Soil & Tillage Research 68 (2002) 17–22

Cowpea yield response to soil compaction under tractor traffic on asandy loam soil in the semi-arid region of northern Nigeria

A. Daudaa,∗, A. Samariba Department of Agricultural Engineering, University of Maiduguri, Maiduguri, Nigeria

b Borno State Agricultural Development Project, Maiduguri, Nigeria

Received 6 June 2000; received in revised form 7 June 2002; accepted 10 June 2002

Abstract

Crops and soils can react to the same level of compaction differently, thus there is a need to assess the impact of compactionunder particular environments. The effect of soil compaction as a result of tractor traffic on cowpea (Vigna unguiculataL.Walps) yield was investigated in a sandy loam soil (Regosol) in the semi-arid region of northern Nigeria for two growingseasons, 1998 and 1999. A randomized complete block design of field plots with treatments of 0, 5, 10, 15 and 20 passes ofa tractor with 31 kPa contact pressure was used. Each treatment was replicated four times. The soil bulk density, penetrationresistance and soil moisture content for each applied load were measured and the yield from each treatment was determined.Agronomic treatments were kept the same for all plots in both 1998 and 1999. Results showed increased soil dry bulk density,penetration resistance and soil moisture content with increased tractor passes. Plant height decreased, plant moisture contentand plant stem diameter increased with increased tractor traffic passes. Highest grain yield was obtained at 10 tractor passeswith a mean bulk density of 1.58 Mg m−3 and penetration resistance of 1.37 MPa with a mean grain yield of 276.5 kg ha−1,while lowest yield was obtained from 20 tractor passes was 555 kg ha−1. Statistical models were used to predict grain yield asa function of bulk density, penetration resistance, soil moisture content, contact pressure, and number of tractor traffic passes.Grain yield with respect to moisture content gave the best prediction(r2 = 0.96).© 2002 Elsevier Science B.V. All rights reserved.

Keywords:Tractor wheel traffic; Soil compaction; Bulk density; Soil moisture; Penetration resistance; Plant growth; Grain yield; Cowpea;Northern Nigeria

1. Introduction

Many field studies have been conducted on theeffects of wheel traffic on soil compaction and on theeffects of crop yield (Mamman and Ohu, 1997; Ohuand Folorunso, 1989; Philips and Kirkham, 1962;Liepiec et al., 1991; Taylor et al., 1981). The degreeof compaction depends on the soil type, moisturecontent, load, tyre dimension, inflation pressure, slip,

∗ Corresponding author.

forward speed and the number of repeated machinerypasses (Soane, 1970; Oni and Adeoti, 1986; Ngunjiriand Siemens, 1995). Raney (1971)reported the char-acteristics of soil compaction as those that controlthe content and transmission of water, air, heat, andnutrients and those that change soil strength.

Raghavan and Mckyes (1983)reported a decreasein the hydraulic characteristics of clay soil due toheavy traffic. Soil compaction leads to increased bulkdensity, soil strength, reduced infiltration, reducedwater movement within the root zone of plants and

0167-1987/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved.PII: S0167-1987(02)00064-8

18 A. Dauda, A. Samari / Soil & Tillage Research 68 (2002) 17–22

reduced nutrient uptake (Negi et al., 1980; Arvidsonand Hakansson, 1991; Grath and Hakansson, 1992).Ohu et al. (1994)reported reduced cotton (Gossyp-ium hirsutum L.) yield due to intensive tractortraffic. Heavy traffic resulted in excessive reduc-tion in maize (Zea maysL.) yield as reported byRaghavan et al. (1979).

Al Adawi and Reeder (1996)reported the reductionof maize and soybean (Glycine maxL. Merr.) yielddue to compaction effects. Considerable research hasbeen conducted to gain an understanding and to quan-tify the effects of soil compaction on crop growth. Amoderate amount of compaction is sometimes consid-ered beneficial (Voorhees, 1982). Swan et al. (1987)reported the level of compaction on crop yields andconcluded that maximum yields are obtained at anoptimum level of compaction. The optimum level ofcompaction changes, however, being dependent on soiltype and weather conditions.

In spite of many studies available in the literatureon the effect of soil compaction on crop production,information relating soil compaction due to machinerytraffic on cowpea production is not available in thesemi-arid region of Nigeria, where cowpea is one ofthe major food and cash crops. The objectives of thisstudy were: (1) to assess the damage done to cowpeaproduction by soil compaction from vehicular traffic,(2) to determine the effect of traffic on sandy loamsoil physical properties and yield of cowpea, and (3)to provide information to farmers for efficient controlof tractor traffic on agricultural lands in the semi-aridregions of northern Nigeria.

2. Materials and methods

2.1. Study site

The experiment was carried out during the grow-ing season in 1998 and 1999 at the University ofMaiduguri Experimental Farm, Maiduguri, on thesame plot of land. The experiment consisted of 20plots with five treatments and four replications usinga complete randomized block design in a uniformfield of sandy loam soil. The soil is classified asa Regosol (FAO/UNESCO) containing 800 g kg−1

sand, 70 g kg−1 silt, and 130 g kg−1 clay determinedby the pipette method (Black, 1965).

2.2. Traffic experiment

The treatments consisted of a 0 traffic, 5, 10, 15and 20 passes of the tractor tyre at 31.0 kPa (the ra-tio of load to contact area) vehicle contact pressure,imposed before seeding following the studies ofOhuand Folorunso (1989)andMamman and Ohu (1997).A Massey Ferguson MF 165 D 2 wheel drive tractorwith rear tyre dimension of 0.43 m× 0.7 m, with aweight of 43.3 kN, and a resulting ground pressureof 31.0 kPa was used for the treatments. The useof 31.0 kPa contact pressure was used based on thestudies ofHakansson and Danfors (1981), Ohu et al.(1989)andMamman and Ohu (1997). Forward trac-tor speed was kept constant at a speed of 6 km h−1

for all the treatments.The plots (10 m × 10 m) were ploughed using

a three bottom disc plough of an average depth of200 mm on 25 August 1998, which produced an av-erage bulk density of 1.41 Mg m−3 in the top 20 cmof the soil. On 26 August 1998, the plots receivedthe specific passes of the tractor, as described above.The whole plot area was covered completely withthe wheeling at an average soil moisture contentof 98 g kg−1, less than the optimum moisture con-tent (i.e. 125 g kg−1) for compacting the same typeof soil as reported byOhu et al. (1989). Manualplanting of cowpea variety IT5S4E-124 seeds wascarried out on 27 August 1998, in a 5 m× 5 m areaof each plot, by placing four seeds in rows 75 cmapart with row spacing of 25 cm. Thinning was car-ried out 14 days later as recommended by BornoState Agricultural Development Project (BOSADP,1994).

In the second year of the experiment (1999), thesame operations were carried out as in 1998 on thesame plots. The plots were ploughed to an averagedepth of 200 mm on 20 August 1999, which pro-duced an average bulk density of 1.42 Mg m−3 in thetop 20 cm of the soil. On 24 August 1999, the plotsreceived the tractor passes as in 1998 at an averagemoisture content of 106.5 g kg−1. On 25 August, theplanting operation was carried out exactly like that ofthe previous year.

In both years, 1.5 kg metochalor and 1.0 kg meto-bromurum (3 l Dual 500EC+ 2.0 potoram 50wp)herbicide per hectare were applied 4 weeks after sow-ing. The herbicide was applied by a knapsack sprayer.

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Superphosphate fertilizer 200 kg ha−1 was appliedduring seedbed preparation.

2.3. Soil and plant measurements

Soil dry bulk density, penetration resistance andsoil moisture content of each plot were determinedat the following crop stages: (i) crop emergence, (ii)flowering, and (iii) harvesting. The soil dry bulk den-sities were measured using the core sampler methodas described byBlake (1965). The dimensions of thesoil core were 7 cm diameter× 7.8 cm height. Thesampling depth was 20 cm. The penetration resistancewas measured using a manually operated soil conepenetrometer (ASAE, 1984) with a cone base diam-eter of 12.8 mm and cone angle of 30◦. The conewas hand-pushed into the soil at a uniform rate of1829 mm/min (as recommended byASAE (1984))immediately after the seeding operation. Four ran-dom penetration resistance measurements were madein each treatment over the entire depth from the soilsurface to 20 cm.

Table 1Mean valuesa at 20 cm depth soil dry bulk density (Bd) in Mg m−3 and penetration resistance (Pr) in MPa and soil water content (MC)in g kg−1 at crop emergence, flowering and harvesting in a sandy loam in northern Nigeria for 1998b

Treatment Crop emergence Flowering Harvesting

Bd Pr MC Bd Pr MC Bd Pr MC

0 Pass 1.29 c 0.64 d 9.2 e 1.32 e 0.50 e 12.8 c 1.30 d 0.60 e 10.0 e5 Passes 1.40 b 1.41 c 9.6 d 1.44 d 1.32 c 3.0 c 1.42 c 1.35 d 10.6 d10 Passes 1.43 b 1.47 bc 10.5 c 1.52 c 1.39 bc 13.5 b 1.45 c 1.42 c 11.2 c15 Passes 1.50 b 1.54 b 11.5 b 1.63 b 1.45 b 14.6 a 1.59 b 1.52 b 12.0 b20 Passes 1.60 a 1.62 a 12.0 a 1.68 a 1.58 a 14.9 a 1.65 a 1.60 a 12.8 a

a Values are means of four replicates.b Values followed by the same letter down the column do not differ significantly atP = 0.05 using Duncan’s multiple range analysis.

Table 2Mean valuesa at 20 cm depth soil dry bulk density (Bd) in Mg m−3 and penetration resistance (Pr) in MPa and soil moisture content (MC)in g kg−1 at crop emergence, flowering and harvesting in a sandy loam in northern Nigeria for the year 1999b

Treatment Crop emergence Flowering Harvesting

Bd Pr MC Bd Pr MC Bd Pr MC

0 Pass 1.40 e 0.60 e 10.7 c 1.52 d 0.41 e 13.5 e 1.45 d 0.56 d 12.1 e5 Passes 1.51 d 1.29 d 10.8 c 1.65 c 1.23 d 13.9 d 1.56 e 1.25 c 12.4 d10 Passes 1.62 c 1.35 c 11.9 b 1.72 b 1.27 c 14.8 c 1.70 b 1.32 b 13.0 c15 Passes 1.69 b 1.45 b 12.5 a 1.74 b 1.32 b 14.9 b 1.71 a 1.41 a 14.2 b20 Passes 1.75 a 1.50 a 12.6 a 1.92 a 1.41 a 15.0 a 1.80 a 1.45 a 14.4 a

a Values are means of four replicates.b Values followed by the same letter down the column do not differ significantly atP = 0.05 using Duncan’s multiple range analysis.

At 7 weeks after emergence, plant height, plantmoisture content, and stem diameter were determined.The crop was harvested in December of each year andthe grain yield determined.

2.4. Statistical methods

Duncan’s multiple range analysis was used to testthe significance(P ≤ 0.05) of the treatments on soilbulk density, penetration resistance, soil moisture con-tent, plant height, plant stem diameter, plant moisture,and grain yield using the MTB statigraphics statisticalcomputer package, Version 3.1.

3. Results and discussion

3.1. Soil compaction

Tables 1 and 2show the mean values of soil dry bulkdensity, penetration resistance and moisture contentfor each treatment at the stages of crop emergence,

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flowering and harvesting for the growing seasons of1998 and 1999.

Dry bulk density, penetration resistance and soilmoisture content increased with increased number oftractor passes at all the three stages of crop growth.The effect of number of tractor passes on soil bulkdensity was significant(P < 0.05). The highest meanvalue of dry bulk density obtained was 1.68 Mg m−3

at 20 tractor passes with a moisture content valueof 14.9 g kg−1. Bulk density increased with increas-ing value of moisture content. Penetration resistanceincreased with increasing number of tractor passes,while it decreased with increase in soil moisture con-tent. These results agree with those obtained bySoane(1970), Ohu et al. (1994)and Mamman and Ohu(1997). The effect of the number of tractor passeson penetration resistance was significant(P < 0.05).Therefore, penetration resistance depends on appliedload (compaction) bulk density and moisture contentamong other properties.

With increased tractor passes, the soil moisture con-tent increased significantly(P < 0.05). The soil mois-ture content in 1998 was higher than 1999. This couldbe attributed to the fact that the amount of total rain-fall was more in 1999 than in 1998. The total rainfallamount for 1998 and 1999 were 692 and 811 mm, re-spectively, in the area of the research plot.

3.2. Crop response

Tables 3 and 4show the mean values of plant height,plant stem diameter and plant moisture content at 7

Table 3Mean valuesa of plant height, plant stem diameter, plant moistureat 7 weeks after plant emergence and grain yield at harvest forthe year 1998b

Treatment Plantheight(cm)

Plant stemdiameter(cm)

Plantmoisture(g kg−1)

Grainyield(kg ha−1)

0 Pass 28.8 a 0.60 a 28.4 a 980 c5 Passes 28.7 d 0.65 d 30.6 d 1050 b10 Passes 25.3 c 0.81 c 35.3 c 1200 a15 Passes 22.1 b 0.92 b 36.8 b 750 d20 Passes 20.7 a 1.01 a 40.0 a 550 e

a Values are means of four replicates.b Values followed by the same letter down the column do not

differ significantly atP = 0.05 using Duncan’s multiple rangeanalysis.

weeks after crop emergence and grain yield at harvestfor 1998 and 1999, respectively.

Treatments had significant effects on plant height(P < 0.05). The average plant height (1998 and 1999)varied from a maximum of 30.4 cm with zero trafficto a minimum of 22 cm for 20 passes of tractor traffic,a decrease in height of 25.6%. However, there was nosignificant difference(P < 0.05) in height betweenthe plants grown on plots with zero and five tractortraffic passes. Similar results have been reported byNgunjiri and Siemens (1995)who found that wheeltraffic significantly decreased maize height. The plantstem diameter increased significantly with increasedtractor passes(P < 0.05). This result is probablya consequence of less root development in the com-pact soils as suggested byOhu et al. (1985). Liptayand Gier (1983)reported an increase in stem diameterof tomato (Lycopersicon esculentumMills) seedlingswhich was attributed to variations in the compressionof the soil in the vertical profile.Mamman and Ohu(1997) also reported that with increased compactionover a soil surface, the stem diameter of millet (Pen-nisetum glaucumL.) plants also increased.

There was a significant difference(P < 0.05) inplant moisture content between treatments. The aver-age maximum plant moisture content value obtainedwas 40.4% from plots with 20 tractor traffic passescompared to 28.8% in the zero traffic plots, a decreaseof 11.67%.

A grain yield of 984.5, 1067.5, 1276.5, 775 and555 kg ha−1 were obtained for the 0, 5, 10, 15, and 20tractor traffic passes, respectively. Plots treated with

Table 4Mean valuesa of plant height, plant stem diameter, plant moistureat 7 weeks after plant emergence and grain yield at harvest forthe year 1999b

Treatment Plantheight(cm)

Plant stemdiameter(cm)

Plantmoisture(g kg−1)

Grainyield(kg ha−1)

0 Pass 32.1 a 0.71 c 29.15 e 989 c5 Passes 31.5 a 0.76 cb 31.7 d 1085 b10 Passes 28.8 b 0.86 ab 36.8 c 1353 a15 Passes 25.7 c 0.99 b 37.75 b 800 d20 Passes 24.6 c 1.31 a 40.8 a 560 e

a Values are means of four replicates.b Values followed by the same letter down the column do not

differ significantly atP = 0.05 using the Duncan’s multiple rangeanalysis.

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10 passes of tractor traffic passes giving the highestsignificant yield value(P ≤ 0.05). For all the treat-ments, the greater grain yield in 1999 than in 1998was probably due to additional rainfall.

Correlations between grain yield and soil bulk den-sity, penetration resistance, soil water content and theproduct of contact pressure and number of tractorpasses were established (r2 = 0.75 for Bd; r2 =0.73 for Pr; r2 = 10.96 for MC; r2 = 0.84 fortµ). A multiple regression equation was establishedas follows:

Yg = 72030+ 4849Bd− 1381Pr

+614MC− 28.4µ, (1)

where Yg is the grain yield (kg ha−1), Bd the bulkdensity (Mg m−3), Pr the penetration resistance(MPa), MC the soil water content (g kg−1), t thenumber of tractor passes, andµ the contact pressure(kPa).

From the results presented, it is evident that in asandy loam soil under the condition investigated, 10passes of tractor traffic soil compaction can be bene-ficial to the cowpea production. Similar investigationson other crops support this report although with differ-ent number of tractor passes (Voorhees, 1982; Swanet al., 1987; Ohu and Folorunso, 1989; Raghavan et al.,1979; Mamman and Ohu, 1997).

4. Conclusions

This study showed that dry bulk density, soil mois-ture content and penetration resistance increased withincreased tractor traffic passes. Significant differencesin grain yield, plant moisture, plant stem diameterand plant height were obtained from tractor passes.Maximum cowpea grain yield was obtained at 10tractor traffic passes. The need for an appropriate se-lection of machine weight, tyre size and traffic timingprogramme for agricultural production efficiency andprofitability is indicated from the results obtained. Itcan therefore be concluded that although excessivesoil compaction is detrimental to crop growth andyield, an optimum level of machinery traffic is ben-eficial to crop production. It is therefore necessaryto achieve optimum level of soil properties to givemaximum yield.

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