ANALYSIS OF THE RANGE OF CHANGES OF SOME MECHANICAL AND TRACTIVE PROPERTIES IN SELECTED TILLAGE TECHNOLOGIES

Włodzimierz Białczyk, Anna Cudzik, Jarosław Czarnecki, Krzysztof Pieczarka
Institute of Agricultural Engineering, Wrocław University of Environmental and Life Sciences, Poland

ABSTRACT

The paper presents results of studies on some mechanical and tractive properties of the soil on the basis on which an analysis was made of the range of their changeability. The plants on which the studies based were peas, winter triticale, sugar beet and winter wheat cultivated by means of traditional methods and a simplified method, which is without ploughing. Comparable values of the yields were found out. Higher values of soil compaction and the maximum contact stress occur in the soil cultivated in a simplified method. Fluctuations of these parameters were observed during the period of vegetation and these were gradually leveled during the harvest. However, it was related to the course of the weather, and especially the soil humidity. The studies showed a relation between the examined technologies of soil tillage and the values of the obtained tractive forces.

Key words: tractive force, compaction, maximum contact stress, soil tillage technologies.

INTRODUCTION

Modern agriculture more and more frequently uses tillage simplifications, consisting in replacing ploughing with the treatments that consume less energy. One of the causes of these efforts is a systematic increase of the demand for the energy of bigger and more efficient machine aggregates. The profitability of simplifications can be considerable since compared to the traditional tillage the expenditure of energy, manpower and time is much smaller. However, the basic reason is seen in ecological factors creating a necessity of looking for solutions preventing degradation, excessive pressures and erosion of the soil [3, 4, 7, 9]. Traditional methods are connected with the fact that a machine has to pass across the field a number of times. The area of the pressure made by machine aggregates is made bigger, which has a negative effect on the soil, which is manifested in deep grooves and changes of the physical, mechanical and tractive properties of the soil [5, 6]. In order to prevent those negative effects, certain tillage treatments are given up, no plough technologies of tillage are introduced and the treatments scarifying the soil before the sowing are even completely eliminated. The shortening of the time of performing particular types of tillage is also of importance since in this way the seed sowing can take place at strictly definite dates [1].

The studies conducted so fat show that the reaction of a plant to simplifications used in tillage technologies, including direct sowing, are much dependent on the plant species. Cases were observed of both a yield drop and no significant differences in yielding. There are no explicit conclusions about it. Accidentally, worsened biological activity, increased density, porosity and compaction of the soil cultivated by means of simplified systems are observed, and on the other hand, an increase of the content of nutritious elements is emphasized. However, there is no doubt that the use of tillage simplifications makes it possible to reduce the costs laid on the soil cultivation even by 60-70%. An additional advantage is a decrease of the number of tillage treatments, decreased soil pressures resulting from the smaller number of times a tractor passes across the field. All this makes it possible to say that the advantages resulting from the use of tillage simplifications can be significant. It is, however, important that there is a lack of univocal conclusions and recommendations concerning the conditions of their use [2, 8].

The above considerations became the basis to undertake studies on simplified technologies of soil tillage, whose main aim was to show in what way the soil reacts to tillage simplifications. It was assumed that such estimation will be made on the basis of studies conducted in the course of vegetation of different cultivated species, which are characterized by a different root system. The detailed purposes realized during the studies included:

1. A comparison of selected technologies of soil tillage in respect of the obtained yields.

2. The analysis of changes of mechanical parameters such as maximum contact stress and soil compaction

3. Recognition of tractive properties on the basis of the analysis of maximum tractive forces.

MATERIALS AND METHODS

The studies were conducted in the experimental fields of the Agricultural Experimental Station Swojec of the Agricultural University in Wrocław. They were a part of the experiment done in the years 1998-2002, the aim of which was to recognize the effect of soil tillage technologies on the conditions of the growth and yielding of plants (table 1). Plants cultivated in the course of the studies included peas ‘Piast’ cv., winter triticale ‘Fidelio’ cv., sugar beet ‘Jupiter’ cv. and spring wheat ‘Elta’ cv. The plants were cultivated in the traditional and the simplified ways. The traditional tillage covered the full range of tillage treatments. In the simplified tillage, skimming was given up but spraying was performed, and next the sowing of mustard, spring rolling and, immediately before the sowing of peas, harrowing with a heavy harrow took place. In the simplified tillage of winter triticale and winter wheat, spraying with herbicide “Roundup”, and next harrowing and sowing were performed.

Table 1. The scheme of the field experiment

Year of experiment	Field
	I	II	III
1998/1999	winter triticale	sugar beet	spring wheat
1999/2000	sugar beet	spring wheat	peas
2000/2001	spring wheat	pea	winter triticale
2001/2002	peas	winter triticale	-

The plants presented in table 1 differ with their root systems, which can cause changes in soil compaction. The applied crop rotation made it possible to show both the effect of the soil tillage technology on the yielding of plants and the effect of plants with different botanical properties (cereal crops, pulse plants and root crops) on changes of the mechanical parameters of the soil.

Studies on recognition of the changeability of selected physical and resistance parameters of the soil were divided into three stages. Table 1 presents the scheme of a field experiment, which was the basis for the realization of all the three stages. Stage one comprises initial studies conducted in the years 1999-2000, the aim of which was to find out the existing dependencies. Stage two, which included the proper studies, was realized in the years 2001-2002. The essence of stage three consisted in tractive measurements in the year 2002, according to the scheme in table 2.

Table 2. Dates of tractive studies and the condition of the field in the course of the studies

No. of measurement	Date of measurement		Tillage
	triticale	peas	traditional	simplified
I	25 VII	25 VII	stubble	stubble
II	25 VII	25 VII	skimming	rotator harrow
III	19 VIII	17 IX	plough	rotator harrow
IV	19 VIII	17 IX	plough rolled once	rotator harrow rolled once
V	19 VIII	17 IX	plough rolled twice	rotator harrow rolled twice

The soil, where the studies were conducted was alluvial soil proper formed from loamy sand, valuation class IVa. The soil moisture was established using a drier method, with the use of a scale-drier WPE – 300S, by means of which the weight moisture of the examined samples was determined. The soil for analysis was taken from the upper soil profile (depth about 0.02-0.07 m) on the day when the measurements of the mechanical and tractive properties were taken. The value of the soil moisture was calculated as the arithmetic means from five samples taken in the site where the measurements of compaction were performed. The total porosity of the soil was also established from the relation (1)

(1)

where: P_o – total porosity, %; γ – soil density proper, g·cm-3; γ_o – real volume density, g·cm^-3

In order to measure the contact stresses, a rotator cutter VANE H-60 by Eijkelkamp company was used. The measurement range of the cutter is 0-260 kPa, with the error of the measurement of 2 kPa. The soil compaction was established indirectly from the penetration resistance measured by means of a conic penetrometer with electronic recording of the force rresistance of soil penetration and the size of the measuring cone (fig. 1).

Figure 1. The scheme of compaction measurer: 1 – tensometric rectifier of power, 2 – the system measuring penetration depth (linear potentiometer), 3 – measuring cone, 4 – power feed, 5 – data agent

The studies made use of a cone with the vertical angle of 30° and the area of the base of 0.0001 m². The penetrometer was driven by an electric engine with big excess of power, which made it possible to maintain the permanent velocity of immersion of the cone during the measurement. It was 0.03 ms^-1. A device for transformation and registration of analog signals (data agent), with six measurement channels and the sampling frequency of 1000 Hz per channel was used in order to simultaneously register the resistance value of penetration and immersion. The measurement of the power of the soil penetration resistance was made by means of a rectifier of power type 3089, with the class of precision 0.1% and the measurement range from 0 to 1 kN. The effect of each measurement was the course of the penetration resistance power in the function of the immersion of the cone of penetrometer. The soil compaction was calculated from the obtained value of penetration resistance on the basis of the relation

(2)

where: Z – soil compaction, Pa; P – penetration resistance, N; S – area of the cone, m².

A stand for the tractive measurement of tyres of micro-tractors in field conditions was used to measure the tractive properties of the examined sub-soil [1]. The construction of this stand is presented in figure 2. The examined wheel was driven by the outer hydraulics of the tractor, thanks to which mobility of the measurement stand was assured and its independence from the outside sources of energy.

Figure 2. The scheme of a stand for tractive measurements of tyres of micro-tractors: 1 – tensometric rectifier of power, 2 – hydraulic servo-motor, 3 – linear potentiometer, 4 – momentometer, 5 – tested wheel, 6 – a system imitating the real driving system of the micro-tractor, 7 – data agent

A wheel bevameter was used in the stand, and its construction made it possible to change the vertical loading with the aim of obtaining different pressures of the wheel on the soil. The drive of the wheel was realized through a hydraulic system with a servo-motor working both sides, which ensured permanent angular velocity and excess of power necessary to make measurements on compact sub-soils and under maximum loading. The applied servo-motor allowed the turn of the wheel by the angle of about ±/4 radians, which was enough to achieve full cutting of the sub-soil.

The studies used a tyre with 4.50–10, whose maximum carrying capacity was 2400 N. The value of horizontal deformation of sub-soil was calculated from the ratio of the static radius of the wheel and the angle of its turn. This measurement was made possible by the use of a revolving potentiometer with linearity of ą25%. A tensometric rectifier of power with the measurement range up to 1 kN and precision 1 N was used to measure the tractive force. A momentometer was installed between the rectifier enforcing the rotary movement of the wheel and the bevameter and thanks to it the measurement of the rotary moment carried to the system was measured. Recording of the studied parameters was made possible thanks to a recorder thoroughly discussed together with the measurements of the soil penetration resistance.

The obtained values were statistically analyzed by means of program Statistica 6.0.Analayzing the results of maximum contact stresses and the soil compaction, a multi-factor analysis of variance and a test of homogenous groups NIR of Fisher were conducted.

RESULTS

A comparison of selected soil tillage technologies was begun with the analysis of the obtained yields. Table 3 contains the yields of the cultivated plants for traditional and simplified tillage in particular years.

Table 3. Yield of cultivated plants in the studied soil tillage technologies

Plant	Year of experiment
	1999		2000		2001		2002
	T	U	T	U	T	U	T	U
Winter triticale	4.0	3.8 -5%	2.9	1.9 -34%	5.3	4.8 -9.4%	5.8	5.4 -6.9%
Sugar beet	60.6	58.2 -4%	48.1	53.7 +11.6%	X	X	X	X
Spring wheat	6.4	6.3 -1.6%	X	X	6.7	6.5 -3%	X	X
Peas	X	X	3.1	3.1 0%	2.4	2.8 +16.7%	1.8*	4.6 +55.6%

* – cultivation damaged by animals, T – traditional tillage, U – simplified tillage % – percentage difference in relation to traditional tillage, X – no tillage

It has to be stated that differences in the yields obtained in the studied technologies of tillage are only slight and generally they do not exceed the level of 10%. Cereal crops reacted to tillage simplification by a yield drop in each of the cases. However, a decisive decrease of the yield by 34% was observed only for the simplified tillage of winter triticale. In the simplified cultivation of peas and sugar beet both a yield drop and its increase was found out. Therefore, it cannot be explicitly stated which of the analyzed soil tillage systems guarantees higher yields. A basic question appears of which factors caused the above-mentioned changes and if these can be explicitly defined. Perhaps the answer to these questions will be obtained on the basis of an analysis of certain physical and mechanical parameters of the examined soils.

Table 4 presents the conditions of soil moisture, and table 5 shows the values of total porosity as yield-forming elements. The values of soil moisture were usually higher for the soil cultivated in the simplified system. In the simplified cultivation of peas a drop of the soil moisture below the level of the value achieved in the traditional tillage was found only at the last date of the measurement after the harvest of this plant. For the soil where winter triticale was cultivated, higher values of soil moisture for the traditional tillage were obtained in June and July, which was directly connected with the course of the weather in that period.

Table 4. Values of soil moisture in [%] determined for the examined fields

	Year of studies	Month	Spring wheat		Sugar beet		Winter triticale		Peas
			T	U	T	U	T	U	T	U
Initial studies	1999	IV	13.2	15.1	12.5	14.2	13.8	15.7	X	X
		VIII	10.5	11.2	12.3	14.1	11.2	12.3	X	X
	2000	III	13.4	14.1	13.5	12.5	X	X	12.6	13.6
		IV	9.2	12.1	9.3	12.9	X	X	9.1	11.7
		VIII	11.5	12.8	12.5	9.8	X	X	10.7	11.4
Principal studies	2001	IV	15.8	13.4	X	X	14.7	16.7	15.0	17.4
		V	6.6	7.2	X	X	12.5	9.1	11.9	12.4
		VI	13.1	15.5	X	X	15.3	16.0	14.0	15.6
		VII	15.3	15.1	X	X	17.2	15.6	14.7	14.1
	2002	IV	X	X	X	X	13.9	16.7	13,8	16.9
		V	X	X	X	X	8.2	7.8	7.4	7.6
		VI	X	X	X	X	10.9	13.1	8.6	9.1
		VII	X	X	X	X	7.1	8.7	8.3	8.1

X – the plant was not cultivated, no parameter

The values of the volume density of the soil, in the case of peas, were higher in the simplified technology of tillage, with an exception of the values measured after the harvest of the plant. For the soil in the cultivation of triticale, higher values of density were achieved at the beginning of the vegetation period, which was probably affected by an earlier date of sowing, that is a longer time when the soil could get densified.

Table 5. The values of total porosity in [%] measured for the examined fields

	Year of studies	Month	Spring wheat		Sugar beet		Winter triticale		Peas
			T	U	T	U	T	U	T	U
Initial studies	1999	IV	13.2	15.1	12.5	14.2	13.8	15.7	X	X
		VIII	10.5	11.2	12.3	14.1	11.2	12.3	X	X
	2000	III	13.4	14.1	13.5	12.5	X	X	12.6	13.6
		IV	9.2	12.1	9.3	12.9	X	X	9.1	11.7
		VIII	11.5	12.8	12.5	9.8	X	X	10.7	11.4
Principal studies	2001	IV	15.8	13.4	X	X	14.7	16.7	15.0	17.4
		V	6.6	7.2	X	X	12.5	9.1	11.9	12.4
		VI	13.1	15.5	X	X	15.3	16.0	14.0	15.6
		VII	15.3	15.1	X	X	17.2	15.6	14.7	17.0
	2002	IV	X	X	X	X	13.9	16.7	13.8	16.9
		V	X	X	X	X	8.2	7.8	7.4	7.6
		VI	X	X	X	X	10.9	13.1	8.6	9.1
		VII	X	X	X	X	7.1	8.7	8.3	8.1

X – the plant was not cultivated, no parameter

Total porosity of the soil was higher in the cultivation of peas, both for the traditional and simplified tillage technologies. In the case of the simplified technology, the soil at the end of vegetation achieved considerable porosity, compared with that of the traditional tillage. It is probably connected with slower distribution of organic substances and the development of microorganisms in the soil which make the latter much looser.

Measurements of the maximum contact stresses were made simultaneously with the measurements of physical parameters of the soil. The first measurements were performed in March of 2000 in the cultivations of sugar beet and spring wheat, which are plants of different soil requirements and differentiated botanic properties (fig. 3). In the case of sugar beet, the greatest changes in the obtained values of stresses were observed in March on all levels of the soil profile. An increase of as much as 276.5% of the stresses was found in the surface layer of the soil cultivated with simplifications as compared to the traditional tillage. Low values of this parameter in the traditional tillage at the beginning of the vegetation season was a result of tillage treatments, the effect of frost and big, 13.5% moisture of the soil. The measurements performed in August showed considerably smaller differences of this parameter in relation to the March measurements. Again, the highest value of stresses was observed at the depth of 0.05 m in the simplified tillage; however, it was only a 44.2% increase. In the deeper layers of the soil the studies observed balanced stressed as far as the depth of 0.15 m, where the differences were insignificant. Sugar beet is a plant that densifies the soil considerably through the intensive increase of contact stresses in the soil.

Figure 3. Values of maximum contact stresses obtained in the cultivation of sugar beet and spring wheat in 2000

Wheat is a plant that, through its bundle root system, should affect gradual loosening of the soil. The measurements confirmed the relations. The greatest differences in the maxim values of contact stresses occurred, like in the case of sugar beet in March, at the depth of 0.05 m. In the simplified tillage the studies observed an increase of as much as 590.1% as compared to the traditional tillage. However, the values of this parameter in the traditional cultivation of wheat were comparable to the traditional cultivation of beet. Much greater compaction of the soil occurred only in the simplified cultivation of wheat, where sugar beet was the fore-crop. Probably, passages of the machines during the harvest of beet caused considerable densification of the sub-soil, and it could not be reduced by means of tillage treatments.

No significant changes of this parameter at particular dates of measurements were noted in the simplified cultivation of wheat. The values of contact stresses remained at a comparable level throughout the vegetation period of wheat. The studies did not also observe any dynamic leveling of this parameter between the applied manners of wheat cultivation. In this case the loosening effect of wheat roots was marked. The smallest differences in the measured values were higher by 33.3% in relation to the traditional tillage.

Figure 4. Values of compaction obtained in the cultivation of sugar beet and spring wheat in 2000

Measurements of penetration resistance of the soil were conducted at the same dates as the measurements of the maximum contact stresses. They gave the basis to calculate the soil compaction (fig.4). The first measurements made in March before beet sowing showed that the soil in the traditional tillage had the compaction of about 297.8% smaller on average as compared to the simplified tillage, which was the result of earlier autumn tillage of the soil and manure fertilization. The measurements carried out in April showed gradual leveling of compaction in the analyzed manners of tillage, and the calculated difference did not exceed 20%. However, it was found out that the soil in the traditional tillage was characterized by greater compaction in the surface layers, i.e. 0.05 m and 0.1 m. Probably the tillage treatments contributed to gradual damage of the soil structure, which in contact with atmospheric factors (big rainfalls and high temperature) underwent greater compaction. This hypothesis was confirmed by August measurements, which showed the compaction in the traditional tillage higher by more than 40% as compared to the simplified tillage in the whole studied soil profile.

Initial studies conducted in 2000 showed significant differences of the analyzed parameters in the studied types of cultivation. The measured parameters were also significantly different at various depths of the soil profile, which testifies to different manners of the formation of soil endurance, depending on the ways of its tillage.

The experiment was repeated in 2001 in a greater number of measurement cycles. Like in the initial studies, plants of different botanic properties were analyzed. Those were peas and winter triticale. The statistical analysis found out a significant effect of the soil tillage, date of studies and the depth of the measurement of the maximum contact stresses and soil compaction. The first measurements were performed after peas sowing in April (fig. 5). The pre-sowing treatments affected considerable loosening of the soil in the traditional cultivation of peas, which was especially visible at the depth of 0.05 m. The measured values were lower by 163% in comparison to the values obtained in the simplified tillage. However, a proceeding process of soil settlement was observed, which resulted in a gradual increase of the values of stresses. Increasing soil densification was especially visible at the successive dates of measurements. The last measurements performed in August showed the smallest values of contact stresses at the depth of 0.15 m and they were 104 kPa, being lower than the ones that were measured in the simplified tillage only by 10 kPa.

The values of this parameter were different in the simplified cultivation of peas. Little changeability of stresses was noted at the greatest measurement depth of 0.15 m. The values measured here were the highest but they did not clearly increase at the successive dates of measurements. This is probably also connected with the structure-forming effect of the cultivated plant, which in the soil with no tillage contributed to the stabilization of the soil’s structure.

Figure 5. Values of maximum contact stresses obtained in the cultivation of peas and winter triticale in 2001

In the next stage of principal studies measurements of soil penetration resistance were performed. The values of soil compaction (fig.6) calculated on this basis correspond to the presented values of maximum contact stresses. The statistical analysis showed a significant effect of the technology of soil tillage and the depth where the measurements were made.

Figure 6. Values of compaction obtained in the cultivation of peas and winter triticale in 2001

The studies observed increased soil densification together with the depth of the measurements in both the traditional and simplified manners of tillage. This process proceeded in a similar way in the cultivation of peas and triticale. Because of these respects, no significant effect of the plant on changes of this parameter was shown. The highest values of compaction were found out at the last date of measurements in the traditional and simplified cultivation of peas and they were, respectively, 3.3 MPa and 3.9 MPa. Contrary to the cultivation of triticale, peas in the last period of vegetation reveals considerable areas of the soil, in this way contributing to its excessive drying out. A decrease of moisture and an increase of mechanical resistance take place, which was reflected in the values of compaction. The highest compaction in the traditional and simplified cultivation of triticale was observed in May at the depth of 0.15 m, respectively, 2.5 MPa and 3.1 MPa, which was directly connected with few rainfalls.

The next stage of the studies was an analysis of tractive properties of the drive-wheel cooperating with the soil in the studied cultivations. The maximum tractive force was taken into consideration for different values of vertical stress. Tractive tests were conducted at the last stage of the plant’s development. The soil in the traditional and simplified manners of tillage was characterized by comparable mechanical resistance. Due to this, no effect of the applied technology of soil tillage was observed on the value of the generated tractive forces. On the other hand, a significant effect of the cultivated plant was noted, which correlates with the above-mentioned parameters of resistance. The effect of the vertilcal loading of the wheel was also significant; however, the greatest increase of tractive forces was observed for the highest loading.

The first to consider were the maximum forces generated by the wheel in the field where winter triticale was cultivated. The obtained results of measurements are presented in figure 7.

Figure 7. Values of maximum tractive forces obtained in the cultivation of triticale

It can be noticed on the basis of the presented data that simplified cultivation of triticale allows to generate higher values of tractive forces, which is especially well visible for higher vertical loading. The use of plough and rotary harrow affects an increase of tractive forces due to a greater area of contact with the sub-soil. On the other hand, conditions of cooperation between the tyre and the sub-soil get worse, especially as far as horizontal deformation is concerned. The highest values of tractive force (425 N) in the traditional tillage was obtained on the soil after performing a plough for the vertical loading of 780 N. In the case of simplified tillage, this value was 560.2 N and it was measured after harrowing by means of a rotary harrow for the loading of 910 N. On the other hand, in the case of measurements performed on the stubble the studies found out no significant changes of tractive forces together with increased loading.

Figure 8. Values of maximum tractive forces obtained in the cultivation of peas

The cultivation of peas (fig. 8) was characterized by different courses of tractive forces. The studies noted higher tractive forces generated on the soil in the traditional tillage as compared to the simplified tillage. In the traditional tillage tractive forces measured on the soil after a plough and on the sub-soil after plough rolled once were significantly higher than the others. The highest maximum tractive force was measured after the plough was rolled once and it was 673.8 N for the loading of the wheel of 780 N. The simplified cultivation of peas is characterized, like the cultivation of winter triticale, by small changes of the measured forces, both for increased loading of the wheel and for the sub-soils after different tillage treatments. The established values of tractive forces are similar to those obtained for the soil in the simplified cultivation of triticale. The maximum value of the force (461.5 N) was obtained on the soil after using a rotary harrow, for the loading of 910 N.

Determination of the optimum loading of the wheel in the cultivation of peas is much more difficult than it was the case for triticale. Changes of the maximum tractive forces together with increased loading are only slight. The studies showed that the greatest tractive forces are obtained in the majority of cases on the soil in the simplified tillage. They depend to a large extent on the condition of the field after a fore-crop as well as the mass and distribution of roots and after-crop residue in the soil profile. Higher tractive forces in the traditional tillage are obtained for peas. In the case of simplified tillage those forces were greater in the cultivation of winter triticale. It was also found out that the cultivated plant significantly influenced the obtained tractive forces both in the traditional and simplified manners of tillage.

CONCLUSIONS

The studies showed that in the examined conditions there is no distinct correlation between the obtained yields and the values of the measured parameters. Contrary to common belief, simplifications in tillage do not have to be associated with a drop of yielding.

The studies found out that the highest values of maximum contact stresses occur in the soil cultivated in a simplified system. It was observed that differences between the values in different technologies of tillage are mostly the highest at the beginning of the vegetation period. In the course of the growth and development of plants, these differences gradually decrease until they are leveled out.

The soil subjected to tillage simplifications is characterized by higher initial compaction. During the vegetation period temporary changes of compaction connected with the course of the weather take place. Increased moisture of the soil led to decreased compaction. Tractors and agricultural machines driving in the field, which is typical of the traditional tillage, often lead to considerable densification of the soil and increased compaction in the traditional tillage, which is especially well visible in the final period of plants’ development.

Simplified technologies of tillage provide a possibility of generating greater tractive forces; however, it is strictly connected with the cultivated plant. It should be also supposed that higher compaction of the soil in this type of tillage has a positive influence on the efficiency of conveying the drive.

REFERENCES

1. Białczyk W., Czarnecki J., Kordas L., Pieczarka K., 2000. Zmiany niektórych właœciwoœci fizycznych i mechanicznych gleby w różnych technologiach uprawy [Changes in physical and mechanical properties of soil in different technologies of tillage]. Inżynieria Roln. 6, 47-53 [in Polish].

2. Blecharczyk A., Skrzypczak G., Małecka I., Piechota T., 1999. Wpływ zróżnicowanej technologii uprawy roli na właœciwoœci fizyczne gleby oraz plonowanie pszenicy ozimej i grochu [Effect of differentiated soil tillage on physical soil properties and yield of winter wheat and pea]. Zesz. Nauk. AR w Krakowie (Fol. Univ. Agric. Stetin) 195 (74), 171-179 [in Polish].

3. Block W., Johson C., 1994. Soil stress measurement under rigid wheel loading. Transactions of the ASAE, 37(6), 1753-1756.

4. Brunotte J., 1999. Trends der Bodenbearbeitung. Landtechnik 54/6, 362-363.

5. Godbole R., Alcock R., 1993. The prediction of tractive performance on soil compaction. Transactions of the ASAE 38(4), 1011-1016.

6. Jun H., Kishimoto T., 1998. Three-directional contact stress distributions for a pneumatic tractor tire in soft soil. Transactions of the ASAE 41(5), 1237-1242.

7. Marsili A., 1998. Changes of some physical properties of a clay soil following passage of rubber- and metal-tracked tractors. Soil & Tillage Research 49, 185-199.

8. Runowska-Hryńczuk B., Hryńczuk B., Weber R., 1999. Aktywnoœć biologiczna gleby w różnych systemach uprawy roli [Biological Activity of soil in different tillage systems]. Zesz. Nauk. AR w Krakowie (Fol. Univ. Agric. Stetin) 195 (74), 59-63 [in Polish].

9. Weber R., Hryńczuk B., Biskupski A., Włodek S., 2000. Zmiennoœć zwięzłoœci gęstoœci i wilgotnoœci gleby w zależnoœci od technik uprawy roli [Variability of compaction, density and moisture of soil as depending on the tillage technique]. Inżynieria Roln. 6, 319-325 [in Polish].

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Włodzimierz Białczyk
Institute of Agricultural Engineering,
Wrocław University of Environmental and Life Sciences, Poland
37/41 Chełmońskiego Street, 51-630 Wrocław, Poland
phone: (+48) 71 320 57 06
fax: (+48) 71 348 24 86
email: wlodzimierz.bialczyk@up.wroc.pl

Anna Cudzik
Institute of Agricultural Engineering,
Wrocław University of Environmental and Life Sciences, Poland
37/41 Chełmońskiego Street, 51-630 Wrocław, Poland
phone: (+48) 71 320 57 28
fax: (+48) 71 348 24 86
email: anna.cudzik@up.wroc.pl

Jarosław Czarnecki
Institute of Agricultural Engineering,
Wrocław University of Environmental and Life Sciences, Poland
37/41 Chełmońskiego Street, 51-630 Wrocław, Poland
phone: (+48) 71 320 57 26
fax: (+48) 71 348 24 86
email: jaroslaw.czarnecki@up.wroc.pl

Krzysztof Pieczarka
Institute of Agricultural Engineering,
Wrocław University of Environmental and Life Sciences, Poland
37/41 Chełmonskiego Street, 51-630 Wrocław, Poland
phone/fax: (+48) 71 320-57-26
email: krzysztof.pieczarka@up.wroc.pl

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