INFLUENCE OF THE NUMBER OF PASSES ON SOIL COMPACTION – A REVIEW
Nicoleta UNGUREANU1, Valentin VLĂDUŢ2, Sorin-Ștefan BIRIȘ1, Dan CUJBESCU2, Daniel Ion VLĂDUŢ1, Neluș-Evelin GHEORGHIȚĂ1, Ioan CABA2
1Politehnica University of Bucharest, Faculty of Biotechnical Systems Engineering, Romania;
2INMA Bucharest, Romania
ABSTRACT
Compaction is the most difficult type of soil degradation and it can be regarded as a mechanical pollution arising as a consequence of poor management of agricultural practices. In mechanized agriculture, soil tillage is often carried out by heavy agricultural machinery, with repeated passes, most often on soils with high moisture content. These machines apply mechanical stresses to the soil, which are further transmitted to different depths, resulting in the artificial compaction of soil, a phenomenon that affects the capacity of plant development and hence the agricultural production. The paper presents a summary of results reported in the literature regarding the influence of the number of passes of agricultural machinery on the artificial compaction, under various conditions, in terms of the variation of cone index and of stresses transmitted into the soil.
1. INTRODUCTION
In the actual context of continuously increasing world population, to cope with the demand for more food it became necessary the intensification of farming and cropping systems. As a result, more and heavier farm machinery and animals per soil area have become common all over the world. Soil compaction is a form of physical degradation resulting in densification and distortion of the soil where biological activity, porosity and permeability are reduced, strength is increased and soil structure partially destroyed [20].
Soil compaction is not a recent phenomenon. It existed in form of hardpans long before the advent of intense agriculture [4]. Nowadays, the risk of soil compaction increased due to the dramatic increase of weight of agricultural machinery, necessary in modern agriculture, that compact the soil with each passage and reduce its production capacity. All tillage operations, starting with seedbed preparation, fertilizer and chemical applications and finally harvesting, increase the risk of degradation of agricultural soil through artificial compaction.
The potential of any soil to compact depends on its physical properties, water content and the nature of forces applied on soil surface. There can be small forces by the size of raindrops, or large forces such as those produced by tractors and agricultural machinery.
In developed countries, all agricultural soils show some degree of compaction. Recent research has shown that compaction is the most widespread type of soil physical degradation in Central and Eastern Europe. It has been estimated that nearly 40% of European soils suffer from compaction, but no precise data are available. More than a third of the soils in Europe are highly susceptible to compaction in the subsurface layers [18].
Concerns related to the environmental effects of soil compaction are increasing worldwide. Many studies have shown that compaction reduces the ability of soil to hold water and air, it prevents water from infiltrating properly in the soil, increases the risk of surface runoff, erosion and leakage of pesticides and nutrients into groundwater, increases the
emanations of exhaust gases which ultimately contribute to the global warming [9, 14]. In terms of agronomic consequences, soil compaction is a common and constant problem on most farms that till the soil, mainly because it impedes root growth, restricts root penetration into the subsoil, decreases the ability of crops to take up nutrients and water efficiently from soil, reduces crop yield potential, increases plowing resistance and fuel consumption [11, 12].
2. METHODOLOGY
Stresses applied by the agricultural machinery are transmitted to different soil depths, through the footprint between the soil and tire, resulting in artificial topsoil and /or subsoil compaction (Figure 1).
Figure 1: Basic mechanism of artificial compaction of soil
Topsoil compaction has a significant effect on crop yield and may last for some years, but it may be alleviated by tillage, drying-wetting and freeze-thaw cycles, and by the action of soil biota. Subsoil compaction (occurring below 25 cm depth) is particularly persistent and the compacted layer cannot be removed by conventional tillage [17].
Figure 2 presents some important factors that have a large influence on the artificial compaction of soil. Literature also mentions other factors, which are related to the agricultural machinery: contact pressure, tire size, speed of machinery, tire slippage and vibrations.
Figure 2: Main factors that influence the artificial compaction of soil
Moreover, the repeated treading by livestock around gateways and watering points also leads to artificial soil compaction. The stress applied by animals trampling can be great, but since the gross mass of the animals is smaller than that of the vehicles, compaction by animals is restricted only to soil surface [15].
The intensity of trafficking, also referred to as the number of passes of agricultural machinery on the soil, plays an important role in soil compaction because the deformation of soil increases with the number of passes. The first pass of a wheel does the most compaction.
Research in tilled soils has shown that approximately 75% of the increase in soil density and 90% of wheel sinking is caused during the first pass [6]. However, the effects of repeated wheeling can still be measured after several passes. Experiments conducted on mineral soils have shown that, for the same vehicle, increasing the number of passes on the same track increases the depth of stress distribution and the volume of soil affected by compaction.
Figure 3: Stress distribution in soil depth depending on the number of passes [19]
Using light tractors involves an increased number of passes on the soil, thus enhancing compaction of topsoil [7]. By using heavy agricultural machinery, the number of passes / soil area / agricultural work is reduced, due to a larger working width, which means that compacted layers of soil occur at lower depth and the degree of subsoil compaction is reduced [1]. The repeated number of wheel passes may also increase the risk of subsoil compaction.
Machinery traffic, repeated annually, may cause cumulative effects if the effects of earlier subsoil compaction have not disappeared before new trafficking [2].
3.RESULTS
Pytka (2005) showed that the most obvious deformation of the soil is caused by the first two passes of the wheeled tractor and soil deformation decreases in the following passages [6]. The findings in [3] showed that 10 passes significantly affected soil properties of the surface layer to 50 cm depth compared to the first pass and no-traffic control treatments.
Jorajuria and Draghi (2000) have determined the influence of the number of passes and the size of external load on subsoil compaction. Two tractors were used, one light and one heavy, and for each of them, the traffic intensity was varied (1, 5 and 10 passes). Tests have shown that 10 repeated passes of a light tractor, on the same track, are needed to produce the same degree of subsoil compaction as a heavy tractor in a single pass [8]. Studies conducted on clayey and organic soils have shown that penetration resistance was 22– 26% greater, the soil water contents were lower, and the soil structure more massive, in plots compacted with four passes than in the control plots [6].
Sakai et. al. (2008) investigated the effects on soil compaction of 1, 8 and 24 passes of an 8-WD forwarder, unloaded and loaded with 9520 kg of timber, equipped with low and high pressure tires, respectively with tracks. They found that compaction occurred during the first passes, and heavy compaction occurred after 8 passes. An increase in contact pressure of 100 kPa caused a decrease in soil porosity of 5.7% at 10-15 cm depth after 24 passes. Maximum increment of cone index of 85 kPa, at depths between 14 and 28 cm, meant a decrease of 1%
in soil porosity at 10-15 cm deep. Increasing the number of passes of the agricultural machinery over the soil increased its porosity and cone index, resulting in topsoil compaction and unsuitable physical soil conditions for seed emergence [16].
Figure 4: Maximum increment of cone index with the number of passes [16]
Botta et. al. (2009) investigated the interaction of cone index and rut depth induced by traffic of two different weight tractors in two tillage regimes: soil with 10 years under direct sowing system and soil worked in conventional tillage system. Treatments included five different traffic frequencies (0, 1, 3, 5 and 10 passes on the same track).
Figure 5: Variation of cone index with the number of passes, in conventional tillage (CT) and direct sowing (DS) [5]
In the topsoil, up to 5 passes of the heavy and light tractors, as in 1 and 3 passes, the cone index responded to the ground pressure being higher for the light tractor. Until the 5th pass, the light tractor caused in both soils higher values in cone index than the heavy tractor.
Between 200–600 mm, compaction increased with the number of passes. For 200–400 mm and 400–600 mm depth, there is a good correlation between tractor passes and cone index in
both tillage regimes. The heavy tractor produced higher cone index than the light tractor.
From 200 mm depth, the higher values for cone index were for the heavy tractor. Highest values of cone index were measured between 400–600 mm for all treatments in both soils conditions. So, below 200 mm depth, tractor load was responsible for subsoil compaction [5].
Over 30% of soil area is trafficked by heavy machinery with tires, even in no-till system (one pass at sowing). Under minimum tillage (2-3 passes) the percentage exceeds 60% and in conventional cropping (multiple passes) it exceeds 100% during one cropping cycle [10].
In another research study [13], stress state transducers were installed in the soil to depths of 15 and 30 cm, to determine the distribution of stress in the soil at the multiple passes of an agricultural tractor of 29 kW. The total mass of the tractor was 2480 kg, with 850 kg load on the front wheel and 1630 kg load on the rear wheel. Two soils were tested: a sandy soil with bulk density of 1.70 g/cm3 and a loess soil with bulk density of 1.63 g/cm3. Figure 6 shows the effect of multiple passes on the same track, on stress distribution in the soil.
Figure 6: The effect of the number of passes over stresses in the soil [13]
The first 2-3 passes had the greatest importance and the increase in peak stress was significantly higher between the first and second pass, than between the second and third pass.
On subsequent passes, the stress increased very slightly. Stresses recorded after the fourth and fifth passes were lower than those measured in the second and third pass, and this may be due to soil consolidation after several passes on the same track [13].
4. CONCLUSIONS
The artificial compaction of soil is due to the technological errors in agriculture, such as excessive traffic on wet soils during the agricultural works. Compaction increases with increasing mass of agricultural machinery, tire inflation pressure, intensity and frequency of
Soil compaction can lead to various undesirable changes in soil properties (cone index, bulk density, porosity, etc.), with significant environmental and agronomic consequences.
Those changes are greatly intensified by the repeated passes during agricultural works.
Most compaction occurs after the first pass of wheeled machinery, and this is obvious both by the changes in soil physical properties and the distribution of stresses in soil depth, but the effects of repeated wheeling can still be measured after several passes.
Acknowledgement
This work has been funded by the Ministry of National Education and Research, through the UEFISCDI, within the project entitled„Conservative tillage technology", contr. 181/2014.
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