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_ ~ d l k ~ K e s e a r c n E LS EV I ER Soil & Tillage Research 32 (1994) 303-311

The effect of soil compaction on mole plough draught

W.C.T. Chamen a,*, R. Cavalli b ~Soil Science Group, Silsoe Research Institute, Wrest Park, Silsoe, Bedford, MK45 4HS, UK

bDepartment of Land and Agricultural and Forestry Systems, Universit?t degli Studi, Via Gradenigo 6, 35131 Padova, Italy

Accepted 14 July 1994

Abstract

Conventional and zero traffic systems were mole ploughed and effects on soil physical properties were compared. Draught of the plough operating at 550 mm depth was mea- sured while it was winched across plots having a 5-year history of different traffic regimes. Results showed that the draught was reduced by about 18% on non-trafficked compared with conventionally-trafficked soil.

Cone resistance measurements, 1 month before and 3 months after mole ploughing, con- firmed that the non-trafficked soil had significantly less strength to a depth of about 400 mm. Bulk density measured at 75 and 175 mm depth 1 month before mole ploughing indicated a similar trend, but clod and bulk densities at 125 mm and 350 mm depth 3 months later, failed to show any consistent differences between treatments.

Keywords: Mole ploughing; Soil compaction; Zero traffic; Draught; Cone resistance; Bulk density

I. Introduction

Mole ploughing is widely used to improve the drainage of clay soils (Hudson et al., 1962 ). Although it is inexpensive compared with the installation of a sim- ilar network o f plastic drains, it remains a relatively costly operat ion because of its high draught demand and low rate o f work. Thus, any soil management tech- nique which leads to a reduct ion in mole plough draught could be of considerable benefit.

It had been observed at the conclusion o f an earlier traffic exper iment (Cha- men et al., 1990) that the draught o f a mole plough, as indicated by t ractor speed

*Corresponding author.

0167-1987/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI0167-1987(94)00426-9

304 W.C.T. Chamen, R. Cavalli ~Soil & Tillage Research 32 (1994) 303-311

and power demand, was affected by the different treatments. Such differences were therefore measured at a later date on the same site, when mole ploughing was carried out on a similar trial in April 1991. The objectives of the study were to determine the differences in mole plough draught requirement between traf- ficked and non-trafficked soil and to identify the reasons for these differences in terms of the soil conditions.

2. Methods

Details of the site are given in Table 1. The treatments (Chamen et al., 1992 ) consisted of a conventional tractor and machinery system and a zero traffic re- gime maintained by a 12-m gantry employing conventional implements (Cha- men et al., 1994). Two primary cultivation techniques were used to deal with either burnt or chopped straw, namely ploughing to 200 mm depth and tine cul- tivation or discing to about 150 mm depth. The gantry and tractor systems were introduced in 1986 and continue to be used to establish, maintain and harvest a rotation of cereal crops. A brief history of the site is given in Table 2. Three rep- lications of each treatment were laid out in a randomised block design on plots 24 m wide by 35 m long.

To minimise wheel compaction on the non-trafficked plots, the mole plough, with a 75-mm diameter mole and 100-ram diameter expander working nominally to 550 mm depth, was mounted on a trailed linkage and winched across the site (Fig. I ). To ensure identical conditions, the winch was used on both gantry and tractor plots. During winching, mean values of draught were estimated over the length of each plot by continuous observation of the analogue dial of a hydraulic dynamometer. As the wheels of the trailed three point linkage were observed to

Table 1 Soil description and textural analysis of the site

Soil constituents Amount (g 100g -~)

Depth range (ram)

0-260 260-400

Clay ( < 2 #in) Silt (2-60 pm) Fine sand (60-600/~m) Coarse sand (600 pro-2 ram) Organic carbon Calcium carbonate Moisture content at lower plastic limit Soil taxonomy (USDA, 1975 ): clayey, mixed, mesic aquic eutrochrept Soil series: Evesham

68 71 23 21

7 6 2 2 3.9 1.9 1.0 4.1

47.3

W.. C. T. Chamen, R. Cavalli / Soil & Tillage Research 32 (I 994) 303-311

Table 2 Recent history of the site

305

Year Details of operations Crop harvested

1986 Mole ploughed, subsoiled and rotary dug a Winter wheat 1987 Straw on cultivated plots burnt Spring barley 1988 Straw on cultivated plots burnt Spring oats 1989 Straw on cultivated plots burnt Winter wheat 1990 Straw on all plots chopped Winter oats 1991 Straw on all plots chopped Spring barley 1991 Mole ploughed in April

a Chamen et al., 1979.

Fig. 1. Winching of the mole plough using a trailed three-point linkage as a stabilizing drawbar.

create different depths o f rut on each of the t rea tments , a straight edge was used to measure the a m o u n t of wheel sinkage.

Just pr ior to mole ploughing, soil cone resistance and bulk density were mea- sured on each o f the plots. Cone resistance was measured to a depth of 350 m m at 12 posi t ions on each plot, with a Bush recording pene t rome te r having a cone with a base d iamete r o f 12.83 m m and semi-angle of 15 °. Bulk density was mea- sured using 75 -mm d iame te r cores taken at three depths: 0 -50 , 50-100 and 150- 200 m m at two r andomly selected posi t ions on each plot. Following harvest o f the spring barley crop, which was accompl ished using the traffic t rea tments al- ready described, fur ther soil measu remen t s were taken, but to a greater depth than those recorded previously. The sampl ing strategy was designed specifically

306 W.C.T. Chamen, R. Cavalli ~Soil & Tillage Research 32 (1994) 303-311

to investigate the reasons for the relatively large differences in mole plough draught observed between the different traffic treatments. Cone penetration resistance and clod and bulk density measurements were made on the tine cultivated plots of both traffic treatments. Clod density was determined from ten clods, each of about 0.2 dm 3 volume, extracted undisturbed from three positions (in the mid- line between mole channels) on each of the sampled plots at approximately 125 mm and 350 mm depth. The clods were coated in Saran resin to determine their density as described by Brasher et al. (1966) and Blake (1965). Clod volume was corrected, assuming normal shrinkage (McGarry and Malafant, 1987), to the value equivalent to the mean moisture content of the complete sample set, which was 275 g'kg- ~. Bulk density was measured at depths of 125-175 and 325- 375 mm.

Cone resistance was measured as before but 20 sets of readings were taken on each plot to a depth of 450 mm.

3. Results and discussion

3.1. Soil density

April 1991 measurements of dry bulk density and water content are presented in Table 3. The effects of traffic and depth were significant at P < 0.01 for both density and water content, but there was an interaction (P< 0.01 ) between cul- tivation and traffic as far as density was concerned. The differences in water con-

Table 3 (a) Soil dry bulk density water content in April 1991

Depth (mm) Bulk density Water content (g kg-~ ) (kg m-3): mean of all Gantry Tractor Mean treatments

25 847 241 367 304 75 936 420 470 445

175 1005 432 483 457 LSD (P< 0.05 ) 47 40 30

(b) Dry bulk density according to treatment (kg m -3)

Traffic Cultivation

Ploughed Cultivated

Gantry 877 832 Tractor 966 1041

LSD (P< 0.05) 46

W. C. T. Chamen, R. Cavalli / Soil & Tillage Research 32 (1994) 303-311

Table 4 Clod dry density and water content in September 1991

307

Depth (mm) Density (kg m -3) Water content (g kg -~ )

125 1804 289 350 1625 261

LSD (P<0.05) 93 11

Table 5 Soil dry bulk density and water content in September 1991

Depth ( mm ) Bulk density (kg m - 3 ) Water content (g kg - ~ )

150 1215 289 350 1300 258

LSD (P<0.05) 62 14.7

tent were confined to the change between 25 mm and 75 mm depth, with no further significant increase between 75 and 175 mm depth. However, bulk den- sity increased significantly between all depths and between gantry and tractor traffic, but the limited depth of measurement meant that it was difficult to relate these data with the draught of the plough (see later section).

Differences in clod density and water content in September were confined to the effect of depth (Table 4). This was surprising and it was postulated that the earlier differences between the treatments were due to a dissimilarity in macro- porosity or structural cracks between clods which would not have shown up in the clod analysis. To check this hypothesis, nine cores were taken from 100-200 mm and 300-400 mm depths on single plots of each of the two treatments. Re- suits (Table 5) again showed that only the effect of depth was significant (P< 0.001 ). It was concluded therefore that wetting and drying cycles of the soil, together with its drier state in September, largely masked the differences that had been found earlier in the year.

3.2. Soil cone resistance

Cone resistance data were analysed as a split plot design over all depths and treatments. In April (Fig. 2) resistance increased significantly (P<0 .001) with depth and on average was significantly greater (P< 0.05) over the whole depth range on the tractor compared with the gantry plots ( 1.22 compared with 1.03 MPa respectively, LSD = 0.01 MPa). There was no evidence to suggest that type of cultivation had an effect. As soil water content on the gantry plots was less than on the tractor plots (Table 3 ), this would have increased cone resistance on the non-trafficked soil. Therefore the contrast between trafficked and non-traf- ficked soil was likely to have been greater than was actually recorded.

308 W..C.T. Chamen, R. Cavalli / Soil & Tillage Research 32 (1994) 303-311

Cone resistance (MPa) 0 1 2

0 D -" -- " 0 D

0 [] 100 o u • •

~ O0 R [] 200 o D • •

v

(3 3 0 0 ~ •

o% • i Tra~or, ~ori11991

400 Gantt)~ April 1991 • • Ttactc¢ Sept 1901 Gantr)i Sept 1991 • •

50C

Standard errors (MPa) D Depth. T Traffic

H D= 0.06~ Apt91 J T=004

H D = 0.071 Sep91 I T=00S

Fig. 2. Soil cone resistance for trafficked (tractor) and non-trafficked (gantry) plots in April and September 1991.

The September measurements of cone resistance (Fig. 2) exhibited a similar trend to those in April, but to a greater extent, with all differences being signifi- cant at P < 0.00 I. The mean resistance on the tractor plots was 2.06 MPa com- pared with 1.60 MPa on the non-trafficked soil (LSD=0.20 MPa). On this oc- casion there was no significant difference in the soil water content between treatments but there was a significant change with depth (Table 5 ).

These results were in contrast to those of clod and bulk density taken in Sep- tember, and it was postulated that differences in cone resistance could have been caused by the small but non-significant differences in soil water content. To test this hypothesis, the following relationship between cone resistance and soil water content derived by Utomo and Dexter ( 1981 ), was assumed:

April lnQ1 = a + bwl ( 1 )

September lnQ2 = a + bw2 (2 )

where QI and Q2 are the mean cone resistances and Wl and w2 the mean water contents at 125 mm depth recorded on the different dates. Values of 1.474 and -3 .181 were obtained for a and b respectively. These values were then used in the same equation with the cone resistances measured in September for trafficked and non-trafficked soil, to predict the soil water content. Taking cone resistances of 2.099 MPa for the trafficked and 1.491 MPa for the non-trafficked plots, the predicted water contents were 230 and 338 g kg- 1 respectively. The mean differ- ence in water content between traffic treatments was only 1 g kg- 1, showing that the differences in cone resistance could not have been accounted for by the vari- ation in water contents that were observed.

3.3. M o l e plough draught

Draught of the mole plough on the tractor plots was significantly greater than on the gantry plots (P< 0.01, Table 6 ), but cultivation method (ploughing or tine

W. C. T Chamen, R. Cavalli / Soil & Tillage Research 32 (1994) 303-311 309

Table 6 Draught requirement and variation in nominal depth of a mole plough when operated on trafficked (tractor) and non-trafficked (gantry) soil at 550 mm depth

Traffic Draught (kN) Wheel sinkage (mm)

Gantry 25.3 44.7 Tractor 29.6 22.5

LSD (P<0.05) 2.1 7.7

cultivation) had no effect. The mean effect of traffic was a 14.5% reduction in draught for the non-trafficked compared with the trafficked soil. This is a con- servative measure of the difference between the treatments, because the wheels of the carriage sank to a greater extent on the gantry plots (Table 6 ). This led to a 22.2-mm increase in depth of operation for the non-trafficked compared with the trafficked soil. According to Godwin et al. (1981), the draught of a mole plough tends to increase as the square of the depth of work. If the draught of the mole plough on the gantry plots is corrected to the equivalent depth on the tractor plots (550+22.2 mm) , the value on the non-trafficked soil is reduced to 24.3 kN. This represents a 17.9% reduction in draught compared with the conven- tional treatment. If an allowance for the increased rolling resistance of the car- riage on the zero traffic plots were to be included, a further reduction in draught for mole ploughing would result.

The relationship between both clod and bulk density and mole plough draught was inconsistent. In April, the bulk density of the non-trafficked soil was about 150 kg m -3 less than the trafficked soil, reflecting the differences in mole plough draught recorded, but in September bulk and clod densities for the different treat- ments were indistinguishable. It is apparent therefore that the density of the soil is not always a good indicator of soil strength. This is probably associated with changes in the structure of the soil, whereby a similar density can exist, but where the pore size distribution is very different. Watts and Dexter (1994), working on this site, found that the geometric mean diameter of aggregates on the gantr3 plots was smaller and the macro-porosity greater'than on the trafficked soil. How- ever, these data only relate to the top 150 mm of soil, and the soil conditions which are likely to have had an effect on the mole plough would almost certainly have existed at a depth of at least 300 mm. It can only be postulated therefore that the dissimilar soil structural conditions near the surface persisted to this depth in the profile.

Below a moling depth of about 300 mm, the leg forces on a mole plough tend to predominate over those from the foot and expander and, in a cohesive soil, the resultant force tends to move up the leg with increasing depth of operation (God- win et al., 1981 ). At 550 mm depth, it is estimated that the resultant force would have been approximately 350 mm deep. As the depth effects of compaction on this soil had in the past generally been confined to the top 300 mm, the magnitude of the differences in draught force between treatments was unexpected. However,

310 W.C.T. Charnen, R. Cavalli / Soil & Tillage Research 32 (1994) 303-311

the rigorous approach to the results of the cone resistance measurements con- firmed that real differences in strength existed at the depth of the resultant draught force. It would appear, therefore, that cone resistance measurements provide a good indication of differences in draught force.

The draught data recorded in this experiment were very similar to that pub- lished by Godwin et al. (1981 ). In their work on an Evesham soil with similar properties, the total horizontal force recorded with a 100-mm expander operating at a comparable depth was 29.2 kN. No mention, however, was made of the effect of soil conditions on the total draught of the plough and there are few other pub- lished data relating to the soil resistance of mole ploughs.

The effect which differences in traffic compaction may have on the persistence of the mole channel is of considerable interest. Although no data have yet been recorded on this aspect, it is hoped that some measurements can be made within the expected life span of the mole system.

4. Conclusions

Zero compared with random traffic on the soil led to a reduction of about 18% in the draught requirement of a mole plough working at 550 mm depth. This reduction was associated with contrasts in soil strength of about 17% in the top 400 mm of the soil profile, as indicated by differences in cone resistance. Soil bulk or clod density was not found to be a reliable indicator for predicting differ- ences in strength or draught requirement on this soil.

Acknowledgements

The authors are grateful to Dr W.R. Whalley of Silsoe Research Institute for making his data on cone resistance and bulk density from April 1991 available to the authors. This work was part of a programme funded by the Ministry of Agri- culture, Fisheries and Food.

Prof. R. Cavalli was supported at Silsoe Research Institute by The British Council/National Research Council Scientific Co-operation Program--Italy.

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