managing traffic-induced soil compaction by using conservation tillage

Soil& Tillage Research, 24 (1992) 319-336 319 Elsevier Science Publishers B.V., Amsterdam

Managing traffic-induced soil compaction by using conservation tillage

C. S o m m e r a n d M. Z a c h Institute of Crop Science and Plant Breeding, Federal Agricultural Research Centre, Bundesallee 50,

D-3300 Braunschweig, Germany

(Accepted 4 May 1992 )

ABSTRACT

Sommer, C. and Zach, M., 1992. Managing traffic-induced soil compaction by using conservation tillage. Soil Tillage Res., 24:319-336.

The term 'Konservierende Bodenbearbeitung' has a somewhat different meaning than conservation tillage as used worldwide. In Germany the term is used not only in relation to the retention of surface residues to reduce erosion but in association with compaction control by carefully timed loosening operations.

Field experiments were conducted from 1985 to 1990 on a loamy sand (Dystric-Luvisol) in north- central Germany. The effect of crop rotation-specific soil loosening on some soil physical properties and crop yields was studied in the presence and absence of wheel-induced soil compaction when growing sugar beet, winter wheat, winter barley and a cover crop. Five tillage treatments were studied in a 3-year crop rotation: sugar beet; winter wheat; winter barley; cover crop. These included conventional mouldboard ploughing, conservation tillage with no loosening and conservation tillage where loosening was carried out with a wide blade chisel plough, ( 1 ) before winter barley, (2) before the cover crop (mustard or California bluebell) and (3) before winter barley and the cover crop.

Wheel-induced compaction decreased the pore space and in most cases eliminated differences due to tillage practice. Pore space on the wheel-tracked plots of the conventional treatment was considerably lower than on the non-wheel-tracked plots. Similar results were obtained for the conservation tillage plots but only where loosening had been carried out within the last 18 months.

In summary of the 6 years experiment, there was in general no evidence that conventional tillage was superior to conservation tillage with respect to the yields of sugar beet, winter wheat, or, within certain limits, winter barley on loamy sand.

Accordingly, conservation tillage with crop rotation-specific non-inverting soil loosening, promises to be a potential strategy not only with regard to reducing soil erosion, but a programme for reducing costs and alleviating traffic-induced soil compaction.

I N T R O D U C T I O N

T h e So i l P r o t e c t i o n C o n c e p t o f t h e G e r m a n F e d e r a l G o v e r n m e n t ( B u n d e s -

m i n i s t e r d e s I n n e r n , 1 9 8 5 ) h a s p r o m p t e d i n t e n s i v e d i s c u s s i o n s a b o u t p r o t e c -

Correspondence to: C. Sommer, Institute of Crop Science and Plant Breeding, Federal Agricul- tural Research Centre, Bundesallee 50, D-3300 Braunschweig, Germany.

320 C. SOMMER AND M. ZACH

tive strategies concerning the need for respective legislation and plans regard- ing further research within the realm of ecologically sustainable agriculture. With regard to erosion, which is a regional rather than a national problem, soil loss and sediment transport from arable land carrying desorbed and ad- sorbed nutrients and pesticides are major points of discussion (Der Rat von Sachverst~indigen f'tir Umweltfragen, 1985). Another problem is nitrogen leaching on sandy soils (Bramm, 1978 ) which leads to the question, which particular agricultural practices and circumstances can negatively influence the quality of our drinking water?

A further aspect of soil protection which has again come to the forefront and has been discussed over the last few years is soil compaction. This problem is a concern in agriculture today because of the use of larger and heavier machines in crop production (Soane, 1983; Olfe and Sch/Sn, 1986). It is mainly a man-made problem created by heavy equipment being driven or pulled repeatedly across the soil. German soils are not completely damaged by soil compaction, but, as the soil becomes compacted, plant roots are re- stricted in their ability to grow vertically. Under severe compaction conditions a crop stand never reaches its full yield potential (Hakansson et al., 1987).

One major objective of tillage is to counteract such compaction by loosening the soil and, thereby, creating an improved soil condition for aeration, water infiltration, crop establishment, and plant growth.

The farmer's aim during primary tillage is to produce and maintain a loosened soil by ploughing once or sometimes twice a year to the full depth of the arable layer (20-35 cm ). This practice has resulted in a special soil compaction problem. It is recognized that many arable soils have a severely compacted layer below the plough depth created by the standard practice of ploughing with two tractor wheels running in the furrow. Farmers try to re- move these layers by periodic deep loosening. Tillage improves the poor ma- cro-soil structure but seldom improves the micro-soil structure (Frede, 1982), while traffic following tillage quickly recompacts the soil (Hartge, 1980, 1981; Sommer, 1985; Horn, 1988 ). A cycle of tillage-traffic-tillage-traffic has de- veloped, but the deeper the tillage the deeper the next wheel traffic compacts the weakened soil structure (Taylor, 1986).

Short crop rotations with 60-70% cereals and 30% root crops are common on German arable soils. Those soils are tilled very intensively and therefore their ability to support traffic is poor. Reduced tillage intensity could be an efficient method in managing soil compaction (Vorhees and Hendrick, 1977 ).

Strategies and methods are therefore necessary for soil protection with respect to soil compaction. To solve the basic problem, research was initiated to investigate two different but complementary possibilities: ( 1 ) the concept of 'controlled traffic' (Taylor, 1986; Chamen, et al., 1990); (2) the concept of'conservation tillage' (Sommer, 1985).

MANAGING TRAFFIC-INDUCED SOIL COMPACTION BY USING CONSERVATION TILLAGE 321

Conservation tillage is a relatively new management concept in Germany (Sommer et al., 1981 a). Impetus for conversion from conventional to conservation tillage is based on observations of soil degradation either from soil erosion or soil compaction (Sommer and Lindstrom, 1987). Therefore, two fundamental ideas are the basis of conservation tillage.

( 1 ) The possibility of reducing or preventing soil erosion by leaving sufficient residues on or near the soil surface. The objective is to maintain surface coverage as long as possible during the year to ensure protection against soil erosion by water or wind. This goal is identical with conservation tillage as defined by scientists in the USA (Mannering and Fenster, 1983 ).

(2) The possibilty of reducing or preventing soil compaction by minimiz- ing primary soil tillage intensity. The objective is to improve the traffickability by reduced crop rotation-specific non-inverting soil loosening. This goal is identical to conservation tillage as defined in the USA concerning the effect of primary tillage intensity on surface coverage. It is a question of determin- ing whether or not the degree of soil compaction influences conservation tillage (Vorhees and Lindstrom, 1983).

Reduced tillage intensity to prevent severe compaction is not extensively discussed in the literature as recent reports from different countries show: Belgium (Frankinet and Roisin, 1989 ); Denmark (Rasmussen, 1989 ); France (Duboin et al., 1989); UK (Ball, 1989); Italy (Bonciarelli et al., 1989); Netherlands (Van Ouwerkerk, 1989).

The objective, within the conservation tillage regime of this experiment, was to study the effect of soil loosening at various times in a 3 year crop rotation. Soil properties of wheel-tracked and non-wheel-tracked plots on a loamy sand were investigated together with yields of sugar beet, winter wheat, and winter barley.

MATERIALS AND METHODS

The experiment was conducted between the autumns of 1984 and 1990 at the Institute of Crop Science and Plant Breeding, Federal Agriculture Re- search Center, Braunschweig-ViSlkenrode (FAL), Germany. The soil was a Dystric-Luvisol (Gehrt, 1988 ). The loamy sand is generally considered to be susceptible to compaction owing to a low organic matter content. Some characteristics of the soil are given in Table 1. The monthly rainfall from April to September 1985-1990 and total precipitation from October to March for 1984-1990 are given in Table 2.

Two tillage systems (conventional tillage with annual mouldboard ploughing, conservation tillage without ploughing) and two compaction treatments (zero traffic and normal traffic) were started in 1984. A sugar beet-winter wheat-winter barley-cover crop rotation was initiated to provide neighbour-

322

TABLE 1

Characteristics of the soil (0-25 cm depth)

C. SOMMER AND M. ZACH

Hori- Depth Particle size distribution Total N pH zon (cm) (°/o w/w) (% w/w) (KC1)

Year Depth Org. Matter (cm) (% w/w)

CONY CONS

Ap 0-25 63-2000/tm: 69.3 0.09 5.7 1984 0-12 1.6 6.3-2/tin: 24.9 1990 0-12 1.64

< 2/tm: 5.8 12-25 1.65 2.04 1.44

CONV, conventional tillage plots; CONS, conservation tillage plots.

TABLE 2

Monthly rainfall and total precipitation for 1985-1990 and long-term average values

Month Rainfall(mm)

1985 1986 1987 1988 1989 1 9 9 0 30-year average 1950-1980

April 71 36 24 14 38 40 45 May 52 52 44 12 2 39 58 June 114 85 109 86 36 63 69 July 69 69 70 79 45 25 70 Aug. 45 66 71 22 83 77 68 Sep. 30 52 75 41 29 100 48 April-Sep. 381 360 393 254 233 344 358 Oct.-March 228 247 348 352 247 281 261

Total April-March 609 607 741 606 480 625 619

ing plots with the main crops sugar beet (SB), winter wheat (WW), and winter barley (WB) every year.

The conventional tillage plots (CONV) (Treatment 1 ) were ploughed (20- 22 cm depth) and cultivated (8-10 cm depth) prior to sowing for each crop. The cover crop plots were ploughed again in spring just before growing sugar beet.

The conservation tillage plots (CONS) received four different soil loosening treatments (Table 3 ). Treatment 2 was not loosened during the course of the experiment and only cultivated to a depth of 8-10 cm. Treatment 3 was loosened to ploughing depth using a non-inverting wide blade sweep (three blades with an 80 cm working width) once in 3 years for winter barley. Treat- ment 4 was loosened similarly once in 3 years for the cover crop and Treat-

MANAGING TRAFFIC-INDUCED SOIL COMPACTION BY USING CONSERVATION TILLAGE

TABLE 3

The t imetable o f soil loosening o f all tillage t r e a t men t s (years due to crop plot 1 )

323

Tillage Year and crop t r ea tmen t

1985 /1986 1986 /1987 1987 1987 /1988 1988/1989 1989 /1990 1990 ( W B ) ( C C ) (SB) ( W W ) ( W B ) ( C C ) (SB)

C O N V ( l ) M P M P M P M P M P MP M P C O N S ( 2 ) . . . . . . . CONS (3) CP - - - CP - - C O N S ( 4 ) - CP - - - CP - C O N S ( 5 ) CP C P - - CP CP -

WB, winter barley; MP, mou ldboa rd ploughing; CC, cover crop; CP, chisel ploughing with wide blades; SB, sugar beet; W W , winter wheat .

ment 5 twice in 3 years, once for WB and once for the cover crop. The operations for growing the main crops are summarized in Table 4.

All the plots were set up with a 2.5 m permanent tramline system designed to provide non-wheel-tracked plots (NWT) in the cropped area (zero traffic ). The normal traffic treatments were prepared by driving a tractor over the whole area following primary tillage (wheel tracked, WT) to simulate culti- vating, planting, and spraying operations. In practice farms have a situation which lies between these two extremes. The tractor rear-wheel axle load was approximately 4.1 Mg and tyre inflation pressure was 1-2 bar.

P205 (70 kg ha -1 ) and K20 (140 kg ha - l ) were applied in autumn. The N-fertilizer was broadcast at three rates in the spring. A common nitrogen policy was adopted by applying 180 kg ha - 1 for SB and 150 kg ha - 1 for cereals. Weeds were controlled on all plots using a range of herbicides.

The CONV cereal plots were seeded with a commercial 21 row drill ( 11.9 cm row spaces) In the CONS plots cereals were sown with a disc drill ( 11.9 cm row spacing). For planting SB, (50 cm row width) discs were mounted directly ahead of the opener to prevent it from becoming blocked by the cover crop residues (mulchplanting without seedbed preparation according to Sommer et al., 1988). The tops and leaves of SB and the chopped straw of cereals were incorporated by using a rotary harrow. Four replications of each tillage treatment were established along the length of every plot. Individual plots were 2.5 m wide× 11.4 m long.

On 29 April 1989, soil samples (100 cm 3, 20 replications, 12-17 and 25- 30 cm depth) were taken for the evaluation of bulk densi ty/pore space, and soil moisture content before planting the SB plots. At the same time cone resistance measurements were made with a recording penetrometer. On 19 and 20 March 1990, soil samples (ten replications) were taken from all the plots.

324 c. SOMMER AND M. ZACH

TABLE 4

Operations for growing sugar beet, winter wheat, and winter barley of all tillage treatments

Crop Operation Tillage / traffic treatment

Sugar beet Harvest winter barley 1-5 Fertilizer (P205, K20 ) 1-5 Stubble mulching 1-5 Ploughing 1 Non-inverting loosening 4-5 Normal traffic 1-5 WT Seedbed preparation 1-5 Drilling cover crop 1-5 Ploughing at spring time 1 Normal traffic 1 WT Planting sugar beet 1 Mulch planting without Seedbed preparation 2-5 N-fertilizer, weed control 1-5

Winter wheat Harvest of sugar beet 1-5 Chopping leaves 1-5 Fertilizer (P2On, K20 ) 1-5 Normal traffic 1-5 WT Ploughing 1 Normal traffic 1 WT Seedbed preparation 1-5 Growing wheat 1-5 N-fertilizer, weed control 1-5

Winter barley Harvest winter wheat 1-5 Fertilizer (P205, K20) 1-5 Stubble mulching 1-5 Ploughing 1 Non-inverting loosening 3 and 5 Normal traffic 1-5 WT Seedbed preparation 1- 5 Growing barley 1- 5 N-fertilizer, weed control 1-5

Grain yield was determined for each entire plot by means of a light com- bine ( 1.25 m width of cut) under dry soil conditions. Yields were calculated on a 14% moisture content basis, sub-samples being taken for grain water content. Sugar beet was harvested by hand in early October to eliminate un- controlled traffic. Root-, leaf- and sugar-yield were determined. The effect of the tramlines on the outer rows was excluded. The quality of the beet tap roots at harvest was estimated by classifying visually about 300 tap roots per plot from Grade 1 (smooth) , Grade 3 (weak rooted), Grade 6 (strong rooted), and Grade 9 (fanged).

MANAGING TRAFFIC-INDUCED SOIL COMPACTION BY USING CONSERVATION TILLAGE

RESULTS

325

Soil properties

Pore space values measured at field capacity (about 100 kPa water suc- tion) in spring 1989 are given in Table 5. In the NWT plots the lowest mean pore space (41.0% v/v) at a depth of 12-17 cm was found in CONS Treat- ment 2 where no soil loosening was done since the experiment started in 1984. Tillage treatment had a considerable effect: the pore space was 43.4% in Treatment 3 which had been loosened 18 months previously, 45.8% in Treat- ment 4, loosened within the last 6 months, and 47.0% in Treatment 5 which had been loosened both 6 and 18 months previously. Mouldboard ploughing with a sub-surface roller, 10 days before the soil samples were taken, resulted in a mean pore space of 44.2% in the CONV Treatment 1. Pore space below the working depth (20 cm) of primary tillage was considerably lower in all treatments. The WT compared with the NWT plots showed large and significant decreases in pore space in both the 12-17 and 25-30 cm depth ranges (Table 6 ).

In spring 1990, similar comparative results were obtained (Table 7 ) for the sugar beet plots with the exception that no further compaction had occurred in the 25-30 cm depth range.

In the NWT plots of winter wheat, the lowest mean pore space values (40%) at a depth of 12-17 cm depth were found in CONS Treatments 2 (no soil loosening) and Treatment 3 (loosened 2.5 years ago). Loosening the soil 18 months previously or twice in 3 years resulted in larger pore space in the top layer: 41.9% for Treatment 4 and 43.5% for Treatment 5. The timing of soil loosening during the crop rotation also affected the pore space values in the WB plots: 41.9% in Treatment 4, loosened 2.5 years ago; 44.8% in Treatment

TABLE 5

Pore space (%, v / v ) in spring 1989

Crop Traffic Depth Tillage treatment plot treatment (cm)

CONS CONV (1) (2) (3) (4) (5)

SB NWT 12-17 44.2 41.0 43.4 45.8 47.0 25-30 38.3 39.9 41.6 39.5 37.1

WT 12-17 36.0 37.0 36.9 37.4 37.5 25-30 33.8 36.0 36.6 36.3 34.6

SB, sugar beet plot; NWT, non-wheel-tracked; WT, wheel-tracked.


TABLE 6

Differences of mean values of pore space (%, v /v) non-wheel-tracked/wheel-tracked plots of sugar beet in spring 1989

Depth Tillage treatment (cm)

CONS CONV (1) (2) (3) (4) (5)

12-17 8.2*** 4.0*** 6.5*** 8.4*** 9.5*** 25-30 4.5*** 3.9*** 5.0*** 3.2*** 2.5**

**Significant at 1% level of probability. ***Significant at 0.1% level of probability.

TABLE7

Pore space (%, v/v) in spring 1990


CONS CONV (1) (2) (3) (4) (5)

SB NWT 12-17 45.6 41.9 44.8 44.5 44.7 25-30 33.3 35.4 34.1 35.6 34.8

WT 12-17 39.8 39.6 39.7 38.6 39.9 25-30 35.3 38.2 36.7 35.0 36.0

WW NWT 12-17 44.8 40.7 40.3 41.9 43.5 25-30 32.9 36.7 40.6 37.0 35.6

WT 12-17 38.7 36.9 36.8 38.9 38.4 25-30 35.4 36.6 36.2 37.9 33.7

WB NWT 12-17 46.1 40.4 44.8 41.9 44.4 25-30 35.1 35.6 34.4 37.4 34.9

WT 12-17 40.6 37.7 38.6 38.9 40.0 25-30 37.3 35.5 35.4 38.2 35.5

SB, sugar beet plot; WW, winter wheat plot; WB, winter barley plot; NWT, non-wheel-tracked; WT, wheel-tracked.

3, loosened 6 months previously; 44.4% in Treatment 5, loosened both 2.5 years and 6 months ago.

Comparing the WT with the NWT plots showed large and significant decreases in the mean volume of pore space in the top layer for all the crops (Table 8). However, there are marked differences with respect to tillage Treatments 1-5. Traffic produced the smallest decreases in pore space in tillage Treatment 2. Large differences between NWT and WT were obtained for the CONV tillage Treatment 1 for all three crops. In the SB plots similar dif-


TABLE 8

Differences of mean values of pore space (%, v /v) non-wheel-tracked/wheel-tracked plots of all crops in spring 1990

Crop Depth Tillage treatment plot (cm)

CONS CONV (1) (2) (3) (4) (5)

SB 12-17 5.8*** 2.3** 5.1"** 5.9*** 4.8*** WW 12-17 6.1"** 3.8** 3.5** 3.8** 5.6*** WB 12-17 5.5*** 2.7** 6.2*** 3.0** 4.4***

**Significant at 1% level of probability. ***Significant at O. 1% level of probability.

TABLE 9

Soil moisture content (%, v/v) of non-wheel-tracked and wheel-tracked plots of sugar beet in spring 1990


CONS CONV (1) (2) (3) (4) (5)

SB NWT 12-17 16.4 16.4 15.9 16.8 17.9 WT 12-17 19.9 19.2 20.4 22.4 19.3 Difference 3.5*** 2.8*** 4.5*** 5.6*** 1.4"

*Significant at 5% level of probability. **Significant at 1% level of probability. ***Significant at 0.1% level of probability.

ferences were found for the CONS tillage Treatments 3, 4 and 5, and also in the WB plots for Treatments 3 and 5, and in the WW plots for Treatment 5.

In both years, soil moisture content at field capacity ( 100 kPa water suc- tion ) of the top layer was higher in the WT than in the NWT plots. Table 9 shows that differences were both large and significant in four out of five tillage treatments in spring 1990. On a volumetric basis, the readily available water was greater on the WT plots compared with the NWT plots.

The penetration resistance at field capacity after 1 year of the experiment was greater in CONS tillage Treatment 2 compared with CONV tillage Treat- ment 1 under NWT conditions (Fig. 1 ). WT plots show higher cone resistance values. Similar results were obtained in 1990 after 6 years of the experiment.


E 10 u

x/ ~_ 2(1

D

3O

~0

( ; ( , l i e

1 2 3 4

l ~ e l | , ~ I . r t t t i o n t - e s i a t a n c , ~ t

I 2 3 4 1 2 3 4

k - ",JWheel track "~ ,,,," (WT)

[ # ~ \ \ :, CONS

Conventional tqllage, Conservation tillage, CONV - NW1- CO/IS-NWT

CONV/CONS - COMPARISON

Fig. 1. Cone penetration resistance at field capacity depending on different forms of tillage (minimum, mean, and maximum values).

Yie/ds

Sugar and grain yield data are summarized in Table 10 for the years 1985- 1990. Different tillage treatments had no detectable effects on sugar and grain yields either under NWT or WT conditions. The differences were often small and insignificant.

Sugar beet In 5 out of 6 years, the NWT plots of CONV Tillage Treatment 1 out-yielded

the WT plots. Only in 1985 did the opposite results occur. Under WT conditions sugar yields were higher, except in 1985 and 1990, for CONS tillage Treatment 2 than for CONV tillage Treatment 1. In general, the greatest yields, within the combination of WT and CONS tillage, were obtained from Treat- ment 3 (soil loosening before WB ).

The taproot quality of SB in 1986 in shown in Fig. 2. There was a tendency for the degree of fanging to increase from CONV Treatment 1 to CONS Treatments 2 and 3 and to decrease from CONS Treatments 4 to 5. The taproot quality of the NWT plots was better than WT plots.

Winter wheat In 4 out of 6 years, the WT plots of CONV Tillage Treatment 1 out-yielded

the NWT plots. In 1987 and 1988, grain yields were comparable under WT and NWT. Under WT conditions, grain yields were higher in 3 out of 6 years for CONV tillage Treatment 1 than under CONS tillage Treatment 2. In 1986, 1987 and 1988, the yields were comparable under Treatments 1 and 2. Gen- erally, no significant differences were obtained between the CONS tillage Treatments 2-5.


TABLE 10

Mean yields of sugar beet (SB) and grain (winter wheat, WW; winter barley, WB) 1985-1990 of five forms of different tillage treatments

Year Crop Traffic Yields ( d t / h a ) treatment

CONS CONV (1) (2) (3) (4) (5)

1985 SB NWT A73.9" A74.5a A74.8b A80.Tb A72.7a WT B85.8" A76.1b A79.7b A74.2bc A68.4c

WW NWT A73.4a A67.7bd A63.9c A66.6 be A70.3d WT a80.8" A70.3b A66.6c A65.6 c A71.9 b

WB NWT A45.6a A45.5~ A46.6a A41.5 b A43.0a WT A45.3" A46.5a a51.0a B45.9" B47.2"

1986 SB NWT A87.7" A87.8a A83.2a A83.3a A89.0a WT B73.7~ n82.6bd A89.8c A80.0 d n85.0b

WW NWT A45.6a a46.3a A44.2a A48.4a A48.6a WT A46.9" A46.1 a A45.2a A46.9 a A45.4~

WB NWT A60.8a A62.5 a A59.9a A56.2a A58.9a WT %3.3" A59.3 a A62.4a 459.1" A62.5"

1987 SB NWT A86.4a A97.3 b A82.2~" A80.4 ac A83.6ac WT a69.0a a77.9b B75.4b B77.3 b B78.6b

WW NWT A60.7 a A61.7 a A56.5 a A60.6 a A60.7 a WT h60.2a A61.8a A58.2a A60.0a A62.9a

WB NWT A59.& A53.9b A52.3c A51.3 c A51.0 c WT A59.1" B50.9b B54.4c A51.1b a49.3 d

1988 SB NWT A55.3a A57.9a A61.0a 462.7" A63.3a WT A53.0a A61.3" A64.3a A61.8" A60.5a

WW NWT A49.8" A50.7a A51.0 a A45.7 a A47.7a WT A50.4a A51.9 a A53.9" A47.7" A49.7~

WB NWT A50.9" A46.0b A46.4b A49.1 ~u A47.7"b WT A49.0" A48.7b A49.5b A47.9 ab A50. l"b

1989 SB NWT A86.5a A88.9a A83.0b A86.4ab 488.3"b WT a70.2" a79.4bd A87.0c A81.3b¢ a76.1d

WW NWT A41.3 a g38.5a A34.2~ A38.2a ~38.1 ~ WT A45.4" A42.9 a A40.1" A42.2" A42.6a

WB NWT A56.9a A53.2" A51.7" A50.3" A61.2 ~ WT A61.0" A58,7" A57.4" A54.4a A61.4 a

1990 SB NWT A72.1" g77.8a A74.8~ A70.6" A75.0~ WT A79.9" A72.7" A68.6" A70.0" A72.2~

WW NWT A65.9" A67.9" A66.1a A67.2a A65.5a WT A73.1" g69.3a A71.0a A71.3" A70.3"

WB NWT A79.4a A68.5a A72.0" A73.4~ A70.9" WT A82.8 ~ A72.9" A73.9" A75.1" A75.7a

CONV, conventional tillage plots; CONS, conservation tillage plots; NWT, non-wheel-tracked; WT, wheel-tracked. In each row (lower case letters) and column (capital letters), the values are significantly different ( P = 0.05 ) when not marked by the same superscripts (using Duncan's Multiple Range Test ).


6"

O" "5 3 £ 13.

[q NWT

9. Tiltage C0NV

1 {1) 12)

WT treatments

- - C O N S - -

(3) It,) (5)

LSD(.05

Fig. 2. Taproot quality of sugar beet for five tillage treatments in 1986. Grade 1 (smooth), Grade 3 (weak rooted), Grade 6 (strong rooted), and Grade 9 (fanged).

120 ] Sugar beet

I LSD {,05) -~,

i v A L r / l ~ i , , i i t l

111 (2) (3} (z,) (51

12o .] Winter wheat

7a lOO 1 -~ r LSD (.05) %,

80 t 60

(1) 12) 13} (~) (s)

!20] Winter barley

noo I LSD (. 05) ~,

• 8o t 6O

k I A I I / i , I r , I ~ 1 ~ 1 I (11 c2) 131 (~(~ --

I--I nonwheel t racked 171 wheel t racked

Fig. 3. Mean annual crop yields of sugar beet, winter wheat and winter barley 1985-1990 relative to CONV NWT plots of five different forms of primary tillage treatments.

Winter barley In 3 out of 6 years, the WT plots of CONV tillage Treatment 1 out-yielded

the NWT plots. In 1985, 1987 and 1988, grain yields were comparable under WT and NWT. In four of the years under WT conditions, grain yields were higher for CONV tillage Treatment 1 than for CONS tillage Treatment 2. In 1985 and 1988, the yields were comparable under Treatments 1 and 2. The greatest yields were generally obtained from CONS tillage Treatments 5 and


3 both of which were loosened just before sowing. The mean annual yields of SB, WW and WB relative to the CONV NWT plots Treatment 1, averaged for the 6 years of the experiment, and the significant differences, as well, are presented in Fig. 3.

The WT beet plots yielded less sugar compared with the NWT plots and the differences were generally significant at the 5% level. Winter wheat and WB yields were lower from the NWT plots, but the differences were often not significant at the 5% level. Regardless of the statistical significance, WB seems to respond more negatively to a lack of loosening under conservation tillage.

D I S C U S S I O N

There is evidence that conservation tillage, with crop specific and soil-specific modifications, has the potential to help manage soil erosion (Allmaras and Dowdy, 1985; Unger and Fulton, 1990) in a range of climates including the humid conditions in Western Europe (Cannell, 1981; Sommer and Lind- strom, 1987; Brunotte, 1990).

'Konservierende Bodenbearbeitung' as defined by Sommer et al. (1981a) has a somewhat different meaning than used in the US. Mannering and Fens- ter ( 1983 ) defined conservation tillage as "any tillage system relative to conventional tillage. Often a form of non-inversion tillage that retains protective amounts of residue on the soil surface". The Conservation Tillage Informa- tion Center (CTIC, 1983) has refined this definition to require a minimum of 30% surface residue cover after planting.

With respect to short crop rotations which are used in Germany, conservation tillage goals include both surface residue cover and reduced primary/ secondary tillage intensity. This intensity is intermediate between highly intensive conventional tillage and zero tillage (Table 11 ).

Farmers who want to follow this concept of conservation tillage commonly have three main problems. Apart from the necessity that non-inversion-til-

TABLE 11 The def ini t ions of tillage systems ( S o m m e r et al., 1981a)

Conven t iona l Conserva t ion Zero tillage tillage tillage

Stubble tillage + +

Pr imary non- inver t ing tillage + soil loosening

if necessary Secondary

tillage + + - Plant ing + mulch seeding


lage implements and planting equipment must be available and chemical weed control must be no more intensive than with conventional tillage, the question arises as to what level of soil loosening has to be carried Out. Indeed, severe compaction in the arable layer needs to be seriously considered if one wants to go into conservation tillage. Such compaction caused by vehicular traffic has a considerable impact on soil physical properties and on crop yields (Sommer et al., 1981 b; Hakansson et al., 1987 ). Furthermore, subsoil compaction can significantly increase the resistance to root growth and affect both nutrient status and crop yields (Vorhees et al., 1986 ). Therefore, it is helpful to evaluate critical load values which lead to damaging levels of soil compaction on different soil types under different conditions (Petelkau, 1986 ). Also a systematic approach with respect to the factors and processes leading to various states of compactness is necessary (Canarache et al., 1984).

However, naturally settled soils with 'moderate compaction' may be adven- titious for crop growth and may also have sufficient shear strength to resist applied axle loads. It is desirable, therefore, that the second idea of conservation tillage, a reduced crop rotation-specific soil loosening operation, is applied on arable land.

The results of this study show that the effect of soil loosening at different times within a 3-year crop rotation had a marked influence on pore space values. Under all crops with conventional tillage and zero traffic, the pore space at a depth of 12-17 cm was about 45%. Pore space on CONS was about the same providing soil loosening had been carried out within the previous 18 months. After wheeling, the pore space at a depth of 12-17 cm was 37- 40% for all crops and tillage treatments. Czeratzki ( 1972 ) considered 40% as an op t imum for sandy soils.

Because of the residual WT effects from the previous years and the fact that no loosening had occurred since the beginning of the experiment, traffic produced the smallest decreases in pore space in the CONS tillage Treatment 2. From the results of the CONS tillage Treatments 3-5, it can be concluded that the rate of pore space reduction caused by traffic decreases with time following loosening.

In relation to these pore space values, the cone index did not exceed 2.0 MPa in the arable layer and therefore was not considered to be in the range that could reduce root growth (Taylor and Gardner, 1963; Ehlers, 1982 ). As these measurements were made at a t ime when water contents were near field capacity, the values of soil strength indeed represent seasonal minimums.

There was a large decrease in pore space from the arable layer (at a depth of 20-22 cm) to the dense untilled soil underneath. No regeneration of soil structure was obtained by reduced tillage intensity on either the WT or NWT soil. This was attributed to the fact that no swelling and shrinkage processes occur on this sandy soil. No further compaction was caused by the WT treatment at a depth of 25-30 cm in 1990. Depth of compaction is expected to be

MANAGING TRAFFIC-INDUCED SOIL COMPACTION BY USING CONSERVATION TILLAGE 3 3 3

greater where larger tractors and fully loaded combines are employed. Some- times pore space was lower than 35% (spring 1990) and close to a level of soil compaction which could have negative effects on soil functions and crop yield (Czeratzki, 1972 ).

These results imply that subsoiling would be beneficial on this loamy sand. However, such tillage would have to be carried out by non-inverting implements such as a wide blade chisel plough or a Paraplough because the soil profile is strongly layered and poor infertile sand could be brought up. Fur- thermore, the improved soil structure created by mechanical loosening would have to be stabilized by growing a cover crop. Mulch planting without seedbed preparation must then be used for growing the next main crop (e.g. SB or maize) to conserve the better traffickability which would otherwise be lost by conventional methods. This means that conservation tillage is the precondi- tion for effective subsoiling. Without this type of management it is doubtful that a previous year's subsoiling will be of any benefit to a succeeding year's crop. Ploughing instead of chiseling does not bring success as Busscher et al. (1986) verified for a Norfolk loamy sand. Last but not least, irrigation is common in this area and Ibrahim and Miller (1989) stated for a similar soil type that "benefits from subsoiling appear to be marginal if irrigation is adequate".

Crop yields were very variable between years owing to differences in weather conditions. Sometimes there were marked yield differences between the tillage treatments. For example, relative yields of WB in 1987 were 86-90%, and those of sugar beet in 1988 were 105-115% compared with CONV treatments. There are some hints that soil loosening was important when a wet rather than a dry season follows ( 1987 and 1989, respectively). However, no correlations were found between yield and weather conditions from year to year.

In summary of the 6 years experiment, there was in general no evidence that conventional tillage was superior to conservation tillage with respect to the yields of SB, WW, and WB on loamy sand. These results are similar to those obtained by Lumkes et al. (1984) who compared loose-soil husbandry and a rational tillage system. However, in this experiment, the beet yielded less sugar in WT plots and the beet quality was inferior compared with the NWT plots. This may be of importance if the harvest is not done by hand but mechanical harvesters are used as is commonly the case.

C O N C L U S I O N S

( 1 ) Non-inverting soil loosening at different dates within a 3-year crop rotation (SB-WW-WB-cover crop ) had a marked influence on the duration of mechanically created pore space. The rate of reduction in pore space caused by traffic decreased with time.


(2) Continuous reduced tillage intensity for 6 consecutive years had no considerable negative or significant effects on the yields of SB and WW. Win- ter barley tends to need regular soil loosening to maintain yield.

(3) Soil loosening could be reduced considerably compared with conventional tillage without loss of yields. It is of importance to time soil loosening specifically in relation to crop rotation. Systems of controlled traffic could initiate further reduction in the amount of mechanical soil loosening, but zero tillage may not be an appropriate management technique owing to increased inputs of chemicals.

(4) Conservation tillage promises to be a potential strategy for further im- provements in crop production efficiency. Its potential value is not only of evident relevance with regard to soil erosion and its related problems, but as a programme for reducing costs and alleviating traffic-induced soil compaction.

ACKNOWLEDGEMENTS

Financial support from the EC Commission by the SCAR Programme on Land and Water Use and Management (Contract No. 5210 ) is gratefully ac- knowledged. C. Sommer would like to express his appreciation to Dr. Mike Lindstrom of the North Central Soil Conservation Research Laboratory, Morris, MN, for his friendly cooperation in the framework of the F.R.G.-U.S. Working Group on Agricultural Science and Technology (Subject: Conser- vation Tillage as a Strategy of Soil Protection in Agriculture) since 1984. Last but not least the authors thank T. Chamen, Silsoe Research Institute, UK, for his kind assistance in correcting the English translation.

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