Compaction can be defined as the response of a soil to external forces and implies a decrease in its volume (or an increase in its density). The extent of the compaction depends on both the soil and the forces applied. Several soil parameters can be used to characterize soil compaction, such as bulk density, void ratio, specific volume, or bulk weight volume . The compactability of the soil (i.e., its response to compaction forces) depends on soil type, moisture content, and initial state of compaction.
The resistence of each soil to compaction (soil strength) decreases rapidly when soil moisture content increases. Soil strength generally increases with depth so that the strength of the subsoil is generally higher than that of the topsoil, which may be loose and soft under certain operative conditions and therefore more compactable. A moderate recompaction of a plowed soil may be favorable to plant growth, but intensive traffic frequently causes excessive compaction. The tilled layer often undergoes an annual cycle of loosening by tillage and recompaction by natural processes and machinery traffic [3-6].
The extent of recompaction by machinery traffic depends on many factors. It tends to increase with the soil moisture content, the wheel load, the inflation pressure of tires, the number of machinery passes, the wheel slip, and the velocity of the vehicle [6-8].
As previously stated, soil compaction for a specific site can be characterized by the bulk density or by the porosity of the soil. However, it is extremely difficult to identify all of the effects of soil compaction on crop growth and yield. So far, the experimental measurement of crop response in field experiments is probably the only way to assess the integrated effect of soil compaction on crop yield. Generally speaking, yields decrease in overly dense soils, probably as a consequence of poor aereation and mechanical impedence to root growth. In overly loose soils, the unsaturated hydraulic conductivity, the volumetric water content, and the concentration of nutrients are often lower. These conditions may restrict water and nutrient transport to roots.
Other soil properties besides bulk density or directly related quantities also have been used for characterizing soil compaction. Among these, penetration resistence is the most common. Provided that soil water content is uniform, penetrometer resistance may be a more sensitive parameter than bulk density . Nevertheless, this parameter has the disadvantage of having high variability and is unsuitable for use in stony soils.
A more universal method for measuring soil compaction was devised by Eriksson et al.  and Hakansson , by normalizing the bulk density values so that they are comparable in their effect on crop production. The maximum bulk density obtained in the laboratory using a long-term, confined, uniaxial compression test and a static pressure of 200 kPa was established as a reference point; the bulk density of the same soil in the field, expressed as a percentage of the reference maximum bulk density, was adopted as the degree of compaction. This parameter has been shown to have biological significance in experiments carried out by Hakansson , which indicated that maximum crop yield can be attained at the same degree of compactness irrespectively of soil type.
Scientists, technicians, and farmers are much more concerned about soil compaction today than in the past, because of the increasing mechanization of crop production in most parts of the world. This trend has enhanced the risks of soil physical degradation when wheels pass over soil used as growing medium for crops [10-12]. Nowadays, farming almost invariably involves the passage of wheeled and/or tracked vehicles for primary and secondary cultivation, sowing, spraying, and harvesting operations. However, soils that have been cultivated for a long time often will not conserve the strength to support the most modern vehicles without considerable compression and rutting. Continuously tilled soils that are less suited to crop growth often need corrective treatments between crops, such as more intensive or deeper primary tillage operations, thus increasing the costs of production. In addition, such operations are rarely completely effective. Compaction is becoming a very serious soil degradation problem where heavy wheeled vehicles are used in crop production, with the possible exception of arid nonirrigated areas. As a result, it has become increasingly necessary to relate the compaction produced by agricultural vehicles to the complete system of soil management.
Crop rotation also may be important in relation to compaction under vehicles. Garwood et al.  found that a free-draining sandy-loam soil was considerably more compact following 20 years of arable cropping then after a similar period of grass pasture (Fig. 4.9).
Wheeled vehicles are becoming larger and more powerful. McKibben  reported that the average mass of tractors increased from 2.71 in 1948 to 4.5 t in 1968. Since then, the average power has increased at a rate of about 5% to 7% per annum. Vehicles used for transporting and spreading lime and slurry now have a mass that sometimes exceds 201 . Furthermore, the development of combine machinery has increased the risk of soil compaction in the actual crop management systems.
Traditionally arable soils pass through an annual cycle of loosening and compaction. Loosening generally is obtained by primary tillage operations or by subsoiling, whereas compaction may occur at several stages of the crop cycle, for instance at seed-bed preparation or at harvesting. In relation to seed-bed preparation, the soil density at the time of sowing is often as high as it is prior to plowing . The mechanism of compaction that occurs during seed-bed preparation is related to the passage of both the implement
Dry bulk density , kg jm1 1000 1200 1400 1600 1800 2000
2 SO 300
Figure 4.9. Soil compaction resulting from two different crop rotation systems. Source: .
itself and the tractor tires. The ability of a soil to support such traffic without structural damage beyond the limits for good crop growth has been used as a definition of the "trafficability" of an agricultural soil .
Subsoil compaction represents a very serious problem in that it is much more permanent and recovery from it is more difficult, and its negative influence on soil hydrology, microbial activity, and root development may be very consistent, especially for deep-rooting crops.
It has been demonstrated that subsoil compaction under a tractor tire is related to the bulk density of the subsoil at the moment of the passage and to the width of the tire . In practical situations, however, the above postulates will be dependent on the relative compactability of the surface soil and of the subsoil.
Under continuous zero-tillage systems, the cycle of compaction and loosening, typical for arable soils subject to annual tillage operations, does not occur. Similarly, in continuous direct drilling systems, there is not evidence of a progressive increase in bulk density or of a variation in relation to soil type . Soils subject to continuous direct drilling for more than two years are likely to become compacted but may acquire sufficient strength to carry normal agricultural traffic without any further compaction.
Dry bulk density , kg jm1 1000 1200 1400 1600 1800 2000
I'll l 1 1 l ' ' I ' 1 I ' 1 ■ 1 i 1 ■ i i 7
soil depth (cm)
■ plowed soil on tractor tracks; +plowed soil between tractor tracks • minimum tillage on tractor tracks; t minimum tillage between tractor tracks
Figure 4.10. Soil compaction as a result of different management systems. Source: .
However, if during harvesting or other mechanical operations the soil is moist, rutting may occur and this may be particularly deleterious if the subsequent crop is still to be planted by direct drilling. Moreover, the decrease of both macropores and permeability below machinery wheel tracks produces much less favorable conditions for crop establishment than wheel-free areas (Fig. 4.10) .
Many soil microorganisms, both beneficial and pathogenic to crops, are known to be sensitive to change in aereation, pore-size distribution and soil-water status, which may result from the passage of vehicle wheels. For instance, a reduction in the number of soybean nodule bacteria has been observed in compacted soils .
Tractor wheeling on silty-loam and clay soils was found to decrease the number of soil fauna (Collembula, Acari, and earthworms) due to direct physical damage to the fauna more that to unfavorable soil physical conditions.
Quirk , in reporting a quotation of Bradfield , emphasized that soil "granulation is flocculation plus" with the intent to stress that soil aggregation depends not only on flocculation of clay particles but on many natural and unnatural factors.
When the stability of the aggregates of different sizes is considered, it can be said that such stability is primarily dependent on the flocculation of the clay particles by electrolites of the soil solution developing a mineral structure, which, in turn, depends, among other things, on the cations adsorbed on the surfaces of clays and on the electrolyte concentration in the soil solution.
Adsorbed iron and calcium cations are highly desirable because they make the clay particles form stable flocculated particles, whereas sodium cation is undesirable because it make clay particles repel and disperse. A high electrolyte concentration also imparts stability to soil aggregates but only as long as the concentration remain high in the soil solution . Mineral soil colloids (clay, oxides), in contributing to aggregate formation, are relatively static in time.
Organic binding agents have been grouped in transient, temporary, and persistent categories according to their endurance in the soil . Moreover, different kinds of organic binding agents tend to locate themself in different scale dimentions, eventually justifying the differentiation between macro- and microstructure. At this point, a soil is structurally stable on the macroscale only if it is structurally stable on the microscale.
On the microscale, roots and microorganisms within the rhizosphere lead to the production of high-molecular-weight polysaccharides, among other decomposition byproducts. Such components act as transient binding agents lasting for periods ranging from a few months to a year. Such compounds are responsible for the stabilization of aggregates greater than about 2 ¡m . Recent reviews have shown a general agreement in considering polysaccharides as the main factor of aggregate stability in cultivated soils [26-28].
Roots and fungal hyphae also act as temporary binding agents at a larger scale, especially mycorrhizae. Their permanency in soil ranges from months to a few years and they are generally associated with stable aggregates larger than 250 ¡m. They are also quite sensitive to changes of soil environmental conditions prompted by tillage operation.
Persistent binding agents are represented mainly by aromatic humic material in association with amorphous iron, aluminium oxides, and aluminosilicates. Such agents bind the smallest microaggregates of sizes from about 0.2 to 250 ¡m [29, 30]. Within these dimensions, chemical bonds and electrochemical forces (mainly London-Van der Walls) control the type and number of bonds formed . For instance, in Ultisols and Oxisols, polycations of iron and aluminium are basic elements in the following microstructural units :
[clay]-[polyvalent cation]-[humified organic matter].
It follows that the removal of these polycations leads to an high degree of aggregate breakdown [32, 33]. The amount of clay particles in the soil relative to soil particles of greater size regulates the frequency of bonding between the two.
In agricultural soils under intensive cultivation, inorganic colloids are more important as elements of structural stability in comparison to undisturbed soils where, apparently, humified organic matter becomes the main agent of soil structural stability [25, 34].
The main unnatural process affecting soil structure, particularly soil macroporosity, is represented by tillage practices. According to Dexter [1, 24, 45], given enough energy input, a soil with a bad physical condition (nonaggregated, massive, hard, anaerobic) can be temporarily turned into a soil with an apparently near-perfect structure (seed-bed of 1- to 5-mm diameter aggregates overlying a loosened, well-drained subsoil) by mechanical manipulation. However, a mechanically produced structure may be consistently unstable. Lacking the intrinsic stability of microstructure, the soil may collapse when wet and/or subject to mechanical actions (rainfall, wind, animal trampling, machinery traffic) and become as bad, if not worse, than it was before the tillage operations.
As a consequence of the tillage operations, a soil that has been sheared and molded by tillage implements and/or beneath vehicle wheels is weaker than an undisturbed soil at the same density and water content. Hence, a soil that has been disturbed mechanically is more susceptible to erosion and compaction than an undisturbed soil. However, if a soil is left after disturbance at constant water content and constant density, then its aggregate water stability and strength will be regained gradually over time. Such recovery is partly due to the rearrangement of the clay particles to new positions of lower free energy and partly to the reformation of cementing bonds between soil particles [52-54]. The rearrangement of clay particles at constant water content leads to a change in the size of the micropore distribution and hence to a change in matrix water potential.
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