long-term reduced cultivation. i. soil strength and stability

Soil & Tillage Research, 15 (1989) 79-90 79 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

Long-Term Reduced Cultivation. I. Soil Strength and Stability

P. SCHJONNING and K.J. RASMUSSEN

Department of Soil Tillage, Soil Physics and Irrigation, Flensborgvej 22, DK 6360 Tingler (Denmark)

(Accepted for publication 6 April 1989 )

ABSTRACT

Schjonning, P. and Rasmussen, K.J., 1989. Long-term reduced cultivation. I. Soil strength and stability. Soil Tillage Res., 15: 79-90.

Topsoil wet aggregate stability was measured during the last 13 years of an 18-year-old field trial with reduced cultivation. Soil strength and soil compressibility were analyzed by a micropen- etrometer and a confined, uniaxial compression test, respectively. The trials were situated on a coarse sandy soil and a fine loam in Jutland, Denmark.

During the trial period, a considerable decrease in topsoil aggregate stability was observed if the soil was ploughed annually and all plant residues were removed from the field. Shallow tillage by tine cultivation to c. 10-cm depth and especially rotovating to only 5-cm depth diminished the structure deterioration induced by the continuous growing of cereals after ploughing. A crop of Italian ryegrass (Lolium multiflorum), grown after harvest of the small grain cereal crop, also stabilized the aggregates in ploughed as well as in shallow-tilled soil, but the differences induced were less than those induced by the tillage systems.

In the top layers of the loamy soil, reduced compressibility was observed in the rotovated soil compared with ploughed and tine-cultivated soil.

Soil strength increased in the non-tilled layers of shallow cultivated soil compared with ploughed soil. A compact soil layer beneath the rotovation depth in the loamy soil (called a "rotovator- pan") was found to have strength values which might depress root development.

INTRODUCTION

The industrialization of Danish society in the 1950s and 1960s influenced agricultural practise. Farmers specialized their production and looked for la- bour-extensive methods of accomplishing the various farming operations. The question arose, whether it was possible to grow a crop without the annual ploughing operation. Further, it was of interest to study how the yield of a small grain cereal crop would develop in time when a monoculture growing system was initiated.

Several field trials concerning these questions were started in the 1960s and

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80 P. SCHJONNING AND K.J. RASMUSSEN

showed in a few years that it was possible to grow a small grain cereal crop year after year in the same field and also to avoid the ploughing operation, especially after chemical weed control was introduced.

Another doubt, however, concerned how the new farming systems influenced soil fertility and soil structure. It was of interest to clarify whether a long-term effect of a growing system with monoculture of spring-sown small grain cereals would be that the soil deteriorated as a growing medium for plants, and whether an input of organic matter from the roots and stubble of a catch crop grown in the autumn could moderate the effect.

This paper intends to highlight the influence of reduced tillage upon soil strength and stability in a monoculture growing system. Soil strength is of interest as an indication of conditions for root growth and as an estimate of resistance to compaction. Structural stability will be dealt with as an expres- sion of the resistance of topsoil aggregates to water disruption, which is im- portant when heavy rain hits the seedbed.

MATERIALS AND METHODS

Soil types and field trial

The investigations were carried out in a field trial located at two soil types which are both situated in the south of Jutland, Denmark. Soil types and topsoil textures are given in Table 1.

The trials were started in the autumn of 1967 and have been carried out for 18 years of continuous growing of small grain cereals. At Jyndevad, a monoculture of spring barley has been grown during the entire trial period except in 1971 when oat was grown.

At Hojer, a crop rotation consisting of wheat, oat and spring barley was practised until 1978, after which a monoculture of spring barley was introduced.

TABLE1

Soil type (Soil Taxonomy System; subgroup (particle size class) ) and topsoil texture (0-20-cm depth) of investigated soils

Location Soil type Texture (% w/w)

Organic Clay Silt Fine sand Coarse sand matter ( < 2 # m ) (2-20#m) (20-200#m) (200-2000/~m)

Jyndevad Orthic Haplohumod 3.5 (coarse sand)

Hojer Mollic Fluvaquent 3.0 (fine loam )

3.1 4.5 13.1 75.8

15.6 15.4 65.1 0.9

REDUCED CULTIVATION. I. SOIL STRENGTH AND STABILITY 81

TABLE 2

Cultivation operations in the field trial

Treat- Cultivation ment

Winter Pre-sowing Post-harvest

Catch crop

A Ploughing 20 cm Harrowing 3-5 cm Harrowing 10 cm - B Ploughing 20 cm Harrowing 3-5 cm None ÷ C None Rotovating 3-5 cm None + D None Rotovating 3-5 cm None - E None Rotovating 10 cm None - F None Harrowing 3-5 cm Harrowing 10 cm -

litalian ryegrass (Lolium multiflorum).

Prior to the trial period, the fields were grown in a normal crop rotation practice, including beets, grass and small grain cereals.

Six different tillage systems were compared in a complete block design with 4 and 2 blocks at Jyndevad and Hojer, respectively.

Table 2 summarizes the cultivation treatments in the trial. Sowing was per- formed with a drill in all treatments. In Treatments B and C the spring-sown catch crop of Italian ryegrass (Lolium multiflorum) was harvested and removed from the plots in late October.

Methods and sampling programme

The research programme comprises 3 lines of investigation.

Investigation 1: aggregate stability In the period 1973-1985, samples of the 0-3-cm topsoil layer were collected

each year from all t reatments and both replications of the Hojer trial. In the period 1973-1980 the replicated samples from each treatment were bulked prior to being analysed. Therefore, it is not possible to elaborate a statistical evalu- ation of results from that period. Sampling took place just after harvesting the cereal. Plant residues were removed from the sample and the soil was air-dried and stored at room temperature until the analysis could take place.

A wet-sieving technique, in principle as suggested by Hartge (1971), was applied to the samples. In brief, the method involves initial partitioning of the dry sample to the aggregate size fractions 2-4, 4-6 and 6-8 mm. The initial size distribution is recorded prior to a gentle process of water saturation of the bulked sample. After a period of 24 h of equilibration, the soil is subjected to a 5-min automized, vertical sieving process in a water-filled container. Finally, a new size distribution can be recorded, using the size fractions 1-2, 2-4, 4-6 and 6-8 mm.


The percentage of 2-8-ram aggregates, which has not been destroyed to aggregates less than 1-mm diameter during the wet-sieving procedure, can be calculated as

SrAB:/- --/IO0 (1)

in which ni and nk denote the weight of aggregate fractions before and after sieving, respectively.

Investigation 2: uniaxial, confined compression In the autumn of 1984 and 1985 undisturbed soil cores were sampled from

all replications in Treatments A, C, D and F (cf. Table 2) at the Hojer and Jyndevad sites, respectively. The cores were taken from the bulk soil in brass cylinders (inner diameter 61.0, height 34.2 ram) holding a total of 100 cm 3 soil. Samples were taken from the very top layer 0-5 cm and the 5-10-cm layer. Twenty-four undisturbed samples (Jyndevad: 4 plot replications X 6 sample replications; Hojer: 2 plot replications X 12 sample replications) were analysed for each treatment and soil layer, making a total of 192 cores from each location.

The samples were placed on top of a sandbox in which a slow water-saturation process took place. Then, after drainage to a water potential of - 100 hPa (~ 100-cm water column), the soil cores were subjected to a simple uniaxial compression test, in principle as suggested by Koolen (1974). The measure- ments were carried out with a strain-controlled stress application at a rate of 1 mm min-1. Strain and stress were automatically recorded. The compression test was run to a maximum load of 800 kPa or stopped manually at a lower force, if the compression expelled water from the sample.

Investigation 3: penetration resistance Sampling for this investigation took place exactly as described for Investi-

gation 2, except that the examined soil layers were 5-10, 15-20 and 25-30 cm. Twenty-four undisturbed samples from each treatment and soil layer were analysed, making a total of 288 cores from each location. Also in this investigation, the samples were drained to a water potential of - 100 hPa.

The penetration experiment was carried out in accordance with the sugges- tion of Whiteley et al. (1981). The samples were placed in an apparatus, in which a number of 5 steel penetrometers (2-ram diameter, 30 ° apex semi- angle) concurrently and automatically were pushed into the soil sample with a rate of 3 mm min- 1. In a soil "depth" of 8 mm (4 times the probe diameter) the penetration resistance was recorded automatically.

REDUCED CULTIVATION. I. SOIL STRENGTH AND STABILITY

R E S U L T S

83

Aggregate stability

From Fig. 1, it appears that the ploughed plot (A) throughout the measuring period has had the poorest aggregate stability compared with Treatments D, E and F with reduced cultivation systems. Almost every year, Treatments E and F, which have been tilled to an intermediate depth of 10 cm, are found to have a stability index intermediate between the ploughed (A) and the shallow cultivated plot (D). The latter has a high and nearly unchanged stability throughout the measuring period.

This pronounced and significant difference between ploughed and shallow rotovated soil is also evident according to Fig. 2. Almost every year, a stabilizing effect of green manure is found in rotovated soil and especially in ploughed soil but the effect is not statistically significant.

Uniaxial confined compression

The data in Table 3 indicate the considerable difference between the two soils investigated. The coarse sandy soil at Jyndevad holds only 11-15% (w/w) of water at a potential approximately similar to the field situation in spring (field capacity), while the fine loam at Hojer contains nearly 30% (w/ w) of water at field capacity.

In the 0-5-cm layer of the Jyndevad soil and in both layers in the Hojer soil there is a tendency for a higher water-holding capacity in the rotovated plots

90 IE \ / o . .

85-{F.... E -o----70 - - - E . \ \ /

E rotovated, 10 ~ , ~ _ _ ~ " ~- F hanowBd, lOcm J i ~ I 60.

1973 1974 1975 1976 1977 1976 1979 1980 1981 lg82 1883 1984 1986 Year

Fig. 1. Stable aggregates for 4 tillage treatments (without catch crop ) through 13 years of trial in the Hojer soil. Bars denote least significant difference (P=0.05).


" .. .~.. /" ,,

"d ", ," " i

. "

65 A rotovl~ed, 5 crlrl "J I/

- "1 ....... ....... !' I - + c ~ n ( m p I I l

60

197'3 1~4 i975 19~'6 19~ 19"/13 I~19 1~I0 1981 1~ I~13 I~ 1~6

Fig. 2. Stable aggregates through 13 years of trial in the Hojer soil. Bars denote least significant difference (P = 0.05 ) for comparison of tillage effects. Catch crop effect is not statistically significant in any year.

TABLE 3

Water content (per cent w/w) in soil samples drained to - 100 hPa water potential and used for the uniaxial confined compression test

Location Depth Treatment ( cultivation depth (cm); _+ catch crop) SE (cm)

A C D F 20; - 5; + 5; - 10; -

Jyndevad 0- 5 11.4 15.5 13.4 12.4 1.3 5-10 11.7 12.8 12.3 12.6 1.0

Hojer 0- 5 27.3 32.6 31.3 29.4 1.3 5-10 25.9 27.3 26.4 25.7 1.2

(C a nd D ). Th i s t r e n d is con f i rmed in a n o t h e r inves t iga t ion in the same tr ial (Schjonning , 1989).

T h e r e is no s igni f icant d i f fe rence in the res i s tance to compac t ion , w h en ex- p ressed as re la t ive sample he igh t a t a no rma l load of 100 k P a (Table 4) . How- ever, a t e n d e n c y for a s t ronger s t ruc tu re appears in the ro to v a t ed plots (C an d D ) whe n c o m p a r e d wi th the p loughed (A) and ha r rowed (F) plots in the Hoje r soil.

In Fig. 3, poros i ty is p lo t t ed towards square root of appl ied pressure for the 3 plots w i thou t ca tch crop a t t he loamy soil a t Hojer . No t r e a t m e n t effects upon the s t ruc tu ra l s tabi l i ty to compress ion could be de tec ted in the general ly less compac t ion- sens i t ive sandy soil a t Jyndevad . T h e reason for the t r an s fo rm a-

REDUCED CULTIVATION. I. SOIL STRENGTH AND STABILITY

T A B L E 4

Relative sample height, %, at a normal load of 100 k P a in the confined compression t e s t

85

Location Depth Treatment (cultivation depth ( c m ) ; ± catch crop) SE (cm)

A C D F 20; - 5; + 5; - 10; -

Jyndevad 0 - 5 91.4 92.9 92.2 92.9 2.2

5 -10 95.4 95.4 95.3 95.9 0.8

Hojer 0 - 5 85.2 87.0 86.9 84.9 4.2

5 -10 92.6 95.2 95.6 93.2 2.0

0- -5 crn 56 ~.

54 ~ . ' ~ ~,3

50

48 " ' .

I I I I 4 l I 2 4 6 8 0 12 14

Square root of preesure ~P (kPa =)

Fig. 3. Soil porosity as a function of square root of applied pressure in the uniaxial confined compression test, Hojer soil. Lines show best linear fit by the least-squares method.

tion of the stress parameter will be given in the Discussion. A reduced slope (numerically) of the lines through the D points compared with A and F points indicates a stabilized structure in the very top layer of shallow tilled soil. In the 5-10-cm layer, the reduced steepness of the D line is perhaps partly owing to an initially lower porosity in the soil.

Penetration resistance

An average penetration resistance exceeding 2 MPa has been found just beneath (5-10-cm soil layer) the rotovating tilling depth in Treatment D, Hojer soil (Table 5). This is a fairly high soil strength at a water potential of - 1 0 0 hPa and could therefore be called a "rotovator-pan". Although not signifi-


TABLE 5

Penetration resistance, MPa, for 2-mm diameter penetrometer in soil drained to - 100 hPa water potential

Location Depth Treatment (cultivation depth (cm); _+catch crop) S.E. (cm)

A C D F 20; - 5; + 5; - 10; -

Jyndevad

Hojer

5-10 0.97 1.13 0.88 1.13 0.25 15-20 0.76 1.23 1.25 1.27 0.25 25-30 1.16 1.07 1.27 1.74 0.31

5-10 1.00 1.68 2.05 1.05 0.55 15-20 1.66 1.82 1.68 1.75 0.38 25-30 1.62 1.68 1.29 1.35 0.19

TABLE 6

Percentage of soil samples having penetration resistance higher than 2 MPa

Location Depth (cm)

Treatment (cultivation depth (cm); + catch crop)

A C D F 20; - 5; + 5; - 10; -

Jyndevad

Hejer

5-10 0 4 0 0 15-20 0 0 0 0 25-30 8 0 4 29

5-10 4 17 63 0 15-20 29 29 25 21 25-30 25 4 0 0

cantly different, the lower strength at the same depth in the C plot, which is also rotovated, might be explained by the annual growth of the catch crop.

The frequency of soil samples showing extreme strength is also of interest. In the coarse sandy soil very few spots in the soil possess strength above 2 MPa (Table 6). Compared with the rotovated plots, perhaps the 8 and 29 percent- ages found in the 25-30-cm layer at the A and F plots, respectively, are expres- sions of the compacting action from wheels and deeper working implements.

Although a mean strength of only 1.62 MPa was detected in the 25-30-cm layer of the ploughed treatment in the Hojer soil (Table 5 ), a quarter (25%) of all samples possess a strength of more than 2 MPa (Table 6).


DISCUSSION

The wet-sieving method used in this investigation only gives relative and empirical results, which are difficult to interpret in relation to practical farming processes. Used as a bare index of relative stability, however, the results can give us an idea of the qualitative effects of the different soil management.

Reid and Goss (1980) reported a significant stabilizing effect of growing ryegrass in a sandy loam soil at approximately the same percentage of clay as in the Hojer soil studied in this paper. This trend is also found in the present investigation (Fig. 2), although the effect is small compared with tillage effects. The stabilization detected is of the same magnitude throughout the period analysed, but it seems to be more pronounced in ploughed soil than in shallow-tilled soil. It cannot be determined whether the effect is caused by a (short-term) influence of the growing ryegrass or to a (more persistent) effect from decaying plant material. What is obvious, however, is the more pronounced and statistically significant difference between tillage systems. This is in accordance with other investigations reporting stabilized topsoil aggregate structure in soil with reduced depth of cultivation (Tomlinson, 1974; Hamblin, 1980; Chaney et al., 1985).

Hamblin (1980) studied soil from field trials with 3-8 years of different tillage, but found no evidence for any correlation between length of time during which the tillage treatment had been imposed and the degree of change in structure. This might be owing to considerable annual variation as it is observed in this investigation (Figs. 1 and 2), which is probably caused by cli- matic differences. In the present investigation, 13 years of repeated measure- ments offer a fine opportunity to analyse the question of whether there is a certain trend in aggregate stability through time.

Linear regressions of aggregate stability to time show that all the treatments except D have given rise to a significant (5% level) decrease in the soil aggregate stability in the period investigated (Table 7). The depth of cultivation highly influences this trend. As experienced, a shallow cultivation (Treatment C and D ) gives nearly unaffected stability through time, while an intermediate cultivation depth (Treatment E and F) and especially the ploughing operation (Treatment A and B ) introduce a decreasing stability of the soil through time.

Larson et al. (1980) suggested the slope of the bulk density-log (pressure) relation for remoulded soil, i.e. the virgin compression curve, as a compression index, C, expressing the compressibility of soils at a given water content. Using soil samples in undisturbed condition, as in this study, the compaction will follow the secondary compression curve until the unknown value of precon- solidation load for the specific soil sample has been reached. To quantify treatment differences in resistance to compaction in these conditions a transfor- mation of the applied pressure was chosen, which produced an approximately linear relation between soil porosity and the stress parameter (Fig. 3). This


T A B L E 7

Stabi l i ty of topsoil aggregates, Hojer soil. Slope, ~, of the l inear regression model STAB = ~ year + fl and p robab i l i t y t h a t ot is zero

T r e a t m e n t Slope, ( cu l t i va t ion d e p t h ( c m ) ; (% agg yea r -1 ) _+ ca t ch c rop)

P ( s l o p e = 0)

A (20; - ) - 1.22 0.007 B (20; + ) - 1.08 0.005 C ( 5; + ) - 0 . 3 5 0.040 D ( 5; - ) - 0 . 2 5 0.204 E (10; - } - 0 . 7 5 0.010 F (10; - ) - 0 . 8 3 0.014

analysis was limited to the pressure range 0-200 kPa owing to samples satu- rated at higher pressures in the loamy soil.

As discussed in the previous section, the tillage treatments have no influence upon compressibility in the coarse sandy soil at Jyndevad. In the aggregated, loamy soil at Hojer the shallow cultivation (plot D) has increased the soil resistance to compaction (Fig. 3). This is in accordance with Horn (1986), who ascribed the stabilization of structure to a more vertically oriented pore geometry. In the topsoil, the increased resistance to compaction should perhaps be explained by the stabilized aggregate structure as it has been detected in this investigation.

Van Ouwerkerk and Boone (1970), Ellis et al. (1977), Hodgson et al. (1977), Pollard et al. (1981), Ball and O'Sullivan (1982) and Chaney et al. (1985) all found increased soil strength in non-tilled soil layers for reduced cultivation systems compared with ploughed soil at the same depths. In the present investigation this trend is confirmed in the 15-20-cm layer of the coarse sandy soil (Table 5). In the fine loamy soil an increase in soil strength is distinct in the 5-10-cm layer of rotovated plots (C and D) compared with the ploughed and harrowed plots (A and F), Tables 5 and 6. Remarkably, however, compared with the annually ploughed soil, the strength in the 15-20-cm layer has not increased in the plots which have not been cultivated for 17 years. Just after the ploughing operation, of course, the strength will be less in the ploughed soil. At the time of sampling (September), the soil strength has been increased in about 10 months to about the same level as appeared in the non-tilled soil layers of the reduced tillage treatments owing to the influence of weather and farming operations. Despite this identical strength, other investigations have shown differences in the pore geometry owing to tillage treatments (Schjonning, 1989).

Dexter (1986), using the same method of strength measurement as in this study, has shown that the proportion of roots penetrating a compacted soil


layer is decreasing with increasing soil strength and halved (compared with optimal conditions) when meeting strengths of 1.2-2.2 MPa, depending on the "anchorage" of the root before penetrating the hard soil layer.

From these findings it can be derived that the strength found in Plot D (rotovated, without catch crop) in 5-10-cm depth of the Hojer soil (Tables 5 and 6) is alarmingly high for optimal root growth. As pointed out by Dexter, the situation is different for mono- and dicotyledonous plants, as the dicoty- ledon has only a single tap root. For instance, when growing sugarbeet, the "rotovator-pan" found in the Hojer soil might be disastrous for the establish- ment of the crop.

CONCLUSION

In a fine loamy soil, which originally has been managed with a crop rotation, continuous growing of small grain cereals can give rise to a considerable decrease in the topsoil aggregate stability when the soil is ploughed annually to a depth of about 20 cm and all plant residues except stubble and roots are removed from the field. Shallow tillage will diminish the structure deterioration induced by this farming system. The growing of a catch crop will further stabilize the aggregates but this effect is smaller and probably less persistent than the tillage effect.

Compared with ploughed soil, increased aggregate stability after a shallow cultivation gives rise to reduced compressibility of the topsoil in a loamy soil, while no effect upon compressibility was found in a coarse sandy soil.

Compared with ploughed soil, the soil strength will increase in the non-tilled layers of shallow cultivated soil. In a loamy soil, many years of shallow rotovation can give rise to a "rotovator-pan", which might depress root development.

REFERENCES

Ball, B.C. and O'Sullivan, M.F., 1982. Soil strength and crop emergence in direct drilled and ploughed cereal seedbeds in seven field experiments. J. Soil Sci., 33: 609-622.

Chaney, K., Hodgson, D.R. and Brain, M.A., 1985. The effects of direct drilling, shallow cultivation and ploughing on some physical properties in a long-term experiment on spring barley. J. Agric. Sci., 104: 125-133.

Dexter, A.R., 1986. Model experiments on the behaviour of roots at the interface between a tilled seed-bed and a compacted sub-soil. I. Effects of seed-bed aggregate size and sub-soil strength on wheat roots. Plant Soil, 95: 123-133.

Ellis, F.B., Elliott, J.G., Barnes, B.T. and Howse, K.R., 1977. Comparison of direct drilling, reduced cultivation and ploughing on the growth of cereals. 2. Spring barley on a sandy loam soil: soil physical conditions and root growth. J. Agric. Sci., 89: 631-642.


Hamblin, A.P., 1980. Changes in aggregate stability and associated organic matter properties after direct drilling and ploughing on some Australian soils. Aust. J. Soil Res., 18: 27-36.

Hartge, K.H., 1971. Die physikalische Untersuchung von BSden. Eine Labor- und Praktikuman- weisung. F. Enke Verlag, Stuttgart, 168 pp.

Hodgson, D.R., Proud, J.R. and Browne, S., 1977. Cultivation systems for spring barley with special reference to direct drilling (1971-1974). J. Agric. Sci., 88:631-644.

Horn, R., 1986. Auswirkung unterschiedlicher Bodenbearbeitung auf die mechanische Belastbar- keit von Ackerb~iden. Z. Pflanzenernaehr. Bodenkd., 149: 9-18.

Koolen, A.J., 1974. A method for soil compactibility determination. J. Agric. Eng. Res., 19: 271- 278.

Larson, W.E., Gupta, S.C. and Useche, R.A., 1980. Compression of agricultural soils from eight soil orders. Soil Sci. Soc. Am. J., 44: 450-457.

Pollard, F., Elliott, J.G., Ellis, F.B. and Barnes, B.T., 1981. Comparison of direct drilling, reduced cultivation and ploughing on the growth of cereals. 4. Spring barley and winter wheat on silt loam soils over chalk. J. Agric. Sci., 97: 677-684.

Reid, J.B. and Goss, M.J., 1980. Changes in the aggregate stability of a sandy loam effected by growing roots of perennial ryegrass (Lolium perenne ). J. Sci. Food Agric., 31: 325-328.

Schjonning, P., 1989. Long-term reduced cultivation. II. Soil pore characteristics as shown by gas diffusivities and permeabilities and air-filled porosities. Soil Tillage Res., 15: 91-103.

Tomlinson, T.E., 1974. Soil structural aspects of direct drilling. Papers of the International Con- gress of Soil Science, 10: 203-213.

Van Ouwerkerk, C. and Boone, F.R., 1970. Soil-physical aspects of zero-tillage experiments. Neth. J. Agric. Sci., 18: 247-261.

Whiteley, G.M., Utomo, W.H. and Dexter, A.R., 1981. A comparison of penetrometer pressures and the pressure exerted by roots. Plant Soil, 61: 351-364.

long-term reduced cultivation. i. soil strength and stability

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