Soil degradation is expressed in quality and quantity of soil nutrients, physical and biological soil characteristics which is linked to stagnation and decline in yields in the most intensive agriculture  . The decline in these soil properties is interconnected to the improper use of inorganic fertilizer and lack of organic fertilization, practices that are now widespread in the most intensive agriculture in developing countries  . For instance, repeated-application of urea under crop production and soil fertility improvement program is not only causing nutrient imbalances, but also negatively affecting the physical and biological properties of the soils  . Thus under common nitrogen fertilizer applications, natural nutrient cycling, microbial biomass, enzymatic activity and water-holding capacity of the soil are dramatically reduced whereas acidification and nitrate accumulation in water bodies increases   .
Therefore, by considering the environmental problems associated with current cropping systems in Ethiopia particularly Oromia region, Bale Zone, it seems reasonable to continue research on the possibilities of growing cereal-legumes intercropping which will rival the current mono cropping systems. Further studies by  indicated that failure by smallholder farmers in Bale zone of Oromia region to implement intercropping system and lack of plant nutrients due continuous mono cropping nutrients uptake, nutrients leaching and soil erosion are the principal causes of low agricultural productivity and food insecurity in Ethiopia.
Cereal/legumes cropping system is advanced as one of the integrated soil fertility management practices consisting of growing two or more crops in the same space at the same time, which have been practiced over the years and achieved the soil fertility restorations and crops yield in agriculture  . Likewise in Ethiopia Highland growing of several crops in association or in mixtures mainly being cereal and legumes is not new a technology for soil fertility improvement and reducing evapotranspiration from the field  . A study by  noted the decline in soil fertility and moisture stress being the major limitation to crops yield in cereal based cropping systems in Eastern and Southern Africa.
Similarly, due to soil and crop moisture stresses are dramatically expending in the country moisture retention structures and planting methods plays a vital role for successful and sustainable crop production. Therefore, planting crops using insitu moisture conservation reduces problems of soil moisture stress. Thus different tied ridge and planting methods with spaced fixed distance apart to form a series of micro catchment basin created to catch and hold rainwater, minimizing surface runoff, improve downward infiltration of water  . Therefore the objective of this research was to evaluate influence of planting methods, tied ridge and cropping system on different soil chemical properties.
2. Materials and Methods
2.1. Description of the Study Area
Madda walabu district located between 5˚45'N, 39˚40'E latitude and longitude respectively. It is one of the districts in the Oromia Region of Ethiopia. In addition it is bounded by Guji zone in the West and south, Gura dhamole district and Delo Mena district in the East, Harena Buluk district in the Northwest. The administrative center of the district is Bidire  .
Thus, the district is located in the south Western parts of the Bale zone. The area of the district leads the zone to have a share of 12.93 percent from the total area of the zone (69,661 km2). The district has a distance of 200 km from zone capital called Robe and 630 km from Finfine. The mean annual temperature of the district is 30˚C. The lowest temperature is 20˚C and highest is 40˚C respectively. The mean annual rainfall is 600 mm whereas the lowest and highest rainfall is 400 mm and 800 mm respectively. Madda Walabu District have two major cropping season which is known as Maher and Belg. Cereals, pulses, Oilseed, fruit, vegetables and spice are the known crop grown in the District. Cereals are the main crops that are produced during Maher season and pulses, oil seed, vegetables, fruit, and spices are followed  .
2.2. Plot Soil Sampling and Analysis
Sampling for soil pH, organic carbon, total nitrogen available phosphorous and Potassium was accomplished. The soil samples were collected from the depth of 0 - 20 cm using auger. Five samples (in zigzag pattern) were collected before land preparation and after harvesting to form composite soil samples (45), which has been transported to the laboratory for further processing. Plant roots and shoots were handpicked and discarded. Then soil samples were air dried, mixed well before analyses. Likewise, soil pH was measured in ratio of soil to water (1:2.5), using pH meter whereas Ec was measured by conductivity meter using suspension of 1:5 soil: water ratio. Organic carbon was determined by following  . Then, percent soil organic matter was calculated by multiplying the percent organic carbon by a factor of 1.724. Total Nitrogen (TN) was determined by the micro-Kjeldahl digestion, distillation and titration method  . Available phosphorus (AP) was determined using the standard Bray II and Olsen test based on the results of pH   . Finally, exchangeable potassium was determined using flame photometer  .
2.3. Data Analysis
All data collected were subjected to Paired Sample T-test using IBM SPSS 20. Significance differences between treatments were delineated by LSD (Least Significant Difference) test at 5% level of significance.
3. Result and Discussion
Effect of Planting Methods, Tied Ridge and Cropping System on Soil Chemical Properties
1) Soil pH
Soil pH was significantly different due to interaction of maize-haricot bean intercropping with both tied ridge and planting methods. Thus the maximum mean of soil pH was observed before planting at sole haricotbean with flat bed whereas the minimum soil pH were observed after planting of maize-haricot bean intercropping with closed end tied ridge and in furrow planting. When before and after planting values of pH was compared by t-test it was indicated that there was reduction due to influence of maize-haricot bean intercropping (Table 1). Relatively lower pH values in the soil after planting due to maize-haricot bean intercropping with tied ridge and planting methods as compared to those sole maize and haricot bean, might be due to depletion of basic cations by the harvested crop biomass, leaching and phosphorus fixation.  has also reported that a lower pH value in intercropping land was attributed to a high rate of organic matter oxidation. This is important to produce organic acids and provide H+ to the soil solution, and thereby reduces soil pH values.
2) Available Potassium
Intercropping of maize-haricotbean with tied ridge and planting methods were highly influenced available potassium in soil which is easily uptake by the crop. So that the mean of available potassium was significantly different due to interaction of maize-haricotbean intercropping with both tied ridge and planting methods. Likely available potassium in the soil was increased after planting due to maize-haricotbean intercropping (Table 1). Similarly, it was observed that by  K levels can be maintained when haricot-bean is grown in intercropping
Table 1. Effects of interaction of maize-haricotbean intercropping on soil fertility with tied ridge, cropping system and planting methods.
IC = intercropping; CE = closed end tiedridge; IF = Infurrow planting; OR = on Ridge Planting; OE = Open end Tiedridge; FB = Flat Bed; SM = Sole maize; NO = Flat Bed; SH = sole Haricotbean.
with cereals. Results obtained by  at Kadawa, Kano clearly indicated that under continuous cultivation with intercropping of maize-haricotbean the available K can be improved.
3) Available Phosphorus
Comparison AP, before and after planting, was significantly different (P ≤ 0.05) under interaction of maize with tied ridge and planting methods (Table 1). Therefore the values of available phosphorus was increased due to intercropping and available soil moisture Reduction of AP could be related to degree of its fixation which occurs at low pH levels where iron and aluminum activity actually increases  .
4) Soil Organic Matter
On plot maize-haricot bean intercropping with tied ridge and planting methods soil organic matter was significantly different (LSD0.05) by their interaction effect. The highest amount of soil organic matter (9.06%) was observed after planting; under maize-haricotbean intercropping with closed end tied ridge and in furrow planting whereas the minimum (4.27%) were observed before planting at sole maize planting on flat bed (Table 1). This result was in line with  that reported the amount f soil organic matter can alter rapidly and drastically as a consequence of legumes-cereals intercropping. In addition,  were obtained availability of soil moisture in arid and semi arid environment improved decomposition of organic matter by 20% in comparison to low temperature areas. These authors support the result of this research because high organic matter observed under closed tied ridge and in furrow planting. Similarly,  reported that intercropping cereals with legumes can address losses of soil organic matter in tropical, rainfed, low-input systems.
5) Total Nitrogen
Before and after planting total soil nitrogen was significantly different by interaction of maize-haricotbean intercropping with both tied ridge and planting methods. This might be because of legumes have capability to fix atmospheric-N through symbiosis (Table 1). It was agreed with works of  that reports, cereal-legumes intercropping patterns’ effect on total soil N as significant. Likewise,  reports legume is grown in a mixture with a cereal, it can improve the N economy of the cereals both by contributing N to the soil for uptake by the cereal or simply by the legume removing less N than if the cereal was grown as a pure stand. Similarly,  were observed amount of mineral-N remaining in the soil after cereal-legume intercropping was greater than that after planting of non-legume crop with an equal amount of N in harvest products. This was might be because of legumes can satisfy part of its N-requirement through atmospheric N-fixation. Likely,  also quantified N-savings on the order of 18 to 23 kg N per ha by Maize-haricotbean intercropping when compared with sole maize. Accordingly, biological nitrogen fixation is the major source of nitrogen in legume-cereal mixed cropping systems when nitrogen fertilizer is limited  . In addition, roots of the legume component can decompose and release nitrogen into the soil where it made available to subsequent crops.
6) Water Use Efficiency
The crop yield and soil fertility status was significantly different (P0.05%) due to tied ridge with intercropping of maize-haricotbean than tied ridge with sole maize and flat bed. Specifically, under closed end tied ridge this difference was mostly observed. This might be because of tied ridge was used as in-situ soil moisture conservation and maize used as a shade to haricotbean from direct sunlight intensity (Table 1). Likely, haricotbean was used as life mulching that can be reduced evaporation from the soil whereas maize was improved haricotbean water use efficiency by minimizing transpiration. The finding reported by  indicates that the advantage of the maize-haricotbean intercropping system under tied ridge is reflected in the water use and the water use efficiency value. The same authors were reported, in 1986 these crops together used 597 mm as opposed to 374 mm for the sole crop of maize and in 1987 total water used by maize and haricotbean in intercropping was 585 mm, in contrast to 398 mm by the sole crop maize.
4. Conclusions and Recommendation
Soil pH was significantly different due to interaction of maize-haricot bean intercropping with both tied ridge and planting methods. Intercropping of maize- haricotbean with tied ridge and planting methods were highly influenced available potassium in soil which is easily absorbed by the crop. Available phosphorus, before and after planting, was significantly different (P ≤ 0.05) under interaction of maize with tied ridge and planting methods. The highest amount of soil organic matter (9.06%) was observed after planting; under maize-haricotbean intercropping with closed end tied ridge and in furrow planting whereas the minimum (4.27%) were observed before planting at sole maize planting on flat bed.
Before and after planting total soil nitrogen was significantly different by interaction of maize-haricotbean intercropping with both tied ridge and planting methods. The soil fertility status was significantly different (P0.05%) due to tied ridge with intercropping of maize-haricotbean than tied ridge with sole maize and flat bed. Specifically, under closed end tied ridge this difference was mostly observed.
Therefore, the use of intercropping and closed end tied ridge in combination with in furrow planting significantly increased soil fertility in the system, which could be promising for sustainable improvement of crop production and productivity for semi arid areas of Bale zone.
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