For this investigation, a two-year field study was conducted at the Omar Almukter Univrsity Center in Albida, Al Jabel Alkder, Libya (Figure 1) (32˚76'272"N, 21˚75'506"W, elevation 590 m) from July to November 2009 and repeated from June to September 2010. The topography was flat (<1% slope). The soil was a

mixed, superactive, calcareous, thermic Typic Xerorthents, Shedd Series (USDA classification), formed in residuum weathered from calcareous soft shale (moderately drained and has slow permeability). This area of the country has a plateau type climate with great rainfall and low temperature and used mostly for range and some grain with alternate fallow.

There was no crop growth and the field was left fallow two years prior to the start of the study. Prior to performing the tillage experiment, soil samples were collected from different locations at a depth of 0 - 20 cm. The soil texture was determined using hydrometer method [16] . Organic matter was determined by the modified Walkey-Black method as suggested by [17] . Available phosphorus (P) and potassium (K) were determined by the method of [18] . The soil in the experimental site was clay loam having an organic matter (6.25%), pH (7.30), available P (1.16 mg∙kg⁻¹), available K (0.05 mg∙kg⁻¹), and EC (0.38 dS∙m⁻¹) (average of five soil samples).

Four machines include of moldboard, chisel, land leveler and ridger were used in this investigation. A description of each machine is given in Table 1. A tractor with 55.1 kw and a mass of 2800 kg was used in both years. The manufacturing company of all machines is Simba Tractors Ltd, Nairobi, Kenya.

The experimental area was divided into three blocks 70 m long and 7 m wide (blocks separated by a 3 m spacing) used for each tillage practice. A small block (10 m long by 5 m wide) in the beginning of each tested block was used prior to the commencement of the actual test runs to enable the tractor and implement to reach the required speed. Three tillage practices were used to represent the standard primary tillage implements most commonly used for seedbed preparation in this region of the country. The tillage practices included conventional tillage (CT) performed using moldboard plow one pass followed by chisel plow one

pass, lander leveler one pass and ridger one pass, ridge tillage (RT) performed using ridger one pass, and zero tillage (ZT). Treatments were replicated four times. The tillage speeds were as follow: 3.8 hr∙km⁻¹ for moldboard plow, 3.6 hr∙km⁻¹ for chisel plow one pass, 4.4 hr∙km⁻¹ for lander leveler, and 4.0 hr∙km⁻¹ for ridger.

After the tillage practices, Zea mays L. cultivar (Arifiye) seeds were obtained from Agricultural Research Institute in Albida (Libya), planted by hand at a rate of 40 kg∙ha⁻¹ in three rows in each block (each block was divided into three rows) in both years. Surface drip irrigation (common in the region) was applied. Tensiometers (Spectrum Technologies, Inc., USA), one in each block, placed at 10 cm to 15 cm below the soil surface to indicate the soil water status. Before field installation, each tensiometer was calibrated. All blocks received the same management (planting, fertilizer, and weed control).

Soil physical properties (penetration resistance (PR) bulk density (BD), total porosity (TP) and saturated hydraulic conductivity (Ks) were measured two times, before tillage (Table 2) and after tillage when the maize was at grain physiological maturity, approximately four months from planting in both years [19] . The soil physical properties were determined from different locations in the field at 20 cm intervals starting from the surface down to a depth of 60 cm.

The PR were determined in the field at depths 0 - 20 cm and 20 - 40 cm and 40 - 60 cm using a hand-pushing penetrometer having maximum measurement range 5000 kPa and 80 cm depth. Soil penetrometer measurements were made by pushing the penetrometer vertically into the soil at each depth. Undisturbed core samples were collected at 0 - 20 cm, 20 - 40 cm, and 40 - 60 cm depths to measure the BD and Ks [14] . The BD and the Ks were determined on separated samples. BD was calculated using the procedure outlined by [20] , then these values were used to calculate the TP using the following equation:

the soil particle density assumed to be 2.65 g∙cm⁻³. The Ks was measured on soil cores in the laboratory using a constant head permeameter [15] .

Percentage of emerged seedlings (PE) was calculated [21] for each tillage practice as follow:

five plants for each tillage practice were sampled randomly at 60, 75, and 120 days after planting to measure plant height, number of leaves, leaf area index and root weight. Plant height was measured as the vertical distance between the ground and the highest living part of the plant with a ruler. Number of leaves per plant was determined by counting all the leaves on each plant. The leaf area index was calculated [22] using the following equation:

for total dry shoot weight determination, the above ground samples were weighed and oven-dried at 80˚C and weighed again.

The dry root mass at various times and depths were determined. Maize roots were sampled with a soil core (a diameter of 10 cm and a height of 10 cm). The soil cores were taken to a depth of 60 cm in each block at four different locations. The cores were then taken to the lab and the soil and the roots were soaked in a solution containing 40 g/liter sodium hexametaphosphate in a 1:5 soil solution ratio [23] . Roots floated to the surface and were skimmed from the surface with a fine wire strainer. By subsequent washing in tap water roots and organic debris were separated. The roots were oven-dried at 80˚C and weighed.

At harvest (120 days after planting), grain and stover (above-ground biomass minus grain) yield were measured. Both the mass of grain and mass of stover were calculated after drying and converted to a per hectare basis at 14% moisture content [24] . Harvest index (%) was calculated on percentage basis by using the following formula:

climate data during the two growing seasons was collected from a weather station located approximately 500 m from the field.

To estimate tillage operation cost, the cost associated with each tillage practice include machinery, fuel and labor was calculated totally as $ ha⁻¹ in both years. Machinery costs were based on the data provided by Agricultural Research Institute in Albida, Libya. They were calculated according to the hours of use including costs for insurance, tax, and average repair costs. Fuel consumption was measured by using a secondary tank with a level marked tube and bulb with volume 140 cm³ [25] . Labor was measured with a stopwatch for each tillage practice. No land costs included since land costs were the same across tillage practices [26] . Production cost ($∙ton⁻¹ for grain + stover) include all variable costs for seed, lime, fertilizer, herbicides, and insecticides for each tillage practices were calculated.

A completely randomized block design was used in this study. Tillage was randomized within blocks. Data across the two years was assessed (residuals were homogeneous across years (Levene’s test) and normal distributed (Shapiro-Wilk test), and then was statistically analyzed using the analysis of variance (ANOVA) to test the effects of tillage practices on the soil physical properties and maize growth and yield. All calculations were performed using SAS (SAS Institute Inc., North Carolina, USA). Year was first tested as a factor and found not significant for all metrics. Therefore, year was considered as a random factor in the model. Differences between the three tillage practices were compared by the Fisher LSD test. Differences between the means were considered to be statistically significant at P < 0.05.

Mean monthly temperature, precipitation and relative humidity during the study period in 2009 and 2010 are shown in Figure 2. The accumulated precipitation during the growing seasons was 7.4 cm in 2009 and 10.7 cm in 2011. The mean temperature during the growing season was 22.9˚C and 24˚C and the highest relative humidity was 72% and 71% in 2009 and 2010, respectively.

The PE differed among tillage practices (p = 0.0206). The highest PE was recoded under CT followed by RT and the lowest under ZT (Figure 5). The topsoil TP of the CT was higher compared to ZT (Figure 4(a)) which facilities free movement of air and moisture in the soil and thus increases the PE under CT. Similarly, [41] [42] found that zero tillage reduce the PE of maize compared to conventional tillage.

The tillage practices showed also significant difference in the maize growth. Plant height, number of leaves, leaf area index and dry weight of shoot at 60, 75, and 120 days after planting were the highest for CT and the lowest for ZT (Table 3).

The positive effect of tillage compared to zero tillage was observed in other maize growth studies. Taller plant [43] [44] , higher number of leaves per plant and higher leaf area index [22] [45] of maize were found in tilled soil compared to zero tillage.

Although the highest root dry mass across all depths at all times was found under CT and the lowest under ZT, these differences were not significant (data not shown). These results are similar to that of [46] who reported higher dry matter of root in conventional tillage compared to zero tillage in sandy loam soil.

The effect of tillage practices on grain and stover at harvest are shown in Figure 6. The ZT presented the lowest grain and stover yield in comparison with the other tillage practices (Figure 6). These results may be due to the lack of soil loosening for providing conditions favorable to crop growth and yield under ZT practice. [47] [22] also reported higher maize yield under CT compared to ZT practice. Soil tillage has a great influence upon the harvest index of maize recording lower harvest index (32%) when CT was applied compared to ZT (62%) and RT (53%). In other words, the physiological effectiveness of maize crop to partition the dry matter into its cost effective (grain) yield than generation of whole-plant biomass increased under ZT and RT compared to CT. These results are not in line with the findings of [48] who stated that higher harvest index was observed where conventional tillage was applied compared to minimum tillage. In other study [49] , disking determined higher values of the harvest index than plowing.

Operation and production costs for each tillage practice are shown in Table 4. Estimates of total machinery, fuel and labor cost were the least for ZT practice compared to RT and CT (Table 4).

The operation tillage costs for CT were approximately $ 76.50 greater than for the RT (Table 4). On the other hand, the production costs were the highest for ZT compared to other practices. Although seed, fertilizer, lime and insecticide expenses were slightly similar among tillage practices, herbicide costs were higher in ZT and RT compared to CT (data not shown), probably due to the fact that zero or minimum tillage production rely exclusively on herbicides for weed control [50] [51] .