0.431
|
0.494
|
0.675
|
0.611
|
0.424
|
0.068
|
0.595
|
SEM
|
0.238
|
0.266
|
0.0118
|
0.407
|
0.0066
|
0.00212
|
0.158
|
The mean value of Av. K relatively increase from upper to lower in the watersheds might be due to larger biomass of grass cover preset on the lower parts so that less leaching of potassium and better soil conditions of soil for micro-organisms to decompose organic nutrients to available nutrients used plant growth.
In general, the overall mean value of available potassium (Av. K) recorded in Jamjam and Koriso micro watersheds was 0.85 ± 0 .023 Cmol (+)/Kg (334.292 mg/Kg). Following FAO, (2006) that the available potassium rating in Cmol (+)/Kg as very high (>1.2), High (0.6-1.2), Medium (0.3-0.6), Low (0.2-0.3) and very low (<0.2). Therefore, that the available potassium present in the soil of the study area was fallen in high. There was no potassium deficiency in the cultivation field of the farmer’s farm plots.