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J. agric. Engng Res. (2001) 78 (4), 359}367 doi:10.1006/jaer.2000.0645, available online at http://www.idealibrary.com on PM*Power and Machinery
A Three-stage Soil Layer Mixing Plough for the Improvement of Meadow Soil, Part 2: Soil Bin Experiments C. Zhang; K. Araya Hejiang Agricultural Research Institute, Jiamusi, Black Dragon, People's Republic of China; e-mail: [email protected] Environmental Science Laboratory, Senshu University, Bibai, Hokkaido 079-01, Japan; e-mail of corresponding author: [email protected] (Received 27 January 2000; accepted in revised form 6 September 2000; published online 22 November 2000)
The objective of this work was to develop a special machine, a three-stage soil layer mixing plough which would achieve longer sustainability of drainage for the improvement of meadow soil. In this paper, the results are presented from soil bin experiments with the three-stage soil layer mixing plough, that were conducted in Japan with the half-scale model ploughs. An optimum plough shape of the second plough body was determined, based on the mechanical properties of the soils presented in Part 1 of this paper. The results showed that the operating depth of the second plough body is small but the operating width is large and the plough body should transfer the tilled soil to the preceding furrow over a large distance of the full furrow width. The bulldozer blade type of plough body was much superior to the mouldboard plough type version. When the working depth of the second plough body is 50 mm, the radius of curvature should be 200 mm, the plough height 300 mm and the throwing angle 603. When the approach angle was more than 453, the transferred soil masses reached a constant value and, hence, the minimum approach angle at which the soil starts slide horizontally was experimentally around 453. When the cutting angle was more than 303, the draught increased steeply and was unsuitable for practical use. Hence, in this model second plough body, the optimum cutting angle should be 303, even though the suction here is slightly negative. 2001 Silsoe Research Institute
1. Introduction Meadow soil producing poor crop yields, is widely distributed on the Three-river plain in the Black Dragon province of the People's Republic of China near the border with Russia which is one of most important grain growing areas in the world. The "rst horizon (Ap) is a humic, black and brown soil which contains abundant organic matter and an aggregate structure, great pore spaces and good permeability, is suitable for plant growth and has a thickness of about 200 mm. The second horizon (A) is also a humic soil and is black, but the availability of the organic matter is less. Hence, the N percentage is low because of high impermeability due to very heavy clay, and it has a thickness of about 300 mm. The third horizon (Cg1) is the gleyed parent material with a thickness of about 400 mm. The permeability of this horizon is also low 0021-8634/01/040359#09 $35.00/0
because of heavy clay. With many impermeable horizons due to heavy clay which consist of extremely "ne soil particles, the ground water and soil surface water are quite isolated. All the precipitation is held only in the Ap horizon. Plants sometimes su!er due to excess moisture. The improvement of meadow soil could be achieved by breaking down A horizon and increasing its permeability because the A horizon consists of extremely "ne soil particles and is quite impermeable. In Part 1 of this paper (Zhang & Araya, 2001), a special three-stage soil layer mixing plough was envisaged, by which the trash on the soil surface of Ap horizon, such as wheat straw and maize stalks, would be mixed into the A horizon of the subsoil. With this treatment, the impermeable A horizon could be broken down by soil disturbance and the permeability could be improved by mixing of the trash because of greater pore spaces created.
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Notation A A A A A a K a N b b c c F V F V F V F V F X F X F X G G g h h h l l M Q
front area of the cut soil block, m side area of the cut soil block, m bottom area of the cut soil block, m front area of the lifted soil block, m bottom area of the lifted soil block, m adhesion of soil}steel, Pa adhesion of soil}polyethylene, Pa width of plough blade, m operating width, m cohesion of non-tilled soil, Pa cohesion of tilled soil, Pa total horizontal force, total draught, N draught of the cut soil block, N draught of the accumulated soil block, N draught caused when soil slides horizontally on mouldboard, N total vertical force, total suction, N positive suction caused by F , N V negative suction caused by friction of soil and mouldboard, N weight of the cut soil block, N weight of the accumulated soil block, N gravitational acceleration, m s\ operating depth, m plough height, m length, m plough share length, m length, m soil mass transferred by plough body, kg m\
The soil bin experiments of the three-stage soil layer mixing plough shown in Fig. 1 were "rst conducted with the half-scale model ploughs in Japan and an optimum plough shape is determined in this paper, based on the mechanical properties of the soils presented in Part 1 of this paper (Zhang & Araya, 2001).
2. Experimental details 2.1. Principle of mixing the soil surface trash layer with the subsoil The three-stage soil layer mixing plough in Fig. 1 consists of three plough bodies. The "rst and third plough bodies run in the same furrow and the second plough body skims an adjacent furrow on the land side. The "rst plough body is a conventional mouldboard plough. The second plough body was designed to cut the soil surface of the Ap horizon at a depth of 50 mm and transfer the
N normal force on the soil surface of the accumulated soil block, N R radius of curvature of bulldozer blade, m S upper area of the cut soil block, m S lower area of the cut soil block, m l travel velocity, m s\ a cutting angle of plough body, deg b angle of slip surface, deg c approach angle of plough body, deg c approximate approach angle at which ? soil starts to slide on plough mouldboard, deg c exact approach angle at which soil C starts to slide on plough mouldboard, deg f throwing angle, deg g dimensionless function k coe$cient of soil}steel friction K k coe$cient of soil}polyethylene friction N k coe$cient of non-tilled soil internal Q friction ks1 coe$cient of tilled soil internal friction m coe$cient o bulk density of non-tilled soil, kg m\ o bulk density of tilled soil, kg m\ p tensile strength of soil, Pa R u angle of non-tilled soil-internal friction, deg u angle of tilled soil-internal friction, deg thin soil slice, together with trash on the soil surface, onto the third plough body which runs at the preceding furrow. The third plough body is a special plough which was developed for mixing two horizons of planosol (Araya et al., 1996b, 1996c). The height of the third plough body is 240 mm (120 mm in the model) and soils are dropped from the end of the third plough body and mixed at random. The height of 240 mm was determined as the optimum shape to till the soil and to drop the soil down from the end of the mouldboard, while obtaining good mixing with the lowest draught. The 303 inclined angle was determined when soil was compressed on the sloping mouldboard, causing slipping of the soil, so that a good mixing was obtained on dropping down (Araya et al., 1996b). The process of mixing the soil surface of the Ap horizon and A horizon using this plough is schematically shown in Fig. 2. The dimensions are for a full-scale furrow. Initially, the "rst mouldboard plough body in Fig. 1 tills the Ap horizon (0}200 mm) shown in stage (1)
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In this part, the development of the second plough body is discussed and hence, the "rst mouldboard plough to till the Ap horizon and the third plough body to till the A horizon in Fig. 1 were not used. The design constraints for the second plough body are that the operating depth is small but the operating width is large and that it should transfer the tilled soil to the preceding furrow over a relatively large distance of the full furrow width. Two half-scale model second plough bodies were "rst designed and tested: the mouldboard type of plough body (not shown) and the bulldozer blade type of plough body in Fig. 3. The soil did not #ow
Fig. 1. Schematic diagram of plough shares of a three-stage soil layer mixing plough (half-scale model): Ap, A and Cg1, meadow soil horizons; a, cutting angle; c, approach angle; f, throwing angle; all dimension in mm
of Fig. 2, and stage (2) is obtained. With the second plough body, the surface layer of 50 mm depth of the Ap horizon of the next furrow on the land side is then precisely cut and is transferred to the preceding furrow as shown in stage (3). The transferred soil surface and the A horizon (200}500 mm) are then tilled together by the third plough body, lifted onto the mouldboard of the third plough body as shown in stage (4), and dropped down from the end of the mouldboard, so that a random mixing is obtained as shown in stage (5). Subsequently, the "rst mouldboard plough body tills the next Ap horizon, inverting the Ap furrow slice, thus covering the mixed soil in the preceding furrow so that stage (6) is obtained. As the fertility of the Cg1 horizon is rather poor, it is not desirable that the Cg1 horizon is mixed into the A horizon. Hence, the working depth of the third plough body should be precisely adjusted not to disturb the Cg1 horizon.
2.2. Apparatus and equipment for soil bin experiments Laboratory plough tests were conducted in a moveable soil bin which is fully described in a previous paper (Araya et al., 1996b). The soil in the soil bin was pseudogley soil, which is a Japanese heavy clay, in place of meadow soil because it was not feasible to transport su$cient meadow soil from China. The soil water content was controlled at about 22% d.b., near the plastic limit. The mechanical properties of the pseudogley soil and meadow soil are also given in Part 1 of this paper (Zhang & Araya, 2001).
Fig. 2. Schematic diagram of the six sequential stages for mixing the surface layer of the meadow soil horizon Ap with the A horizon; all dimensions are for a full-scale furrow in mm
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Fig. 3. Half-scale model of second plough body
over the mouldboard type of plough body, but clogged in front of the plough body and hence, the bulldozer blade type of plough body was much superior to the mouldboard plough. In the half-scale model bulldozer blade type of plough body in Fig. 3, the approach angle c which is the angle from the travel direction, and the cutting angle a (see Fig. 1) could be adjusted. In this paper, the optimum value for c and a were determined such that the soil surface of the Ap horizon could be precisely cut and transferred to the preceding furrow. Hence, setting the working depth h constant at 25 mm and the cutting angle a at 303, which is a common value for the bulldozer blades, c was varied to "ve angles: 30, 45, 60, 75 and 903. In Fig. 1 at smaller c, the working width of the second plough body becomes less than 250 mm. To prevent this, in the model plough body in Fig. 3, the length of the blade can be freely adjusted and the working width was always made 250 mm regardless of the approach angle. The soil transferred by the second plough body to the preceding furrow was collected in a box of 300 mm length; the soil mass was measured by a balance. After the determination of the optimum approach angle c, setting c at this value, the cutting angle a was then varied to four values: 15, 30, 45 and 603. The smaller cutting angle would be suitable for cutting the soil surface of the Ap horizon because it has plant roots and stalks. However, at the smaller cutting angles, the vertical force produced on the plough body would become negative, upwards, and the tilling operation becomes unsteady because of the lack of suction force. The minimum cutting angle where the vertical force is positive, downwards and there is a suction force, was determined.
Two to four experimental replications were made for each setting. The main dimensions of the half-scale model second plough body in Fig. 3 are shown in Table 1.
3. Development of second plough body The second plough body in Fig. 1 and 3 has a mouldboard with a special curve similar to that of bulldozer blades. The shape of the mouldboard was inferred from the shape of bulldozer blades which have been experimentally determined by trial and error (Fujimoto, 1984; Nakajima, 1980; Torihi, 1969). Such a blade consists of one or two simple curves. As the mouldboard of the second plough body is much smaller than a bulldozer blade, the curve of the plough mouldboard consists of one radius R only. Here, the relation between the working depth, the curvature radius of the mouldboard and the plough height was determined by Araya's design method (Araya et al., 1996a). As a result, when the working depth h is 50 mm (25 mm in the model plough body), the curvature radius R should be 200 mm (100 mm in the
Table 1 Dimension of half-scale model, second plough body Operational width b , m Operating depth h , m Plough height h , m Plough share length l , m Approach angle c, deg Cutting angle a, deg Travel velocity l, m s\
0)25 0)025 0)15 0)04 30}90 15}60 0)016
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prism is formed and pushed by the plough body. It is assumed that the angle between the inclined plane of the triangular prism and the horizontal soil surface is the same as the angle of soil-internal friction u of the tilled and loosened soil.
3.1. Forces acting on the cut soil block The forces acting on the cut soil block 1 should be analysed by the theory of Reece and Cook (1996) because the uniform load of the lifted soil block 2 acts on the cut soil block 1. However, here, the working depth h is small, only 50 mm and the plough height h is also small; hence, the e!ect of the uniform load by the lifted soil block 2 can be neglected and the forces acting on the cut soil block 1 can be analysed by the method of Jia et al. (1998). The outline is as follows. The draught F required when the cut soil block 1 is V tilled is Fig. 4. Schematic diagram of forces acting on second plough body tilling the soil surface; A21, front area of the lifted soil block; A23, bottom area of the accumulated soil block; a , soil}polyethyN lene adhesion; b, width of plough blade; b0, operating width; c1, cohesion of tilled soil; Fx2, draught of the accumulated soil block; G1, weight of the cut soil block; G2, weight of the accumulated soil block; h1, operating depth; h2, plough height; h3, length; l0 , plough share length; l1, length; N2, normal force of the soil surface; a, cutting angle of plough body; b, angle of slip surface; c, approach angle of plough body, k , coezcient of soil}polyethylene friction; N ks1, coezcient of tilled soil internal friction; u1, angle of tilled soil internal friction
model) and the plough height h 300 mm (150 mm in the model). The throwing angle f (see Fig. 1) is 603. The operating width of the second plough body is 500 mm (250 mm in the model) because the operating width of the "rst mouldboard plough body is 460 mm. As the meadow soil is extremely sticky, the value of the approach angle c in Fig. 1 at which the soil slides horizontally on the mouldboard and is transferred to the preceding furrow, and the value of the cutting angle a in Fig. 1 at which the vertical force (suction) becomes positive were evaluated. Further, the draught requirement of the plough body was analysed. Figure 4 shows the forces acting on the second plough body which is cutting the soil at the working depth of h . The soil block 1, given by area AFDI, is formed when a plough share with the cutting angle of a tills the soil at the working depth of h . The soil block 2, given by area ABCH, is the tilled and lifted soil block along the plough mouldboard. The soil block 3, given by area CHDE, is the accumulated soil of the soil block 2 in front of the plough body, and the shape of the curved triangular
G 1 c A F " #f (p )# V R g g sin b#k cos b Q a A 1 K # (1) g sin a#k cos a K Equation (1) is the summation of four forces, the upheaval resistance, tensile resistance, the cohesive resistance and the adhesion resistance, where: G is the weight of the cut soil block 1, f (p ) is the tensile resistance, R p is the tensile strength of soil, c is the soil cohesion, R A is the area of the slip surface, b is the angle of the slip surface, k is a coe$cient of soil internal friction, k is Q K a coe$cient of soil}steel friction, a is the soil}steel K adhesion, A is the surface area of the plough share, a is the cutting angle and g is a dimensionless function given by
cos a!k sin a cos b!k sin b Q K g" # (2) sin a#k cos a sin b#k cos b K Q The acceleration resistance was negligibly small (Jia et al., 1998) and, hence, it is not considered here. The angle of the slip surface b is produced by a passive earth pressure and hence is
u b"453! 2
(3)
where u is the angle of non-tilled soil internal friction. In Fig. 4, the area of the slip surface A is b h A " sin b
(4)
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In Fig. 4, the weight of the cut soil block 1 G is a truncated pyramid and hence, is related to bulk density o, the upper area S and the lower area S of the cut soil block 1: h G "og (S #(S S #S ) 3
(5)
S "(2l #b )(2l #l ) (6) S "b l (7) where g is the gravitational acceleration, l is the plough share length, b is the operational width, l and h are the lengths shown in Fig. 4. h "h
sin(a#b) sin b
(8)
l "h
cos(a#b) sin b
(9)
The function for the tensile force f (p ) acts on one side R and the bottom of the cut soil block 1 in Fig. 4 and is f (p )"(A #A )p (10) R R where A is the bottom area of the cut soil block 1 and A is the side area of the cut soil block 1. They are A "h (l #l ) (11) A "b l cos a (12) 3.2. Forces acting on the accumulated soil block In Fig. 4, the accumulated soil block 3 of weight G , is a soil loosened by tillage which slides along the soil surface, DE. The friction produced is the horizontal force (draught) F of the soil block 3. A normal force N acts V on the soil surface, DE. On the vertical surface CH, a vertical force which is caused by the sliding friction of the lifted soil block 2 acts. The force balance in the vertical direction of the soil block 3 is N "G !(ks1F #c A ) (13) V and the force balance in the horizontal direction of the soil block 3 is F "ks1N #c A (14) V where ks1 is a coe$cient of soil internal friction of the tilled soil, c is the soil cohesion of the tilled soil, A is the surface area of soil surface, DE and A is the surface area of the vertical surface, CH.
The draught of the soil block 3 F , is obtained from V Eqns (13) and (14) as ks1G !c ks1A #c A F " V 1#ks1
(15)
In Fig. 4, it is assumed that the length, BC can be negligibly small because of the small working depth h and the curved line, AB is approximately a straight line. Hence, the soil block 3 is a straight triangular prism and its weight G is hb o g G " (16) tan u where h is the plough height and o is the bulk density of the tilled soil. The surface area A of the soil surface, DE is h b (17) A " tan u The surface area A of the vertical surface, CH is A "h b (18) 3.3. Condition where the soil slides horizontally on the plough mouldboard In Fig. 4, the soil blocks 2 and 3 starts to slide horizontally along the plough mouldboard caused by the draught F of the soil block 3 becomes larger than the V friction force along the plough mouldboard as F cos c'k F sin c#a A (19) V N V N where k is a coe$cient of soil}plastic friction (such as N polyethylene) and a is the adhesion of soil}plastic. N The exact approach angle at which the soil starts to slide on the plough mouldboard c is C !a A N !m c "sin\ (20) C F (1#k V N where the coe$cient m is
!1 (21) k N If the adhesion of soil}plastic a is small enough (i.e. N a "0), an approximate approach angle c at which the N ? soil starts to slide on the plough mouldboard is obtained from Eqn (19). m"tan\
1 c "tan\ ? k N
(22)
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3.4. Horizontal force (draught) acting on the second plough body In Fig. 4, when the lifted and accumulated soil blocks 2 and 3 start to slide horizontally, a new horizontal force (draught) along the travel direction F is produced and V is F "cos c(k F sin c#a A ) (23) V N V N Hence, the total draught of the second plough body F is V obtained from Eqns (1), (15) and (23) as F "F #F #F V V V V
(24)
3.5.
(27)
4. Results and discussion 4.1. Optimum approach angle The surface of the plough mouldboard was preliminarily made of steel, but the pseudogley soil adhered on the plough surface regardless of the approach angles; the soil did not slide horizontally on the plough mouldboard but accumulated in front of the plough body. The adhesion of the meadow soil is greater than that of the pseudogley soil, and a greater soil volume would be accumulated in front of the plough body. As a measure of this problem, a polyethylene plate, which would have the least coe$cient of soil}plastic friction, was coated on the plough mouldboard as shown in Fig. 3. The approach angle was then varied to "ve stages, and the soil masses transferred by the second plough body to the preceding furrow per unit travel distance were determined as shown in Fig. 5. The transferred soil mass M increased with approach angles smaller than 903 (at Q right angles to the travel direction). When the approach
Fig. 5. Transferred soil mass per travel distance as a function of approach angle; pseudogley soil versus polyethylene; cutting angle a"303; c and c , approximate and exact approach angles, ? C respectively: , measured; , predicted
angle was more than 453, the transferred soil masses reached a constant value, and hence, the minimum approach angle at which the soil starts slide horizontally would be experimentally around 453. In Fig. 5, the exact approach angle at which the soil starts to slide c of Eqn (20) and the approximate C approach angle c of Eqn (22) are predicted for the ? polyethylene and shown by chain lines. The values in Table 1 for the plough dimensions and the values of Part 1 of this paper (Zhang & Araya, 2001) for the soil properties were used for the calculation. The value for c of 50.83 nearly agreed with the experiC mental value. The value for c of 68.23 obtained by ? neglecting of the adhesion a did not agree with the N experimental value. Hence, the value of the adhesion of soil}polyethylene should not be neglected. The optimum approach angle could be predicted from the value of c of C Eqn (20) for a proper soil.
4.2. Draught of the half-scale model, second plough body The draughts of the model second plough body predicted from Eqns (1), (15), (23) and (24) are shown in Fig. 6 by dotted lines. The cutting angle a is constant at 303. The draught of the accumulated soil block 3 F is V the largest, followed by the draught of the cut soil block
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Eqn (24) are shown as a function of the cutting angle a in Fig. 7 by dotted lines. The approach angle c is constant at 453. As a result, the value of a should be more than 403 where the suction is positive. However, when a is more than 303, the draught increases steeply and is not suitable for practical use. Hence, in this model second plough body, the optimum cutting angle a should be 303 even though the suction is slightly negative here. The suctions and draught measured in the soil bin experiments are also shown in Fig. 7. The increasing trends of both measured suction and draught with the greater cutting angles were similar to those of the
Fig. 6. Predicted and measured draught of half-scale model second plough body as a function of approach angle; cutting angle a"303; F , total draught; F 1, draught of cut soil block; F 2, V V V draught of accumulated soil block; and F 3, draught caused when V soil slides on mouldboard: , measured; , predicted
1 F , and the draught caused when the soil slides horiV zontally on the mouldboard F . The forces F and V V F are constant regardless of the approach angle, but V F varies with the values of the approach angle. The V total draught F is maximum around 303 for c. V The values of the draught forces measured in the soil bin experiments are somewhat scattered in Fig. 6. This occurred because the soil mass accumulated in front of the plough body was not always the same for the same experimental parameters. The measured draught forces did not decrease when the approach angle reached 903 as expected from the predicted draught. This is because in the predicted value, the weight of the soil block 3 accumulated in front of the second plough body in Fig. 4 is constant, and hence, F is also constant, regardless of the V approach angle c. However, the weight of the accumulated soil block 3 actually increased in the soil bin experiments when c became greater and the soil did not slide horizontally. In an extreme case, the accumulated soil spilled over the top of the plough body. Hence, F would V not actually be constant but would increase at the greater approach angles.
4.3. Suction of the half-scale model, second plough body The vertical forces, suctions F , F and F predicted X X X from Eqns (25)}(27) and the draught F predicted from V
Fig. 7. Predicted and measured suction and draught as a function of cutting angle; approach angle c was 453; F , total draught; F , V X total suction; F 1, positive suction caused by Fx1; Fz2 , negative X suction caused by friction on mouldboard: , measured; , predicted
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predicted values. However, both slopes of the measured values for F and F to the cutting angle in Fig. 7 were V X smaller than those for predicted values. This is because in the predicted value of Eqn (1), the upheaval and cohesive resistances acting on the cut soil block 1 in the draught F increase steeply with the greater cutting angles a, V while in the soil bin experiments, the cut soil block 1, AFDI in Fig. 4 became actually more crumbled with the greater cutting angles, and hence, the weight of the cut soil block 1 showed little increase. As a result, the total draught F and the suction F also did not increase V X steeply.
5. Conclusions (1) The constraints on the design of the second plough body are that the operating depth is small but the operating width is large and it should transfer the tilled soil to the preceding furrow over a relatively large distance of the full furrow width. The bulldozer blade type of plough body was much superior to the mouldboard plough version. (2) When the working depth of the second plough body is 50 mm, the curvature radius should be 200 mm, the plough height 300 mm and the throwing angle 603. (3) When the approach angle was more than 453, the transferred soil masses reached a constant value, and hence, the minimum approach angle at which the soil starts to slide horizontally was experimentally around 453. (4) The value of the predicted optimum approach angle of 50)83, when the adhesion of soil}polyethylene was not neglected, nearly agreed with the experimental value. (5) Based on the draught prediction of the model second plough body, the draught of the accumulated soil block was the largest, followed by the draught for soil
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cutting, and the draught caused when the soil slides horizontally on the mouldboard. (6) The cutting angle should be more than 403 because the suction should be positive. However, when the cutting angle was more than 303, the draught increased steeply and became unsuitable for practical use. Hence, in this model second plough body, the optimum cutting angle should be 303 even though the suction is slightly negative here. References Araya K; Kudoh M; Zhao D; Liu F; Jia H (1996a). Improvement of planosol solum, part 2: optimization of design of roll-in ploughs in soil bin experiments. Journal of Agricultural Engineering Research, 63, 261}268 Araya K; Kudoh M; Zhao D; Liu F; Jia H (1996b). Improvement of planosol solum, part 5: soil bin experiments with a three-stage subsoil mixing plough. Journal of Agricultural Engineering Research, 65, 143}149 Araya K; Kudoh M; Zhao D; Liu F; Jia H (1996c). Improvement of planosol solum, part 6: "eld experiments with a three-stage subsoil mixing plough. Journal of Agricultural Engineering Research, 65, 151}158 Fujimoto Y (1984). Draught of bulldozer blades with sand. Transactions of Construction Machines, 20th Anniversary, 98}105 Jia H; Liu F; Zhang H; Zhang C; Araya K; Kudoh M; Kawabe H (1998). Improvement of planosol solum, part 8: analysis of draught of a three-stage subsoil mixing plough. Journal of Agricultural Engineering Research, 70, 185}193 Nakajima M (1980). Development of lined blades. Journal of Komatsu Industries, 26(4), 12}23 Reece A R; Cook P E R (1996). Theory of bulldozer action in friable soil. Proceedings of Second International Conference of ISTVS Torihi T (1969). Capacity of bulldozer blades. Journal of Komatsu Industries, 5(4), 42}51 Zhang C; Araya K (2001). A three-stage soil layer mixing plough for the improvement of meadow soil, part 1: mechanical properties of soils. Journal of Agricultural Engineering Research, 78, in press.
A Three-stage Soil Layer Mixing Plough for the Improvement of Meadow Soil, Part 2: Soil Bin Experiments C. Zhang; K. Araya Hejiang Agricultural Research Institute, Jiamusi, Black Dragon, People's Republic of China; e-mail: [email protected] Environmental Science Laboratory, Senshu University, Bibai, Hokkaido 079-01, Japan; e-mail of corresponding author: [email protected] (Received 27 January 2000; accepted in revised form 6 September 2000; published online 22 November 2000)
The objective of this work was to develop a special machine, a three-stage soil layer mixing plough which would achieve longer sustainability of drainage for the improvement of meadow soil. In this paper, the results are presented from soil bin experiments with the three-stage soil layer mixing plough, that were conducted in Japan with the half-scale model ploughs. An optimum plough shape of the second plough body was determined, based on the mechanical properties of the soils presented in Part 1 of this paper. The results showed that the operating depth of the second plough body is small but the operating width is large and the plough body should transfer the tilled soil to the preceding furrow over a large distance of the full furrow width. The bulldozer blade type of plough body was much superior to the mouldboard plough type version. When the working depth of the second plough body is 50 mm, the radius of curvature should be 200 mm, the plough height 300 mm and the throwing angle 603. When the approach angle was more than 453, the transferred soil masses reached a constant value and, hence, the minimum approach angle at which the soil starts slide horizontally was experimentally around 453. When the cutting angle was more than 303, the draught increased steeply and was unsuitable for practical use. Hence, in this model second plough body, the optimum cutting angle should be 303, even though the suction here is slightly negative. 2001 Silsoe Research Institute
1. Introduction Meadow soil producing poor crop yields, is widely distributed on the Three-river plain in the Black Dragon province of the People's Republic of China near the border with Russia which is one of most important grain growing areas in the world. The "rst horizon (Ap) is a humic, black and brown soil which contains abundant organic matter and an aggregate structure, great pore spaces and good permeability, is suitable for plant growth and has a thickness of about 200 mm. The second horizon (A) is also a humic soil and is black, but the availability of the organic matter is less. Hence, the N percentage is low because of high impermeability due to very heavy clay, and it has a thickness of about 300 mm. The third horizon (Cg1) is the gleyed parent material with a thickness of about 400 mm. The permeability of this horizon is also low 0021-8634/01/040359#09 $35.00/0
because of heavy clay. With many impermeable horizons due to heavy clay which consist of extremely "ne soil particles, the ground water and soil surface water are quite isolated. All the precipitation is held only in the Ap horizon. Plants sometimes su!er due to excess moisture. The improvement of meadow soil could be achieved by breaking down A horizon and increasing its permeability because the A horizon consists of extremely "ne soil particles and is quite impermeable. In Part 1 of this paper (Zhang & Araya, 2001), a special three-stage soil layer mixing plough was envisaged, by which the trash on the soil surface of Ap horizon, such as wheat straw and maize stalks, would be mixed into the A horizon of the subsoil. With this treatment, the impermeable A horizon could be broken down by soil disturbance and the permeability could be improved by mixing of the trash because of greater pore spaces created.
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Notation A A A A A a K a N b b c c F V F V F V F V F X F X F X G G g h h h l l M Q
front area of the cut soil block, m side area of the cut soil block, m bottom area of the cut soil block, m front area of the lifted soil block, m bottom area of the lifted soil block, m adhesion of soil}steel, Pa adhesion of soil}polyethylene, Pa width of plough blade, m operating width, m cohesion of non-tilled soil, Pa cohesion of tilled soil, Pa total horizontal force, total draught, N draught of the cut soil block, N draught of the accumulated soil block, N draught caused when soil slides horizontally on mouldboard, N total vertical force, total suction, N positive suction caused by F , N V negative suction caused by friction of soil and mouldboard, N weight of the cut soil block, N weight of the accumulated soil block, N gravitational acceleration, m s\ operating depth, m plough height, m length, m plough share length, m length, m soil mass transferred by plough body, kg m\
The soil bin experiments of the three-stage soil layer mixing plough shown in Fig. 1 were "rst conducted with the half-scale model ploughs in Japan and an optimum plough shape is determined in this paper, based on the mechanical properties of the soils presented in Part 1 of this paper (Zhang & Araya, 2001).
2. Experimental details 2.1. Principle of mixing the soil surface trash layer with the subsoil The three-stage soil layer mixing plough in Fig. 1 consists of three plough bodies. The "rst and third plough bodies run in the same furrow and the second plough body skims an adjacent furrow on the land side. The "rst plough body is a conventional mouldboard plough. The second plough body was designed to cut the soil surface of the Ap horizon at a depth of 50 mm and transfer the
N normal force on the soil surface of the accumulated soil block, N R radius of curvature of bulldozer blade, m S upper area of the cut soil block, m S lower area of the cut soil block, m l travel velocity, m s\ a cutting angle of plough body, deg b angle of slip surface, deg c approach angle of plough body, deg c approximate approach angle at which ? soil starts to slide on plough mouldboard, deg c exact approach angle at which soil C starts to slide on plough mouldboard, deg f throwing angle, deg g dimensionless function k coe$cient of soil}steel friction K k coe$cient of soil}polyethylene friction N k coe$cient of non-tilled soil internal Q friction ks1 coe$cient of tilled soil internal friction m coe$cient o bulk density of non-tilled soil, kg m\ o bulk density of tilled soil, kg m\ p tensile strength of soil, Pa R u angle of non-tilled soil-internal friction, deg u angle of tilled soil-internal friction, deg thin soil slice, together with trash on the soil surface, onto the third plough body which runs at the preceding furrow. The third plough body is a special plough which was developed for mixing two horizons of planosol (Araya et al., 1996b, 1996c). The height of the third plough body is 240 mm (120 mm in the model) and soils are dropped from the end of the third plough body and mixed at random. The height of 240 mm was determined as the optimum shape to till the soil and to drop the soil down from the end of the mouldboard, while obtaining good mixing with the lowest draught. The 303 inclined angle was determined when soil was compressed on the sloping mouldboard, causing slipping of the soil, so that a good mixing was obtained on dropping down (Araya et al., 1996b). The process of mixing the soil surface of the Ap horizon and A horizon using this plough is schematically shown in Fig. 2. The dimensions are for a full-scale furrow. Initially, the "rst mouldboard plough body in Fig. 1 tills the Ap horizon (0}200 mm) shown in stage (1)
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In this part, the development of the second plough body is discussed and hence, the "rst mouldboard plough to till the Ap horizon and the third plough body to till the A horizon in Fig. 1 were not used. The design constraints for the second plough body are that the operating depth is small but the operating width is large and that it should transfer the tilled soil to the preceding furrow over a relatively large distance of the full furrow width. Two half-scale model second plough bodies were "rst designed and tested: the mouldboard type of plough body (not shown) and the bulldozer blade type of plough body in Fig. 3. The soil did not #ow
Fig. 1. Schematic diagram of plough shares of a three-stage soil layer mixing plough (half-scale model): Ap, A and Cg1, meadow soil horizons; a, cutting angle; c, approach angle; f, throwing angle; all dimension in mm
of Fig. 2, and stage (2) is obtained. With the second plough body, the surface layer of 50 mm depth of the Ap horizon of the next furrow on the land side is then precisely cut and is transferred to the preceding furrow as shown in stage (3). The transferred soil surface and the A horizon (200}500 mm) are then tilled together by the third plough body, lifted onto the mouldboard of the third plough body as shown in stage (4), and dropped down from the end of the mouldboard, so that a random mixing is obtained as shown in stage (5). Subsequently, the "rst mouldboard plough body tills the next Ap horizon, inverting the Ap furrow slice, thus covering the mixed soil in the preceding furrow so that stage (6) is obtained. As the fertility of the Cg1 horizon is rather poor, it is not desirable that the Cg1 horizon is mixed into the A horizon. Hence, the working depth of the third plough body should be precisely adjusted not to disturb the Cg1 horizon.
2.2. Apparatus and equipment for soil bin experiments Laboratory plough tests were conducted in a moveable soil bin which is fully described in a previous paper (Araya et al., 1996b). The soil in the soil bin was pseudogley soil, which is a Japanese heavy clay, in place of meadow soil because it was not feasible to transport su$cient meadow soil from China. The soil water content was controlled at about 22% d.b., near the plastic limit. The mechanical properties of the pseudogley soil and meadow soil are also given in Part 1 of this paper (Zhang & Araya, 2001).
Fig. 2. Schematic diagram of the six sequential stages for mixing the surface layer of the meadow soil horizon Ap with the A horizon; all dimensions are for a full-scale furrow in mm
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Fig. 3. Half-scale model of second plough body
over the mouldboard type of plough body, but clogged in front of the plough body and hence, the bulldozer blade type of plough body was much superior to the mouldboard plough. In the half-scale model bulldozer blade type of plough body in Fig. 3, the approach angle c which is the angle from the travel direction, and the cutting angle a (see Fig. 1) could be adjusted. In this paper, the optimum value for c and a were determined such that the soil surface of the Ap horizon could be precisely cut and transferred to the preceding furrow. Hence, setting the working depth h constant at 25 mm and the cutting angle a at 303, which is a common value for the bulldozer blades, c was varied to "ve angles: 30, 45, 60, 75 and 903. In Fig. 1 at smaller c, the working width of the second plough body becomes less than 250 mm. To prevent this, in the model plough body in Fig. 3, the length of the blade can be freely adjusted and the working width was always made 250 mm regardless of the approach angle. The soil transferred by the second plough body to the preceding furrow was collected in a box of 300 mm length; the soil mass was measured by a balance. After the determination of the optimum approach angle c, setting c at this value, the cutting angle a was then varied to four values: 15, 30, 45 and 603. The smaller cutting angle would be suitable for cutting the soil surface of the Ap horizon because it has plant roots and stalks. However, at the smaller cutting angles, the vertical force produced on the plough body would become negative, upwards, and the tilling operation becomes unsteady because of the lack of suction force. The minimum cutting angle where the vertical force is positive, downwards and there is a suction force, was determined.
Two to four experimental replications were made for each setting. The main dimensions of the half-scale model second plough body in Fig. 3 are shown in Table 1.
3. Development of second plough body The second plough body in Fig. 1 and 3 has a mouldboard with a special curve similar to that of bulldozer blades. The shape of the mouldboard was inferred from the shape of bulldozer blades which have been experimentally determined by trial and error (Fujimoto, 1984; Nakajima, 1980; Torihi, 1969). Such a blade consists of one or two simple curves. As the mouldboard of the second plough body is much smaller than a bulldozer blade, the curve of the plough mouldboard consists of one radius R only. Here, the relation between the working depth, the curvature radius of the mouldboard and the plough height was determined by Araya's design method (Araya et al., 1996a). As a result, when the working depth h is 50 mm (25 mm in the model plough body), the curvature radius R should be 200 mm (100 mm in the
Table 1 Dimension of half-scale model, second plough body Operational width b , m Operating depth h , m Plough height h , m Plough share length l , m Approach angle c, deg Cutting angle a, deg Travel velocity l, m s\
0)25 0)025 0)15 0)04 30}90 15}60 0)016
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prism is formed and pushed by the plough body. It is assumed that the angle between the inclined plane of the triangular prism and the horizontal soil surface is the same as the angle of soil-internal friction u of the tilled and loosened soil.
3.1. Forces acting on the cut soil block The forces acting on the cut soil block 1 should be analysed by the theory of Reece and Cook (1996) because the uniform load of the lifted soil block 2 acts on the cut soil block 1. However, here, the working depth h is small, only 50 mm and the plough height h is also small; hence, the e!ect of the uniform load by the lifted soil block 2 can be neglected and the forces acting on the cut soil block 1 can be analysed by the method of Jia et al. (1998). The outline is as follows. The draught F required when the cut soil block 1 is V tilled is Fig. 4. Schematic diagram of forces acting on second plough body tilling the soil surface; A21, front area of the lifted soil block; A23, bottom area of the accumulated soil block; a , soil}polyethyN lene adhesion; b, width of plough blade; b0, operating width; c1, cohesion of tilled soil; Fx2, draught of the accumulated soil block; G1, weight of the cut soil block; G2, weight of the accumulated soil block; h1, operating depth; h2, plough height; h3, length; l0 , plough share length; l1, length; N2, normal force of the soil surface; a, cutting angle of plough body; b, angle of slip surface; c, approach angle of plough body, k , coezcient of soil}polyethylene friction; N ks1, coezcient of tilled soil internal friction; u1, angle of tilled soil internal friction
model) and the plough height h 300 mm (150 mm in the model). The throwing angle f (see Fig. 1) is 603. The operating width of the second plough body is 500 mm (250 mm in the model) because the operating width of the "rst mouldboard plough body is 460 mm. As the meadow soil is extremely sticky, the value of the approach angle c in Fig. 1 at which the soil slides horizontally on the mouldboard and is transferred to the preceding furrow, and the value of the cutting angle a in Fig. 1 at which the vertical force (suction) becomes positive were evaluated. Further, the draught requirement of the plough body was analysed. Figure 4 shows the forces acting on the second plough body which is cutting the soil at the working depth of h . The soil block 1, given by area AFDI, is formed when a plough share with the cutting angle of a tills the soil at the working depth of h . The soil block 2, given by area ABCH, is the tilled and lifted soil block along the plough mouldboard. The soil block 3, given by area CHDE, is the accumulated soil of the soil block 2 in front of the plough body, and the shape of the curved triangular
G 1 c A F " #f (p )# V R g g sin b#k cos b Q a A 1 K # (1) g sin a#k cos a K Equation (1) is the summation of four forces, the upheaval resistance, tensile resistance, the cohesive resistance and the adhesion resistance, where: G is the weight of the cut soil block 1, f (p ) is the tensile resistance, R p is the tensile strength of soil, c is the soil cohesion, R A is the area of the slip surface, b is the angle of the slip surface, k is a coe$cient of soil internal friction, k is Q K a coe$cient of soil}steel friction, a is the soil}steel K adhesion, A is the surface area of the plough share, a is the cutting angle and g is a dimensionless function given by
cos a!k sin a cos b!k sin b Q K g" # (2) sin a#k cos a sin b#k cos b K Q The acceleration resistance was negligibly small (Jia et al., 1998) and, hence, it is not considered here. The angle of the slip surface b is produced by a passive earth pressure and hence is
u b"453! 2
(3)
where u is the angle of non-tilled soil internal friction. In Fig. 4, the area of the slip surface A is b h A " sin b
(4)
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In Fig. 4, the weight of the cut soil block 1 G is a truncated pyramid and hence, is related to bulk density o, the upper area S and the lower area S of the cut soil block 1: h G "og (S #(S S #S ) 3
(5)
S "(2l #b )(2l #l ) (6) S "b l (7) where g is the gravitational acceleration, l is the plough share length, b is the operational width, l and h are the lengths shown in Fig. 4. h "h
sin(a#b) sin b
(8)
l "h
cos(a#b) sin b
(9)
The function for the tensile force f (p ) acts on one side R and the bottom of the cut soil block 1 in Fig. 4 and is f (p )"(A #A )p (10) R R where A is the bottom area of the cut soil block 1 and A is the side area of the cut soil block 1. They are A "h (l #l ) (11) A "b l cos a (12) 3.2. Forces acting on the accumulated soil block In Fig. 4, the accumulated soil block 3 of weight G , is a soil loosened by tillage which slides along the soil surface, DE. The friction produced is the horizontal force (draught) F of the soil block 3. A normal force N acts V on the soil surface, DE. On the vertical surface CH, a vertical force which is caused by the sliding friction of the lifted soil block 2 acts. The force balance in the vertical direction of the soil block 3 is N "G !(ks1F #c A ) (13) V and the force balance in the horizontal direction of the soil block 3 is F "ks1N #c A (14) V where ks1 is a coe$cient of soil internal friction of the tilled soil, c is the soil cohesion of the tilled soil, A is the surface area of soil surface, DE and A is the surface area of the vertical surface, CH.
The draught of the soil block 3 F , is obtained from V Eqns (13) and (14) as ks1G !c ks1A #c A F " V 1#ks1
(15)
In Fig. 4, it is assumed that the length, BC can be negligibly small because of the small working depth h and the curved line, AB is approximately a straight line. Hence, the soil block 3 is a straight triangular prism and its weight G is hb o g G " (16) tan u where h is the plough height and o is the bulk density of the tilled soil. The surface area A of the soil surface, DE is h b (17) A " tan u The surface area A of the vertical surface, CH is A "h b (18) 3.3. Condition where the soil slides horizontally on the plough mouldboard In Fig. 4, the soil blocks 2 and 3 starts to slide horizontally along the plough mouldboard caused by the draught F of the soil block 3 becomes larger than the V friction force along the plough mouldboard as F cos c'k F sin c#a A (19) V N V N where k is a coe$cient of soil}plastic friction (such as N polyethylene) and a is the adhesion of soil}plastic. N The exact approach angle at which the soil starts to slide on the plough mouldboard c is C !a A N !m c "sin\ (20) C F (1#k V N where the coe$cient m is
!1 (21) k N If the adhesion of soil}plastic a is small enough (i.e. N a "0), an approximate approach angle c at which the N ? soil starts to slide on the plough mouldboard is obtained from Eqn (19). m"tan\
1 c "tan\ ? k N
(22)
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3.4. Horizontal force (draught) acting on the second plough body In Fig. 4, when the lifted and accumulated soil blocks 2 and 3 start to slide horizontally, a new horizontal force (draught) along the travel direction F is produced and V is F "cos c(k F sin c#a A ) (23) V N V N Hence, the total draught of the second plough body F is V obtained from Eqns (1), (15) and (23) as F "F #F #F V V V V
(24)
3.5.
(27)
4. Results and discussion 4.1. Optimum approach angle The surface of the plough mouldboard was preliminarily made of steel, but the pseudogley soil adhered on the plough surface regardless of the approach angles; the soil did not slide horizontally on the plough mouldboard but accumulated in front of the plough body. The adhesion of the meadow soil is greater than that of the pseudogley soil, and a greater soil volume would be accumulated in front of the plough body. As a measure of this problem, a polyethylene plate, which would have the least coe$cient of soil}plastic friction, was coated on the plough mouldboard as shown in Fig. 3. The approach angle was then varied to "ve stages, and the soil masses transferred by the second plough body to the preceding furrow per unit travel distance were determined as shown in Fig. 5. The transferred soil mass M increased with approach angles smaller than 903 (at Q right angles to the travel direction). When the approach
Fig. 5. Transferred soil mass per travel distance as a function of approach angle; pseudogley soil versus polyethylene; cutting angle a"303; c and c , approximate and exact approach angles, ? C respectively: , measured; , predicted
angle was more than 453, the transferred soil masses reached a constant value, and hence, the minimum approach angle at which the soil starts slide horizontally would be experimentally around 453. In Fig. 5, the exact approach angle at which the soil starts to slide c of Eqn (20) and the approximate C approach angle c of Eqn (22) are predicted for the ? polyethylene and shown by chain lines. The values in Table 1 for the plough dimensions and the values of Part 1 of this paper (Zhang & Araya, 2001) for the soil properties were used for the calculation. The value for c of 50.83 nearly agreed with the experiC mental value. The value for c of 68.23 obtained by ? neglecting of the adhesion a did not agree with the N experimental value. Hence, the value of the adhesion of soil}polyethylene should not be neglected. The optimum approach angle could be predicted from the value of c of C Eqn (20) for a proper soil.
4.2. Draught of the half-scale model, second plough body The draughts of the model second plough body predicted from Eqns (1), (15), (23) and (24) are shown in Fig. 6 by dotted lines. The cutting angle a is constant at 303. The draught of the accumulated soil block 3 F is V the largest, followed by the draught of the cut soil block
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Eqn (24) are shown as a function of the cutting angle a in Fig. 7 by dotted lines. The approach angle c is constant at 453. As a result, the value of a should be more than 403 where the suction is positive. However, when a is more than 303, the draught increases steeply and is not suitable for practical use. Hence, in this model second plough body, the optimum cutting angle a should be 303 even though the suction is slightly negative here. The suctions and draught measured in the soil bin experiments are also shown in Fig. 7. The increasing trends of both measured suction and draught with the greater cutting angles were similar to those of the
Fig. 6. Predicted and measured draught of half-scale model second plough body as a function of approach angle; cutting angle a"303; F , total draught; F 1, draught of cut soil block; F 2, V V V draught of accumulated soil block; and F 3, draught caused when V soil slides on mouldboard: , measured; , predicted
1 F , and the draught caused when the soil slides horiV zontally on the mouldboard F . The forces F and V V F are constant regardless of the approach angle, but V F varies with the values of the approach angle. The V total draught F is maximum around 303 for c. V The values of the draught forces measured in the soil bin experiments are somewhat scattered in Fig. 6. This occurred because the soil mass accumulated in front of the plough body was not always the same for the same experimental parameters. The measured draught forces did not decrease when the approach angle reached 903 as expected from the predicted draught. This is because in the predicted value, the weight of the soil block 3 accumulated in front of the second plough body in Fig. 4 is constant, and hence, F is also constant, regardless of the V approach angle c. However, the weight of the accumulated soil block 3 actually increased in the soil bin experiments when c became greater and the soil did not slide horizontally. In an extreme case, the accumulated soil spilled over the top of the plough body. Hence, F would V not actually be constant but would increase at the greater approach angles.
4.3. Suction of the half-scale model, second plough body The vertical forces, suctions F , F and F predicted X X X from Eqns (25)}(27) and the draught F predicted from V
Fig. 7. Predicted and measured suction and draught as a function of cutting angle; approach angle c was 453; F , total draught; F , V X total suction; F 1, positive suction caused by Fx1; Fz2 , negative X suction caused by friction on mouldboard: , measured; , predicted
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predicted values. However, both slopes of the measured values for F and F to the cutting angle in Fig. 7 were V X smaller than those for predicted values. This is because in the predicted value of Eqn (1), the upheaval and cohesive resistances acting on the cut soil block 1 in the draught F increase steeply with the greater cutting angles a, V while in the soil bin experiments, the cut soil block 1, AFDI in Fig. 4 became actually more crumbled with the greater cutting angles, and hence, the weight of the cut soil block 1 showed little increase. As a result, the total draught F and the suction F also did not increase V X steeply.
5. Conclusions (1) The constraints on the design of the second plough body are that the operating depth is small but the operating width is large and it should transfer the tilled soil to the preceding furrow over a relatively large distance of the full furrow width. The bulldozer blade type of plough body was much superior to the mouldboard plough version. (2) When the working depth of the second plough body is 50 mm, the curvature radius should be 200 mm, the plough height 300 mm and the throwing angle 603. (3) When the approach angle was more than 453, the transferred soil masses reached a constant value, and hence, the minimum approach angle at which the soil starts to slide horizontally was experimentally around 453. (4) The value of the predicted optimum approach angle of 50)83, when the adhesion of soil}polyethylene was not neglected, nearly agreed with the experimental value. (5) Based on the draught prediction of the model second plough body, the draught of the accumulated soil block was the largest, followed by the draught for soil
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cutting, and the draught caused when the soil slides horizontally on the mouldboard. (6) The cutting angle should be more than 403 because the suction should be positive. However, when the cutting angle was more than 303, the draught increased steeply and became unsuitable for practical use. Hence, in this model second plough body, the optimum cutting angle should be 303 even though the suction is slightly negative here. References Araya K; Kudoh M; Zhao D; Liu F; Jia H (1996a). Improvement of planosol solum, part 2: optimization of design of roll-in ploughs in soil bin experiments. Journal of Agricultural Engineering Research, 63, 261}268 Araya K; Kudoh M; Zhao D; Liu F; Jia H (1996b). Improvement of planosol solum, part 5: soil bin experiments with a three-stage subsoil mixing plough. Journal of Agricultural Engineering Research, 65, 143}149 Araya K; Kudoh M; Zhao D; Liu F; Jia H (1996c). Improvement of planosol solum, part 6: "eld experiments with a three-stage subsoil mixing plough. Journal of Agricultural Engineering Research, 65, 151}158 Fujimoto Y (1984). Draught of bulldozer blades with sand. Transactions of Construction Machines, 20th Anniversary, 98}105 Jia H; Liu F; Zhang H; Zhang C; Araya K; Kudoh M; Kawabe H (1998). Improvement of planosol solum, part 8: analysis of draught of a three-stage subsoil mixing plough. Journal of Agricultural Engineering Research, 70, 185}193 Nakajima M (1980). Development of lined blades. Journal of Komatsu Industries, 26(4), 12}23 Reece A R; Cook P E R (1996). Theory of bulldozer action in friable soil. Proceedings of Second International Conference of ISTVS Torihi T (1969). Capacity of bulldozer blades. Journal of Komatsu Industries, 5(4), 42}51 Zhang C; Araya K (2001). A three-stage soil layer mixing plough for the improvement of meadow soil, part 1: mechanical properties of soils. Journal of Agricultural Engineering Research, 78, in press.