Agricultural and Food Science in Finland 333 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Vol. 8 (1999): 333–351. © Agricultural and Food Science in Finland Manuscript received June 1999 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Vol. 8 (1999): 333–351. Review Subsoil compaction due to wheel traffic Laura Alakukku Department of Agricultural Engineering and Household Technology, PO Box 27, FIN-00014 University of Helsinki, Finland. Current address: Agricultural Research Centre of Finland, Crops and Soil, FIN-31600 Jokioinen, Finland, e-mail: laura.alakukku@mtt.fi The article reviews those major soil properties and traffic factors, which together influence subsoil compaction resulting from the passage of agricultural vehicles. Likewise, the effects of subsoil com- paction on soil properties, processes and crop growth are discussed on several levels, from methods of measuring to the persistence of compaction effects. The risk of subsoil compaction exists whenev- er moist soils are loaded with heavy axle load and moderate to high ground contact stress. Subsoil compaction tends to be highly persistent. To avoid the risk of long-term deterioration, limits for the induction of mechanical stresses in the subsoil should be established through international team- work. Key words: axle load, ground contact stress, long-term effects, soil physical properties Introduction Soil compaction has become a problem of ma- jor proportions in agriculture to day (Soane and Ouwerkerk 1994a) causing reduced yield, eco- nomic and environmental damage and poorer soil workability. According to Håkansson (1994a), the harmful compaction of arable soils is main- ly attributable to wheel traffic where heavy ma- chines are used in unfavourable soil conditions. As mechanization has intensified, machinery power and weight and implement size have in- creased. As an example, the increase in power and weight of newly sold tractors in Finland during 1976—1996 is shown in Fig. 1. In Ger- many, the proportion of newly registered trac- tors larger than 44 kW jumped from 33% in 1976 to 77% in 1992 (Renius 1994). In the United States the average power of new tractors in- creased from 60 to 85 kW between 1973 and 1986 (Eradat Oskoui and Voorhees 1991), and the gross weight of a 85 kW tractor was 6 Mg. Today in the United States, agricultural tractors commonly have a total weight of 20 Mg. Like- wise, in Western Europe, fully loaded combine and sugar beet harvesters may weigh more than 20 and 35 Mg, respectively, and slurry tankers mailto:laura.alakukku@mtt.fi 334 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Alakukku, L. Subsoil compaction due to wheel traffic may weigh as much as 25 Mg (Håkansson and Petelkau 1994). The continuous increase in the weight of farm machinery has increased the potential for pro- gressive subsoil damage. In the present paper, subsoil refers to soil below normal primary till- age depth. Although Hadas (1994) argues that subsoil compaction constitutes a problem with- in the realm of soil compaction in general and should not be a special field of research, some important differences between topsoil and sub- soil compaction nevertheless exist. Normal till- age does not loosen the subsoil and the effects of compaction are difficult to correct. Thus, the effects of subsoil compaction often persist for a long time. The present paper reviews, subsoil compaction due to field traffic with the follow- ing objectives: (1) to determine the internal and external param- eters that influence the capability of wheel traffic for subsoil compaction (2) to evaluate existing stress, axle load and farming recommendations for avoiding sub- soil compaction (3) to discuss the long-term effects of subsoil compaction on agriculture Compaction of subsoils by wheel traffic According to Koolen and Kuipers (1983), soil compaction in agriculture is usually accompa- nied by deformation in addition to compression lateral movement. The machinery traffic that causes soil compaction, which may be defined as an increase in bulk density, also causes non- volumetric changes in soil structure. Factors in- fluencing the compaction capability of wheel traffic include the soil conditions during the traf- fic, the type and intensity of forces applied, the Fig. 1. Tractors of different power and weight as proportion of new tractors sold in Finland in 1976– 1996. Calculated from the statis- tics of annual tractor sales com- piled by the Institute of Agricul- tural Engineering of the Agricul- tural Research Centre. 335 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Vol. 8 (1999): 333–351. number of loading events and the duration of loading (Fig. 2). In this section we look at some of the major soil properties (internal parameters) and traffic factors (external parameters) relevant from an agricultural engineering point of view. Internal soil strength Internal topsoil and subsoil strength resists the external stresses induced by wheel traffic. Inter- nal soil strength varies temporally, horizontally and vertically owing to differences in soil tex- ture, content and kind of organic matter, struc- ture, root density and especially moisture con- tent/potential (Fig. 2). The effects of these dif- ferent parameters on the compressibility and compaction behaviour of soils have been re- viewed among others by Horn and Lebert (1994). Soil moisture content and/or potential is the dominant property affecting soil strength during compaction (Dawidowski and Lerink 1990). As soil moisture content increases, the strength of an unsaturated soil drops rapidly, increasing the risk of soil compaction. Thus, the same stress compacts a subsoil more when it is moist than when it is dry (Salire et al. 1994). Pure sandy soil may be weak when either dry or saturated, however, since soil resists loading best when the capillary cohesion is highest. According to Akram and Kemper (1979), the soil compaction due to a given stress is greatest when the soil is at field capacity (ψ m = –10 kPa). Saturated soil cannot be compacted without the water draining out from the soil. Saturated soil may smear, how- ever, with resultant puddling and subsequent soil compaction when the homogenized soil dries (Guérif 1990). Fig. 2. Field traffic factors and soil properties affecting the soil compaction process (Canarache 1991, Soane and Ouwerkerk 1994b). 336 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Alakukku, L. Subsoil compaction due to wheel traffic Stresses induced by forces applied to subsoil During field traffic, downwards acting forces due to dynamic wheel load (vertical normal forces), shear forces imposed by wheel slip and vibra- tion forces transmitted from the engine through wheel or track are exerted on the soil. A towed wheel applies mainly vertical normal forces to the soil, while the driven wheel also exerts shear forces. Downwards acting forces have been the main concern in connection with subsoil com- paction. Raghavan and McKyes (1977) never- theless found in laboratory studies that up to 50% of topsoil compaction was attributable to shear stress owing to wheel slip. Likewise, at high slips, smearing damages the soil. According to Koolen et al. (1992), the shear effect vanishes relatively rapidly with depth. However, Kirby (1989) concluded from a model evaluation that shear damage may extend to a depth of 1.5 to 2 times the tyre width in a soil profile of uniform strength. Even though the shear effect may van- ish rapidly, it can damage the subsoil markedly, especially during ploughing, owing to furrow wheel slipping. Davies et al. (1973) suggest a slip maximum of 10% to avoid topsoil damage owing to shear. The same limit is probably ap- propriate for subsoils. The contribution of vibration effects to the compaction below pneumatic tyres has seldom been documented for arable soils (Soane et al. 1981). Wong and Preston-Thomas (1984) pro- pose that a tracked tractor compacts topsoil more than expected because of the vibration transmit- ted through the track to the soil. With powerful tracked tractors in increasing used in agriculture, study is needed of the extent of the vibration they transmit to the soil and the possible effects of the vibration on subsoil. Average ground contact stress The average ground contact stress (wheel load divided by ground contact area between tyre and soil) estimates the vertical stress exerted on the soil surface by wheel or track. The contact stress is often evaluated from the tyre inflation pres- sure. The relationship between the average ground contact stress and the inflation pressure of a tyre depends, however, on tyre stiffness and soil conditions. For stiff agricultural tyres, tyre walls carry a considerable proportion of the to- tal load, and on rigid surfaces the contact stress is higher than inflation pressure (Plackett 1984). Burt et al. (1992) found that the dynamic aver- age ground contact stress below an 18.4R–38 tractor tyre on rigid soils was closely approxi- mated by the inflation pressure, whereas on un- compacted soils the contact stress was clearly lower than the inflation pressure. Burt et al. (1992) and Tijink (1994) offer detailed introduc- tions for the determination of contact stress. The average ground contact stress denotes the calculated average value of the vertical stress in the tyre/track- soil contact area. The stress is not, however, uniformly distributed over the contact area. Stress distribution beneath the wheel is complex because of the tyre lug patterns and because the tyre itself can be deformed. Thus, the maximum ground contact stress under lugs or stiff tyre walls on a firm soil or at the centre of the contact area of a hard tyre on a soft soil may be twice or even four times the estimated average ground stress (Smith 1985, Burt et al. 1992). Likewise, the ground contact stress un- der a track will concentrate under the roadwheels (Wong 1986). The effect of uneven ground con- tact stress distribution on soil compaction has been little investigated. It is believed that the effect of the unevenness is limited to the upper part of the soil profile. However, Akker et al. (1994) found that at a depth of 0.35 m the peak stress with a low pressure tyre was about 60% of that with normal tyre even though the infla- tion pressure of the low pressure tyre (80 kPa) was only 33% that of the normal tyre (240 kPa). They proposed that the ground contact stress was concentrated below the stiff sides of the low pres- sure tyre. Extent of stresses In unsaturated soils, external stresses are trans- mitted three-dimensionally via solid, liquid and 337 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Vol. 8 (1999): 333–351. gaseous phases. Most of the analytical models for the propagation and distribution of stresses in the soil profile describe the stress distribu- tion under a point load acting on a homogene- ous, isotropic, semi-infinite, ideal elastic medi- um. The theoretical solution was proposed by Boussinesq (1885, ref. Söhne 1953). Fröhlich (1934) later modified the original solution by introducing a concentration factor (ν) to account for the increase in Young’s modulus with soil depth due to overburden stress. Söhne (1953, 1958) and Gupta and Raper (1994) review the equations that describe stresses on a soil element as a result of a point load. According to the theory, a vertical normal stress (σ z ) under a point load (P) at a given depth (z) can be expressed as [1] where R is the radial distance from the point load to the subsoil point under consideration and β is the angle between the vertical line from the loaded point and R (Gupta and Raper 1994). An analytical solution for the soil vertical normal stress beneath the centre of a uniformly loaded circular ground contact area can be cal- culated as a function of depth using the equa- tion given by Taylor et al. (1980): [2] where p is the average ground contact stress (tyre load/ground contact area) acting on the soil -tyre contact area, and α is the half aperture an- gle between the point at depth z and the edge of the contact area. The stress described in the equations corre- sponds to that in an elastic (Poisson ratio 0.5), isotropic body if the concentration factor n is equal to 3 (Gupta and Raper 1994). Söhne (1953, 1958) assumed the values 4, 5 and 6 for hard, average and soft soil conditions, respectively. Soil firmness he described as a combination of looseness (bulk density) and moistness. Horn et al. (1987) report more fully that the concentra- tion factor depends on the moisture content, den- sity, load history, structure and texture of the soil. According to Horn (1980), ν varies from 1 for strong material to 10 for soft soil, and it is lower in aggregated than in non-aggregated soil (Horn 1986). In summary it can be stated that in strong and/or dry soils with low ν, vertical normal stress is distributed more horizontally, in a shallower soil layer, but in weak and/or wet soils with high ν it is transmitted to deeper depths. The analytical solution has limitations. Esti- mation of the concentration factor is problemat- ic and expensive (Gupta and Raper 1994). Fur- thermore, the soil conditions assumed above are seldom encountered in the field since soil is more a plastic medium than an elastic one, and the conditions vary spatially. Likewise, any non- uniformity in the soil profile tends to distort the stress distribution. Thus, Taylor et al. (1980) found that a plough pan in the soil profile, which is a common situation in agriculture, caused higher stress above the pan and lower stress be- low it than was the case for a uniform soil pro- file. Hard-pan tends to act like an elastic bridge, spreading the load over a wider area by reduc- ing the concentration factor (Dexter et al. 1988). Thus, finite-element models are more often used, to take into account the heterogeneity of a soil profile (Kirby 1999). Likewise, the analytical solution described above is based on static stress. Hadas (1994) states that the results may be ex- pected to be different when the stress is dynam- ic (short duration stress changing during driv- ing). Nor is stress ever uniformly distributed over the ground contact area, as equation [2] assumes (see sect. Average ground contact stress). The analytical solution shows that the stress in the soil under a loaded wheel decreases with depth (Fig. 3). From this, a highly simplified conclusion can be drawn: the stress in the top- soil depends on the average ground contact stress, but the stress in the subsoil is determined mostly by the wheel load (Söhne 1958, Carpen- ter et al. 1985). The same has been found in field and soil bin experiments (Danfors 1974, Taylor et al. 1980, Bolling 1987, Fig. 3). Hadas (1994) and Olsen (1994) criticize the generalization, however. On the basis of analytical calculations, Olsen (1994) concludes that the decrease of in- 338 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Alakukku, L. Subsoil compaction due to wheel traffic duced vertical normal stress with depth in the upper subsoil (0.10–0.30 m to 1 m depth) de- pends on both ground contact stress and wheel load, and below 1 m solely on wheel load. The criticism is justified since, based on equation (2), reduction in the average ground contact stress reduces the magnitude of stresses transmitted to the soil. The wheel load determines the normal stress level deep in the soil profile, but the stress level will never exceed the maximum ground contact stress level. From the analytical solution and experimen- tal results the following conclusion can be drawn on the effects of wheel load and contact stress on the soil stress and compaction. As the wheel load is increased with the same tyre and contact area, the stress at a specific depth increases and a given stress is transmitted deeper (Danfors 1974). When the wheel load is increased, even though the contact stress is kept unchanged by increasing tyre dimensions or the number of tyres (dual, triple), a given isostress is transmitted deeper into the soil and a greater soil volume is stressed (Fig. 3, Hadas 1994). When the number of tyres is increased, the extent of a given isos- tress may, however, be reduced by spacing the tyres widely apart to avoid any interaction be- tween them (Olsen 1994). Lebert et al. (1989) found in in situ stress transmission measurements at the same wheel load but increasing contact area that vertical stresses can be reduced in the top- soil by using larger tyres, but in the subsoil the Fig. 3. Calculated isostress lines as a function of wheel load (WL) with constant tyre inflation pressure (TIP) in a homogeneous soil (left, redrawn from Söhne 1953), and measured vertical stress distribution as a function of depth at constant wheel load in a silt soil (right, Lebert et al. 1989). A represents contact area and p represents average ground contact stress. Figures reprinted with kind permission from Institut für Biosystemtechnik, Bundesforschungsanstalt für Landwirtschaft and Kluwer Academic Publishers. 339 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Vol. 8 (1999): 333–351. reductions are less significant (Fig. 3). Likewise, Danfors (1994) reported that a reduction in in- flation pressure from 150 to 50 kPa (axle load > 8 Mg) reduced the compaction of moist clay soils only down to a depth of 0.30–0.40m, not in the deeper layers. In summary it can be stated that risk of subsoil compaction exists whenever a moist soil is loaded by a high wheel load and moderate to high ground contact stress. Number of wheel passes The number of passes affects the number of load- ing events and the coverage, intensity and dis- tribution of wheel traffic. In field experiments on mineral soils, an increase in the number of passes in the same track increased subsoil com- pactness (Gameda et al. 1987, Schjønning and Rasmussen 1994, Alakukku and Elonen 1995a) and the depth of the compacted layer (Sommer and Altemüller 1982, Alakukku 1996a, b). An increase in the number of random passes would normally increase the spatial coverage of traffic in a field. Heavier machines usually reduce the number of passes per unit of land area per oper- ation because of increased working width. Olfe and Schön (1986) report that a doubling in trac- tor power from 60 to 120 kW clearly reduced the area of soil covered by wheel tracks in cere- al cultivation. At the same time, however, the intensity of field traffic increased from 176 to 216 Mg km ha-1. Due to the use of heavier ma- chines. Thus, the risk of subsoil damage may increase, even though the percentage of the field covered by wheel traffic is reduced by increased working width. Driving speed When the velocity of a machine is increased, the duration of the loading is reduced. Bolling (1987) measured the effects of velocity on the maximum soil stress below a wheel centre with two wheel loads (0.82 Mg, tyre inflation pressure 160 kPa and 1.5 Mg, 170 kPa) on sandy loam soil and found that an increase in velocity from 2 km h-1 to 10 km h-1 decreased stress at 0.30 m depth below the wheel centre. The effect of velocity was greater on loose than on dense soil. Similar results were reported by Horn et al. (1989). Ear- lier, however, Danfors (1974) found that higher velocity caused bouncing, which transmitted high point stresses to the subsoil. An increase in velocity would seen to reduce the stress trans- mitted to upper subsoil layers. The effects of velocity on the stress in deeper layers and the practical importance of velocity to subsoil com- paction have seldom been documented, howev- er. The highest velocity tested has been 8–12 km h-1, which is the normal speed in field operations. Extent of subsoil compaction The most serious source of subsoil compaction tends to be tractor wheels running in the open furrow during mouldboard ploughing because the tractor wheels then run directly on the upper part of the subsoil. As summarized in Table 1, field traffic on the soil surface with an axle load higher than 6 Mg and with tyre inflation pres- sure greater than 50 kPa compacts moist miner- al soils to 0.50 m depths or deeper. Under unfa- vourable soil conditions, an axle load less than 5 Mg also compacts the subsoil. The depth of compaction in organic soils has seldom been in- vestigated because the effects of organic soil compaction have not been considered harmful. An axle load higher than 6 Mg has been found to compact organic soils to 0.40–0.50 m depths (Table 1). In view of the persistence of subsoil compaction (see sect. Long-term effects of com- paction in agriculture), Håkansson and Petelkau (1994) regard compaction to depths more than 0.40 m as unacceptable. Stress, axle load and farming recommendations With the aim of avoiding subsoil compaction, recommendations have been given for maximum 340 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Alakukku, L. Subsoil compaction due to wheel traffic values of average ground contact stress and stress at 0.50 m depth (Table 2) and for axle loads. For moist soils, it is sometimes recommended that ground contact stress should not exceed 50 kPa (Table 2). Carpenter and Fausey (1983) suggest that the maximum ground contact stress with Table 1. Maximum depth of soil compaction due to field traffic with different axle loads and soil conditions. Axle Depth of Soil load TIP1) compaction Type Moisture (Mg) (kPa) (m) Reference Coarse sand Sand Sand Sand Sand Loamy sand Loam Loam Silt loam Silt loam Silt loam Silt loam Silty loam Silt Sandy clay Silty clay loam Silty clay Clay loam Clay loam Clay loam Clay loam Clay Clay Clay Clay Clay Fen Mull Sedge peat5) 1) TIP = tyre inflation pressure, FC = field capacity, PW = permanent wilting point, SS = saturation 2) tandem axle 3) average ground contact stress 4) total load 5) sedge peat mixed with clay from 0.20 m to 0.40–0.50 m depths, and underlain by gythia Schjønning and Rasmussen (1994) Danfors (1974) Håkansson (1985) Akker (1988) Dumitru et al. (1989) Lipiec et al. (1990) Gameda et al. (1984) Lipiec et al. (1990) Schuler and Lowery (1984) Blackwell et al. (1986) Hammel (1988) Wood et al. (1993) Stewart and Vyn (1994) Alakukku (1997) Gameda et al. (1984) Voorhees et al. (1978) Schuler and Lowery (1984) Voorhees et al. (1986) Danfors (1994) Alakukku and Elonen (1995a) Alakukku (1997) Danfors (1974) Eriksson (1976) Aura (1983) Håkansson (1985) Alakukku (1996a, b) Schmidt and Rohde (1986) Pietola (1995) Alakukku (1996a, b) At FC1) Near FC Near FC – – 60% of FC 0.22 g g-1 78% of FC 0.16 g g-1 –14– –60 kPa Near FC 72% of SS1) Near FC < 0.3 m FC 0.23 g g-1 – 0.20 g g-1 > 0.3 m PW1) Near FC Near FC Near FC 85–90% of FC Near FC – Near FC > FC Near FC Near FC – > FC > FC 222) 162) 4 6 162) 6.4 2.5 1.7 10/20 1.7 12.5 13.2 10/20 15.2 12 11 10/20 4.5 12.5 9/18 9/18 8 102) 5 212) 162) 4 6 404) 3 3 162) 192) 6 3 162) 2203) 400 400 400 300 240 300 60 414/414 60 220 118 200/330 210 145 600 414/414 2503) 220 150/2003) 150/2003) 50/150 50/150 150/300 800 400 400 400 – 140 140 300 700 – 1203) 700 0.60 0.60 0.20 0.40 0.50 0.80 0.40 0.20–0.40 0.60 0.20–0.40 0.40 0.50 0.75 0.40 0.35 0.50 0.60 0.45 0.40 0.30 0.50 0.50 0.50 0.35 0.40–0.45 0.60 0.20 0.40 1.00 0.20 0.40 0.50 0.50 0.50 0.35 0.40–0.50 341 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Vol. 8 (1999): 333–351. high wheel loads should not exceed the stress allowed in the subsoil. Few data exist, however, to allow assessment of the maximum allowable subsoil stress in different conditions, and this area should be addressed in future studies. From a practical point of view it is relevant that the recommendations for ground contact stress are close to the recommendations for the maximum tyre inflation pressures given by Dw- yer (1983, 50 kPa for moist soil, 100 kPa for dry soil) and Perdok and Tijink (1990, 50 kPa for moist soil, 250 kPa for dry soil). When the contact stress is to be minimized, the technical solution will depend on the demands of the op- eration in which a machine is used. The tyre inflation pressure should, however, always be the lowest allowable in the prevailing situation (tyre loading capacity, velocity, traction), and the tyre -soil contact should be uniform. For a detailed discussion of tyre factors see, among others, Tijink (1994); for track factors see Er- bach (1994). Table 2. Recommendations for maximum average ground contact stress and vertical soil stress at 0.50 m depth in different soil conditions to prevent soil compaction. Ground contact Stress at 0.50 m stress (kPa) depth (kPa) Summer/ Summer/ Soil Spring autumn Spring autumn Reference Sand Loam Clay Arable land Clay MC > 90% of FC2) MC 70–90% of FC MC 60–70% of FC MC 50–60% of FC MC < 50% of FC Sand, Sandy loam MC > 90% of FC MC 70–90% of FC MC 60–70% of FC MC 50–60% of FC MC < 50% of FC Silts, 14–27% clay ψ m –30 kPa2) ψ m –100 kPa Mineral soils MC ≥ FC 1) moisture content < 70% of field capacity 2) moisture content (MC) of field capacity (FC), ψ m is matric potential 3) the stress at which irreversible deformation occurs at 0.45 m depth Soil with high bulk density has higher value than soil with low bulk density 4) cited by Lipiec and Simota (1994) 30 30 35 45 50 30 30 35 45 50 Petelkau (1984) Vermeulen et al. (1988) Rusanov (1994) Salire et al. (1994) Bondarev et al. (1988)4) 50 80 150 50 80 100 120 150 180 95 120 145 180 215 40–50 801) 1501) 2001) 100 100 120 140 180 210 120 145 170 215 250 25 25 30 35 35 25 25 30 35 35 64–2713) 119–356 342 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Alakukku, L. Subsoil compaction due to wheel traffic To avoid soil compaction below normal pri- mary tillage depth (0.2–0.3 m), single axle loads not exceeding 4 to 6 Mg have been recommend- ed for moist mineral soils (Danfors 1974, 1994, Voorhees and Lindstrom 1983, Petelkau 1984) even when the tyre inflation pressure is 50 kPa (Danfors 1994). For tandem axle loads on moist soils Danfors (1974, 1994) proposes a limit of 8–10 Mg. On dry soil, the axle load may be high without causing subsoil compaction (Table 1). Farmers cannot, however, easily estimate mois- ture conditions in the subsoil. Moreover, heavy field traffic on moist fields often cannot be avoid- ed owing to the time constraints of given field operations. Thus, recommendations need to be set with a view to the wettest conditions prevail- ing during the normal use of a machine. While stress and axle load limits may be con- sidered to be the engineering tools for the con- trol of soil compaction, it should be remembered that the farming system, and the way of using machinery as part of that system, will markedly influence the effects of the field traffic on soils and crops. For instance, if the drainage does not work, a larger tractor or larger tyres will not es- sentially reduce the workability or trafficability problem and may even intensify the compaction problem. Moreover, spring application of slurry manure before seedbed preparation is recom- mended as the most effective way of conserving plant nutrients. Yet slurry tankers exert substan- tial axle loads on soils which are usually wet, and may induce significant compaction. Thus, Håkansson and Danfors (1988) calculated that the compaction cost of spreading liquid manure on wet soil may exceed the economic value of the nutrients supplied by the manure. Further- more, the area covered by wheel traffic can be reduced without increasing working width by using combined implements or linked operations or by concentrating field traffic to the same tracks. An example is combined seedbed prepa- ration, fertilization and sowing. Likewise, the transport traffic on moist soil should be concen- trated to as few tramlines as possible, with these located perpendicular to the subsurface drains or to headland roadways. Long-term effects of compaction in agriculture Compaction induced by field traffic has both short- and long-term effects on soil and crop pro- duction. Short-term (1–5 years) effects are main- ly associated with topsoil (0–0.30 m) compac- tion, which is largely controlled by tillage oper- ations, field traffic and the way in which these operations are adapted to soil conditions. The topsoil compaction is alleviated by tillage and natural processes of freezing/thawing, wetting/ drying and bioactivity. Aura (1983) found that the compaction of clay soils within the plough layer due to traffic with an axle load of 3 Mg in spring was alleviated by ploughing and frost by the following spring. When the plough layer is severely compacted, the recovery of heavy clays may take three (Alakukku 1996b) or even five years (Arvidsson and Håkansson 1996) despite annual ploughing and frost. This section discuss- es the effects of wheel traffic on subsoil proper- ties and on crop production in field conditions. Subsoil properties and behaviour Many studies have found that compaction mod- ifies the pore size distribution of mineral sub- soils mainly by reducing the macroporosity (> 30 µm) (Eriksson 1976, Ehlers 1982, Blackwell et al. 1986, Alakukku 1996a, 1997). Besides the volume and number of macropores, compaction may also affect their continuity. Modifications in soil macroporosity are very important since they affect other soil properties and soil behav- iour. Subsoil compaction has been found to have harmful effects on many soil properties relevant to soil workability, drainage, crop growth and environment. Subsoil compaction due to heavy field traffic increased the dry bulk density, vane shear resistance and penetrometer resistance of subsoils with clay content 0.06–0.85 g g-1 (Ta- ble 3). Compacted subsoils had a more massive 343 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Vol. 8 (1999): 333–351. and coarse structure (Pollard and Webster 1978, Alakukku 1997). Likewise, compaction reduced the saturated hydraulic conductivity (Table 3), CO 2 and O 2 exchange (Simojoki et al. 1991) and air permeability (Horn et al. 1995) of subsoils. The likelihood of drainage problems increas- es when compaction reduces the permeability of the subsoil and may lead to waterlogging prob- lems in rainy years. Poorly drained soil may also dry slowly, reducing the number of days availa- ble for field operations. Furthermore compaction may increase surface runoff and topsoil erosion by impeding water infiltration (Fullen 1985). Likewise, the reduction in drainage rate attrib- uted to subsoil compaction can be expected to increase the emission of green house gases from the soil, for instance, by increasing denitrifica- tion. The environmental consequences of soil compaction have been reviewed by Soane and Ouwerkerk (1995). However, there is little in- formation as yet on the implications of machin- ery induced subsoil compaction for the hydrau- lic processes in the soil profile and the quality of the environment. These subjects are in need of further study. Normal tillage does not loosen the subsoil (below about 0.30 m). The effects of subsoil compaction may persist for a very long time. For example, in spite of cropping and deep frost, the effects of subsoil compaction due to high axle load traffic were measurable in soils with clay contents of 0.06–0.85 g g-1 three to 11 years af- ter application of heavy loading (Table 3, Gault- ney et al. 1982, Voorhees et al. 1986, Gameda et al. 1987, Etana and Håkansson 1994, Lowery and Schuler 1994, Wu et al. 1997). Subsoil compac- tion still persisted six years after the traffic in a cropped sandy loam (Pollard and Webster 1978) Table 3. Change in dry bulk density (BD), total porosity (TP), penetrometer resistance (PR), vane shear resistance (VR), macroporosity (MP), saturated hydraulic conductivity (K sat ) and number of cylindrical pores (NB) in the subsoil of mineral soils after compacting treatments. Results are expressed relative to the control. Change in soil properties due to loading (%) Soil Loading BD TP PR VR MP K sat NB Reference Sandy loam Sandy loam Loam Loam Silt loam Silty clay loam Silt Clay loam Clay loam Clay loam Clay loam Silty clay Clay Heavy clay 6 years earlier by tractor 4 passes 38 Mg sugar beet harvester 9 years earlier, 4 passes 16 Mg tandem axle load 4 passes 38 Mg sugar beet harvester 1 pass 13.2 Mg axle load Traffic during one season 3 passes 10 Mg single axle load 9 years earlier, 4 passes, 750 kPa contact stress 7 years earlier, 4 passes 18 Mg axle load 3 passes 20 Mg tandem axle load 4 passes 18 Mg axle load 4 passes 12.5 Mg axle load 9 years earlier, 4 passes 16 Mg tandem axle load Several years 40 Mg –32 –17 –7 –67 –38 Pollard and Webster (1978) Arvidsson (1998) Alakukku (1996b) Arvidsson (1998) Blackwell et al. (1986) Voorhees et al. (1978) Alakukku (1997) Blake et al. (1976) Logsdon et al. (1992) Alakukku (1997) Voorhees et al. (1986) Lowery and Schuler (1994) Alakukku (1996b) Eriksson (1976) 10 3 4 20 5 11 3 4 2 6 –7 –8 200 12 400 15 –1 25 23 18 63 12 –33 –15 –45 –8 –14 –42 –64 –67 –84 –80 –78 –90 57 13 –48 –14 344 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Alakukku, L. Subsoil compaction due to wheel traffic and ten years after the traffic in a cropped clay (Duval et al. 1989). In all these investigations, the effects of compaction persisted for the dura- tion of the experiment. Håkansson and Petelkau (1994) concluded that subsoil compaction tends to be highly persistent, and in non-swelling sandy soils and tropical areas it may be permanent im- mediately below the tillage depth. As shown above, subsoil compaction can be quantified by measuring many different soil properties. A common approach is to determine the changes in bulk density. Bulk density is also included in most models predicting subsoil strain due to external stresses (Smith 1985). The use- fulness of bulk density determinations for agri- cultural purposes is limited, however, since the bulk density does not take into account the non- volumetric changes of subsoil strcture due to the loading and it does not determine the behaviour of subsoils. Horn et al. (1996) demonstrated un- der idealized sandy loam soil bin conditions that up to 60% of soil deformation can be determined as a volumetric compression and up to 40% has to be defined as volume-constant deformation. Changes in soil behaviour properties, such as mechanical behaviour, water retention and wa- ter and air flow, better express the effects of com- paction on soil than does bulk density (Gupta et al. 1989, Carter 1990). Crop response The harmful effects of subsoil compaction are reflected not only in soil properties but in crop growth and yield. Soil compaction to depths of 0.50 m or more by axle load traffic greater than 9 Mg decreased the yield of grain (Håkansson et al. 1987, Fig. 4) and nitrogen uptake (Alakukku and Elonen 1995b), and delayed (Gaultney et al. 1982, Gameda et al. 1987) or accelerated (Alakukku and Elonen 1995b) the ripening of crops up to several years after the loading. The greatest decrease in yield occurred in the first three years after the compaction (Fig. 4), proba- bly mainly owing to plough layer compaction (Håkansson and Reeder 1994). By degrees, an- nual ploughing and freezing alleviated the plough layer compaction, and after the first few years the reduction in yield became less (Fig. 4). It may be that the subsoil compaction is not alleviated, however, and its long-term effect on the crop growth and yield will then depend on the climatic conditions of the growing season. Voorhees (1992) found that subsoil compaction resulting from loading with an 18 Mg axle load decreased maize yield significantly on clay loam in relatively dry and rainy years but not in years with normal precipitation. Lowery and Schuler (1994) reported that the subsoil compaction due to traffic with a 12.5-Mg axle load reduced maize yield considerably only in a rainy year. Likewise, Alakukku and Elonen (1995b) found that, when the beginning of the growing season was dry, subsoil compaction in clay soil had little effect on the grain and nitrogen yields of annual crops whereas in rainy years the yields were clearly reduced. They also observed that extended drought reduced the yields in all plots. Further- more, subsoil compaction may have little effect on crop yield if the crop does not need to extract Fig. 4. Crop yields relative to control treatment (=100%) for 12 successive years (1–12) after loading with four pass- es by vehicles having a single axle load of 10 Mg or tandem axle load of 16 Mg and tyre inflation pressure > 200 kPa. Mean values from field experiments in seven countries in Europe and North America. Data for the last four years (Alakukku 1999) are included in the figure, which is redrawn from Håkansson and Reeder (1994). 345 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Vol. 8 (1999): 333–351. water from the subsoil. Montaga et al. (1998) reported that the root growth of vegetable crops in the subsoil was reduced by compaction, but there was no direct effect on crop shoot growth or yield when water and nutrients were adequate- ly supplied to the soil. A common approach in examinins the effects of subsoil compaction is to determine the quan- titative changes in yields. Yield quantity alone does not, however, always show how harmful the influence of soil compaction on crops may be. Interestingly, nitrogen yield has been found to be a more sensitive measure of the influence of topsoil and subsoil compaction than the grain yield. Douglas et al. (1995) found that the aver- age dry matter yield and nitrogen uptake of grass silage over a period of four years were 13 and 18% lower, respectively, in conventional traffic plots than in zero traffic plots. Likewise, taken as a mean of the first nine years after a clay soil was compacted to 0.50 m depth by a single heavy loading, the grain yield of annual crops in com- pacted plots was reduced by 3% but the nitro- g e n y i e l d h a r v e s t e d i n g r a i n y i e l d b y 7 % (Alakukku and Elonen 1995b). The long-term effects of subsoil compaction on crop quality and nutrient uptake have seldom been studied, how- ever. The reasons for yield reduction are not al- ways well quantified. Yet in any attempt to mon- itor the consequences of compaction it is impor- tant to quantify the effects on the subsoil/plant/ weather system. Likewise, mainly annual grain crops were grown in field experiments. Yet crop sensitivity to topsoil compaction varies with plant species (Brereton et al. 1986) and even variety. In summary it should be emphasized that field experiments on the effects of subsoil com- paction on crop production in different crop ro- tations need to be carried out over a sufficiently long period of time to account for iteractions between variations in weather conditions and crop response. Moreover, to better quantify sub- soil/plant/weather interactions, measurements of plant development, water and nutrient uptake, root growth and soil and weather conditions need to be made during the growing season. Cumulation of subsoil compaction Since the alleviation of a severe subsoil com- paction takes many years, if it occurs at all, a heavy loading repeated in the same place each year may increase soil compactness and yield losses year by year. The area of compacted soil may also increase if the heavy field traffic is not concentrated annually in the same tracks. Soil compaction may thus become more harmful as time goes on, even though the effect of a single pass by a heavy vehicle tends to be rather small (Håkansson 1994a). Arvidsson and Håkansson (1996) report that repeated loading of 350 Mg km ha-1 on wet clay soils before ploughing in- creased the yield losses of spring cereals annu- ally during the first four years. After that, the yields levelled out. They evaluated, however, that the effects of compaction were mainly caused by compaction in the topsoil. Gameda et al. (1987) did not observe cumulative yield losses with maize even though heavy field traffic with axle loads 10–20 Mg increased the maximum soil density each year during the first three years. Repeated field traffic does not always increase soil compaction cumulatively. Alblas et al. (1994) loaded sandy soils with an axle load of 10 Mg twice a year for four years but did not find any cumulative subsoil compaction or cu- mulative losses in silage maize yield. In separate studies, Håkansson et al. (1996) and Fenner (1997) quantified the cumulative compaction effects on the subsoil in farmers fields induced by machinery traffic in recent decades. Håkansson et al. (1996) found that soil penetration resistance at 0.40 m depth was an average 40% higher in 17 fields (subsoil clay content 0.01–0.58 g g-1) with intensive potato and sugar beet production and where large quanti- ties of slurry were spread than in the control fields under permanent grass without field traf- fic in the past 35 years. In eight fields (subsoil clay content 0.35–0.60 g g-1) in a cereal produc- tion region with less intensive machinery traf- fic, the corresponding increase in penetration resistance was 10%. Håkansson et al. (1996) es- timated persistent crop yield reductions caused 346 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Alakukku, L. Subsoil compaction due to wheel traffic by subsoil compaction in the 17 and eight fields to be 6% and 1.5%, respectively. According to these two studies the state of compactness of arable subsoils and soil productivity are depend- ent on the intensity of heavy field traffic. Fen- ner (1997) reported similar results for penetra- tion resistance on loess soil in two neighbour- ing fields with maximum axle loads of 4 Mg and 8.9 Mg during the past 20 years. General discussion and conclusions Results reviewed in the present paper show that heavy machinery traffic increases the risk of long lasting soil physical degradation and crop yield losses, particularly in temperate, moist zones. Threats to soil productivity need to be minimized in future. As world population is forecasted to grow 8 billion and the demand for food to in- crease by 64% during the next 25 years (World- watch Institute 1996), world food production must necessarily be increased. Yet little addition- al arable land is available. In general, this means that food production per unit area will have to be increased rather than be allowed to decrease. Furthermore, the impact of agriculture on the environment must be diminished. Likewise, in developing countries, improved and modern farming systems are needed to enhance crop pro- duction. Thus, it is necessary to continue devel- oping technical solutions to avoid detrimental topsoil and subsoil compaction induced by field traffic and to find methods to improve already compacted subsoils. The risk of subsoil compaction is high when moist soils are loaded with high axle load traffic with moderate to high ground contact stress. Håkansson (1994b) notes that the gradually in- creasing weight of agricultural machinery rais- es concern about a possible permanent deterio- ration of the subsoil. With the aim of avoiding subsoil compaction, some recommendations have been given for axle loads and for maximum permissible ground contact stress and stress at 0.50 m depth. To avoid long-lasting soil physi- cal degradation below normal tillage depth, in- ternational limits should be established for me- chanical stresses in the subsoil. Håkansson and Reeder (1994) suggest that these could be sim- ple maximum axle load limits, or combined axle load -ground contact stress limits. Since agricul- tural machinery and farming practices are simi- lar world-wide, the recommendations for maxi- mum mechanical stress limits should be arrived at through international teamwork. Our under- standing of the subsoil compaction process is incomplete, however, and our modelling of com- paction in soil/machine/plant systems imperfect. Thus, before appropriate stress limits can be set, areas such as the bearing capacity of subsoils under different conditions should be investigat- ed. While large and heavy machinery is often blamed for soil compaction problems, it should be noticed as well that the farming system is not usually changed when the machinery size in- creases. Larger machines, often equipped with large, low-pressure and/or dual tyres, are used in the same soil conditions (macroporosity, drainage) and in the same tillage and other work- ing practices as smaller ones. Added to this, in- creased engine power and/or enlarged tyre con- structions make it possible to work in worse con- ditions. Large machines do not, however, com- pensate for the poor drainage or other misman- agement of soil structure. Thus, in examining the possible ways to avoid subsoil compaction due to field traffic, the whole farming system needs to be considered, not just the machinery. Soil compaction affects virtually all physi- cal, chemical and biological soil properties and processes by modifying soil macroporosity. Yet the effects of heavy loading on subsoils and the effects of subsoil compaction on drainage, crop growth and environment are typically investigat- ed with a limited number of soil, crop and envi- ronmental parameters. When these effects are being evaluated, it is relevant to take the eco- nomics into account as well. Voorhees (1991) has 347 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Vol. 8 (1999): 333–351. pointed out that even though a given machinery system decreases crop yield a little, it still may make economic sense to use it. Eradat Oskoui et al. (1994) reviewed the economics of modifying machinery to minimize soil compaction. The ef- fects of subsoil compaction on the farm econo- my have seldom been documented, however. Further studies will be needed to obtain suffi- cient information on the long-term and cumula- tive effects of machinery induced subsoil com- paction on soil hydrology, crop productivity, farm economics and environment. The risks of subsoil compaction should be analysed on the basis of the findings of these studies, and the probabilities of harmful consequences of subsoil compaction in different situations should be eval- uated. Subsoil compaction has been documented to be long-lasting and difficult to correct. Deep loosening is expensive and will seldom amelio- rate the compacted structure completely. More- over, the loosened soil is often recompacted within two or three years, with even worse phys- ical properties (Kooistra and Boersma 1994). Thus, it is better to avoid subsoil compaction in the first place than to rely on alleviating the com- pacted structure afterwards. The alleviation of soil compaction through biological tillage by earthworms and plant roots is in need of further study. Whalley and Dexter (1994) note that bi- opores are more stable than the macropores pro- duced by mechanical tillage. Acknowledgements. This writing of this paper was finan- cially supported by the Commission of the European Com- munities, Agriculture and Fisheries (FAIR) specifiec RTD programme, FAI5–CT97–3589, “Experiences with the im- pact of subsoil compaction on soil, crop growth and envi- ronment and ways to prevent subsoil compaction”. In prin- ciple the presented data do not necessarily reflect its views and in no way anticipates the Commission’s future policy in this area. References Akker, J.H.H. van den 1988. Model computation of sub- soil stress distribution and compaction due to field traffic. Proceedings 11th International Conference of ISTRO, 11–15th July, Edinburgh, Scotland 1: 403– 408. – , Arts, W.B.M., Koolen, A.J. & Stuiver, H.J. 1994. Com- parison of stresses, compaction and increase of pen- etration resistance caused by a low ground pressure tyre and a normal tyre. Soil & Tillage Research 29: 125–134. Akram, M. & Kemper, W.D. 1979. Infiltration of soils as affected by the pressure and water content at the time of compaction. Soil Science Society of America Journal 43: 1080–1086. Alakukku, L. 1996a. Persistence of soil compaction due to high axle load traffic. I. Short-term effects on the properties of clay and organic soils. Soil & Tillage Research 37: 211–222. – 1996b. Persistence of soil compaction due to high axle load traffic. II. Long-term effects on the proper- ties of fine-textured and organic soils. Soil & Tillage Research 37: 223–238. – 1997. Properties of fine-textured subsoils as affect- ed by high axle load traffic. Acta Agriculturae Scan- dinavica. Section B. Soil and Plant Science 47: 81– 88. – 1999. Response of annual crops to subsoil compac- tion in a field experiment on clay soil lasting 17 years. International Conference on Subsoil Compaction. Christian-Albrechts-Universität zu Kiel, Kiel, Germa- ny 24.–26.03.1999. Abstracts. p. 13–15. – & Elonen, P. 1995a. Cumulative compaction of a clay loam soil by annually repeated field traffic in autumn. Agricultural Science in Finland 4: 445–461. – & Elonen, P. 1995b. Long-term effects of a single compaction by heavy field traffic on yield and nitro- gen uptake of annual crops. Soil & Tillage Research 36: 141–152. Alblas, J., Wanink, F., Akker, J. van den & Werf, H.M.G. van der 1994. Impact of traffic-induced compaction of sandy soils on the yield of silage maize in The Netherlands. Soil & Tillage Research 29: 157–165. Arvidsson, J. 1998. Soil compaction caused by heavy sugar beet harvesters measurements with tradition- al and new techniques. In: Märländer, B. et al. (eds.). Soil compaction and compression in relation to sug- ar beet production. Advances in Sugar Beet Research 1: 35–42. – & Håkansson, I. 1996. Do effects of soil compaction persist after ploughing? Results from 21 long-term field experiments in Sweden. Soil & Tillage Research 39: 175–197. Aura, E. 1983. Soil compaction by the tractor in spring and its effect on soil porosity. Journal of the Scientif- 348 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Alakukku, L. Subsoil compaction due to wheel traffic ic Agricultural Society of Finland 55: 91–107. Blackwell, P.S., Graham, J.P., Armstrong, J.V., Ward, M.A., Howse, K.R., Dawson, C.J. & Butler, A.R. 1986. Compaction of a silt loam soil by wheeled agricultur- al vehicles. I. Effects upon soil conditions. Soil & Till- age Research 7: 97–116. Blake, G.R., Nelson, W.W. & Allmaras, R.R. 1976. Per- sistence of subsoil compaction in a Mollisol. Soil Science Society of America Journal 40: 943–948. Bolling, I. 1987. Bodenverdichtung und Triebraftverhalten bei Reifen-Neue Mess- und Rechenmethode. Forsc- hungsbericht Agrartechnik Arbeitskreises Forschung und Lehre Max-Eyth-Gesellschaft (MEG) 133: 1–274. Bondarev, A.G., Sapozhnikov, P.M., Utkaieva, V.F. & Shepotiev, V.N. 1988. Izmeneniye fizicheskykh svoys- tv I plodorodya pochv pri ikh uplotnieny dvizhytela- my sielskokhazyaystvennoy tyekhniki. (Changes in soil physical properties and productivity as related to compaction by agricultural vehicles, in Russian.) Sbornik Nauchnykh Trudov, VIM, Moscow, U.S.S.R. 188: 46–57. Boussinesq, J. 1885. Application des potentials à l‘etude de l‘equilibre et du mouvement des solides elas- tiques. Gauthier-Villais, Paris, France. 30 p. Brereton, J.C., McGowan, M. & Dawkins, T.C.K. 1986. The relative sensitivity of spring barley, spring field beans and sugar beet crops to soil compaction. Field Crops Research 13: 223–237. Burt, E.C., Wood, R.K. & Bailey, A.C. 1992. Some com- parison of average to peak soil-tire contact pressures. Transaction of American Society of Agricultural En- gineers 35: 401–404. Canarache, A. 1991. Factors and indices regarding ex- cessive compactness of agricultural soils. Soil & Till- age Research 19: 145–164. Carpenter, T.G. & Fausey, N.R. 1983. Tire sizing for min- imizing subsoil compaction. American Society of Ag- ricultural Engineers. Paper no. 83/1058. 23 p. – , Fausey, N.R. & Reeder, R.C. 1985. Theoretical ef- fect of wheel loads on subsoil stresses. Soil & Till- age Research 6: 179–192. Carter, M.R. 1990. Relative measures of soil bulk densi- ty to characterize compaction in tillage studies of fine sandy loams. Canadian Journal of Soil Science 70: 425–433. Danfors, B. 1974. Packning i alven. Summary: Compac- tion in the subsoil. Swedish Institute of Agricultural Engineering. Special report S24: 1–91. – 1994. Changes in subsoil porosity caused by heavy vehicles. Soil & Tillage Research 29: 135–144. Dawidowski, J.B. & Lerink, P. 1990. Laboratory simula- tion of the effects of traffic during seedbed prepara- tion on soil physical properties using quick uni-axial compression test. Soil & Tillage Research 17: 31– 45. Davies, D.B., Finney, J.B. & Richardson, S.J. 1973. Rel- ative effects of tractor weight and wheelslip in caus- ing soil compaction. Journal of Soil Science 24: 399– 409. Dexter, A.R., Horn, R., Hollaway, R. & Jakobsen, B.F. 1988. Pressure transmission beneath wheels in soils on the Eyre Peninsula of south Australia. Journal of Terramechanics 25: 135–147. Douglas, J.T., Crawford, C.E. & Campbell, D.J. 1995. Traffic systems and soil aerator effects on grassland for silage production. Journal of Agricultural Engineer- ing Research 60: 261–270. Dumitru, E., Colibas, I., Ludusan, V., Nicolae, H., Nicu- lescu, R., Ion, P., Seitan, L. & Canarache, A. 1989. Effects of wheel traffic under drought conditions on soil and yield in Romania. International Conference in Soil Compaction, 5–9 June, Lublin, Poland. Ab- stracts. p. 63–64. Duval, J., Raghavan, G.S., Mehuys, G.R. & Gameda, S. 1989. Residual effects of compaction and tillage on the soil profile characteristics of a clay-textured soil. Canadian Journal of Soil Science 69: 417–423. Dwyer, M.J. 1983. Soil dynamics and the problems of traction and compaction. Agricultural Engineering 38: 62–68. Ehlers, W. 1982. Die Bedeutung des Bodengefüges für das Pflanzenwachstum bei moderner Landbewirt- schaftung. Mitteilungen Deutsche Bodenkundliche Gesellschaft 34: 115–128. Eradat Oskoui, K, Campbell, D.J., Soane, B.D. & Mc- Gregor, M.J. 1994. Economics of modifying conven- tional vehicles and running gear to minimize soil com- paction. In: Soane, B.D. & Ouwerkerk, C. van (eds.). Soil compaction in crop production. Developments in Agricultural Engineers 11. Elsevier Science B.V., The Netherland. p. 539–567. – & Voorhees, W.B. 1991. Economic consequences of soil compaction. Transaction of American Society of Agricultural Engineers 34: 2317–2323. Erbach, D.C. 1994. Benefits of tracked vehicles in crop production. In: Soane, B.D. & Ouwerkerk, C. van (eds.). Soil compaction in crop production. Develop- ments in Agricultural Engineering 11. Elsevier Sci- ence B.V., The Netherlands. p. 501–520. Eriksson, J. 1976. Influence of extremely heavy traffic on clay soil. Grundförbättring 27: 33–51. Etana, A. & Håkansson, I. 1994. Swedish experiments on the persistence of subsoil compaction caused by vehicles with high axle load. Soil & Tillage Research 29: 167–172. Fenner, S. 1997. Langjährige Verdictungswirkung durch unterschiedliche Achslasten auf einem Löß-Acker- standort. Zeitschrift für Pflanzenernährung und Bod- enkunde 160: 157–164. Fröhlich, O.K. 1934. Druckverteilung im Baugrunde. Ver- lag von Julius Springer, Wien, Austria. 178 p. Fullen, M.A. 1985. Compaction, hydrological processes and soil erosion on loamy sands in east Shropshire, England. Soil & Tillage Research 6: 17–29. Gameda, S., Raghavan, G.S.V., McKyes, E. & Theriault, R. 1984. Soil and crop response to the high axle load subsoil compaction – Recovery and accumulation. American Society of Agricultural Engineers. Paper no. 84/1542. 24 p. – , Raghavan, G.S.V., McKyes, E. & Theriault, R. 1987. Subsoil compaction in a clay soil. I. Cumulative ef- fects. Soil & Tillage Research 10: 113–122. Gaultney, L., Krutz, G.W., Steinhardt, G.C. & Liljedahl, J.B. 1982. Effects of subsoil compaction on corn 349 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Vol. 8 (1999): 333–351. yields. Transaction of American Society of Agricul- tural Engineers 25: 563–569, 575. Guérif, J. 1990. Factors influencing compaction-induced increases in soil strength. Soil & Tillage Research 16: 167–178. Gupta, S.C. & Raper, R.L. 1994. Prediction of soil com- paction under vehicles. In: Soane, B.D. & Ouwerk- erk, C. van (eds.). Soil compaction in crop produc- tion. Developments in Agricultural Engineering 11. Elsevier Science B.V., The Netherlands. p. 71–90. – , Sharma, P.P. & DeFranchi, S.A. 1989. Compaction effects on soil structure. Advances in Agronomy 42: 311–338. Hadas, A. 1994. Soil compaction caused by high axle loads-review of concepts and experimental data. Soil & Tillage Research 29: 253–276. Håkansson, I. 1985. Swedish experiments on subsoil compaction by vehicles with high axle load. Soil Use Management 1: 113–116. – 1994a. Soil tillage for crop production and for protec- tion of soil and environmental quality: a Scandinavian viewpoint. Soil & Tillage Research 30: 109–124. – 1994b. Subsoil compaction caused by heavy vehi- cles-a long-term threat to soil productivity. Soil & Till- age Research 29: 105–110. – & Danfors, B. 1988.The economic consequences of soil compaction by heavy vehicles when spreading manure and municipal waste. Swedish Institute of Ag- ricultural Engineering. Report 96(13): 1–10. – , Grath, T. & Olsen, H.J. 1996. Influence of machinery traffic in Swedish farm fields on penetration resist- ance in the subsoil. Swedish Journal of Agricultural Research 26: 181–187. – & Petelkau, H. 1994. Benefits of limited axle load. In: Soane, B.D. & Ouwerkerk, C. van (eds.). Soil com- paction in crop production. Developments in Agricul- tural Engineering 11. Elsevier Science B.V., The Neth- erlands. p. 479–500. – & Reeder, R.C. 1994. Subsoil compaction by vehi- cles with high axle load – extent, persistence and crop response. Soil & Tillage Research 29: 277–304. – , Voorhees, W.B., Elonen, P., Raghavan, G.S.V., Low- ery, B., Wijk, A.L.M. van, Rasmussen, K. & Riley, H. 1987. Effect of high axle load traffic on subsoil com- paction and crop yield in humid regions with annual freezing. Soil & Tillage Research 10: 259–268. Hammel, J.E. 1988. Influence of high axle loads on sub- soil physical properties and crop yields in the Pacific northwest, USA. Proceedings 11th International Con- ference of ISTRO, 11–15th July, Edinburgh, Scotland. 1: 275–280. Horn, R. 1980. Die Ermittlung der vertikalen Druckfortp- f l a n z u n g i m B o d e n m i t H i l f e v o n D e h n u n g s - messstreifen. Zeitschrift für Kulturtechnik und Flurb- ereitung 21: 343–349. – 1986. Auswirkung unterschiedlicher Bodenbearbei- tung auf die mechanische Belastbarkeit von Acker- böden. Zeitschrift für Pflanzenernährung und Bod- enkunde 149: 9–18. – , Blackwell, P.S. & White, R. 1989. The effect of speed of wheeling on soil stresses, rut depth and soil phys- ical properties in an ameliorated transitional red- brown earth. Soil & Tillage Research 13: 353–364. – , Burger, N., Lebert, M. & Badewitz, G. 1987. Druck fortpflanzung in Böden unter fahrenden Traktoren. Zeitschrift für Kulturtechnik und Flurbereitung 28: 94– 102. – , Domzal, H., Slowinska-Jurkiewicz, A. & Ouwerkerk, C. van 1995. Soil compaction processes and their effects on the structure of arable soils and the envi- ronment. Soil & Tillage Research 35: 23–36. – , Gräsle, W. & Kühner, S. 1996. Einige theoretische Überlegungen zur Spannungs-und Deformations- messung in Böden und ihre meβtechnishe Realisie- rung. Zeitschrift für Pflanzenernährung und Boden- kunde 159: 137–142. – & Lebert, M. 1994. Soil compactability and compress- ibility. In: Soane, B.D. and Ouwerkerk, C. van (eds.). Soil compaction in crop production. Developments in Agricultural Engineering 11. Elsevier Science B.V., The Netherlands. p. 45–70. Kirby. J.M. 1989. Shear damage beneath agricultural tyres: a theoretical study. Journal of Agricultural En- gineering Research 44: 217–230. – 1999. Soil stress measurement: Part I. Transducer in a uniform stress field. Journal of Agricultural Engi- neering Research 72: 151–160. Kooistra, M.J. & Boersma, O.H. 1994. Subsoil compac- tion in Dutch marine sandy loams: loosening practic- es and effects. Soil & Tillage Research 29: 237–247. Koolen, A.J. & Kuipers, H. 1983. Agricultural soil mechan- ics. Advanced Series in Agricultural Science 13. Springer-Verlag, Berlin Heidelberg, Germany. 241 p. – , Lerink, P., Kurstjens, D.A.G., Akker, J.J.H. van den & Arts, W.B.M. 1992. Prediction of aspects of soil- wheel systems. Soil & Tillage Research 24: 381–396. Lebert, M., Burger, N. & Horn, R. 1989. Effects of dy- namic and static loading on compaction of structured soils. NATO Advanced Science Institutes Series. Series E: Applied Science 172: 73–80. Lipiec, J., Kania, W. & Tarkiewicz, S. 1990. Effect of wheeling on physical characteristics of soils and root- ing of some cereals. Zeszyty Problemowe Postepow Nauk Rolniczych 385: 105–114. – & Simota, C. 1994. Role of soil and climate factors in influencing crop responses to soil compaction in cen- tral and eastern Europe. In: Soane, B.D. & Ouwerk- erk, C. van (eds.). Soil compaction in crop produc- tion. Developments in Agricultural Engineering 11. Elsevier Science B.V., The Netherlands. p. 365–390. Logsdon, S., Allmaras, R.R., Nelson, W.W. & Voorhees, W.B. 1992. Persistence of subsoil compaction from heavy axle loads. Soil & Tillage Research 23: 95– 110. Lowery, B. & Schuler, R.T. 1994. Duration and effects of compaction on soil and plant growth in Wisconsin. Soil & Tillage Research 29: 205–210. Montaga, K.D., Conroy, J.P. & Francis, G.S. 1998. Root and shoot response of field-grown lettuce and broc- coli to a compact subsoil. Australian Journal of Agri- cultural Research 49: 89–97. Olfe, G. & Schön, H. 1986. Bodenbelastung durch Sch- lepper- und Maschineneinsatz in der Landwirtschaft. KTBL-Schrift 308: 35–47. 350 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Alakukku, L. Subsoil compaction due to wheel traffic Olsen, H.J. 1994. Calculation of subsoil stresses. Soil & Tillage Research 29: 111–123. Perdok, U.D. & Tijink, F.G.J. 1990. Developments in IMAG research on mechanization in soil tillage and field traffic. Soil & Tillage Research 16: 121–141. Petelkau, H. 1984. Auswirkungen von Schadverdichtun- gen auf Bodeneigenschaften und Pflanzenertrag sowie Massnahmen zu ihrer Minderung. Tag.-Ber. Akad. Landwirtsch. – Wiss. DDR, Berlin, Tagungs- ber. 227: 25–34. Pietola, L. 1995. Effect of soil compactness on the growth and quality of carrot. Agricultural Science in Finland 4: 139–237. Plackett, C.W. 1984. The ground pressure of some agri- cultural tyres at low load and with zero sinkage. Jour- nal of Agricultural Engineering Research 29: 159– 166. Pollard, F. & Webster, R. 1978. The persistence of the effects of simulated tractor wheeling on sandy loam subsoil. Journal of Agricultural Engineering Research 28: 217–220. Raghavan, G.S.V. & McKyes, E. 1977. Laboratory study to determine the effect of slip-generated shear on soil compaction. Canadian Agricultural Engineering 19: 40–42. Renius, K.Th. 1994. Trends in tractor design with partic- ular reference to Europe. Journal of Agricultural En- gineering Research 57: 3–22. Rusanov, V.A. 1994. USSR standards for agricultural mobile machinery: permissible influences on soils and methods to estimate contact pressure and stress at a depth of 0.5 m. Soil & Tillage Research 29: 249– 252. Salire, E.V., Hammel, J.E. & Hardcastle, J.H. 1994. Com- pression of intact subsoils under short-duration load- ing. Soil & Tillage Research 31: 235–248. Schjønning, P. & Rasmussen, K. 1994. Danish experi- ments on subsoil compaction by vehicles with high axle load. Soil & Tillage Research 29: 215–227. Schmidt, W. & Rohde, S. 1986. Untersuchungen zur wirkung von Raddruck auf die Lagerungsdichte von Niedermoorboden. Archiv Acker- und Pflanzenbau und Bodenkunde 30: 37–44. Schuler, R.T. & Lowery, B. 1984. Subsoil compaction ef- fect on corn production with two soil types. American Society of Agricultural Engineers. Paper 84/1032. 13 p. Simojoki, A., Jaakkola, A. & Alakukku, L. 1991. Effect of compaction on soil air in a pot experiment and in the field. Soil & Tillage Research 19: 175–186. Smith, D.L.O. 1985. Compaction by wheels: a numerical model for agricultural soils. Journal of Soil Science 36: 621–632. Soane, D.B., Blackwell, P.S., Dickson, J.W. & Painter, D.J. 1981. Compaction by agricultural vehicles: A review. II. Compaction under tyres and other running gear. Soil & Tillage Research 1: 373–400. – & Ouwerkerk, C. van 1994a. Soil compaction in crop production. Developments in Agricultural Engineering 11. Elsevier Science B.V., The Netherlands. 662 p. – & Ouwerkerk, C. van 1994b. Soil compaction prob- lems in world agriculture. In: Soane, B.D. & Ouwerk- erk, C. van (eds.). Soil compaction in crop produc- tion. Developments in Agricultural Engineering 11. El- sevier Science B.V., The Netherlands. p. 1–21. – & Ouwerkerk, C. van 1995. Implications of soil com- paction in crop production for the quality of the envi- ronment. Soil & Tillage Research 35: 5–22. Söhne, W. 1953. Druckverteilung im Boden und Boden- verformung unter Schlepper Reifen. Grundlagen der Landtechnik 5: 49–63. – 1958. Fundamentals of pressure distribution and soil compaction under tractor tires. American Engineer- ing 39: 276–281, 290. Sommer, C. & Altemüller, H.J. 1982. Direkt- und Nach- wirkungen starker verdichtungen auf das Bodenge- füge und den Pflanzenertrag. Mitteilungen Deutsche Bodenkundliche Gesellschaft 34: 187–192. Stewart, G.A. & Vyn, T.J. 1994. Influence of high axle loads and tillage systems on soil properties and grain corn yield. Soil & Tillage Research 29: 229–235. Taylor, J.H., Burt, E.C. & Bailey, A.C. 1980. Effect of total load on subsurface soil compaction. Transaction of American Society of Agricultural Engineers 23: 568– 570. Tijink, F.G.J. 1994. Quantification of vehicle running gear. In: Soane, B.D. & Ouwerkerk, C. van (eds.). Soil com- paction in crop production. Developments in Agricul- tural Engineering 11. Elsevier Science B.V., The Neth- erlands. p. 391–416. Vermeulen, G.D., Arts, W.B.M. & Klooster, J.J. 1988. Perspective of reducing soil compaction by using a low ground pressure farming system; selection of wheel equipment. Proceedings 11th International Conference of ISTRO, 11–15th July, Edinburgh, Scot- land 1: 329–334. Voorhees, W.B. 1991. Compaction effects on yield – are they significant? Transaction of American Society of Agricultural Engineers 34: 1667–1672. – 1992. Wheel-induced soil physical limitations to root growth. Advances in Soil Science 19: 73–95. – & Lindstrom, M.J. 1983. Soil compaction constraints on conservation tillage in the northern corn belt. Jour- nal of Soil Water Conservation 38: 307–311. – , Nelson, W.W. & Randall, G.W. 1986. Extent and per- sistence of subsoil compaction caused by heavy axle loads. Soil Science Society of America Journal 50: 428–433. – , Senst, C.G. & Nelson, W.W. 1978. Compaction and soil structure modification by wheel traffic in the north- ern corn belt. Soil Science Society of America Jour- nal 42: 344–349. Whalley, W.R. & Dexter, A.R.1994. Root development and earthworm movement in relation to soil strength and structure. Archiv für Acker- und Pflanzenbau und Bodenkunde 38: 1–40. Wong, J.Y. 1986. Computer aided analysis of the effects of design parameters on the performance of tracked vehicles. Journal of Terramechanics 23: 95–124. – & Preston-Thomas, J. 1984. A comparison between a conventional method and an improved method for predicting tracked vehicle performance. Proceedings 8th International Conference of International Socie- ty for Terrain-vehicles Systems, 5–11th August, Cam- bridge, England. p. 361–380. 351 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Vol. 8 (1999): 333–351. SELOSTUS Raskaan peltoliikenteen aiheuttama pitkäaikainen maan tiivistyminen Laura Alakukku Maatalouden tutkimuskeskus Maataloudessa käytettävien koneiden tehot ja painot ovat kasvanut viime vuosikymmenien aikana jatku- vasti. Tämä on lisännyt pohjamaan tiivistymisriskiä. Tässä kirjoituksessa tarkastellaan, miten maan omi- naisuudet ja peltoajoon liittyvät tekijät vaikuttavat pohjamaan tiivistymiseen. Lisäksi selvitetään, miten pohjamaan tiivistymisen vaikuttaa maahan, peltovil- jelyyn, satoon ja ympäristöön. Pohjamaan tiivistymisriski on suuri, kun koste- alla pellolla ajetaan raskaalla kalustolla, jonka pin- tapaine on kohtalainen tai suuri. Tiivistymisen eh- käisemiseksi on annettu yksittäisiä suosituksia akse- lipainon, pintapaineen ja maassa 0,50 metriin ulot- tuvan jännityksen ylärajaksi. Tekniset suoditukset pit- käaikaisten tiivistymishaittojen ehkäisemiseksi tuli- si kuitenkin laatia kansainvälisenä yhteistyönä. En- nen suositusten laatimista on tutkittava mm. pohja- maiden kuormituksen kestävyyttä vaihtelevissa olo- suhteissa. Pohjamaan tiivistyminen vaikuttaa lähes kaikkiin maan fysikaalisiin, kemiallisiin ja biologisiin ominai- suuksiin ja prosesseihin. Se on todettu pienentävän myös kasvien satoa ja typenottoa. Tutkimusten mu- kaan pohjamaan tiivistymisen vaikutukset säilyvät pitkään. Karkeissa maissa ne voivat olla jopa pysy- viä. Wood, R.K., Reeder, R.C., Morgan, M.T. & Holmes, R.G. 1993. Soil physical properties as affected by grain cart traffic. Transaction of American Society of Agri- cultural Engineers 36: 11–13. Worldwatch Institute 1996. State of the World. World- watch Institute, Washington D.C., USA. 209 p. Wu, L., Allmaras, R.R., Gimenez, D. & Huggins, D.M. 1997. Shrinkage and water retention characteristic in a fine-textured mollisol compacted under different axle loads. Soil & Tillage Research 44: 179–194. 352 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Alakukku, L. Subsoil compaction due to wheel traffic Title Introduction Compaction of subsoils by wheel traffic Long-term effects of compaction in agriculture General discussion and conclusions References SELOSTUS