Soil Nutrient Stock Evaluation under Different Land Use Types in the Smallholder Farming Systems of Jimma Zone, Ethiopia
The threats of soil nutrient depletion and food insecurity due to unsustainable farming practices are the problems for most of developing countries including of Ethiopia. This investigation was conducted to determine the status of soil nutrient stock under different land use types in the smallholder farmers and to evaluate the relationship between N, P and K stocks, different soil parameters and land use types. Soil samples were collected from each land use types and tested in laboratory for bulk density, % sand, % silt, % clay, total N, total P and total K. Soil N, P and K stocks for the land use types were calculated through multiplying the concentration of N, P and K with their respective bulk density and sampling depth. Mean comparisons were carried out to soil bulk density, % sand, % silt, % clay, total N, total P and total K. One way ANOVA was used to determine N, P and K stocks significant difference (p = 0.05) under different land use types. Also, 2-tiled Pearsons correlation coefficient was used to determine the relationship between soil nutrient stocks, soil parameters and land use types. Accordingly, the mean values of soil physico-chemical properties were varied among land use types but none significant difference (p<0.05) was observed for N, P and K stock. Because, soil nutrient management practices applied to each land use types are not adequate to improve soil nutrient stocks and compensate losses. There was a positive correlation between some soil physico-chemical properties, nutrient stocks and land use types. Therefore, to improve soil nutrient stocks and availability of nutrients for plants in order to achieve food security intensive soil nutrient management practice across land use types are compulsory.
Received: May 13, 2011;
Accepted: August 25, 2011;
Published: October 31, 2011
Soil nutrient stock is the reserve of N, P, K and other essential elements
in the soil that can be available to plants during time scale of 5 to 10 years
(Sanchez and Palm, 1996). Several studies conducted
in Sub Sahara African (SSA) countries showing N, P and K socks have been declining.
The major causes of nutrient stock depletions are less attention paid to the
rural areas, land degradation, inadequate nutrient replenishment, late adoption
of improved soil fertility management technologies, poor agronomic practices
and lack of ample natural land resources management policies (Sanchez
et al., 1997; Stoorvogel and Smaling, 1990).
Consequently, per capita food production is diminishing in the continent and
thousands of peoples are depending on food aid (FAO, 2001).
Nutrient balance studies conducted in different parts of Ethiopia at various
levels such as field, farm sections, farm, watershed, regional and national
showing high N, P and K depletion is takes place in the smallholder farm (Elias
et al., 1998; Haileslassie et al., 2005;
Aticho, 2011). To overcome the problems of soil nutrient
and ensure food security, Ethiopian Ministry of Agriculture and Rural Development
(EoMARD) as well as some national and international nongovernmental organizations
has been involved in different soil nutrient management activities through offering
training, extension and rewarding early technology adopting farmers.
Resembling other Sub Sahara Africa (SSA) Ethiopian farmers are less aware to
the existing problems as a result the acceptance levels of new soil nutrient
management technologies is varying among farmers and locations (Braun
et al., 1997; Haileslassie et al., 2005).
Also, the introduced technologies such as rate mineral fertilizer (DAP and urea)
and combination of mineral and organic fertilizer addition per unit of area
are not supported with adequate field tests at national level (Kebebe
et al., 2007). Consequently, farmers are complaining on the response
of fertilizer applied to crops because the same amount and types of fertilize
applied on same land and crop at different growing season but, yield is decreasing
through time. This is happened due to soil nutrient (N, P and K) stock depletion,
soil toxicity (high H+, Al3+ concentration) and rainfall
variability (Elias, 2002). N, P and K nutrients are required
by plants in higher concentrations than other elements. The imbalance in the
supply of one of these nutrients compromise plant growth, affecting root development,
cell division, crop quality, crop yield and resistance to disease and drought.
In the study area N, P and K stock under different land use type is not studied.
Therefore, this study was conducted to determine the status of soil nutrient
stock under different land use types in the smallholder farmers and to evaluate
the correlation between N, P and K stocks, different soil parameters and land
MATERIALS AND METHODS
Description of the study area: The study was conducted in the Segno
Gebeya Village (Kebele), Seka Chekoruisa District (Woreda), Jimma Zone, Southwestern
Ethiopia. Which is 390 km away from Addis Ababa and 35 km from Jimma town. It
has an elevation ranges from 1580-2560 m above sea level (m.a.s.l), mean annual
rainfall 1592 mm and temperature 15°C (Aticho, 2011).
The dominant soils are Nitisol and cambsoil (FAO, 1994).
A land survey conducted at Jimma Zone showing that land use of the study district
are 45.3% arable, 6.1% pasture (communal), 25.8% forest and the remaining 22.8%
is considered swampy and/or degraded (SEPJZG, 2006).
Integration of livestock with crop cultivation is the major agricultural activity
in the area. Enset (Enset ventricosum), Wheat (Triticum spp.),
Oat (Avena sativa), Barley (Hordeum vulgare) and Tef (Eragrostis
tef) are the major agricultural crops while animals like cow, ox, calf,
sheep, goat, donkey, horse and mule. Lands are used for crop and grazing in
rotation due to limited accesses of communal grazing land (Aticho,
2011; Aticho et al., 2011). Livestock play
key role in crop production (draught power, manure supply, transportation, income
generation etc.) in the study area (Yisehak, 2008).
Data collection and analysis: Composite soil samples were collected from 15 cm depth from the following land use types; Enset (Enset ventricosum), Wheat (Triticum spp.), Oat (Avena sativa), Barley (Hordeum vulgare) and Tef (Eragrostis tef). Also, an undisturbed core samples were collected from each land use type to determine soil bulk density. The total numbers of soil samples collected for the analysis were fifteen (each land use type has three replications i.e., 5x3 = 15).
The collected soil samples were air dried, sieved by 2 mm sieve and analyzed
by following standard laboratory procedures. Laboratory analysis was carried
out in Hawassa University and research center. Soil texture was determined by
Bouyoucos method (Jaiswal, 2003). Bulk density was determined
by measuring the wet weight of each core and converting to dry mass equivalent
per unit volume of soil using moisture data and the known volume of the cores
(Klute, 1986). Total N was determined by Kjeldahl method
(Houba et al., 1989). Both total P and K concentrations
in the soil were extracted with fluoroboric acid digestion method and the stock
of P and K in the solution were determined Atomic Absorption Spectroscopy. Finally,
soil N, P and K stocks (kg ha-1) were calculated according to Bond
Statistical analysis: All statistical analyses were carried out using SPSS version 16. Mean comparisons were carried out to the physico-chemical properties of soils under different land use types. One way analysis of variance (ANOVA) was employed to determine N, P and K stocks significant difference (p = 0.05) under different land use types. The 2-tiled Pearsons correlation coefficient was used to determine the relationship between soil nutrient stocks, soil parameters and land use types.
RESULTS AND DISCUSSION
Soil physico-chemical properties: The mean values of soil particle size distribution showed land used for Enset (Enset ventricosum) and Wheat (Triticum spp.) were mostly clay loam soil (Table 1). While, the lands use for Barley (Hordeum vulgare) and Tef (Eragrostis tef) were silt clay loam. Different land use types with the same soil texture found in the study area demonstrates land use types or changes have no effect on soil texture. But, the cause of difference or similarity in soil texture is due to similarity or difference in parent materials from which they developed.
The mean value of bulk density under Enset (Enset ventricosum), Wheat
(Triticum spp.), Oat (Avena sativa), Barley (Hordeum vulgare)
and Tef (Eragrostis tef) were 1.01±0.03, 1.25±0.11, 1.21±0.02,
1.23±0.11 and 1.27±0.05 g cm-3, respectively (Table
1). This is agreed with the findings of Landon (1991),
bulk density of clay loam and silt loam topsoil is ranges from 1.00-1.60 g cm-3
depending up on the management condition. That is why in this study soils of
same texture has differences in bulk density. Increase in bulk density inhibits
plants ability to exploit the available resources in the environment through
restricting the root system. In the study area soil bulk density related problems
were not observed thus it should be mentioned through the possible management
practices like organic matter addition, crop rotation, fallowing and decreasing
compaction caused by overgrazing.
|| Mean value of selected soil physico-chemical properties in
topsoil (0-15 cm) in different land use type
|±Values are Mean±SD, BD: Soil bulk density, TN: Total
nitrogen, TP: Total phosphorous, TK: Total potassium
||Soil N, P and K stock under different land use types (LUTs).
(LUT 1): Enset (Enset ventricosum), LUT 2: Wheat (Triticum
spp.), LUT 3: Oat (Avena sativa), LUT 4: Barley (Hordeum vulgare),
LUT 5: Tef (Eragrostis tef)
When the mean values of bulk density compared with land use types the smaller
value was observed under Enset (Enset ventricosum) crop. This is associated
with the land used for Enset (Ensete ventricosum) plantation was not
subjected to grazing and intensive mulching with organic materials for the whole
years. The result of this study is agreed with the findings of Seneviratne
et al. (2006) and Benjamin et al. (2007).
But, other land use types were used both for grazing and crop production rotationally
consequently, crop residues are removed from field through grazing, burning
to clean land from weeds, bush growth and debris in order to facilitate cultivation
then soil organic matter is depleted and bulk density under annual cropping
system becoming higher than the perennial.
As illustrate in Table 1, the mean value of total N was relatively
higher on lands used for Enset (Ensete ventricosum) and Tef (Eragrostis
tef) than the other land use types. This result is agreed with the findings
of Havlin et al. (2005). According to Landon
(1991) rating the levels of N under Enset (Enset ventricosum) and
Tef (Eragrostis tef) is medium while in Wheat (Triticum spp.),
Oat (Avena sativa), Barley (Hordeum vulgare) and Tef (Eragrostis
tef) low. This happened due to continuous mulching of Enset (Ensete ventricosum)
garden with organic matter which is the potential source of soil N.
The mean value of total P was 314.39±53.97 in Enset (Enset ventricosum)
and 318.43±63.08 under Wheat (Triticum spp.) which is comparatively
higher than other land use types. Because of nutrient recycling through manure
addition or litters fall. According to Barber (1995) rating
the total P concentration in soils of the study site is found in the normal
range. According to Havlin et al. (2005) rating
the total K in the soils of the study area is very small. This happened due
to, soil of the study area is highly weathered; absence of K based fertilizer
addition and adsorbed in large quantity by plants next to N. Generally there
was difference in soil physico-chemical properties under different land use
types, which is agreed with Onweremadu (2007).
N, P and K stocks under different land use types: Total N, P and K (kg
ha-1) stock in the top 15 cm depth were varied for different land
use types. When we compare the stock depletion more P and K losses were observed
on the land that was used for Barley (Hordeum vulgare) production than
others Enset (Enset ventricosum), Oat (Avena sativa), Tef (Eragrostis
tef) and Wheat (Triticum spp.) (Fig. 1). Because,
P and K removed with Barley (Hordeum vulgare) biomass was a bit higher
than other crops. The stock of N was similar for all land use types due to its
high demand by all crops for vegetative growth and loss caused by environmental
condition (leaching due to high rain fall and gaseous loss). The total N, P
and K stock has not any relationship with the amount currently available to
plants; it will be available for plants progressively 5-10 years (Sanchez
and Palm, 1996). Soil N, P and K stock improvement is not to maximize their
stocks in soil but rather to maintain the minimal quantity that will maximize
service flows for sustainable crop production.
Statistical analysis for N, P and K stocks for different land use types showed
none significant (p = 0.05) differences between and within groups (Table
2). This revealed soil of the study area was developed from similar parent
material with analogous climatic and biotic influences. Consequently, the existing
soil nutrient management practices have miniature influence on the total amount
of N, P and K stock. Because, the quantity of manure and crop residues once
or gradually added to the soil was inadequate and not able to increase soil
nutrient stock. Study conducted at the site showed high amount of N, P and K
are removed from agricultural lands either with service and wastage (leaching,
erosion and gaseous lose) flow (Aticho et al., 2011).
Correlation between nutrient stock, land use types and other soil parameters:
Correlation analysis of nutrient stock, land use types and other soil parameters
(bulk density, % sand, % silt and % clay) indicated strong relationship (Table
|| One-way ANOVA for soil nutrient stock in 15 cm depth under
different land use types
|NS = None Significant difference at p = 0.05
||Pearson Correlation matrix for Bulk Density (BD), % sand,
% silt, % clay nutrient stock (N, P and K kg ha-1) and land use
|*Correlation is significant at the p = 0.05 and **at p = 0.01
Positive correlation was observed between bulk density and N, P and K stock
at correlation coefficient (r) of 0.03, 0.37 and 0.23, respectively. This illustrates
improvement in soil bulk density improves N, P and K stocks. The available options
used by smallholder farmers to improve soil bulk density were organic matter
addition and reducing grazing pressures (grazing is the cause of soil compaction
since machineries were not used for cultivation). Therefore, organic matter
added to soil to improve bulk densities and N, P and K stocks. Also, significant
(p = 0.01) correlation (r = 0.62) between land use types and bulk density was
observed. This exhibits soil bulk density has a close relation with land use
types. Soils with high bulk density is less preferred for crop production since
high soil bulk density (compacted soils) limits nutrient, moisture and air flow
to plant root which results in yield reduction, this idea is agreed with the
findings of Awodun (2007). Despite of this, negative
correlations were observed between land use types and % silt and clay, % sand
and clay, % clay and bulk density.
CONCLUSION AND RECOMMENDATIONS
The types of soil nutrient management practices used by smallholder farmers in the study area for different land use types has no longer contribution to improve soil N, P and K stocks. Because, nutrients added to the soil is not yet adequate to compensate the annual loss with service (nutrient with biomass) and wastage flows (nutrient loss with leaching and water erosion). To ensure food security at household and country level soil nutrient stock improvement and increasing the availability of nutrients for crops should be improved through nutrient recycling and external inputs (mineral and organic fertilizer) addition. To do this, site specific researches should be done in different parts of the country on the rate of soil nutrient depletion, amount of nutrients required to stop depletion and increase the stock, factors that limits the availability of nutrient stocks to plant and how to overcome these restrictive factors.
Data presented in this paper was generated during the M.Sc thesis data collection of the first author. Therefore, the authors would like to thanks the sponsor for financial and material support of this study.
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