Compaction by agricultural vehicles: A review III. Incidence and control of compaction in crop production

Soil & Tillage Research, 2 (1982) 3--36 Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

COMPACTION BY AGRICULTURAL

3

VEHICLES: A REVIEW

III. I N C I D E N C E A N D C O N T R O L O F C O M P A C T I O N IN C R O P PRODUCTION

B.D. SOANE, J.W. DICKSON and D.J. CAMPBELL

Scottish Institute of Agricultural Engineering, Bush Estate, Penicuik, Midlothian EH26 OPH (Great Britain) (Accepted 1 October 1981)

ABSTRACT Soane, B.D., Dickson, J.W. and Campbell, D.J., 1982. Compaction by agricultural vehicles: a review. III. Incidence and control of compaction in crop production. Soil Tillage Res., 2: 3--36. Modern systems of crop production are tending to increase both the number of passes and the loads carried on the wheels of agricultural vehicles. Therefore, compaction problems may arise, especially in seedbed preparation, spraying and harvesting operations. Because of the difficulty and cost of subsoil cultivation it appears likely that more importance will be attached to the avoidance of subsoil compaction since there is widespread evidence that such compaction may persist for many years even when deep freezing is a regular occurrence in winter. Compaction from wheel traffic has often been found to influence adversely all stages of crop growth, responses being particularly marked in the early phases of establishment. However, in some situations crop responses to compaction are beneficial. In both cases crop responses show marked interaction with weather conditions, particularly water status, during the growing period of the crop. Opportunities exist for reducing the compaction from vehicles. Apart from the combination of field operations to permit fewer wheel passes there would be additional benefits from reductions in load and tyre inflation pressure and by confining some or all traffic to pre-arranged strips for use solely as unplanted wheel tracks ("controlled traffic"). Changes in the demand for traction and in the amounts of applied sprays, amendments and fertilisers may permit a radical departure from current tractor design which could greatly reduce the incidence of compaction problems. The financial disadvantages attributable to the incidence of compaction in crop production are increasingly recognised but quantitative information is rarely sufficient to permit a cost/benefit analysis to be undertaken for those techniques which allow compaction to be avoided rather than ameliorated.

INTRODUCTION In this third a n d last part of this paper c o n c e r n i n g the p r o b l e m s of c o m p a c t i o n by agricultural vehicles, we have a t t e m p t e d to review s o m e r e c e n t

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work on three basic questions concerning compaction in crop production, namely: (1) In what circumstances and to what extent does wheel traffic on agricultural softs induce compaction? (2) To what extent does such compaction give rise to problems in crop production? (3) What measures can be adopted to overcome these problems within the economic constraints of commercial practice? The present concern about compaction in arable soils

Crop production today almost invariably involves the passage of wheeled or tracked vehicles for primary and secondary cultivation, sowing, spraying and harvesting operations. However, soils which have been cultivated to provide a medium for root growth will not generally have the strength subsequently to support most modern vehicles without considerable compaction and rutting. These effects render the soil less suited to crop growth and hence necessitate corrective treatments prior to the next crop such as more intensive or deeper primary tillage which will increase the costs of production and are rarely completely effective. It has become increasingly necessary to relate the compaction produced Dry bulk d e n s i t y , 1000 0

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Fig. 1. E f f e c t o f 20 years o f arable cropping and pasture on bulk density o f a s a n d y l o a m soil (after Garwood et al., 1977).

by agricultural vehicles to the complete system of soil management. Such studies have to be multi-disciplinary so that all aspects of machine/soil/plant interactions and their economic significance can be included. The subject is complicated by the fact that for certain soil and climatic conditions undercompaction is also known to reduce crop establishment and yield (Eriksson et al., 1974). It seems unlikely that compaction is ever unimportant where wheeled machinery is used in crop production with the possible exception of arid non-irrigated areas. The crop rotation may also be of importance in influencing compaction under vehicles. Garwood et al. {1977) found that a free draining sandy loam was considerably more compact following 20 years of arable cropping than after a similar period of pasture grass (Fig. 1). Wheeled vehicles are showing a marked upward trend in power and mass. McKibbon (1971) reports that the average mass of tractors ~ncreased from 2.7 t in 1948 to 4.5 t in 1968, while since then the average power of tractors has been rising at about 5--7% per annum. Dvortsov and Polyak (1979) show that the average power of tractors in both U.S.S.R. and U.S.A. has increased by approximately 30% over the decade 1965 to 1975, and they report that in the U.S.S.R., where many farms exceed 4000 ha, 200-kW tractors are already in mass production. They consider that tractors of 750 kW and over " m a y be expected to make their appearance in the very near future". Such a vehicle would be expected to have a mass of 30--36 t. However, tractors often do not represent the heaviest equipment to run over farm soils. Vehicles used for transporting and spreading lime and slurry now have a mass which frequently approaches and may sometimes exceed 20 t (H~kansson, 1979).

Compactive and loosening phases in crop production Arable soils cultivated traditionally pass through an annual or perennial cycle of loosening and compaction. Loosening is only obtained by primary tillage or subsoiling whereas compaction may occur at several stages in the cycle, for instance, at seedbed preparation. As a result soil density at the time of sowing often is as high as it had been prior to ploughing (Kuipers and Van Ouwerkerk, 1963}. The mechanism for compaction occurring during seedbed preparation is related to the passage of both the implement itself and the tractor tyres. The ability of a soil to support such traffic without structural damage beyond the limits for good crop growth has been used as a definition of the trafficability of agricultural soils (Paul and De Vries, 1979). TRAFFIC DISTRIBUTION A clear distinction must be drawn between the distribution of traffic and the incidence of compaction. Kononov and Garbar {1973) calculated that during a single cereal growing season in Byelorussia, U.S.S.R., more than 80% of the field was covered by the wheelings of tractors. The areas sub-

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Fig. 2. A graphical technique for indicating the distribution of: (A) tractor wheels, (B) implement wheels, (C) all wheels, during eleven operations over 9 m width of a cereal field with cumulative number and area of wheel passes (after Kononov and Garbar, 1973). jected to between 0 and 9 passes of tractor and implement wheels are shown in Fig. 2. During sugar beet p r o d u c t i o n some parts of the field even received eleven wheelings. Because tractor wheel tracks are often obscured or eradicated by following implements or drilling operations a record of wheelings can indicate a m uch higher coverage than is often suspected from observation o f the surface. An example for a typical traditional series of seedbed operations for barley in Scotland is shown in Fig. 3. Voorhees (1977a) f ound that for six operations in planting maize, the tractor's dual tyres, each 450 mm wide, will cover the field approximately twice. Eriksson et al. (1974) reported that over a year the total wheel traffic coverage over a cereal farm can be 4 or 5 times the field area while higher total coverage will occur where non-cereals are grown. The influence of different types o f vehicles on the com pa c t i o n risks was taken into account by calculating the term 'ton-kilometers per hectare per year'. Traffic is very heavy on crops such as lucerne which in warm climates is harvested several times a year. Sheesley et al. (1974) estimate that in California, wheels of harvesting equipment cumulatively cover 75% of the area harvested at each cutting. Many plants will be subjected to up to 20 wheel passes a year. Traffic distribution for a variety of cropping systems was studied by Ar n dt and Rose {1966) in terms of a repeated module of track width having within it a n u m b e r of c o m p o n e n t bands of varying traffic intensity. Apart from the regular traffic associated with crop product i on there are certain operations such as limespreading which, although infrequent, may involve very heavily loaded wheels covering some 12 to 15% of the ground surface. Slurry spreading also represents a severe com pact i on risk due both to high loads and f r e que nc y of traffic. A weakness of m os t systems of traffic analysis is that there is no opport u n ity to relate the passage of weels on each occasion to the compactability o f the soil, which can vary widely with soil water content. A not her factor is

4

9m

9m

Fig. 3. Example of the pattern of tractor wheel tracks during traditional seedbed preparations for spring barley in Scotland (fertiliser distribution, harrowing twice, sowing, rolling) giving 91% coverage including overlap (after Soane, 1975). that subsequent passes over the same area causes less com pact i on than first passes. The use of 2-wheel drive for tractors over 60 kW is considered to be a potential cause of increased compaction as a result of both high cont act pressure and excessive slip at high draught. COMPACTION IN RELATION TO THE HYDROLOGY OF FIELD SOILS The soil water status may be modified through irrigation and drainage and, in view of the extremely sensitive relationship between compactability and soil water status, bot h these operations are related to the likely incidence of co mp ac t i on problems. Irrigated soils, particularly those producing two crops a year, are very prone to compaction problems. K uet her (1977) f o un d th at co mpact i on and mobility problems are serious in rice soils in the Philippines if 37-kW 4-wheel drive tractors are used in place of 7.5-kW 2-wheel walking tractors or water buffaloes for cultivating operations. He attributed this largely to the high residual moisture c o n t e n t at the time when one crop is harvested which is closely followed by cultivation for the following crop.

Erosion problems associated with compaction R u n o f f and erosion, bot h of which have been of longstanding importance in the tropics, are increasingly observed in t em perat e regions. Reduced permeability in wheel ruts has been observed to lead to water erosion problems

in Britain (Reed, 1979} and Norway (Gaheen and NjOs, 1978}. In some countries wind erosion control may depend on the ability to create cloddy tilths during the part of the year when no crop is present. Ploughing land which has previously been subject to sufficient wheel traffic to increase its bulk density and/or aggregate size can produce some degree of wind erosion control (Voorhees et al., 1979).

Interactions between drainage and compaction Soil water matric potential and hence drainage are dominant factors controlling the increase in density during wheel traffic. Steinhardt and Trafford (1974) reported negligible compaction and wheel sinkage when the matric potential was maintained below - 2 5 to - 3 5 kPa at the b o t t o m of the plough layer of a clay soil by ensuring that water-tables remained below 500 to 600 mm depth. This critical depth of water-table has been found to vary with soil texture (Paul and De Vries, 1979} and values of 4 5 0 , 5 3 0 and 600 mm were reported for a silty clay loam under grass, a muck soil and a cultivated silty clay loam, respectively. The presence of a plough pan in the cultivated soil appeared to influence the soil water/strength relations. On many soils in the U.K. good artificial drainage is required to maintain water-tables below 500 mm depth during most of the year (Trafford, 1978). An artificial drainage system is however not the sole means of removing excess water. For instance, autumn ploughing can reduce the soil water content in spring and may therefore reduce the risks of compaction from wheel traffic in seedbed preparation (Ljungars, 1977). The water-table level has a marked effect on the bearing capacity of peat soils under grass in the Netherlands (Boxem and Leusink, 1978). A minimal bearing capacity of about 500 kPa was required to prevent serious damage to the sward during fertiliser application, rolling and pasture harrowing, even when dual rear wheels and wide section front wheels were used on the tractor. For slurry distribution, in which the wheel loads were higher, a bearing capacity of about 600 kPa was required. These bearing capacity values could be achieved by lowering the water table to about 300 mm below the surface. Compaction will reduce soil permability and the opportunity to remove excess water by drainage may diminish (Gaheen and Nj~bs, 1977). Thus compaction may both induce drainage problems and be influenced by the effectiveness of soil drainage. The thousand-fold reductions in permeability which can be introduced readily as a result of wheel traffic (Chancellor, 1976) may either intensify a drainage problem or create one. PERSISTENCE OF COMPACTION Heinonen (1977) has drawn attention to the inconsistencies in the information on the persistence of soil compaction. In North America, Dickerson

(1976) found evidence that compaction in forest soils after logging operations declined with time at a rate that would lead to full recovery in 12 years, whereas it has been claimed to persist for 16 years by Froehlich (1979) and for 50 years in South Australia (Greacen and Sands, 1980). Where a certain minimum clay content is present swelling and shrinkage due to changes in soil water content can cause a reduction in compaction, but most workers attribute a decline in bulk density to frost action. Gill (1971} stated that in the northern U.S.A. compaction was not a problem as shown by the decreases of bulk density in Michigan which were attributed to freezing by Krumbach and White (1964). Voorhees et al. (1978) showed that wheel effects produced during the growing season were essentially alleviated by autumn tillage and natural weathering in Minnesota for the 0--150 mm depth but persisted at greater depths. However, in the same area compaction resulting from seedbed operations in spring persisted throughout the summer (Voorhees, 1977a). Blake et al. (1976) in a 9-year study in Minnesota found that freezing and thawing did not alleviate compaction below the depth of ploughing in a clay loam soil, even though the 0°C isotherm usually reaches a depth of 900 mm in winter. In Sweden, where frost usually penetrates to 600--800 mm, Eriksson et al. (1974) compared the persistence of compaction in the subsoil with the m o n t h l y changes in water content and frost incidence. After 2 or 3 years there was little or no change in the persistence of compaction from very heavy vehicles in the subsoil but the compaction in the 450--500-mm layer resulting from intense traffic by a 30-t vehicle could be detected after nine years (H~kansson, 1979). It appears that the increasing mass of vehicles in recent years may result in a level of compaction which will persist throughout a winter even in the northern temperate zone whereas previously this was not the case (Saini, 1978). In areas in which soil freezing in winter is only slight or absent the effects of wheel traffic are likely to persist for several years. In Western Australia Smith et al. (1969) studied vineyard soils previously heavily compacted by interrow traffic. The hard-pan in general persisted unchanged during a 4-year "recovery" period. In California changes in the structure and strength of aggregates were detected by Vomocil and Flocker (1965) 6 years after wheel traffic. In the Netherlands, Van Ouwerkerk (1968) found that there was no change in pore space or air content at 180--400 mm depth over a period of 6.5 years. In the U.K., Pollard and Elliott (1978) have shown that compacted zones in a controlled traffic experiment on a sandy loam soil persisted for at least 2 years in the absence of any further traffic. In a subsequent study at the same site (Pollard and Webster, 1978) 6 years after the original compaction treatment was applied, it was found that a severely compacted layer persisted at 200--300 m m depth. At 230 cm depth the cone resistance was about 4 MPa compared with only 1.6 MPa for plots which had not received the initial compaction treatment.

10 COMPACTION IN THE SUBSOIL In this context, the subsoil is considered as that soil which lies below the depth of primary cultivation. Compaction within the subsoil may occur as a result of tractor wheels running in the plough furrow or of wheel traffic on the soil surface. Subsoil c o m p a c t i o n during p l o u g h i n g

The presence of a compacted layer immediately below the depth of ploughing was observed even in the days of horse ploughing. The effect is attributable partly to the tractive element running in the furrow {hoof or wheel) and partly to the passage of the plough sole (Sack, 1962). Trouse (1966) found that the permeability of a hidrol-humic latosol in Hawaii below 300 mm was 160 m m / h before ploughing and 57 m m / h after ploughing with a loaded wheel running in the furrow bottom. Czeratzki (1966) found a progressive decline of pore volume below the depth of ploughing for three soils during a 6-year period as a result of annual ploughing operations. Birecki (1966) reported that shallow ploughing (< 150 mm) caused an increase of penetrometer resistance at a depth of 150--250 mm which would normally show a maximum concentration of plant roots. Soane et al. (1970) found some evidence for the development of a compacted zone just below the furrow depth when a regime of shallow ploughing was introduced on land previously ploughed to greater depth. The combination of generally higher water content and lower organic matter content of soil below the furrow than in the topsoil and the extra loading due to axle inclination, gives rise to a severe risk of pan formation during in-furrow wheel traffic. However, compaction and smearing below the depth of ploughing may not occur, even at a high soil water content, if the subsoil is already compact (Gooderham, 1977). Subsoil c o m p a c t i o n resulting f r o m traffic on the surface

Fekete {1972) showed that compaction in the subsoil was related both to the bulk density of the subsoil and to the width of the tyre. Where the initial bulk density of a sandy loam subsoil was comparatively high (1450--1580 kg/m 3) the application of a tyre with a contact pressure of 50 kPa increased the bulk density to only 1500--1660 kg/m 3. A qualitative representation of the influence of tyre width and the depth of loose surface soil overlying firm subsoil is shown in Fig. 4. Three conditions (1, 2 and 3) are illustrated depending on whether the zone of maximum stress concentration will be respectively above, at, or below the depth of transition from loose surface soil to undisturbed firm subsoil. In condition (1), in which a > z + b/2, the subsoil will probably not be compacted to any measurable extent whereas in condition (2) a = z + b/2 and some compaction can be expected in the upper

11

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part of the subsoil. In condition (3) a < z + b/2 and considerable distortion to the normal distribution of compaction can be expected. All the soil lying between the b o t t o m of the rut and the upper limit of the subsoil is likely to be compacted to a greater extent than in conditions (1) and (2). However, these qualitative postulations need to be confirmed for specific situations and will be dependent on the relative compactability of surface soil and subsoil. Voorhees et al. (1978) showed that repeated wheel traffic caused significant increases in bulk density at 150--300 mm depth in 4 out of 5 experimental years but no differences at 300--450 mm depth. However, cone resistance values were significantly higher under wheel tracks to a depth of 600 mm in one year and always so to a depth of 300 ram. The effect of repetitive traffic of very heavy tracked military vehicles (up to 50 t) over a period of 30 years was established by Eriksson (1975/76) by comparing compacted profiles with those nearby which had received no traffic. Marked differences in visual structure, the frequency of earthworm holes, macroaggregation, water permeability and pores > 0.03 mm were found to a depth of 500 mm and certain effects could be traced to a depth of 1 m. The extension of compaction effects to such a depth was attributed to the very high mass of the vehicles. The effects of subsoil compaction from wheel traffic may be ameliorated by intensive deep cultivation and appreciable increases of yield have resulted from this technique (see for example McEwen and Johnston, 1979; Rowse and Stone, 1980). However, in some cases the origin of the unfavourable physical conditions in the subsoils may have been pedological rather than attributable to compaction under wheels. INCIDENCE AND EFFECTS

OF COMPACTION IN THE PRODUCTION

OF CEREALS

S e e d b e d and harvest traffic on c o n v e n t i o n a l l y tilled soils

During fertiliser distribution, secondary cultivation and sowing, soil strengh is generally low as a result of the loosening during primary cultiva-

12

tion and the soil is usually moist. At such times tractors can cause appreciable compaction (Figs. 5 and 6). Zones of compacted soil formed then are likely to remain throughout the life of the crop. The risks of this situation have long been recognised by farmers who frequently employ cage or dual wheels, tracked vehicles or additional cultivating tines in the wheel tracks to nullify the effect. Ljungars (1977) found that the soil water content and the number of wheel passes were the factors primarily responsible for the compaction resulting from seedbed traffic. Tractor mass appeared to be less important, presumably due to the tendency for tyre sizes to increase with increasing tractor mass, thus limiting the increase of contact pressure. Dry Bulk Density, kg / m 3 1000 1200 0

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d_<. Fig. 5. The variation of dry bulk density with depth before and after the passage of a 33 kW (1.6 t) tractor over recently cultivated sandy loam soil (after Soane, 1970). Dry bulk density, k g / m 3 0

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Fig. 6. Soil bulk density for three occasions after deep ploughing for barley in comparison with the long-term equilibrium values for bulk density following zero-tillage on sandy loam soil (after Soane et al., 1977).

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Fig. 7. The influence of combine harvester wheels on the distribution of bulk density for four cereal cultivation systems on sandy loam soil (after Pidgeon and Soane, 1978). Where soils are generally dry and of high strength at cereal harvest (e.g. Southern and Eastern Europe) traffic then may result in little change in soil structure. However, in northern latitudes not only is the cereal harvest later but the date of return to field capacity is earlier and pronounced soil responses to harvest traffic may occur (Figs. 6 and 7) to a depth of 150 mm (Soane et al., 1970; Habegger, 1971; Pidgeon and Soane 1978). Low (1972) attributed to harvest traffic a reduction of total porosity from 58 to 49% (v/v) in an old arable field. As this effect was based on random sampling, the compaction occurring below the individual wheel tracks would be considerably greater. Domsch (1959) considered the tyres fitted to combine harvesters in the Soviet Union too small and too highly inflated.

Compaction in zero-tillage systems Under continuous zero-tillage systems the cycle of compaction and loosening found in arable soils subject to annual tillage no longer occurs. Mechanical loosening is not imposed other than that which may be caused by the drilling operation itself or any subsequent harrowing. Observations at

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Fig. 9. The variation of air-filled porosity with depth for a sandy clay loam (water content 32%, w/w at 180 ram) under barley stubble with respect to the wheel tracks of a combine harvester operating eight weeks earlier (after J.W. Dickson, personal communication, 1980).

a number of sites indicate that under direct drilling a pronounced increase of bulk density develops at 0--150 mm depth (Pidgeon and Soane, 1977; Gantzer and Blake, 1978; Ellis et al., 1979; Cannell et al., 1980). In long-term zero-tillage trials, attempts have been made to examine the changes of bulk density with time. On a sandy loam soil Pidgeon and Soane (1977) found that an equilibrium condition was established after 1 year at 150 and 270 mm depth but at 60 mm depth increases of bulk density continued for about 2--3 years (Fig. 8). However, following the introduction of continuous direct drilling at other sites there was either no evidence of progressive increase of bulk density (Ellis et al., 1979) or a variable effect depending on soil type (Cannell et al., 1980).

15 Soils subjected to continuous direct drilling for more than 2 years are likely to be in a precompacted state and may have acquired sufficient strength to carry normal traffic without further compaction. However, if during harvesting or other operations the soil is moist, rutting may still occur and this will be particularly deleterious if the subsequent crop is again to be planted by direct drilling. After such drilling the passage of any wheel, such as that of the drill itself or a tractor engaged in post-drilling harrowing, may severely compact the soil in the vicinity of the seed and thus reduce emergence. The decrease of both macro-pores and permeability below combine harvester wheel tracks gave rise to much less favourable conditions for crop establishment than was the case out of the wheel tracks (Fig. 9; J.W. Dickson, personal communication, 1980).

Establishment and root growth of cereals The spacing and depth of sowing are of critical importance for a uniform plant establishment and to obtain this uniform soil physical properties within the seedbed are necessary. Traffic maps for seedbed operations and data on compaction under individual tractor wheels indicate that the uniformity of seedbeds is likely to be very poor and this is often borne out by the appearance of the emerging crop. While direct drills often penetrate to inadequate depth in wheel tracks from previous operations giving poor establishment and yields, the effect may also be found for conventional drills. After drilling cereals in the U.S.S.R., the proportion of seeds uncovered along the wheel tracks of tractors was reduced from 13.1% to 2.4% by fitting dual wheels in place of single wheels (Soloveitchik et al., 1977). Although on light textured soils in a dry spring seedling emergence is generally superior in the wheel tracks, wheat may show reduced emergence when the surface soil has a high bulk density and strength (Chancellor, 1976). The germination and early growth of silage maize on clay soil subject to wheel traffic at sowing were studied for two contrasting seasons by Raghavan and McKyes {1978) and Raghavan et al. {1979a, 1979b). In the first (wet) season emergence was delayed for five to six days on heavily compacted plots compared with uncompacted plots, due to the higher moisture content and consequently lower temperature of the surface soil on the compacted plots. In the second (dry) season the time to emergence and the early growth responses to the same levels of traffic showed a positive quadratic relationship to the product of the number of passes and the contact pressure, with the fastest growth at a cumulative contact pressure of about 600 kPa. The delayed germination under very light compaction was attributed to lack of contact between soil and seed and hence reduced uptake of water. R o o t responses to compaction may be complex owing to the numerous ways in which compaction can modify the physical properties of the soil. There have been many attempts to find critical values of bulk density, soil strength or permeability which might correlate with limits to root growth or function {Scott Russell, 1977). Laboratory studies using simplified media

16 can often produce clear evidence for these relationships but in the field the complexity of soil structure and the widely varying soil water status will result in poorer or negligible correlations. Moreover, reduced root activity in one part of the soil profile may be compensated for by increased root growth elsewhere. Field observations in the vicinity of wheel tracks have shown that root penetration may be seriously retarded (Voorhees, 1977b). In contrast, root growth on soils of low bulk density tends to be relatively unbranched which may result in sub-optimal exploration of the soil and hence deficiences of some macro- and minor-nutrients. Eriksson et al. (1974) reported that the root growth of wheat seedlings was progressively reduced when the soil was subjected to surface pressures in excess of 200 kPa and the limiting penetration resistance for root growth was reported to be between 0.8 MPa and 5 MPa. The importance of penetrometer size, penetration rate, root size and soil type in influencing the relationship between penetration resistance and root elongation has been discussed by Gooderham (1977). He concluded that with standardisation of techniques, penetration resistance may well find increased value in studies of root growth on compacted soils. Root distribution of maize has been found to be closely associated with both the number of passes and the contact pressures of tyres running over the soil either before or after seeding (Raghavan and McKyes, 1978; Raghavan et al., 1979c). The depth to which dense rooting (> 0.1 mg roots per g soil) extended was 900 mm in the absence of traffic at sowing whereas for 1, 5 and 15 passes of a wheel with a contact pressure of 62 kPa dense rooting was restricted to 6 0 0 , 4 5 0 and 250 mm respectively.

Cereal yield responses The literature on the relationship between plant growth and soil compaction has been reviewed by Eriksson et al. (1974), Cannell (1977) and others. Plant growth and yield are likely to show optimum responses at a certain level of soil compactness. The position of this optimum however is related to soil type, crop growth stage and climatic conditions. These interacting effects represent part of the explanation for the varying responses observed in different experiments and for the discrepancies between experimental results obtained in the laboratory and the field. In some experiments heavy traffic prior to or during seedbed operations for wheat, sorghum and maize has given no decrease in yield (Chancellor, 1976) and may in some cases increase yield largely as a result of better continuity of water filled pores, leading to better plant establishment. Even the apparently very damaging effect of sugar beet harvesting under wet conditions has been found to have no effect on the yield of wheat grown subsequently after ploughing (Jaggard, 1974; Jaggard, 1975). However, Eriksson et al. {1974) estimated that cereal yields on clay soils in Sweden would be

17 increased by about 6% in the absence of compaction from wheel traffic while in the U.S.S.R. Dvortsov and Polyak (1979) reported a 15% drop in the yield of oats when the bulk density of a chernozem soil was increased by only 100 kg/m 3 above the optimum by the passage of heavy machinery. The likely yield response to traffic-induced compaction will depend to a considerable extent on the soil water status at the time of the traffic. Where the matric potential at 50 mm depth was ~ - 5 kPa at the time of heavy traffic prior to the drilling of barley and oats, the mean grain yield was 36% lower than when the matric potential at the same depth was between - 7 and - 5 0 kPa (Nj~bs, 1978). The water regime during the growing season may be important also. Czeratzki (1966) reported results on the influence of wet and dry years on the yield of summer wheat in relation to tctal porosity. Similarly in roofed microplot studies by H~kansson (1966), with three levels of watering and four compaction treatments, barley yield showed a pronounced interaction between wetness and bulk density. The seasonal change in the optimum 'degree of compactness' for yield of cereals was shown by Eriksson et al. {1974). The 'degree of compactness' quotient was independent of soil characteristics although yield responses on fine-textured soils tended to be more sensitive than those on coarse-textured soils. The mean optimum degree of compactness for spring sown cereals was generally between 85 and 90%. For winter cereals the mean optimum value was between 78 and 84%. Voorhees (1977b) reported that wheat planted in wheel tracks in a relatively dry year in Minnesota yielded 0.9 t/ha more than on untracked soil but 1 t/ha less in a wet year. Differences in yield in both years were attributable to slower germination and to reduced plant population in the wet year. The temperature of a relatively dry soil in the US Corn Belt can be increased by 1.0--1.7°C by compaction (Voorhees, 1977b). Frost damage to winter wheat is reported to be more marked in the wheel tracks of tractors running over the soil before ploughing due to the cloddy nature of such soil at the time of drilling (Domsch, 1959). Pollard and Elliott (1978} related barley grain yield to compaction of a sandy loam soil over a 2-year period. In the first year the yield on the plots of bulk density 1540 kg/m 3 at 150--200 mm depth was 4.86 t/ha compared to 3.00 t/ha on the plots compacted to a bulk density of 1980 kg/m 3 at the same depth. However, in the following year the yield from the compacted plots (4.71 t/ha) was less than but not significantly different from that of the non-compacted plots (5.16 t/ha). They concluded that differences in the first year were attributable to excessive moisture on the compacted plots due to restricted permeability and heavy rain during the first 20 days after drilling. Seasonal effects also influenced the relationships between vehicle traffic and the yield of silage maize (corn) on a clay soil (Raghavan et al., 1978, 1979a, 1979c, 1979d; McKyes et al., 1979). Traffic was applied, either before or after sowing, with either 0, 1, 5, 10 or 15 passes of wheels with

18 c o n t a c t pressures of 31, 41 or 62 kPa. In 1976, with a high summer rainfall, severe traffic resulted in a 50% reduction in yield (Raghavan et al., 1978) although the bulk density at 0--200 mm had reached only 1120 kg/m 3. An inverse linear relationship was obtained between yield and the natural logarithm of the p r o d u c t of the n u m b e r of passes (n) and the c o n t a c t pressure (p). In the following year summer rainfall was 35% below average and the yield response to the same traffic treatments was entirely different in that it showed a quadratic relationship to np with a m a x i m u m yield at n p = 500 kPa. For n p values of 50 and 900 kPa the yield was reduced by 28 and 14%, respectively (Raghavan et al., 1979d). The m a x i m u m yield was ob-

Fig. 10. The effect o f excess wheel traffic at the time of seedbed preparation on the unif o r m i t y and ripening characteristics o f spring barley on a sandy loam at SIAE.

19 tained at a dry bulk density of 1050 kg/m 3 at 50--200 mm depth and was reduced by a b o u t 30% at bulk densities below a b o u t 920 kg/m 3 and above about 1100 kg/m 3. These results give further evidence for the wide fluctuation of o p t i m u m bulk density according to weather conditions and also illustrate h o w low the optimum value can be for a fine textured soil. Compaction is likely to lower the quality and uniformity of many crops: cereals m a y show delayed or irregular ripening (Fig. 10) which can seriously reduce the market value of the grain. INCIDENCE AND E F F E C T S OF COMPACTION IN THE PRODUCTION OF CROPS OTHER THAN CEREALS

Traffic in annual crops

In many non-cereal crops wheel traffic both before and after sowing is traditionally confined to the interrow zone either to avoid compaction or direct mechanical damage to the crop. Crops which are subjected to frequent spraying operations after planting are particularly at risk from traffic damage. Mogilevets and Khallyyev (1977) reported that, for one cotton crop, the tractor rear wheels passed 22 times down the interrow zone. Where the crop is grown in ridges, as with potatoes, the passage of tractor wheels in the furrow may cause compaction in the sides of the ridge (Fig. 11} as well as in the soil below the furrow (Nj~bs and Nordby, 1966). These effects will reduce root and tuber growth, increase the draught forces required during harvesting operations and increase the amount of clods which have to be separated from potatoes on the harvester (Campbell, 1980). A reduction in tyre width and an increase in p o t a t o row spacing may overcome this problem though marked reductions in tyre width can results in excessive slip, sinkage and side movement on gradients (Nj~bs and Nordby, 1966). SURFACE

SURFACE

,.,1-"

1300

400

1400 l

I

I

i

200

I

I

I

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400

i

l

I

I

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600

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I

800

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i

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1000

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1200

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Fig. 11. The distribution of dry bulk density in a cross-section of two potato ridges after the passage of a tractor wheel during spraying on a sandy loam soil (after Soane, 1970).

20 The harvesting of vegetable crops (Vomocil and Flocker, 1965) and sugar beet (Jaggard, 1974; 1975) may also give rise to considerable soil damage. As with potatoes the harvesting of sugar beet may involve compaction of soil close to the roots and if the soil water content is high this may increase the amount of soil lifted with the crop (Wayman and Maughan, 1966). The harvesting of non-cereals may involve very heavy equipment, such as 15 t pea-podders. In the case of sugar beet and potatoes, the harvest may occur when the soil water content is much more critical than at cereal harvest. Moreover, crops such as vegetables involve repetitive passes of harvesters and transport vehicles.

Traffic in perennial crops The traffic in perennial row crops, often along identical tracks for m a n y years may result in very intense compaction {Miller et al., 1963). In apple orchards the repetitive spraying of pesticides generally has to be carried out shortly after rain when the soil is most likely to be compacted by the wheels of the heavy machinery used (Raghavan et al., 1976). Wheel traffic effects in raspberries on a loam soil were reported by Soane et al. {1975). The bulk density at 100 mm depth in the wheel tracks of the tractor used for repetitive interrow cultivation for weed control was 1520 kg/m 3 compared with 1160 kg/m ~ between the wheel tracks. There were indications that the horizontal distribution of raspberry roots was affected by this wheeling. Smith et al. {1969) showed that on a sand soil in Western Australia 80% of the soil between vine rows had a bulk density of at least 1750 kg/m 3 at a depth of 0--300 mm. The harvesting of forage crops, either as hay, silage or in a fresh condition, may also involve soil compaction. The effect of wheel traffic from grass harvesting operations over 4 years has been reported by Eriksson et al. {1974). Three levels of traffic (zero, normal and double normal) were applied immediately after each hay harvest. There was a build-up of compaction on the normal and double normal traffic treatments and a loss of 3.6 and 5 t hay, respectively, for these treatments over a 4-year period compared with the zero traffic treatment. They considered that changes in haymaking machinery management may be desirable to avoid these effects. The need to reduce sinkage of trailers on grassland was recognised by K o o y m a n {1969) who advocated the use of large wheel diameters and tandem arrangements. Similar problems arise with lucerne and Sheesley et al. {1974) considered that the crop should have a well established root system before any harvest traffic occurs, preferably to below 450 mm which was the depth to which compaction from wheel traffic extended.

Establishment and root growth of non-cereals While some irregularity in emergence may be tolerated in cereals, the

21 uniform establishment of non-cereals is of paramount importance. Compaction due to wheel traffic close to the seed position prior to or after planting of sugar beet, c o t t o n and vegetable crops is likely to delay or prevent seedling emergence. Durrant {1972) found that wheel traffic prior to sowing resulted in compaction to 300 mm which induced reduction in the early growth of several crops. Maximum leaf cover of sugar beet and potatoes was reduced by 5 and 47%, respectively, on the compacted plots. The root growth in non-cereals is often strongly influenced by compaction under wheels. A 57% reduction in lucerne root concentration has been recorded within the 0--600 mm depth in traffic zones compared with zerotraffic zones (Sheesley et al., 1974}. R o o t growth showed a strong negative correlation with cone resistance at field capacity, each 700 kPa increase of soil strength corresponding to a 15--20% reduction in root concentration. De Haan and Van der Valk (1970) found that the growth of tulip roots was strongly influenced by the passage of a crawler tractor. At the highest level of compaction very few roots penetrated below 150 mm whereas for the lowest level of compaction roots penetrated to 400 ram. A critical total porosity of about 42--43% was suggested for root growth on the sand soil studied. Roots of bulbs appear to be much more sensitive to adverse porosity than roots of cereals. Trouse (1966) found that for certain soils in Hawaii sugar cane root penetration was restricted by bulk densities as low as 700 kg/ m 3 whereas on other soils roots were able to penetrate a density twice that figure. On compacted soils root crops such as carrots and sugar beet may have unacceptable shapes and with potatoes a high incidence of clods in the ridge will reduce the speed of harvesting operations and increase tuber damage {Campbell, 1980). Yield responses in crops other than cereals

The yields of non-cereals are generally more sensitive to compaction than those of cereals. Mogilevets and Khallyyev (1977) found that cotton rows in which wheels passed on both sides yielded less than those which received traffic on one side only. However, as with cereals, soil water status can influence this response. For example, Voorhees (1977b) found that in Minnesota during years when precipitation was less than normal, the yield of soyabeans was 25% higher where wheel traffic had occurred on both sides of the row than where it had occurred on one side only. However, he found a 35% reduction in the yield of potatoes as a result of wheel traffic. Different cultivation techniques were used by Jaggard {1972) for the preparation of sugar beet seedbeds to provide plots with bulk density values at 0--160 mm depth between 1300 and 1650 kg/m 3. The yield of sugar at the highest bulk density was 0.9 t/ha less than on the least compacted plots. In a subsequent experiment (Cooke and Jaggard, 1974) the effect of similar levels of compaction was to reduce sugar yield by 1.9 t/ha and yield of tops by 7 t/ha.

22 Cotton experiments at many locations in the U.S.A. have shown a wide range of yield responses to increasing compaction within the root zone, sometimes positive, sometimes negative and sometimes without effect (Chancellor, 1976). Often there were marked seasonal differences associated with interactions with available water supply to crop roots. Yield reductions attributable to wheel traffic have been observed for many other crops including ornamental bulbs (De Haan and Van der Valk, 1970) and lettuce (D.J. Harrison, personal communication, 1980} where a 28% reduction in yield as a result of 4 wheel passes side by side was also accompanied by a marked increase in the incidence of tip burn and a delay in maturity. The yield of potatoes has been found to be particularly sensitive to the passage of tyres down the furrows (Nj~bs and Nordby, 1966). In four out of five experiments the tuber yield following one pass of a tractor was 3.4 t/ha higher than following two passes; the effect was greater when the traffic took place under comparatively wet conditions in late summer. Although we are here concerned with agricultural crops there have been notable advances in the understanding of the role of vehicle traffic in influencing the growth of forest species (Greacen and Sands, 1980).

Compaction and soil micro-organisms o f importance to non-cereal crops Many soil micro-organisms, both beneficial and pathogenic to crops, are known to be sensitive to changes in aeration, pore size distribution and soil water status such as may result from the passage of vehicle wheels. These effects may therefore exert indirect influences on crop responses to compaction. The numbers of soyabean nodule bacteria was found to be reduced by 30% in rows which received wheel traffic on both sides compared with one side trafficked rows (Voorhees et al., 1976). Burke et al. (1972) showed that bean root rot disease was reduced and yield was increased when subsoiling was undertaken beneath the bean rows to reduce compaction before planting. Nematode attack on sugar beet has sometimes been found to be more prevalent on coarse textured soils when in a loose condition and to be reduced in tractor wheel tracks (Whitehead et al., 1971). However, Cooke and Jaggard (1974) showed that the sugar yield from sugar beet grown on compacted sandy soil (dry bulk density 1550 kg/m 3 at 0--160 mm) was reduced by 1.4 t/ha compared to the uncompacted soil (1310 kg/m3), but following the addition of a nematicide, the loss of yield was only 0.3 t/ha. In the absence of nematicide the compact seedbeds showed an increased number of severely misshapen roots. These results illustrate the important interactions between compaction and soil-living plant pathogens in influencing crop growth. Tractor wheelings on silt loam and clay grassland soils have been found to decrease the number of soil fauna particularly springtails (Collembola), mites (Acari) and earthworms due to direct physical damage to the fauna rather

23

than to unfavourable soil physical conditions. A recovery of numbers was observed in six to ten months time (Aritajat et al., 1977). OPTIONS F O R R E D U C I N G C O M P A C T I O N U N D E R WHEELS

There are three primary ways of reducing the overall compaction of field soils by agricultural vehicles: (1) reduction of the number of passes of con-

CompactionProblemsin AgriculturalSoils

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Cultivation Factors

I I EconomicFactors I I WheelandVehicieFactors

Optionsfor reducingcompactionproblems

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Vehiclesof Vehiclesof very high high mass (c 10t) ~~l~/mass(>20t)

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Widerwheels Reductionin load Reductionin

Restricted

inflation pressure New wheel systems

Prohibited

or

%_ Fig. 12, A simplified diagramatic representation of some of the options available for reducing c o m p a c t i o n in relation to the factors affecting the cultivation system as a whole (after Soane et al., 1979).

24 ventional machinery; (2) reduction of the vehicle mass and the contact pressure of wheel systems; and {3) confinement of traffic to permanent or temporary wheel tracks {controlled traffic). A diagrammatic representation of these options in relation to the types of vehicles is shown in Fig. 12. Traffic reduction can be achieved by combining in one pass operations such as cultivation and spraying or certain types of harvesting operations using currently available machinery and common sense attitudes to machinery management. The introduction of large reductions in contact pressures or the adoption of controlled traffic systems however, may require the use of vehicles or wheel systems which in some cases are still in a development stage. R e d u c t i o n in vehicle mass and c o n t a c t pressure

The opportunities for reducing the mass of farm machinery has been under consideration for many years. Amos (1918) considered that heavy tractors (3--5 t) " m a y do untold damage" to the soil especially to fine-textured soils when wet. He advocated the use of steel instead of iron to reduce the mass of tractors. Although his advice was adopted, the demand for greater power and complexity of operation during the past 20 to 30 years has given rise to machines three or four times heavier than the tractors with which Amos was concerned. The possibility of restricting the axle load of agricultural vehicles has been raised by H~kansson (1979). He considered that axle loads should be limited so that no compaction causing significant yield decreases will occur deeper than 400 mm. This conclusion was based on the results of nine long-term field experiments on contrasting soil types applying, on a single occasion when the soil was close to field capacity, either nil, one or four passes of a vehicle having a total mass of 26 t (10 t on single front axle, 16 t on tandem rear axle). In subsequent farming operations the maximum axle load was restricted to 4 t. One to two years after the treatments it appeared that compaction extended to a depth of at least 500 mm and was most obvious at about 400 mm depth. Adoption of new technology can sometimes result in opportunities for reducing the mass of vehicles used in field operations (Elliott, 1979). Particularly striking reductions in the loaded mass of spraying vehicles can be obtained by the adoption of low volume and controlled drop application. However, for some crops the development of new but much heavier harvesting machinery can give the opposite effect. Elliott (1979} has pointed out that for non-agricultural operations vehicles already exist which, with special tyres or tracks, can travel successfully over a variety of very soft surfaces such as snow, mud and sand. He also stated that it is technically possible to carry all the loads (seed, fertiliser, chemicals, grain) involved in cereal production using inflation pressures of only 35--70 kPa. Although current farm transport practice generally employs vehicles

25 with inflation pressures considerably in excess of this range there has been rapid acceptance of low ground pressure vehicles for post-emergence spraying operations for winter cereals. Deflation o f tyres below r e c o m m e n d e d minimum inflation pressures has been examined experimentally as a possible way of reducing com pact i on on seedbeds. Following a single pass of a tractor wheel over the whole plot area before drilling winter barley, crop emergence (Fig. 13) was found to increase from 131 to 189 plants per m 2 when front and rear tyre inflation pressures were reduced from normal (220 and 90 kPa, respectively) to approximately half normal values (D.J. Campbell, personal communication, 1981). Where no wheel traffic preceded drilling the emergence was 315 plants per m 2. The opportunities of using open, lugged wheels to overcome the problems of vehicle mobility on very soft soils such as rice paddies have been investigated by Gee-Clough and Chancellor (1976). In contrast with conventional tyres the load is carried by only a very small cont act area and loads of 10 kN or more can be carried by such a wheel on loose seedbeds with very little change in bulk soil properties (Dickson et al., 1979). Wide section f r ont tyres and trailer tyres together with dual rear tyres for tractors pulling slurry tankers have been found to be essential for spreading operations on soft soils with a high water table in the Netherlands (Boxem and Leusink, 1978) to maintain mobility and to prevent excessive sinkage

i

:

Fig. 13. The influence of inflation pressure of rear (13.6--36) and front (7.50--16) tyres fitted to a 41 kW (3.2 t) tractor on the emergence and early growth of winter barley on a sandy clay loam after one pass of the tractor wheels over the whole plot area prior to drilling. Left: r.ormal pressure (rear 90 kPa, front 220 kPa). Right: low pressure (rear 40 kPa, front 100 kPa). (After D.J. Campbell, personal communication, 1981.)

26 and sward damage. However, with row crops the use of wide low pressure tyres or duals may not be satisfactory if the wheel tracks pass over or close to the planting row (Gill and Trouse, 1972). Controlled traffic on permanent wheel tracks The concept of controlled traffic is not new. Towards the end of the 19th century, winch-operated ploughs and other cultivation implements were widely used with steam traction engines which, due to their considerable mass (about 15--20 t) and poor manoeuvrability, were confined to the headlands. Halkett (1858) advocated the use of permanent railway lines to provide support for steam-powered platforms from which all farm operations would be conducted. The track width used in preliminary trials was 9 m but 'no difficulty' was anticipated in extending this to 15 m or more. It is likely that the enterprising farmer of the future will ensure that the a m o u n t and distribution of vehicle traffic within his fields is subject to strict control. Already the practice of confining certain types of traffic to headlands is increasing, some farmers insisting that combine harvesters discharge grain there, thus eliminating trailer traffic from the field. The tramlining of sowing and spraying traffic in cereals in the U.K. has resulted in greater uniformity of crop in the zones free of traffic but the wheel tracks created are essentially temporary, being destroyed by subsequent cultivation for the next crop. The permanent bed system of crop production, which has long been used in certain vegetable and fruit crops, is now being examined experimentally for large scale agricultural crops. Initially the approach was to use conventional machinery with the axles extended to about 3 m. Marked improvements in soil conditions and cotton yields were obtained at the US National Tillage Machinery Laboratory (Cooper et al., 1969; Dumas et al., 1974). Particular attention was given to obtaining optimum chemical and physical conditions of the subsoil before the start of a permanent wheel track regime. An electronic guidance system was used to ensure the greatest possible precision in the alignment of the wheel tracks, a maximum displacement of 25 mm being obtained. Cotton grown at 1.03-m row spacing gave a yield increase of 48% where permanent wheel tracks were used with previous deep tillage to 460 mm depth compared with normal traffic without deep tillage. Where wheel traffic was excluded the volume of soil having a cone resistance below 1.4 MPa was much greater than where normal traffic had occurred. This looseness persisted from one season to the next which confirmed the value of the permanent wheel tracks. Increases in yield of several crops have been found with zero levels of post-planting traffic. For instance, Raghavan et al. (1978) found that the average yield of silage maize in zero-traffic plots was 30% higher than yields with standard commercial management. Controlled traffic may have particular significance under reduced tillage systems. The yield of soya beans was

27 increased by 16% where traffic was eliminated on deep ploughed plots but by 25% on shallow cultivated plots (W.E. Nelson, quoted by Gill and Trouse, 1972). The adoption hitherto of controlled traffic for the commercial production of row crops has been inhibited by the lack of standardisation of working width of implements of different manufacture (Gill and Trouse, 1972). Accurate alignment is particularly important in view of the lateral spread of compaction which may extend to 300 mm from the side wall of the tyre. Some of the problems of experimental testing of controlled traffic were discussed by Voorhees et al. (1978). Heestermans (1976) described some of the problems associated with the development of a wide wheel-track tool carrier with 3- to 4-m track width and a 3-m wheel base and the use of a standard tractor (60 kW) with the track width extended to 3.4 m for controlled traffic work. Harvesting controlled traffic plots may present problems. Colvin (1976) described techniques for applying varying amounts of traffic over a number of years to maize (corn) grown in 770 mm row spacing. To permit the wheel traffic treatments to be harvested without interference from the combine wheel tracks maize combines with 3, 5 and 7 row head frames had to be used. To overcome the problems of compaction from lucerne harvesting machinery on commercial farms, Sheesley and Grimes (1977) proposed the use of permanent wheel tracks which would reduce the area covered by wheels at each cut from about 70% to about 20%. Evidence for the potential benefits of controlled traffic in potatoes has been given by Soane (1975). A tool carrier was employed with wheels set at 2.8 m for planting, cultivating, ridging and spraying potatoes. The wheel tracks spanned four rows at 700-mm spacing so that the two inner rows were without wheel traffic prior to and throughout the life of the crop. In the first year of the study yield increases of 24% were reported for the zerotraffic treatments. The use of permanent concrete tracks in a 2-year controlled traffic experiment has been described by Pollard and Elliott (1978). The tracks were designed to allow a 46-kW tractor with 2.64-m wheel track to operate with a range of suitably modified cultivating and drilling equipment. The soil of the 0.05-ha experimental area was a free draining loam which was prepared with a loose surface layer overlying a compacted layer at either 100 or 230 mm depth. Grain yield of barley, drilled with a triple-disc direct drill, was depressed in both years by the presence of the compacted layer at shallow depth. In one year which was wet during the early growth period, the yield on these plots was 62% of those where the depth of the loose soil was 230 mm. Concrete tracks were also used by Rice, in Georgia, quoted by Gill and Trouse (1972), to eliminate wheel traffic in the interrow zones, and as a result maize showed a 20% increase in yield over conventional traffic. Field machines with working widths much in excess of 3 m, encounter severe problems on public and farm roads and through gateways. However, a

28 suitable wheel system would allow the machine to run longitudinally on roads and headlands and laterally in the field. Such a machine has been described by Rutherford (1979) and a design for an advanced multi-purpose vehicle with a 12-m track width operated from a 100--150 kW power unit has been set out by Chamen et al. (1980). Maximum advantage would be taken of micro-computer controls to improve the efficiency of steering, depth of working etc. High draught operations such as ploughing and subsoiling would be beyond the traction capabilities of the machine but could be achieved, if needed, by a winch system with mobile anchors on the headland. An advanced controlled traffic system for the field production of potatoes, wheat, sugar beet and onions under polder conditions in the Netherlands has been described by Perdok (1979). The area covered was approximately 4 ha and an underground electric leader cable for vehicle guidance was used throughout with all machinery operating on a 3-m wheel track distance. Ploughing was undertaken with a 2.5 m wide skim plough modified so that the open furrow could be filled without disturbing the permanent wheel tracks. A " n o r m a l " traffic treatment, to compare with the traffic-free treatment, was simulated in half the area by driving a tractor and trailer over the plots between the wheel tracks. Wheat was harvested with a standard combine with extended axles, while potatoes, onions and sugar beet were lifted with trailed equipment fitted with extended side-conveyors discharging into trailers on 3-m wheel tracks, running in next-but-one plots. Potatoes, wheat and onions gave a 5% increase of yield in the absence of traffic whereas sugar beet yielded 4% less, presumably because for this crop the soil was too loose. Even in the very limited duration of the treatments there were already considerably differences between the traffic free and " n o r m a l " traffic treatments for which the air volumes, at 36% field moisture content, were 21.5 and 3.9% (v/v), respectively, while the cone resistances were 0.75 and 1.45 MPa, respectively, in the layer receiving secondary cultivation. The soil beneath the wheel tracks had reached a cone resistance of 3.5 MPa and this high strength provided good mobility of vehicles even at times of year when traffic would have not been normally possible. Yield increases have not always been obtained with cotton as a result of adopting controlled traffic. Colvin (1976) found that in one out of three years the yields from controlled traffic were lower than from normal traffic. Although experience with controlled traffic systems is still very limited it is clear that they provide the potential for far-reaching improvements in soil and crop management. ECONOMIC ASPECTS OF COMPACTION

A N D ITS C O N T R O L

Value of yield reductions associated with compaction The annual losses in crop yields due to soil compaction in Sweden have

29 been reported to be worth about Skr 80 million (Eriksson et al., 1974) while in the U.S.A. the corresponding figure is $1180 million {Gill, 1971). Compaction also reduces crop yields in the U.S.S.R. (Dvortsov and Polyak, 1979). Yield reductions will vary with respect to location, season, soil type and crop. The most serious losses can be expected with non-cereals grown under irrigation. In California 800,000 ha of cropland, much of it under irrigation, were estimated to be affected by compaction to the point where the yields of many crops were reduced (Flocker, 1976). Sheesley and Grimes (1977) calculated that comparatively minor modifications to the wheel traffic patterns in lucerne fields in California could result in an increased annual income of about $ 63 million per annum or $ 1 5 7 / h a for the farmers concerned.

Energy/cost requirement for ameliorative cultivation A soil which has been subjected to intense wheel traffic will require the input of considerably more energy to bring it to a given level of looseness and c o m m i n u t i o n for a seedbed than would otherwise be the case. This energy may be applied by using a more intensive method of cultivation or by making a greater number of passes or both. For a given operation, for instance ploughing, the specific soil resistance increases rapidly as the level of compaction increases (Chancellor, 1976). Voorhees and Hendrick (1977) found that in Illinois the draught for ploughing a silty clay soil increased by 92% as a result of previous heavy compaction. The power requirement increased by an even greater proportion due to increased wheel slip. On a clay loam in Minnesota the rear wheel slip when ploughing increased from 15 to 20% when the proportion of the land previously tracked by tractor wheels increased from 20 to 40%. Fuel consumption for ploughing increased from 25.6 to 30.5 1/ha, an increase of 19% as a result of one wheel pass previously (Voorhees, 1979). Considerable additional energy is needed to break up the clods produced during ploughing compacted soils. Up to 16-fold increases in energy were needed to pulverise a compacted soil to the same degree of fineness as an uncompacted one (Chancellor, 1976). The increasing inflationary cost of fuel and tractive machinery is in sharp contrast to the much smaller increase in the value of most crops and Elliott {1979) has suggested that, as a result, deep soil cultivation will have to be abandoned for all but the most valuable crops. The extent to which fuel and machinery costs for cultivation could be reduced by the adoption of zerotraffic zones for crop production still has to be substantiated. The savings could be considerable.

Compaction and tool wear Cultivator tines and drill coulters which run in the same line as the tractor rear wheel are subject to greater wear than tools running in soil which has

30

not been tracked by a wheel. Wheel slip may increase when compacted soils are cultivated and Voorhees and Hendrick (1977) reported that a 5% increase in slip may result in 20% greater tyre wear. The overall additional wear of soil engaging tools and tyres used for primary and secondary cultiVation and drilling which results from previous compaction from wheel traffic must be considerable but no quantitative assessments of the costs of this appear to have been made.

Costs of adopting options for reducing compaction The costs of adopting the various options for reducing the intensity of compaction in field soils vary widely as does their effectiveness. Improved management of existing machinery in the field to reduce the distribution and number of passes of wheels can be adopted with very little cost. The fitting of cage wheels and dual wheels can also be undertaken at modest cost though it is rare to find them fitted to vehicles other than tractors. Wide section and flotation tyres can be readily fitted to lightweight vehicles but tend to be very expensive if fitted to larger tractors and harvesters. Domsch (1959) attempted to show that the numerous advantages of oversize tyres can be achieved without excessive cost and stated "The higher cost of oversize tyres would hardly account for 5% of the cost of the tractor, an amount which will surely be gladly paid by every farmer taking care of his soil". However, his views do not yet seem to be widely supported. The costs of adopting controlled traffic systems, especially those requiring very wide wheel tracks and automatic steering systems, is likely to be considerable initially but may well be fully justified as such systems have the potential for increasing yields, decreasing cultivation costs, tyre rolling resistance, wheelslip and draught requirement and utilising a greater degree of automatic control than is the case for conventional equipment. With controlled traffic systems it is possible that cereals could be established by broadcasting or scratch-drilling techniques which would show very large reductions in machinery and fuel costs (Pidgeon, 1981). While a realistic and fully quantitative assessment of the economic aspects of compaction has y e t to be made, enough is known to indicate that the subject is of considerable importance in commercial crop production. Techniques for reducing or eliminating compaction under wheels are already available although their adoption by farmers and manufacturers has been very limited. In the future these techniques may be found to be economically very desirable especially in reduced or zero-tillage systems. CONCLUSIONS

(1) There is little difficulty in demonstrating the occurrence of compaction due to the passage of agricultural vehicles. The effect appears to be widespread under modern systems of crop production in which high stresses are applied to the soil at frequent intervals during the year.

31

(2) Although compaction may either reduce or increase crop establishment, growth and yield depending on complex interactions between the crop, soil type and weather conditions, there is increasing evidence for widespread harmful effects of traffic by modern agricultural vehicles. (3) The reduction of compaction within the economic constraints of practical farming appears to be a worthwhile objective in many parts of the world and research has already indicated a number of approaches in the use and design of wheeled vehicles which could materially reduce compaction. (4) Further quantitative information is needed on the economic consequences of traffic in commercial crop production and the benefits to be obtained from the adoption of new wheel and vehicle systems, especially in the context of reduced or zero-tillage systems. ACKNOWLEDGEMENTS

The authors are grateful to Dr. B.C. Ball (Scottish Institute of Agricultural Engineering) and Dr. P.S. Blackwell (Letcombe Laboratory) who assisted in reading the manuscript and made helpful suggestions. We also acknowledge, with thanks, the permission granted to reproduce copyright material as follows: Fig. 1, Grassland Research Institute, Hurley, England; Fig. 2, Byelorussian Agricultural Academy (founded in 1840), Gorkie, Mogilev Region, U.S.S.R.; Fig. 3, Ministry of Agriculture, Fisheries and Food, London, England; Fig. 4, A. Fekete, Agricultural Machinery Research Institute, Gfdflltl, Hungary; Figs. 5, 11, Institution of Agricultural Engineers, Silsoe, England; Fig. 6, Soil and Water Management Association Ltd., Stoneleigh, England; Fig. 8, Cambridge University Press, E~gland.

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