Compressibility of soils in a long term ®eld experiment
with intensive deep ripping in Romania
A. Canarache
a,*, R. Horn
b, I. Colibas
caResearch Institute for Soil Science and Agrochemistry, Bd. Marasti 61, Bucharesti 71331, Romania
bInstitute for Plant Nutrition and Soil Science, Christian-Albrechts UniversitaÈt, Olshausenstrasse 40-60, 24118 Kiel, Germany cAgricultural Research Station Oradea, Sos. Aradului 4, Oradea, Romania
Received 23 March 1999; received in revised form 7 March 2000; accepted 28 June 2000
Abstract
Laboratory compressibility tests were done on soil samples taken from a ®eld experiment 21 years old, located on a Stagnic Luvisol, with deep ripping performed with various frequencies: no ripping, ripping every 8, 4 and 2 years, and ripping yearly. Precompression stress was found to increase with depth of the soil pro®le down to some 60 cm, and somewhat decreasing at the depth of 70±75 cm, which corresponds to the Bt horizon. Due to ripping, the values of the precompression stress decreased; for soils from experimental treatments with different periodicity of ripping operations, the differences were small, and not in a very de®nite direction. The estimation procedures suggested by Lebert to predict precompression stress for ``normal'' arable soils could not be applied to ameliorated soil samples investigated in this paper because repeated ripping prevents a continuous aggregate formation and results mainly in structural texture dependent relations. The compression index showed an increase down to 60 cm and a decrease in the Bt horizon (70±75 cm). In the different experimental treatments, it showed a less clear variation, although some trend of increasing with increased number of rippings may be considered. As inferred from these parameters, soil strength and compressibility do not affect directly crop yields.#2000 Elsevier Science B.V. All rights reserved.
Keywords:Precompression stress; Compression index; Stagnic Luvisol; Deep ripping
1. Introduction
1.1. General review
Deep ripping of soils with a heavy textured, com-pact and impervious subsoil (Schulte-Karing, 1970; Wildman, not dated; Zaidelman, 1985) is expanding nowadays in many countries, including Romania (Canarache, 1978). In this country, due to speci®c
soil and climatic conditions, the positive effect of deep ripping is rapidly decreasing, leading to the need of repeating this practice every 4±6 years (Stanga et al., 1975; Colibas et al., 1989).
Mechanical soil properties are less studied in this context. The differentiation of soil strength by mechanical and physical measurements always includes a ``normal'' strength increase by aggregate formation, as well as the soil reaction to external forces creating an anthropopgenic strength increase. Thus, the determination of the precompression stress is a measure to quantify this ``summed'' effect. The effect of subsoiling however coincides with a
loosen-*Corresponding author. Tel.:40-1-2271331; fax:40-1-2225979.
E-mail address: fizica@icpa.ro (A. Canarache).
ing or deterioration of the aggregate strength, which especially in hardsetting and in overcompacted soils results in a weaker, less stable soil, which is even more susceptible for a subsequent intense overcompaction. There are several papers, mentioned above, which report the bene®ts of such loosening on crop yields, however it is unclear how far you can quantify the ``reloosening'' and ``reaggregation'' processes by means of the precompression test.
Shear strength and stress distribution were deter-mined in a slit-plough ®eld experiment on a silty sandy Luvisol (Horn et al., 1998). Laboratory compressibil-ity tests are frequently used as related to studies on soil compaction (Horn, 1981). Curves describing the rela-tionship between applied load (logarithmic) and bulk density (or void ratio) are well known to consist of two parts: a linear one, the virgin compression line of which slope is called the compression index (Terzaghi and Peck, 1967), and a non-linear one resulting from re-compression of the previously loosened soil. The intercept of the two components of the curve is the precompression stress (Casagrande, 1936), also called reference normal stress (McNabb and Boersma, 1996), widely accepted as a good index to describe soil compressibility. Estimation of the precompression stress from current soil properties has been suggested by Lebert and Horn (1991) for agricultural soils with-out often repeated aggregate deterioration by deep loosening techniques.
1.2. Earlier research in the ®eld experiment used in this paper
The current technique practiced in Romania, with repeated deep ripping every 4±6 years as mentioned above, was considered as possibly having some long range effects, perhaps negative ones. To get some knowledge in this sense, a ®eld experiment simulating such changes was established at Sanmartin (north-western Romania) in the fall of 1977 and conducted until 1998. Results on crop yields, some current physical and chemical properties, moisture regime, etc. from this experiment have been published 6 and 14 years after starting the experiment (Colibas et al., 1985, 1994). It was concluded that at those stages no negative effects occur, while earlier conclusions show-ing signi®cant increases in crop yield and net eco-nomic output due to deep ripping, as well as the need
to repeat this practice every 4±6 years, were con-®rmed.
1.3. Objectives
This paper refers to new data resulting from the long term ®eld experiment with repeated deep ripping mentioned earlier:
results of compressibility tests, not undertaken previously, performed on soil samples from the Sanmartin intensive deep ripping experiment;
results on some physical soil properties from the same field experiment, not included in earlier publications, and relationships of these data with the results of the compressibility tests;
final data on crop yields for the 21 years, thus
completing earlier data which referred to only the first and the second stage of the experiment, and relationships of crop yields with the soil physical properties and with the results of the compressi-bility tests in the various treatments.
2. Materials and methods
The Sanmartin intensive deep ripping ®eld experi-ment was located on a pseudogleyed Albic Luvisol (Romanian classi®cation; approximate WRB equiva-lent: Stagnic Luvisol). The soil (Table 1) is a silt loam in the upper horizons, and a silty clay in the lower ones. Bulk density is high all-over the soil pro®le. Saturated hydraulic conductivity is very low, espe-cially in the lower horizons. Yearly average climatic data (Fig. 1) are characterized by a temperature of 10.58C, rainfall of 635 mm, potential evapotranspira-tion of 689 mm, and length of the frost-free season of 180 days.
grain maize was generally used during the 21 years of the experiment.
Samples used for laboratory consolidation tests were taken only from four of the ®ve treatments. Sampling was done in October 1993, under dry soil conditions, in the centre of the ripper traces, at four depths: 10±15, 30±35, 50±55, and 70±75 cm. Cores with a volume of 200 cm3(6.6 cm in diameter, 5.8 cm high) were used. Twelve cores were sampled from each plot and at each soil depth.
The ®ve treatments in the ®eld experiment were:
nonripped;
ripped every 8 years (three rippings during the
1977±1998 period; two rippings, the last one in 1985, before sampling for the consolidation tests);
ripped every 4 years (five rippings during the 1977± 1998 period; four rippings, the last one in 1989, before sampling for the consolidation tests);
ripped every 2 years (11 rippings during the 1977± 1998 period; no samples for consolidation tests taken);
ripped every year (17 rippings during the 1977±1998
Table 1
Soil properties of the Stagnic Luvisol at Sanmartin
Horizon Depth (cm)
Soil texture (%, w/w) Texture
class (German)
Bulk density (g cmÿ3)
Saturated hydraulic conductivity (mm hÿ1)
Humus (%)
pH in water <0.002 mm 0.002±0.05 mm 0.05±0.2 mm 0.2±2 mm
Ap 0±21 23.7 69.8 5.4 2.1 Lu 1.41 0.26 2.50 5.5
El 21±42 25.6 65.0 6.9 2.5 Lu 1.49 0.09 1.52 5.6
E/B 42±58 37.3 55.7 4.9 2.1 Tu 1.52 0.11 1.27 5.5
Bt 58±122 40.7 51.6 6.4 1.3 Tu 1.55 0.09 0.90 5.5
period; 13 rippings, the last one in 1992, before sampling for the consolidation tests).
Laboratory consolidation tests (Hartge and Horn, 1992) were performed following saturation of the soil cores and successive equilibration on a suction plate at pF 1.8. The tests were done under uniaxial con®ned and drained conditions. Eight cores from each treat-ment and soil depth, randomly selected, were com-pressed during 2 h with normal stresses of 20, 40, 70, 100, 150, 200, 400 and 800 kPa. Soil bulk density before compression was determined, settlement of the soil in each core following compression was mea-sured, and soil bulk density after compression was calculated. Precompression stress and compression index were measured on usual log stress/bulk density graphs.
Statistical processing of the consolidation test data took into consideration the speci®c procedure used in such determinations. On one hand, each test including eight different cores, it offers some information about soil variability. On the other hand, there is formally no replication, as all eight cores contribute to only one ®nal results (either precompression stress or compres-sion index). To deal with these facts, at least as a ®rst approximation, the linear regressions describing the virgin compression lines were used. The con®dence intervals corresponding to these lines of each of the precompression tests were calculated (Snedecor and Cochran, 1967), and these con®dence intervals were
used to compare any of the precompression stress values between them. As for the compression index, the square deviations of the slope of each virgin compression line were used in the same way.
Current data on soil moisture content, resistance to penetration (®eld determination with a drop-cone penetrometer), soil structure, water retention curve, and pore-size distribution, determined on soil samples taken either at the same time with sampling for the consolidation tests, or in previous years, were also available.
Crop yields were registered every year. The data presented here refer to the average of all crops used, mainly winter wheat and grain maize, also including 1 year of oats and 1 year of clover seeds. Data on grain yield included always refer to standard moisture content.
3. Results and discussion
3.1. Results of the compressibility tests
Data on precompression stress are presented in Table 2. The data, varying between ca. 20 and 150 kPa, are comparable to those frequently found in literature, on various, sometimes very different, soils. Differences between depths of soil samples and, in part, between experimental treatments were obvious, showing de®nite trends, generally signi®cant
Table 2
Precompression stress (Log kPa) of soil samples from the intensive deep ripping ®eld experiment at Sanmartin (sampled October 1993) after predrying atÿ60 hPa
Depth (cm) Treatment
Nonripped Ripped every 8 years Ripped every 4 years Ripped every year
Average and
10±15 1.46 0.09 1.34 0.10 1.26 0.06 1.63 0.10
1.37±1.55 1.24±1.44 1.20±1.32 1.53±1.73
30±35 2.01 0.02 1.81 0.02 1.49 0.03 1.69 0.04
1.99±2.03 1.79±1.83 1.46±1.52 1.65±1.73
50±55 2.15 0.04 1.85 0.05 2.00 0.04 1.94 0.14
2.11±2.19 1.80±1.90 1.96±2.04 1.80±2.08
70±75 2.00 0.05 1.85 0.04 1.85 0.01 1.61 0.06
from the statistical point of view according to the approximate methodology which could be used here. Precompression stress was increasing with depth of the soil pro®le down to approximately 60 cm: as an average for all experimental treatments, it was 25 kPa at the depth of 10±15 cm, 60 kPa at 30±35 cm, and 95 kPa at 50±55 cm. It decreased to 70 kPa at the depth of 70±75 cm, which corresponds to the Bt horizon. Considering precompression stress as an index of soil strength (KeÂzdi, 1969) such data con®rm the increase in strength in the deeper soil horizons, with a higher clay content and a higher bulk density in the case of the Sanmartin soil. The load of the soil itself down to 58 cm, i.e. to the beginning of the original Bt horizon, results in 8.4 kPa, which contri-butes only slightly to the higher strength of the lower horizons. The decrease in precompression stress in the 70±75 cm depth soil layer can be explained by the often described smaller strength of soil samples with increasing clay content, if the drying intensity is not very pronounced. As it can be derived from some moisture content data, this soil depth remains always wetter than the upper ones and is consequently also less stable. Thus, the pore water pressure component, m, of the effective stress equation becomes less impor-tant with increasing depth, and the remaining stress is primarily to be attributed to the mechanical compo-nent.
Concerning the deep ripping experimental treat-ments, not studied before as far as we know from this point of view, the trend of decreased values of the precompression stress following ripping can be seen: from 90 kPa (average for all soil depths) in the nonripped plot to 50±55 kPa in the ripped plots. For the soils from experimental treatments with different periodicity (and number) of ripping operations there was a reduced trend of change, and this trend was not in a very de®nite direction. There was a decrease in strength due to ripping, but it was about the same whether this technique was applied 2 or 13 times during the 15 years of the experiment, or whether in this period the soil has been last time ripped 1 or 8 years before sampling. These ®ndings can be explained by the time necessary to rearrange particles by swelling and shrinking or by biological activity, and by the effect that any regain in structure strength has in including the two components: (a) the increase in number of contact points, and (b) an increase in
shear strength per contact point. Consequently, only merely slight differences are to be expected. The high sensitivity of soils for structure deterioration, which does not result in a complete strength regain even after several years, is furthermore underlined.
The estimation procedures suggested by Lebert (1989) to predict precompression stress for nonripped normal agricultural soils proved to be of little use for the soil samples investigated in this paper (Fig. 2). Apart from the fact, that the method was until now only applied to ``natural'' soils with no deep ripping induced structure deterioration, the input parameters required were only partly available. We had no data on shear parameters, and as such we had to use only the estimation procedures not needing this input para-meters, procedures considered by their author as less accurate. We did determine the saturated hydraulic conductivity as one of the input parameters, but this determination was done according to the method used in Romania which is different and leads to somewhat different results than the one used in Germany. Esti-mation procedures for silts were used, best ®tted to the determined data, the soil samples from Sanmartin having a texture of silty loams or silty clays.
To summarize the effect of stress induced changes in the hydraulic conductivity in combination with structure deterioration, the slope of the virgin com-pression line, i.e., the comcom-pression index, also not investigated earlier in this ®eld experiment, was
calculated. The values (Table 3) ranged, with two exceptions, between 0.150 and 0.450. The two excep-tions (0.083 for the 30±35 cm deep sample of the treatment with ripping every 4 years, and 0.860 for the 50±55 cm soil depth from the treatment with ripping every year) may be explained either by an increased variability in the ripped soils, or by the retarded drainage of the excess water during the 2 h of the soil compression test. In principle, we have to consider that any deterioration of the pore system results in an intensely reduced hydraulic conductivity.
The variation with depth of the soil pro®le showed an increase of the compression index down to 60 cm (average values for all experimental treatments: 0.208 at 10±15 cm depth, 0.246 at 30±35 cm, and 0.461 at 50±55 cm), and a decrease in the Bt horizon (70± 75 cm), where the average of the compression index was 0.204. These data show a trend to require a higher load for a given compaction effect in the lower soil layers, which is an agreement with the natural higher initial compaction of these layers. It is well known that as much as the time dependent change of the pore water pressure as a stabilizing phase for static loading is slower, the more are the samples homogenized as a consequence of repeated structure deterioration. Con-sequently, the hydraulic conductivity dependent change in height (settlement) is reduced, and the compression index is increased.
The compression index in the different experimen-tal treatments showed a less clear variation, although
some trend of increasing with increased number of rippings may be considered. As an average for all soil depths, the compression index was 0.237 in the nonripped soil, 0.319 in the soil ripped every 8 years, 0.160 in the one ripped every 4 years, and 0.406 in the treatment with yearly ripping. These trends are also con®rmed by Horn and Baumgartl (1999) and under-line the fact, that soil strength and soil compressibility are affected also by pore water pressure changes and the drainage off as a function of stress and time.
3.2. Other soil physical properties and their relationships to the results of the compressibility tests
Bulk density was clearly decreasing, and certainly the void ratio was increasing, in the ripped as com-pared to the nonripped subsoil (Table 4). Nevertheless, this decrease was more evident in the treatments less frequently ripped, and much lower in the treatment ripped every year. This had been expected, because each year ripping coincides with a continuous rede-terioration of newly formed aggregates. The effect of time on structure reformation exceeds for sure even the longer time intervals in between ripping (see also Horn, 1998).
There was an obvious decrease in the mean weighted diameter of the subsoil dry structural aggre-gates with frequency of ripping (Table 4), with an evident decrease in the percentage of aggregates <1 mm in diameter, and an increase in the content
Table 3
Compression index of soil samples from the intensive deep ripping ®eld experiment at Sanmartin (sampled October 1993) after predrying at ÿ60 hPa
Depth (cm) Treatment
Nonripped Ripped every 8 years Ripped every 4 years Ripped every year
Average Square
10±15 0.156 0.071 0.171 0.119 0.212 0.051 0.295 0.139
0.085±0.227 0.052±0.290 0.161±0.263 0.156±0.434
30±35 0.289 0.036 0.356 0.034 0.083 0.033 0.256 0.048
0.253±0.325 0.322±0.390 0.050±0.116 0.208±0.304
50±55 0.315 0.099 0.455 0.097 0.216 0.071 0.860 0.088
0.216±0.414 0.358±0.552 0.145±0.287 0.772±0.948
70±75 0.190 0.098 0.293 0.046 0.128 0.022 0.213 0.096
of aggregates 2±5 mm in diameter in the ripped treat-ments as compared to the nonripped one, and with no consistent differences between the various ripping treatments. Changes in results of wet sieving of the subsoil were also noticed, namely a decrease in water stability, an increase in dispersion, and as a conse-quence an increase in the structure instability index. Again, these trends were less obvious in the treatment ripped every year.
The water retention curves, as well as the pore-size distribution (Table 5), showed quite small differences between the average of the ripped soil and the non-ripped one (no data were available for each of the individual ripped treatments).
The soil moisture content at the beginning of the growing period, in April, was sharply increased in the ripped treatment (Fig. 3) as a consequence of loosen-ing, even if this loosening was not so well evidenced by the bulk density data. Such ®ndings are very essential in principle, because as long as the soil volume is not alleviated, there is no mass per volume change. Thus, the rearrangement of soil clods or particles in a given volume does not result in a bulk density decrease. At the end of the growing period,
in October, the soil moisture content was almost the same in the ripped and the nonripped treatments, with perhaps some decrease in the ripped soil, due to better crop development and higher water consumption
Table 4
Bulk density and soil structure in the experimental ®eld with deep ripping at Sanmartin (sampled in October 1990)
Structure parameter Treatment and depth of sampling
Nonripped Ripped every 8 years Ripped every 4 years Ripped every year
Topsoil Subsoila Topsoil Subsoila Topsoil Subsoila Topsoil Subsoila
Settlement status
Bulk density (g cmÿ3) 1.43 1.59 1.47 1.50 1.48 1.50 1.48 1.56
Void ratio 0.86 0.67 0.80 0.75 0.79 0.74 0.79 0.70
Dry sieving in the field(percentage of structural elements with specific diameter, mm)
<1 16.5 29.2 17.0 18.0 10.7 21.0 12.2 14.9
1±2 12.4 16.8 11.2 18.9 14.5 18.4 12.2 20.9
2±5 16.7 19.5 20.3 28.4 21.0 27.0 23.0 29.5
5±15 26.4 19.7 23.7 21.5 27.5 22.2 25.2 21.7
15±40 13.0 7.8 10.3 6.9 15.0 8.5 16.0 7.4
40±80 2.9 2.0 2.7 2.5 2.8 2.0 3.4 4.0
>80 12.4 5.0 8.0 4.4 7.0 5.0 8.7 4.0
Mean weight diameter 35.0 52.7 35.5 43.2 30.6 46.2 31.4 40.9
Wet sieving in the laboratory
Water-stable aggregate (%) 9.0 28.8 5.3 21.9 7.5 22.0 11.3 24.0
Dispersion (%) 7.1 5.4 7.4 7.1 7.3 6.8 5.8 6.4
Structure instability 0.85 0.40 1.44 0.40 1.04 0.41 0.58 0.32
aVolume of subsoil effectively ripped.
Table 5
Water retention curve and pore-size distribution in the Sanmartin experimental ®eld (sampled in 1986)
Parameter Treatment and depth of sampling
Nonripped Ripped
Topsoil Subsoila Topsoil Subsoila
Parameters of the Van Genuchten closed-form equation
ys 0.439 0.435 0.489 0.441
yr 0.000 0.072 0.000 0.055
a 0.0418 0.0034 0.0911 0.010
n 1.211 1.638 1.206 1.997
m 0.174 0.364 0.171 0.342
Volume of pores with diameter(mm)of
<0.2 16.0 24.0 15.2 22.4
0.2±3 7.1 11.6 6.6 12.0
3±60 4.3 1.9 4.1 3.7
60±300 5.1 1.6 5.0 3.7
>300 3.1 0.6 3.3 1.3
in this soil. In an earlier paper (Canarache, 1980) the important role of the improved water balance due to ripping on crop yields was discussed in more details.
The resistance to penetration (Fig. 3) showed a decrease in the subsoil of the ripped treatments. In April, when the soil was very wet, this decrease was smaller, but present almost all-over the ripped
subsoil, while in October, under a dry soil status, this decrease was much more evident, but evident mainly in the middle part of the ripped subsoil.
Much of the differences in soil compression data are not in good agreement with earlier ®ndings, dis-cussed by Canarache (1980) and in part brie¯y presented here, which had shown that the effect of deep ripping on other soil properties, on its moisture regime and on crop yields is rapidly decreasing in a few years following ripping. There is a better agree-ment with the resistance to penetration data as an integral of all soil properties, although these ones are in part due to differences in the soil moisture content at the time when penetration was determined. More detailed and prolonged investigations seem to be needed in order to also quantify the effect of structure deterioration due to ripping on crop yield by the altered pore continuity and the theoretically improved accessibility of ion exchange sites and adsorbed exchangeable cations due to ripping affected homo-genization.
Correlations between various soil physical proper-ties and the compressibility indices are shown in
Table 6. It has to be speci®ed that the amount of data available to calculate these correlations was quite reduced: 16 for those involving bulk density (and void ratio), and eight when structure characteristics or resistance to penetration was used as the independent variable. Because of this low number of data, only ®rst degree regressions are presented, even if in some cases the spreading of points on the graph was suggesting more elaborated regression curves.
Most correlations related to precompression stress are, even with such a reduced amount of data, sig-ni®cant at the 0.95 or even at the 0.99 level. Precom-pression stress increases with increasing bulk density (and certainly with decreasing void ratio), with increasing mean weighted diameter of the soil aggre-gates, with increasing aggregate water stability, with decreasing dispersion and structure instability, and with increasing resistance to penetration (both under dry soil and wet soil conditions). Correlations with bulk density, with void ratio, and with resistance to penetration may be considered as showing a basic interrelationship between these properties, while cor-relations with the various structure indices might be
Table 6
Correlations between soil physical properties, precompression stress, and compression index
Soil property Regression equation Correlation coefficient
Correlations for precompression stress(kPa)
Bulk density (g cmÿ3) PS ÿ572418
Structure instability index PS76ÿ30:13 SII R ÿ0:59 Resistance to penetration (MPa), October PS1222:28 RP R0:79** Resistance to penetration (MPa), April PS11115:3 RP R0:95**
Correlations for compression index
Bulk density (g cmÿ3) CI ÿ0
Structure instability index CI0:234ÿ0:0001 SII R0:01 Resistance to penetration (MPa), October CI0:1570:0245 RP R0:56* Resistance to penetration (MPa), April CI0:2170:0474 RP R0:24
Correlation between precompression stress(kPa)and compression index
Compression index CI0:2170:0006 PS R ÿ0:26
due to the fact that both characteristics are ®nally determined by some other soil characteristic.
The correlations between the same soil physical properties and the compression index were all non-signi®cant which was expected, showing a lack of interrelationship between these indices. In fact the correlation between the two compressibility indices was also non-signi®cant, which can be explained by the time dependency of water release from the stressed samples which indeed acts under static measurement conditions as a stabilizer.
3.3. Crop yields
Table 7 shows the average crop yields for the entire 21 years of the experiment, thus completing earlier papers published before the end of the ®eld experi-ment, for each of the crops used. These data con®rm earlier results, but here for a much longer period of the ®eld experiment. There was a signi®cant increase in
yield with deep ripping repeated every about 4 years: 10±15% for winter wheat, and 20±30% for grain maize. As shown in earlier papers (e.g. Colibas et al., 1989), these yield premiums are sound from the economic point of view, being appreciably higher than the cost of the technique performed. Repeating deep ripping every 8 years decreased crop yield differences below the level of signi®cance. Increasing the frequency of deep ripping did not signi®cantly increase crop yield and, of course, the net pro®t was decreased or even annulled. Such high frequency of deep ripping did not decrease crop yields, thus con-®rming that repeated ripping, up to 17 rippings in 21 years in this case, did not have for the time being adverse consequences (Fig. 4).
Crop yields were signi®cantly correlated (Table 8) to bulk density (negative correlation), void ratio (posi-tive correlation), and mean weighted diameter of soil aggregates (positive correlation). Correlations with the other structure characteristics and with the
Table 7
Summary of crop yields (q haÿ1) in the ®eld experiment with repeated deep ripping at Sanmartin
Treatment Crop, number of years cropped
Winter wheat, 8 Grain maize, 11 Oats, 1 Grain clover, 1
q haÿ1 % q haÿ1 % q haÿ1 % q haÿ1 %
Nonripped 37.4 100 38.1 100 34.8 100 7.0 100
Ripped every 8 years 40.1 107 42.2 111 37.3 107 7.8 111
Ripped every 4 years 41.8 112 47.2 124 39.6 114 8.8 126
Ripped every 2 years 42.5 114 46.8 123 37.2 107 8.2 117
Ripped every year 42.7 114 49.2 129 37.5 108 7.9 113
LSD 0.05 2.96 8 5.22 14 3.50 10 1.1 16
Table 8
Correlations between soil physical properties (average for the 0±80 cm depth) and crop yield (q haÿ1, grains of standard moisture content, average of various crops)
Soil physical property Regression equation Correlation coefficient
Bulk density (g cmÿ3) Y238ÿ13:04 BD R ÿ0:95*
Void ratio Y ÿ43111:2 e R0:95*
Mean weighted diameter of structural aggregates (%) Y79ÿ0:9661 MWD R0:95 * Water stable aggregates (%) Y49ÿ0:5070 WSA R0:37
Dispersion Y43ÿ0:1671 D R0:03
Structure instability index Y46ÿ7:206 IIS R0:39 Precompression stress (kPa) Y385:834 PS R0:19
Compression index Y46ÿ18:80 CI R ÿ0:24
compressibility indices were not signi®cant. Resis-tance to penetration could not unfortunately be corre-lated to crop yields, as it was determined only in two treatments.
4. Conclusions
1. Deep ripping signi®cantly decreased the precom-pression stress, and showed some trend for increasing the compression index. However, these soil parameters did not differ consistently with ripping operations frequency or with the time elapsed since the last ripping.
2. There was a signi®cant correlation between several soil physical properties (bulk density, structure, resistance to penetration) and the precompression stress, but not a signi®cant one between these soil physical properties and the compression index.
3. Data on crop yields con®rmed earlier results, but now for a longer ®eld experiment, on feasibility of deep ripping on such soils, on the need of repeating deep ripping every 4±5 years, and showed that a high number of repeated rippings, in this case resulting from a frequency of deep ripping operations higher than actually needed,
did not induce negative changes on soil properties and on crop yields.
4. Soil bulk density and size of soil aggregates were signi®cantly correlated with crop yields, while the two compressibility indices used here did not affect directly crop yields.
Acknowledgements
The authors are highly indebted to the German Research Foundation (DFG) for the ®nancial support to the ®rst author during his stay in Kiel, without which this project could not have been carried out in 1994.
References
Canarache, A., 1978. Estimation of cropland area requiring combination of deep ripping with other improvement techni-ques. St. Sol. 4±6, 13±25 (in Romanian).
Canarache, A., 1980. Erhaltung und VerfuÈgbarkeit des Wassers in schweren BoÈden, sowie deren VeraÈnderung durch Tie¯ocker-ung. Akademnie der Deutschen Landwirtschaftswiessenschaf-ten der DDR, Tagungsbericht Nr. 180, Berlin, 1980, pp. 89±97. Casagrande, A., 1936. The determination of precompression stress and its practical signi®cance. In: Proceedings of the First Conference on Soil Mech. Found. Eng., Vol. 3, Cambridge, MA, pp. 60±66.
Colibas, I., Colibas, M., Postolache, T., Simota, C., Canarache, A., 1985. VeraÈnderung von hydrophysikalische Bodeneigenschaf-ten und der Produktion unter dem Ein¯uss der wiederholBodeneigenschaf-ten Tie¯ockerung. Tag. Ber. Akad. Landwirtsch. DDR 231, 325± 335.
Colibas, I., Stanga, N., Dumitru, E., Canarache, A., 1989. Integrated technology of deep ripping and subsoiling. Prod. Veg., Cer. Pl. Tehn. 5, 3±11 (in Romanian).
Colibas, I., Colibas, M., Canarache, A., 1994. Effects of repeated deep ripping of a surface waterlogged Luvisol. In: Proceedings of the 13th International Conference of ISTRO, Vol. 2, July 24± 29, 1994, Aalborg, Denmark, pp. 899±903.
Hartge, K.H., Horn, R., 1992. Bodenphysikalisches Praktikum, 3rd Edition. Enke Verlag, Stuttgart, 177 pp.
Horn, R., 1981. Die Bedeutung der Aggregierung von BoÈden fuÈr die mechanische Belastbarkeit in dem fuÈr Tritt relevanten Au¯astbereich und deren Auswirkungen auf physikalische BodenkenngroÈssen. Habilitationsschrift, Schriftenreihe des FB 14 TU, Vol. 10, Berlin, 200 pp.
Horn, R., 1998. Assessment, prevention and rehabilitation of soil degradation caused by compaction and surface sealing. In: Blume, et al. (Eds.), Towards Sustainable Land Use. Adv. Geoecol. 31, 527±538.
Horn, R., Baumgartl, T., 1999. Dynamic processes in structured soils. In: Sumner, M., et al. (Eds.), Handbook of Soil Science, 2000S. CRC Press, Boca Raton, pp. A19±A46. (ISBN 0-8493-3136-6).
Horn, R., Kretschmer, H., Baumgartl, T., Bohne, K., Neupert, A.,
Dexter, A.R., 1998. Soil mechanical properties of a partly-reloosened (split plough system) and a conventionally-tilled overconsolidated Gleyic Luvisol derived from glacial till. Int. Agrophys. 12, 143±154.
KeÂzdi, A., 1969. Handbuch der Bodenmechanik. VEB-Verlag, Bauwesen, Berlin, 842 pp.
Lebert, M., 1989. Beurteilung und Vorhersage der mechanischen Belastbarkeit von AckerboÈden. Diss., Univ. Bayreuth, Bayr-euther Bodenkd. Berichte, Band 12, 133 pp.
Lebert, M., Horn, R., 1991. A method to predict the mechanical strength of agricultural soils. Soil Till. Res. 19, 275±286. McNabb, D.H., Boersma, L., 1996. Non-linear model for
compres-sibility of partly saturated soils. Soil Sci. Soc. Am. J. 60, 333±341. Schulte-Karing, H., 1970. Die meliorative Bodenwirtschaftung. Dr.
WaÈhrlich, Ahrweiler, 169 pp.
Snedecor, G.W., Cochran, W.G., 1967. Statistical Methods, 6th Edition. Iowa State College Press, Ames, IO, 593 pp. Stanga, N., Colibas, I., Colibas, M., 1975. Contributions to the
technique of prevention and controlling rain-induced water-logging on clay-illuvial soils. A. ICPA 41, 311±324 (in Romanian).
Terzaghi, K., Peck, B.R., 1967. Soil Mechanics in Engineering Practice. Wiley, New York, 566 pp.
Wildman, W.E., not dated. Slide Set. University of California, Davis.
