Effects of furrow packing at ploughing on light soils

Soil & Tillage Research, 16 (1990) 203-218

203

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

Effects of F u r r o w P a c k i n g at P l o u g h i n g on Light Soils J.K. KOUWENHOVEN

Wageningen Agricultural University, Tillage Laboratory, Diedenweg 20, 6703 GW Wageningen (The Netherlands) {Accepted for publication 10 March 1989)

ABSTRACT Kouwenhoven, J.K., 1990. Effects of furrow packing at ploughing on light soils. Soil Tillage Res., 16: 203-218. Mechanical reconsolidation by packing at ploughing on light soils was studied on sand in a soil bin, and on reclaimed peat soil in the field. Packing increased soil bulk density in the whole arable layer, but the maximum increase was found at an average depth of 0.11 m. The maximum bulk density after packing was about 1300 kg m -3 on sand and about 865 kg m -3 on reclaimed peat soil. This low density was reflected in a cone index of only 200-500 kPa. Consequently, packing reduced the depth of the ruts caused by subsequent wheel traffic (drilling) by only 25%, i.e. from about 0.08 m to 0.06 m. Moreover, packing considerably reduced soil surface roughness, especially in the direction of travel. This effect was stronger when the packers were followed by rollers. With increasing weight of the packer the density obtained by packing increased and, at drilling, rut depth decreased. The depth of maximum reconsolidation increased with an increase in weight and a decrease in the rim angle of the packer. Furrow packing had only a slight influence on emergence and growth of sugar beet and no influence on yield. Probably, this was caused by the only small effect of furrow packing on soil structure, and by favourable weather during the growing season.

INTRODUCTION

In The Netherlands, as a rule, light soils are ploughed just before drilling of spring-sown crops such as sugar beet or silage maize. Consequently, there is no time available for desirable reconsolidation of the ploughed soil by natural forces, such as rain and gravity. To obtain sufficient reconsolidation, more single or double furrow packers, sometimes combined with rollers, are used (Fig. 1 ). This system of mechanical reconsolidation aims at: (1) reducing the depth of the ruts by subsequent field traffic and at decreasing rolling resistance (Kowalewsky, 1987; Lumkes, 1987 ); (2) producing a sufficient surface roughness to resist wind erosion and a shallow seedbed of constant depth (Bouman, 1966); (3) improving the capillary rise of water for satisfactory crop emergence, growth and yield (Kouwenhoven et al., 1981; Van der Werf et al., 1983 ). 0167-1987/90/$03.50

© 1990 Elsevier Science Publishers B.V.

J.K. KOUWENHOVEN

204

u

I

Fig. 1. Rear view of a single furrow packer (top), side view of a double packer plus roller (centre), and rear view of a single spiral roller (bottom) (Hietbrink, 1986).

A furrow packer is supposed to reconsolidate the central and lower part of the tilled layer, whereas the roller is supposed to reconsolidate the upper part (Heuser, 1928; Rilbensam and Rauhe, 1964; Bernacki et al., 1972 ). To investigate how far furrow packers come up to expectations, research was carried out in 1978 in a soil bin filled with sand. In 1986 and 1987 field experiments were carried out with sugar beet on reclaimed peat soil in co-operation with the Research Station for Arable Farming and Field Production of Vegetables at Lelystad (Hietbrink and Kouwenhoven, 1987; Kouwenhoven and Maljaars, 1988). In these experiments a total of 23 furrow packers and packer plus roller combinations were involved. MATERIALS AND METHODS

Soils Soil data are shown in Table 1. The sand studied in 1978 can be described as a cover sand. Reclaimed peat soil is essentially a sand soil with a high organic

FURROW PACKING AT PLOUGHING ON LIGHT SOILS

205

TABLE 1 Characteristics (mean values) of the soils involved in the furrow-packing experiments Year

Soil type

Specific weight (kgm -3)

Organic matter content (%,w/w)

Moisture content at packing (%, w/w)

1978 1986 1987

Cover sand (M50--= 180/~m) Reclaimed peat soil Reclaimed peat soil

2580 2440 2434

4.1 11.1 11.5

15.6 37.0 40.1

matter content. Both fields studied on reclaimed peat soil were subsoiled to about 1-m depth and, therefore, were rather heterogeneous with respect to organic matter content and soil moisture content.

Furrow packers Furrow packer experiments were carried out just after or in conjunction with rotadigging or ploughing to about 0.26-m depth, with a forward speed of about 1.5 m s - 1. Some common packers are depicted in Fig. 1. All furrow packers and packer plus roller combinations used in the experiments, and their effects, are specified in Tables 2 and 3. These implements are characterized by: (1) diameter and weight per ring; (2) width of the packer rings and angle of the rim; (3) configuration of the rim (smooth or serrated); (4) single or double type (total number of rings per m working width); (5) rollers behind the packer or not. The diameter of the packers used ranged from 0.5 to 0.9 m; the majority had a diameter of 0.7 m. The spacing of the rings varied from 0.1 to 0.2 m and the width of the rings from 1.4 to 7 cm. The narrower the rings (Zu Jeddeloh, 1986) and the smaller the rim angle (Hann et al., 1987; Hietbrink and Kouwenhoven, 1987), the deeper the penetration of the rings will be and, thus, the deeper the maximum reconsolidation will be found (Kowalewsky, 1987).

Sur[ace pressure The surface pressure under rollers can be calculated by a formula of Atwood (1953), cited by Aboaba ( 1969): W

15= b v / ~ Z where: i6=mean surface pressure (kPa); ,W--weight of roller (kg); b = w i d t h of roller (m); r = r a d i u s of roller (m); Z--sinkage (m). Sinkage was measured only in 1978 and in 1987. Calculated mean surface pressures are presented in Table 4. The tyre inflation pressure was about 150 kPa for the drilling tractor and about 140 kPa for the seed drill (Table 5 ). Thus, if the surface pressure equals

206

J.K. KOUWENHOVEN

TABLE2 Characteristics of furrow packers and furrow packer combinations Year

1978

No.

1 2 3 4

1986

5 6 7 8 9 10 11 12 13 14 15 16 17 18

1987

19 20 21 22 23

Implement 1

Diameter Width of rings of rings (cm) (cm)

Rim Number of angle rings m -1 (°) Apiece Total

Packer Packer Plain roller + water Packer +packer + crumbier roller + 7 spring tines

70 90 62

3.2 7,0

30 40

70 50 16

3.2 4.0

30 ?

Packer Packer Serrated packer Packer Double packer Double packer Double packer Packer + cam roller Packer + croskill roller Serrated packer + serrated roller Packer + serrated roller Packer + croskill roller Packer + spiral roller Double spiral roller

70 70 70 90 70 70 90 70 55 70 45 70 38 70 38 90 45 70 48 48

4.5 5.1 2.0 6.0 4.5 5.1 6.0 4.5 3.2 4.5 3.0 2.0 1.5 4.5 1.5 6.0 3.0 4.5 6.0 6.0

30 90 90

Spiral roller + extra weight Packer + cam roller Serrated packer + serrated roller Packer + crumbier roller Double packer + cam roller

45

6.0

90

8

70 55 70 40 70 28 90 55

4.5 2.7 2.0 1.4 5.0

30

5 6 9 11 11

Weight (kg m -1) Apiece Total

7 7

275 558 396

7 8

30 45 45 30 45 45 30 30

30 45

5 5 4 5 7 5 11 9 11 5 11 5 11 5 8

15

5 5 9 5 10 10 8 12 16 20 16 16 13 14

267 142 30 85

256 253 446 287 227 287 187 188 125 287 125 6O8 187 287 143

524 287 290 188 608 511 506 892 514 474 313 412 795 430 253

377

?

11 20 11

5.5 2.7

30

7 6

13

276 232 277 65 490 60 754 232

508 342 550 986

IMark: 2: Tigges UPS II; 4: Becker KSPH 300; 5-16, 20, 23 Lemken; 17, 18: Robertus; 19: Samex; 21: Zinger; 22: K~ckerling.

FURROW PACKING AT PLOUGHING ON LIGHT SOILS

207

TABLE 3 Effects of furrow packing on t h e soil Year N o ?

1978

Maximum CI 2 at d e p t h

25

(kPa)

70/120 130/180

Surface roughness

(R)3

(ram) X

Y

X+Y

Pore space (%, v / v ) at depth (mm)

Air-filled pores (%, v / v ) at depth (mm)

70/120

130/180 70/120

50 49 50 49

50 49 51 50

49 57

50 57

130/180

1

-

-

1 5 0

-

-

2 3 4

-

-

-

180 110 70

-

-

-

-

-

1 3 0

-

-

-

-

260

-

390 310 310 410 490 540 580 470 460 410 360 550 460 420

370 340 370 480 460 430 580 390 390 400 370 590 420 410

410 320 420 500 570 620 630 520 500 410 400 610 500 420

120 100 150 150 110 100 120 70 90 130 90 120 110 70

8 0 12 2 11 3 13 12 8 1 1 0 1 0

45 41 23 47 41 33 51 23 14 13 12 10 25 19

59 62 66 62 65 62 59 66 67 66 65 64 68 67

61 62 68 61 66 62 61 68 69 67 67 65 68 69

33.3 32.0 32.2 33.2 31.5 29.0 30.0 28.7 27.0 30.9 31.5 31.7 25.7 26.2

36.6 33.5 36.6 32.2 33.4 30.2 33.2 34.6 38.3 34.8 35.3 34.5 31.2 30.0

200 440 160 200

429 200

490 220

110 260

5 26 28 23 35 38

64 72

65 73

30.9 41.3

34.6 44.6

180 230 220 170 220

270 270 260 280 360

270 260 270 300 350

270 290 270 300 370

90 70 50 100 120

1 21 20 3 7 11 0 14 16 1 4 4 2 8 14

66 65 66 66 66

65 66 66 65 66

32.7 32.6 30.9 33.1 31.0

33.1 35.5 34.9 33.4 32.3

200 290 100 130

290 140

300 140

90 260

1 11 13 11 27 28

66 67

66 67

32.1 35.3

33.8 38.2

Mean Control 1986

Cone Index ( k P a ) at d e p t h (ram)

5 6 7 8 9 10 11 12 13 14 15 16 17 18

Mean Control 1987 19 20 21 22 23 Mean Control

-

160 160 90 160 100 150 100 260 190 300 260 270 290 360

44 41 20 46 41 32 51 20 11 12 10 6 24 18

1See Table 2. 2CI = Cone Index. 3 R = 100 log s; X in t h e direction of travel, Y at right angles to t h e direction of travel, X + Y overall surface roughness.

tyre pressure × 1.2 (Perdok and Arts, 1986), the pressure on the soil surface under the tyres would have been about 180 kPa.

208

J.K. KOUWENHOVEN

TABLE 4

Influence of surface pressure on reconsolidation by packing and on rut depth (mean values) Year

1978 1986 1987

Surface pressure i

Reconsolidation

(kPa)

(mm)

24.5 36.2

Rut depth (ram)

39 31

Packed

Control

66 52

87 72

1Calculated according to Atwood (1953) cited by Aboaba (1969); tyre pressure of the drilling tractor was about 1.5 bar in 1987 and the travelling speed was 1.5 m s - 1. TABLE 5

Tyre size and inflation pressure, and static load on the wheels of the tractor and the drill in 1987 Equipment

Type

Tyres Place

Tractor

Zetor 6945

Front

4X4

Rear 1

Rear 1 Seed drill

Vicon 6 rows

Static load per wheel

Size 12.4-24 13.6-30 9.5-42 400-16

Width

Pressure

(m)

(kPa)

0.31 0.35 0.24 0.11

120 140 160 140

(kN) 0.85 1.05 0.25

1Dual tyres.

Measurements Each experimental year had some special features. 1978. Laboratory soil bin experiments with sand (Nos. 1-4 in Table 2). Reconsolidation was measured as surface depression and was also determined by means of gamma ray attenuation in undisturbed soil columns with a length of 1.6 m and a diameter of 0.12 m. 1986. A large number of packers and packer combinations (Nos. 5-18 in Table 2) were studied in 2 replications in a field experiment on reclaimed peat soil. Soil porosity was measured by means of core sampling and the cone index (CI) was determined with a Bush electronic penetrometer. 1987. Only 5 packers were studied (Nos. 19-23 in Table 2), but the experiment was laid out in 4 rather than 2 replications to improve the reliability of the results. Reconsolidation by packing of the reclaimed peat soil was established by means of measurements with a reliefmeter, by core sampling, and by determination of the CI. In all experiments, soil bulk density and soil surface roughness, as influenced by packing, were regarded as the most important characteristics.

F U R R O W PACKING AT P L O U G H I N G O N LIGHT SOILS

209

Soil bulk density was measured by means of 100-cm 3 core samples from the 0.07-0.12 m and 0.13-0.18 m depths in 1978. Comparison of the results of g a m m a ray measurements with the core sampling resultsobtained showed that the freshly ploughed and very loose soil was considerably compacted by the sampling action itself,i.e.by about 200 kg m -3 on non-packed soiland about 100 kg m -3 on packed soil.However, as the g a m m a ray method is very laborious, it was not applied in the fieldexperiments. Pore spaces are mentioned for reference only, as in realitythey were higher than indicated in Table 3. Density was also characterized with a Soil Test pocket penetrometer in 100c m 3 samples in the laboratory (1978) and in the field (1986 and 1987) with the Bush electronicpenetrometer (cone angle 30 °; base area 3.2 cm 2) ( A S A E A-standard; in: Tijink and Vaandrager, 1983 ). Cone resistancewas measured at 15 depths at intervalsof 0.025 m just after ploughing, and at 0.01 m in the seed rows in summer. Just after ploughing, a variation in moisture content in the verticaldirection could not be expected. Therefore, differences in cone resistance can be attributed mainly to differences in bulk density (Kowalewsky, 1987; Perumpral, 1987). By comparison of these resultswith the resultsobtained from samples which were compacted under standardized moisture conditions, penetrometer readings could be translated into true bulk densities. Reconsolidation by packing can also be calculated from the difference in surface level measured before and after packing. This was done in 1978 and in 1987. The depth of the ruts which the tractor and the seed drill(Table 5 ) made during sowing were measured with a ruler. Surface roughness afterploughing and afterpacking was measured by means of two types of reliefmeter. (I) A horizontal plank of 2-m length, fitted with vertical measuring rods with 0.10-m spacing. Thus, from 20 measurements 400 height figureswere obtained. From these figures,soilsurface roughness (R) was calculatedaccording to the formula: R = 100 log s, where s is the standard deviation of the height differences between separate rods (cm) (Kuipers, 1957). Reconsolidation evaluations were obtained from the same figures. (2) With an electronic reliefmeter (Van Ouwerkerk et al.,1982) surface roughness was measured in the direction of travel (related to the coarseness of the soil),and at square angles to the direction of travel (relatedto the surface shaping by the packers). These figurescombined give an indication of the overall surface roughness (relatedto resistance to wind erosion ). RESULTS

Density According to Klenin et al. (1985), packing and rolling would increase soil bulk density by 30-40%, and, thus, cancel the upheaval caused by ploughing

210

J,K. KOUWENHOVEN

(Kouwenhoven, 1986); the maximum reconsolidation would occur at 0.090.10-m depth. In 1986 (Nos. 5-18 in Table 3), owing to packing, pore space decreased by at least 8% (v/v) and the air-filled pore space by 10% (v/v). In 1987 (Nos. 19-23) the measured increase in bulk density was much less, but this could be due to incorrect core sampling. Especially on reclaimed peat soil, but also on sand, pore space and air-filled porosity were still very high after packing. This was reflected in the low values of the CI, though the CI was roughly doubled to tripled by furrow packing. On reclaimed peat the average maximum CI was about 400 kPa at about 0.1-m depth, which accords with the results of Klenin et al. (1985). Kowalewsky (1987) found a CI of 200-500 kPa. Figure 2, which presents some typical penetrometer results, obtained immediately after ploughing in spring and in the seed row in summer, shows that furrow packing increased soil bulk density in the whole tilled layer. The degree of reconsolidation increased with an increase in weight of the 0 0

500 •

i,

50

2

L

Cl (kPo) 1000 1500 . . . .

500 0

,7...,,.

x\

so

CONTROL

CONTROL

1oo

..r

E

CI (kPQ) 1000 1500

V-11 (:3 150

150

tu

200

20O

250

250

,-L--18

\ \

SPRING

Ci (kPa) 1000

500 ,

5O

500

1500

CI (kPa) 1000 1500

0

ON RO

50

~ ~CO.,ROL

o, 1oo

150

~910

10

~ 100 150

(

~-I1

',~"

' 18

SUMMER

Fig. 2. Cone Index (CI) just after spring ploughing plus furrow packing (top) and in the plant row in summer (bottom) 1986. Implement numbers refer to Tables 2 and 3. Control=ploughed only.

FURROW

PACKING

AT PLOUGHING

ON LIGHT

SOILS

211

~65 surface pressure

~'~-~

= 50 ~°a 2

~

~

(~. ~ 600

~. \

"

~j 500 "~

~ : 5 .c Q 0 Q.

~

L,O0

N

300

z ~ o

200

~

100

////

~

i/I

50:

~'~'/ --~'J.

20

.

. . . . . ~"~'~'~' 25 30 35 40 H01STURE CONTENT (%, wl w}

•

~ 45

200

÷

~

r2=0.97

o~ 0

t

i

0 100

,

i

i

,

,

L

300 500 700 PACKERWEIGHT {kg.m -1)

e1986 01987 i i ,

900

Fig. 3 (left). Influenceof moisture content of the tilled layerand surfacepressure (kPa} exerted by furrowpackers on pore spaceof reclaimedpeat soil (1987); arrowsindicate moisturecontents in the fieldduringpacking in 1986 and in 1987. Fig. 4 (right). Influenceof packerweighton Cone Index (CI) in 1987. packer and the level of maximum reconsolidation was deeper down when the rim angle decreased from 90 ° to 30 °. This was also found by Kowalewsky (1987) and is demonstrated by the superficial reconsolidation caused by the spiral packer (No. 18). To investigate the influence of moisture content and surface pressure during reconsolidation on pore space, a laboratory experiment was carried out in which reclaimed peat soil was compacted in 100-cm 3 core samples, at a speed of 3 mm s - 1 and at a range of moisture contents and pressures. The results show that the influence of moisture content was relatively small up to a level of 32% (w/w) (Fig. 3). Extrapolation of the pressure to about 30 kPa, as exerted by packers, and a moisture content of about 40% ( w / w ) , as present in the field in 1987, resulted on reclaimed peat soil in a pore space of about 58% (v/v). Because its reconsolidating action is concentrated near the surface, the spiral packer (Fig. 1, bottom) should be considered as a roller. Values for CI found in the seed row in summer were higher than those found just after ploughing, which is mainly caused by the lower moisture content of the soil. The influence of packing in spring could still be noted, and the order of magnitude was more or less similar. The results shown in Fig. 4 demonstrate that the weight of a furrow packer is the most important factor in mechanical reconsolidation (Klenin et al., 1985; Kowalewsky, 1987).

Reconsolidation and rut depth

As a result of the stronger reconsolidation by increased packer weight, the depth of the ruts made by subsequent trafficdecreased with increasing packer

212

J.K. KOUWENHOVEN ~0~ALtt4 RuT

90 80

/

r2=0.86

70, 9O A@O E

~ 70

E

L... c~

"""---L

=

"- ..,

•

y=-0.OlSx+7;. ~

Z o

w

~ 50

y= -0.007x.56 r 2 =0.71

I--

~4o 30

L~

0

I

100

=

I

I

~

60

~"~

g ~~ 50 LC

r2 =0.71 /

3(3

/

• 1986 o 1987 I

300 500 700 PACKER WEIGHT Ikg.m-l)

h

=

900

10 I

IN RUT

/

~

X

*

1

8

/

/

CI

0

100

300 500 700 PACKER WEIGHT {kg,m-~)

900

Fig. 5. Influenceof packer weight on the depth of the rut made by singletyres in 1986and by dual tyres in 1987. Fig. 6. Total reconsolidationin the rut made by dual tyres after packing during drillingof sugar beet in 1987. weight (Fig. 5 ). Packing reduced rut depth by about 25% or 0.02 m. The total reconsolidation in the wheel tracks made after packing with packers of various weights is shown in Fig. 6. The heaviest packer caused a reconsolidation of 0.03 m. In that case the compaction owing to wheeling was 0.06 m. Thus, the total reconsolidation in the rut amounted to 0.09 m. When ploughing at a depth of 0.26 m, the upheaval was about 0.09 m (Kouwenhoven, 1986). Consequently, after drilling, in the ruts the original upheaval had disappeared completely, whereas outside the wheeltracks about 0.06 m still remained. This means that the effects of packing were rather small. This is probably due to the low surface pressure: about 30 kPa (when calculated per m working width) and about 50 kPa (when calculated per ring ) (Table 4 ). The contact pressure of the tractor tyres was about 180 kPa, i.e. 3-6 times as much. This explains the small increase in soil bulk density by packing and the small decrease in rut depth.

Surface roughness Generally, soil surface roughness was decreased by furrow packing, especially in the direction of travel (Table 3 ). In some cases, packing without rollers resulted in increased surface roughness (e.g. Nos. 5, 6, 8 and 11). Figure 7 shows the effect of some packers on surface roughness in the direction of travel (bottom) and in cross-section (top). Generally, differences in surface roughness (R) of 5 units are significant at the 5% level. The differences in roughness between packer combinations Nos. 19-23 were significant at the 1% level. Soil surface roughness was negatively related with the number of packer

FURROWPACKINGA T PLOUGHINGON LIGHT SOILS

1001

213

CO/~FROL

i 80'

"~60 g

\',..:/

',.,\.._',_./ 20

x;

...,-" . ~.~,:..~.~,.~. ...................... _,:/..............'"'. . . . . . . . ~ ' ~ -23 i ~ i i .J

20 0

0

50

100

150

200

250

HORIZONTALDISTANCE(mini Fig. 7. Cross-section through the surface layer after ploughing ( R y = 27), after packing with a spiral roller (No. 19; R y = 2 1 ) , after packing with a serrated p a c k e r + s e r r a t e d roller (No. 21; R y = 14), and after packing with cam rollers and crumbler rollers (No. 20 R x = 3; No. 22; R x = 1; No. 23; R x = 2; Kouwenhoven and Maljaars, 1988).

~60

PACKERS PACKERS,,ROLLERS

SO -I-

~0

~....~= 0.79

g3o Q

uJ 20 ~o ~

i

i

i

i

i

i

i

a

4 8 12 16 TOTAL NUMBER OF RINGS.m "1

Fig. 8. Influence of the number of rings m - 1 width on soil surface roughness (R). TABLE 6 Effect of rollers following a furrow packer on surface roughness, cone index and depth of ruts caused by subsequent field traffic Implement No. 1

1-3 4 5-11 12-18 19 20-23

Roller

. + + +

.

Surface roughness

M a x i m u m CI

X

(kPa)

D e p t h (ram)

500 480 270 300

150 70 120 90 90 80

. 7 3 1 1

Y

X+ Y

39 14 21 8

40 17 20 11

.

Rut depth (mm)

69 64 54 33

1See Table 2.

rings per m working width (Fig. 8). The effect of rollers following the packers on surface roughness and on bulk density is shown in Table 6. Rollers gave a smoother surface, but had hardly any influence on soil bulk density or on rut depth.

214

J.K. KOUWENHOVEN

~oo

CONTROL

.~9o I~ ~o, z

ff 70~ i

15,05

i

.

~.05

i

12,0657 DATE

Fig. 9. Influence of packing on the number of plants h a - ~ after wind erosion following drilling on 21 April 1987. TABLE7 Influence of packing on plant density, yield and tare (mean values) Year

1986 1987

Plants h a - l

Yield (t ha - I )

Tare (%, w / w )

Packed

Control

Packed

Control

Packed

Control

62 000 93 700

51 300 98 400

52.0 55.1

52.7 ~

11.5

?

Plant establishment and yield of sugar beet Plant establishment was disturbed by wind erosion after drilling, especially on the packed fields. The numbers of plants on 15 and 21 May were significantly lower than the final number on 12 June (Fig. 9). However, on the control plots (ploughed only), the number of plants was already nearly at the final level on 15 May. In June, the final number of plants on the packed plots was only slightly lower than on the control plots (Table 7). This was probably caused by uprooting and covering of seedlings by wind erosion. No significant influence of packing on crop yield and on soil tare was found. The relatively wet growing seasons of 1986 and 1987 and the wind erosion after drilling in 1987 may have dominated the possible effects of packing on crop growth and yield. Similar results were found in the literature. Only Van der Werf et al. (1983) found a 2% higher yield of silage maize after packing. DISCUSSION

Pore space and air-filled porosity figures shown in Table 3 should be distrusted because of uncontrolled compaction of the loose soil during sampling. Even so, pore space figures are high. Packing increased bulk density from 1100 to 1330 kg m -3 on sand (1978) and from 670 to 860 kg m -3 on reclaimed peat

FURROW PACKING AT PLOUGHING ON LIGHT SOILS

215

soil (1986). On sandy soil Sprong (1979) found an average increase in bulk density from 1090 to 1200 kg m -3. The still low bulk density might be one of the reasons that packing had no effect on crop establishment and crop yield. Another reason might be that both experimental growing seasons were rather wet and no shortage of water occurred. In 1987 CI was generally low (Fig. 4 ). This is related to the fact that the ruts made by subsequent tractor traffic were relatively shallow, owing to the use of dual tyres (in 1986 single tyres were used). Stronger mechanical reconsolidation could be obtained by a sheepfoot roller, as used in road building, which exerts foot pressures of 1-5 MPa (Nichols, 1976). Another possibility is offered by the Falcon tyre roller, mounted in front of the tractor (Fig. 10). The total width of this combination is 2.4 m and the surface pressure is about 100 kPa, i.e. 2-3 times higher than the surface pressure exerted by common packers. Both options will reduce subsequent rut depth and rolling resistance considerably. On a ploughed, non-reconsolidated soil, rolling resistance of a tractor with 16-inch {about 0.4-m wide) tyres can easily amount to 2-5 kN {Inns and Kilgour, 1978; Tijink, 1988). Sometimes packing is carried out by means of tractor wheels. On low lying, wet sandy soils, Van der Werf et al. (1983) found a 2% decrease in silage maize yield after a single track to track wheeling with dual tyres, and a yield reduction of 11% after track-to-track wheeling three times with single tyres. However, on high and dry sandy soils, Kouwenhoven et al. (1981) found a positive effect of track-to-track reconsolidation, especially in dry springs. A laboratory experiment with the same soil indicated that, when growing silage maize, bulk density should not exceed 1450 kg m - 3 at 25 % (v/v) moisture content (Stibbe and Terpstra, 1982 ). Results of surface roughness measurements on the control plot (ploughing only) could be erroneous, as here the figures obtained with the electronic reliefmeter with a rod spacing of 0.025 m were not independent as they should be (Kuipers, 1957). In 1987, when a reliefmeter with a rod spacing of 0.1 m was used, in nearly all cases (Nos. 20-23 in Table 2) surface roughness was lower after packing than after ploughing. Obviously, another reason for low surface roughness is the use of rollers, as in 1986 (Nos. 12-18 in Table 2 ). 500

~

.I~00

2

500

•

Fig. 10. Reconsolichtion by a Falcon tyre roller (R) mounted in front of a tractor (T = rear tyres; dimensions in m m ).

216

J.K. KOUWENHOVEN

An increase in soil surface roughness increases the chance of variation in drilling depth and, thus, uneven crop establishment. On the other hand, on a rough surface, wind erosion causes less damage so that crop establishment will generally be better than on a smooth surface. Bouman (1966) stated that wind erosion after packing is generally less severe than might be expected. However, in my opinion, rollers, which contribute little to reconsolidation, but cause a strong decrease of soil surface roughness, should not be used. CONCLUSIONS

(1) All packers and packer plus roller combinations studied in 1986 and 1987 compacted the tilled layer to ploughing depth, but left the surface layer in a loose condition. (2) After mechanical reconsolidation soil bulk density was still low. Therefore, packing reduced the depth of the ruts made by subsequent traffic by only 25%, i.e. from about 0.08 m to 0.06 m. (3) The degree of reconsolidation increased with an increase in packer weight. (4) After packing, the maximum reconsolidation was found at about 0.11-m depth, i.e. in the centre of the tilled layer. The depth of maximum reconsolidation increased with a decrease in the rim angle and with an increase in the weight of the packer. {5) Soil surface roughness generally decreased by packing. The decrease was larger with a larger number of rings per m working width. The largest decrease in soil surface roughness was produced by packer plus roller combinations, especially in the direction of travel. (6) Little or no crop response to packing was noticed. This may be caused partly by insufficient reconsolidation and partly by favourable (wet) weather after drilling. (7) On soils prone to wind erosion, the use of rollers behind packers should be discouraged. ACKNOWLEDGEMENTS The author acknowledges with thanks the help of his co-workers B. Kroesbergen, P. Looijen and R. Terpstra, and the Agricultural University students O. Hietbrink, J.M. Maljaars and L.A. van Sonsbeek. The author is grateful for the cooperation of J. Alblas and L.M. Lumkes, Research Station for Arable Farming and Field Production of Vegetables at Lelystad, and their State Agricultural College students H. Nieboer and A. Waalkens. J. Alberts and J. Schreuder of the EHF Valthermond provided valuable assistance in the layout of the field experiments. The firms of Lemken, K~ckerling, Robertus, Samex and Zinger made a sub-

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stantial contribution to the success of the experiments by providing the necessary equipment.

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