Soil & Tillage Research, 6 (1985) 17--29 17 Elsevier Science Publishers B.V., Amsterdam --Pr in ted in The Netherlands

COMPACTION, HYDROLOGICAL PROCESSES AND SOIL EROSION ON LOAMY SANDS IN EAST SHROPSHIRE, ENGLAND

MICHAEL A. FULLEN

Environmental Sciences Section, School of Applied Sciences, The Polytechnic, Wolverhampton WV1 1L Y (Gt. Britain)

(Accepted 2 April 1985)

ABSTRACT

Fullen, M.A., 1985. Compaction, hydrological processes and soil erosion on loamy sands in east Shropshire, England. Soil Tillage Res., 6 :17- -29 .

Field investigations on loamy sands in east Shropshire show that compaction by agricultural machinery increases soil bulk density and soil erodibility, and decreases infiltration rates. Structural and hydrological changes, in combination with runoff con- centration in cultivation lines, can contribute to serious erosion of arable soils. Com- pacted soils are also more responsive to rainfall and evidence is presented that intensities as low as approximately 1 mm h " can be erosive. Evidence suggests that compacted subsoils impede infiltration and so contribute to surface runoff and serious topsoil erosion.

INTRODUCTION

Water erosion of arable soils is increasingly recognized as a problem in agricultural areas of Britain (Evans, 1980; Morgan, 1980; Fullen, 1985). Studies in east Shropshire showed that erosion rates on runoff plots con- siderably exceeded tolerable levels when rotovated softs were allowed to compact by raindrop impact and splash (FuUen, 1984).

Soil compaction by agricultural machinery tends to decrease infiltration rates (Trouse, 1966; Gaheen and Nj~bs, 1977), so water has a greater pro- pensity to run off slopes, increasing the likelihood of erosion. Systematic photographic surveys (Evans, 1980) and reports of erosion in various regions of Britain (Fullen, 1985) suggest both the areal extent and the magnitude of erosion are increasing. Moreover, recent discussions on rainfall intensity have proposed increasingly lower thresholds at which rainfall becomes erosive. Hudson (1967, 1971) considered falls of 1 inch per hour (25.4 mm h- ' ) to be necessary to cause erosion, and as such intensities are con- sidered rare in temperate climates the erosion hazard in Britain was thought to be low. Field studies on sandy softs in Bedfordshire led Morgan (1978) to suggest that falls of 10 mm h -' can produce splash erosion, but studies in

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Shropshire suggested that rates as low as approximately 2.0 mm h-* can initiate erosion on crusted loamy sands and that severe damage is inflicted by falls > approximately 10 mm h-* (Fullen, 1984). Reed (1979) con- sidered intensities as low as 1 mm h -* to be erosive on compacted arable Shropshire soils. Increases in the areal extent and magnitude of erosion and the declining thresholds of erosivity could reflect the effects of soil compaction.

Given the present tendency for greater use of increasingly heavy farm machinery (Soane, 1981) compaction-related problems can be expected to increase. Fur thermore, cultivation during very moist conditions or with blunt ploughs can smear the soil, producing a compact subsoil with a platy structure. U.S. agriculturalists have long been aware of plough pans (Kohnke and Bertrand, 1959), but pans have only recently been recognized as a problem on European softs (Ehlers et al., 1980). This paper focuses upon the structural and hydrological changes consequent upon soil compaction and their effects on soil erosion.

FIELD STUDIES AND METHODS

Field investigations were conducted within an area of approximately 100 km 2 in east Shropshire, mainly within the parishes of Claverley, Rudge and Worfield (Fig. 1). Most soils are freely drained sandy loams and loamy sands of the Newport and Bridgnorth Associations, consisting primarily of medium- and fine-grained sand with a low organic content of 1--2%. Fallow arable topsoil samples taken throughout the area from these series had a mean organic content of only 1.90% by weight (-+ S.D. 0.93, N = 61, determined by loss-on-ignition). The mean textural characteristics of 8 Bridgnorth series Ap horizon samples were sand {2000 83 urn) 80.2% by weight, silt (63--2 Urn) 16.0% and clay (< 2 ~m) 3.8%.

Detailed erosion studies were conducted at a field station near Hilton, east Shropshire (NGR SO 778952) (Fig. 1). Soil structural conditions were investigated using a Vicksberg penetrometer , with a 1.58-cm diameter 30 ° cone, and a bulk density corer, which placed 5-cm-deep soil samples into 220-cm3-volume tins. Samples were then oven dried at 105°C for 24 h, to determine dry bulk density.

The infiltration rate of ponded water proved a very sensitive measure of soil structure and was determined using double-ring flooding infiltrometers (Hills, 1970). Water was carefully poured into both rings, onto a plastic sheet which acted as a baffle to prevent soil disturbance and sealing with fines, after which the sheet was carefully removed. When the rates of falling head had settled to a reasonably constant level, usually between 20 min and 2 h after water appication, rates of lowering of head were measured using a hook gauge accurate to 0.01 ram.

Two replicate infiltrometers were inserted on pasture and 4 on uncom- pacted fallow soils on the Hilton site. Direct insertion of infiltrometers into

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crusted softs broke the crust and produced unrepresentative results, so infil trometers were inserted into freshly-rotovated raked soils which were then allowed to crust by raindrop impact, thus sealing the infi l t rometer. Wheelings were formed by a 4.5-t t ractor on the flat top of the Hilton site in November 1982 and 2 infi l t rometers were forced approximately 5 cm into them. Infi l t rometers were inserted into a plough pan that had been exposed by gully erosion in July 1983 near Hilton village (NGR SO 776955), 0.5 km north of the Hilton site (Fig. 1). Pene t rometer readings showed the pan to persist to between 20 and 40 cm depth. Soil above the pan was carefully cut away, and at 2 sites < 2 m apart, 2 infi l t rometers were inserted.

RESULTS

Table I shows that significant increases in topsoil bulk density resulted from the packing of soil by raindrop impact and splash after the soils had been cultivated by mechanical rotovat ion. Artificial compact ion o f soils

TABLE I

Summary of bulk density values of east Shropshire soi ls (g cm -3)

Soil condition Max Min Mean S.D. N t value*

1 Freshly rotovated

2 Crusted

1.29 0.89 1.08 0.09 50 lv2 --9.49

1.45 1.01 1.24 0.09 50 2v3 --8.43

3 Artificially c o m p a c t e d 1.71 1.12 1.41 0.11 50 lv3 --15.78

*All t values P < 0 . 0 0 1 ; d.f. = 98 .

TABLE II

Inf i l trat ion rate m e a s u r e m e n t s on east Shropshire soi ls ( m m h -I )

Cond i t i on M a x i m u m M i n i m u m Mean S.D. N Mean Vicksberg penetrom- eter c o n e res istance (kPa)

Pasture 807 147 343 192 23

Crusted 78.60 5.15 30.16 17.85 88 < 180

Plough Pan 5 . 7 4 1 .57 3 . 3 0 1 .30 32 5069 at 30 cm

Tractor whee l ing 0 . 3 7 0 . 0 1 6 0 . 1 3 0 . 0 7 1 2 6 2 0 9 0 at 15 cm

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by farm machinery resulted in further and significant increases in bulk density.

Infiltration rates were strongly affected by degree of soil packing. Due to the presence of an organic mat and root channels penetrating the surface, infiltration rates on permanent pasture were high (Table If), similar to those reported for old permanent pasture on U.S. silt foams (Musgrave and Holtan, 1964; Skidmore et al., 1975). Infiltration rates on crusted soils were con- siderably lower, but the measured rates were quite high compared with the 1--61 mm h-' range measured on U.S. agricultural soils (Dunne, 1978). The distribution of rates reported in Table II accord with the 25--50 mm h -I range reported for loamy sands (Kohnke and Bertrand, 1959).

Mechanical compaction of soils resulted in a drastic decrease in infiltration rates. On the Hilton tractor, wheeling rates were so low that 3 days infiltra- tion were required to obtain a representative estimate. Table II shows that rates were reproducibly low, with a mean rate on crusted soils exceeding that within the wheelings by more than 2 orders of magnitude. Field obser- vations during dry weather showed that these measurements were generally applicable because ponded water frequently remained in wheelings for several days, and so rates were < 1 mm h -~. Standing water within wheelings was particularly noticeable on compacted headlands, where soils were not only subject to heavy trafficking, but were also smeared by wheel-slip.

Table II shows the notable decrease in infiltration rate on the pan, from approximately 30 mm h-' on crusted soils to approximately 2 mm h -1. Reduced infiltration was confirmed by the frequent observation of standing water on the pan. These pools were usually in a very turbid state, with sam- pled sediment concentrations of 4000--6000 mg 1-'. As the pools dried out a wet clay skin remained on the pan surface which further decreased in- filtration rates, thus adding a textural barrier to infiltration to the pre- existing structural barrier.

Soil compaction increases the water-erodibility of soils, as can be illustrated by calculating erodibility (K) values from the Universal Soil Loss Equation (Wischmeier, 1977). K was estimated using 8 particle-size analyses of loamy sands of the Bridgnorth series, 61 soil organic matter determinations, ob- servations of soil structure in topsoils, soil pits and cuttings, supported by bulk density and penetrometer surveys, and by field measurements and observations of profile permeability. On crusted soils mean computed K was 0.08, comparable with K values reported for U.S. sandy loams (Olson, 1981). K increased to 0.20 on mechanically-compacted soils; thus, soil compaction increases the water erodibility of the soils which, because of their sandy nature and low organic content , are already very erodible. In Bavaria, Becher et al. (1980) reported comparable K values of 0.3 and 0.35 on uncompacted ridges, and 0.4 and 0.45 in adjacent compacted furrows on brown earths and podzols respectively.

Fullen (1984) reported that rainfall intensities as low as 1.4 mm h- ' initiated runoff from an array of 8 runoff plots while soils were in a crusted

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TABLE III

Observed low-energy erosive events on compacted soils

Date Time of event (GMT) Duration Fall (mm) Intensity equivalent (ram h -1)

9.12.82 1327--1410 43" 1.4 1.95

14.03.83 1130--1150 20" 0.4 1.2

15.09.83 1415--1420 5" 0.4 8.0

10.10.83 1035--1045 10" 0.3 1.8

30.01.84 1300--1400 a 1' 1.0 1.0

aObservations over 1 h during a prolonged low-intensity event.

condition. Given the soil structural changes consequent upon compaction, lower erosivity thresholds and increased erosion rates are probable on compacted soils. Table III shows events where plot runof f did not occur, though runoff took place on tractor wheelings. Short blustery showers of only 0 . 3 - 0 . 4 mm in 5--10 min generated low-energy flow down tractor

Fig. 2. Active gully erosion, Grange Farm, Hilton, 16 January 1984. Note the presence of the compact shoulder at 15--20 cm depth.

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wheelings on very moist soils. Water recurrent ly accumulated on flat surfaces in depressions, and where the local slope was greater than approximate ly 5 ° it over topped and integrated causing runof f down t rac tor wheelings. On such saturated soil very small increments of rainfall were necessary to maintain erosional processes.

Gully erosion on 16 January 1984 at Hilton (NGR SO 782952) resulted from a moderate fall on snow-melt-saturated soil. Only 5.6 mm of precipita- t ion fell during a 4-h period at consistent intensities ranging between 1.0 and 1.8 mm h- ' , but this was sufficient to reactivate a gully in which there was severe topsoil and subsoil compact ion. Rainfall intensity when Fig. 2 was taken was 1.8 mm h -~, and serious erosion occurred, al though only a little low-energy flow was observed on the runof f plots.

Reed (1979) suggested that rainfall intensities of only 1 mm h -~ are erosive on compacted arable soils in east Shropshire, and such falls are very frequent . Between 1965 and 1983 rain fell at intensities > 1 mm h -I for a total of 3541 hours ( a xr 196.7 hours per year) at Shawbury, north Shropshire (Fullen, 1984). Field observations also provided evidence that artificial soil compact ion and runof f concent ra t ion within cultivation lines were contr ibut ing to a serious erosion problem. Water erosion was identified at over 1000 sites in the West Midlands between 1965 and 1983 and soil compact ion and up and down slope cultivation lines were considered as major con t r ibu tory factors in over 95% of cases (Reed, 1983). Drill lines

Fig. 3. Turbulent flow down drill lines, Westbeech Farm, 26 April 1983.

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Fig. 4. Turbulent flow down drill lines, Westbeech Farm (close up). Sampled sediment concentration of runoff 7.01 x 10' mg 1 -~.

Fig. 5. Active gully erosion, Wrottesley Lodge Farm, 27 April 1983.

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Fig. 6. Active gully erosion, Wrottesley Lodge Farm (close up). Sediment concentrat ion of runoff (mean of 2 samples) 10.96 × 104 mg 1-L

and tractor wheelings of ten acted as initial axes of erosion, incising as rills into the Ap horizon. Frequently, rills broke ou t from cultivation lines and integrated into a dendritic or trellisized pat tern of deeper rills, sometimes incising gullies up to 1.0--1.5 m deep.

The number of t ractor passes, orientat ion of wheelings in relation to slope and the direction of movement in relation to slope, were recurrent con- t r ibutory factors. Deeper rill features f requent ly formed in secondary or tert iary wheelings, and adjacent primary wheelings and drill lines acted as receptors of splashed material or developed smaller rills. Wheelings where the apex of the V-shaped lug pat tern was orientated downslope were more prone to subsequent erosion as the tyre impressions allowed water to ac- cumulate in the lug depressions. With modera te amounts of rainfall these would overtop and integrate with other ponded depressions. Such integra-

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tion along the centre of each wheeling encouraged rill formation by incision into the wheeling.

The erosional significance of both topsoil and subsoil compact ion was well illustrated during the wet spring months of April and May, 1983. A total of 146.0 mm of rainfall was received at the Hilton site in April and 70.9 mm in May. In England and Wales, April totals were 200% and May totals 173% of average (Weather, 1983). Fallow arable soils throughout the area underwent serious erosion. Figure 3 shows the channelling of runoff in drill lines and tractor wheelings after the passage of a storm on 26 April 1983 at Westbeech Farm (NGR SJ 827004) . Of note is the moderate slope (4 ° ) and short length between the slope crest and point of runoff initiation. Turbulent flow is shown to be transporting considerable amounts of sedi- ment within drill lines (Fig. 4).

Compacted subsoils appeared to contr ibute to gully erosion by impeding deep percolat ion of the low intensity rainfall of early April, a conclusion supported by the low infiltration rates measured on the Hilton pan. Con- sequently, intense storms in late April fell upon soils where the upper part of the profile was saturated partly due to subsoil compact ion. Observations of deep gullies of ten revealed the presence of a platy structured compact shoulder at 20--30 cm depth, characteristic of a plough pan. Active erosion of a gully at Wrottesley Lodge Farm (NGR SJ 830016) (Figs. 5 and 6) exposed a compacted subsoil. Mean cone resistance on top of the pan was 1205 (-+ 490) kPa (N = 10) and the maximum was 6733 kPa, although of

Fig. 7. Gully, Wrottesley Lodge Farm, May 1983.

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Fig. 8. Gully, Wrottesley Lodge Farm, May 1983 (close up).

necessity these readings were taken in very moist condit ions and so readings were lower than on equivalent dry soils. The gully expanded by headward retreat, side-wall collapse and evacuation to form the large-scale feature shown in Figs. 7 and 8. Thus, one might agree with Ehlers et al. (1980) that compact subsoil zones are hydrologically active and influence slope runoff when topsoils achieve saturation.

DISCUSSION

Field measurements in east Shropshire demonstrate that soil compact ion by agricultural machinery can drastically reduce infiltration of rainfall into soil. Results are comparable to measurements on Hawaiian latosols subject to compact ion by sugarcane trucks (Trouse, 1966). As in Shropshire, bulk density increased only moderate ly (from 1.02 g cm -3 before trafficking to 1.20 afterwards), but infiltration rates were reduced from 173 to 3 mm h -I. Similar observations have been made in flower bulb beds in The Nether- lands (De Haan and Van der Valk, 1970), Norwegian loams (Gaheen and Nj~s, 1977) and U.S. soils (Akram and Kemper, 1979; Lindstrom and Voorhees, 1980). Diminished infiltration increases the tendency for slope runof f in response to frequently recurring low rainfall intensities. Conse- quent ly, soil erodibility and the probabil i ty of accelerated erosion are increased, especially if cultivation practices provide efficient conduits for concentrated runoff in the form of t ractor wheelings and drill lines. More-

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over, soil compact ion can decrease crop growth, a theme reviewed by Cannell (1977), and so diminish the protective effects of crop cover.

The extent and severity of soil compact ion can often be underestimated, which may partly explain the insufficient at tention devoted to compact ion-- erosion relationships. Because tractor wheel marks are often obscured or eradicated by subsequent operations, a record of wheelings can indicate a much higher coverage than is of ten suspected from observations of the surface. Soane (1975) reported 91% areal coverage of t ractor wheels within a field in Scotland during traditional seedbed preparation for spring barley {fertilizer distribution, harrowing twice, sowing, rolling).

Subsoil compact ion is a much neglected phenomenon, mainly because it is "invisible", at least until erosion reveals the problem. As noted in the field studies, deeper gully features of ten exhibited signs of severe subsoil compact ion, and some of the extreme soil erosion features discussed earlier might have been smaller if subsoil compact ion had been identified and rectified. Identification of compacted subsoils and appropriate remedial action might assist in preventing some of the more extreme cases of gully erosion. The penet rometer has proved a robust and informative instrument in investigations of subsoil structure and its widespread use in agriculture is recommended. Penetrometer surveys could identify the extent , depth and severity of subsoil compact ion in the field and recommend appropriate subsoiling operations. To be effective, subsoiling must break the plough pan rather than simply smear it further, as is of ten the case with shallow subsoiling on deeply compacted soils.

ACKNOWLEDGMENTS

I would like to thank Dr. J.A. Catt, Dr. A.H. Reed, Dr. J.P. Smith and Dr. R.V. Dackombe for their advice and encouragement and K. Muggleston and A. Morris for their help with field work.

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