The state and stability of soil structure under different management systems are strictly connected with several natural processes. The most important are the hydrology and the biological activity of the soil [24, 35].
An important distinction can be made between natural and human-induced processes in relation to the amelioration of soil physical conditions dependent on soil structure.
Human actions such as tillage practices, crop rotation, crop fertilization, herbicide and pesticide applications, and use of soil conditioners can affect soil processes. On the other hand, soil structural features are directly responsible of soil physical conditions influencing plant growth and crop production as well as soil physical degradation due to compaction, erosion, and soil crusting and sealing.
In more detail, the status and stability of soil structure influence soil hydrology and aeration through the pore-size distribution in the soil mass. It follows that soil structural characteristics also can be assessed by an evaluation of pore-size distribution of the soil .
Porosity assumes different features when a volume of soil is considered. Microp-orosity is connected to pore-size dimensions ranging from 10-7 to 10-4 m and can be practically separated from macroporosity, which is associated with pore-size dimentions greater than 10-4 m. Such a division appears to be functionally useful because micro-porosity is the soil domain where capillary forces prevail whereas macroporosity is the domain where gravitational forces prevail in terms of movement of water within the soil.
The other important physical parameter strictly connected with soil structural conditions is soil cohesion, which plays an important role in relation to plant germination and seedling sprouting and to soil erodibility.
Considering natural processes of structural recovery, cycles of soil wetting and drying are responsible for macropore dynamics. According to Dexter , when a soil of medium to high clay content dries, it shrinks and vertical desiccation cracks form. If the drying is rapid, the cracks will be closely spaced and narrow. If the process is slow, the cracks will be spaced more widely. Cracks form important pathways for rapid water infiltration, aereation, and deep penetration of roots through the soil layers, which may otherwise exert mechanical impedence. When these vertical (primary) cracks become wider than about 4 mm, significant convection of air currents may occur within them so that drying occurs on their faces, producing secondary cracks at right angles to the primary cleavage direction. Occasionally, tertiary cracks also can form on the surface of secondary cracks in the same way.
When the soil becomes wet, it swells and the desiccation cracks close. The rate at which cracks close varies widely. In some soils it happens almost immediately after wetting whereas in others it may take days or even weeks before some cracks finally close. When a soil is wetted rapidly, for example during an intense rainstorm, the combined effect of differential swelling and pressure buildup in entrapped air can cause mechanical failure of aggregates, sometime to the point of complete slaking into microaggregates and/or primary particles <250 ¡m. Such a phenomenon was described many years ago for clay soils and designated as hydromolecular aggregate disruption [36-38]. With slower wetting, complete slaking may not occur. Instead, there may be only partial slaking or mellowing in which arrays of microcracks form throughout the soil mass, which retains its coherence and shape . Soil that has been wetted rapidly appears to offer less resistence to root penetration . Slaking of a compacted layer caused by rapid wetting seems to make the soil more penetrable to growing roots.
Cycles of freezing and thawing also influence soil structural dynamics. When soil water content is less than 20%, ice crystals that form during freezing are often smaller than pores and therefore cause little disruption of particle-to-particle bonds. However, when water contents are higher and freezing takes place slowly, some of the crystals become larger than the pores and exert stresses that break many of the bonds between soil particles.
In fall, as the soil surface cools, moisture moves to the surface layers from both the atmosphere and the underlaying warmer soil layers. Consequently, the water content of the soil near the surface, at the moment when it starts to freeze in winter, is often higher than 20%, even in arid climates. Major disruption of the soil aggregates, therefore, may take place.
Plant roots have an important role in soil structural dynamics, directly affecting the macrostructure by the formation of biopores. The mean soil density remains constant while the volume of roots is accommodated by the loss of pore space of the sorrounding soil. Thus, the soil around the roots can be compacted to some extent for a distance of the order of one root diameter beyond the surface of the root.
When roots decompose, which usually occurs within about one year for the nonlig-nified tissues of annual crops, a biopore remains. The amount of roots and, hence of biopores, can be quite impressive. For the soil profile of wheat crops in South Australia, a development of 15 km of roots was estimated per square meter of soil surface.
Roots play a double role in aggregate formation and soil stabilization. A well-known example is the increase in aggregation of soil particles observed under grass-based pastures . In such systems, an improvement of aggregation occurs as a consequence of the high density of the grass roots that bind the aggregates. In addition, soil structural stabilization is enhanced as a consequence of the organic-matter turnover in a well- balanced aerobic/anaerobic soil environment. This is the case of the grass/legumes perennial swards, which favor the production of humified organic compounds by microbial activity, capable of binding soil particles at the microscale level [42,43].
Besides grass roots, fungal hyphae also may play an important role in binding larger aggregates. Moreover, exudates from roots are important in the stabilization of microaggregates (<250 ¡m), whereas other exudates also are produced by bacteria and fungi involved in the decomposition of root material.
Biopores formed by one crop often can provide channels for deep rooting of a subsequent crop . There is also evidence that roots grow preferentially toward biopores under conditions of poor aeration .
Soil fauna is another important natural factor in structural amelioration. Earthworms can have a profound effect on soil structure both via their production of burrows (macrostructure) and via the casts that they excrete (microstructure). There are numerous species of earthworms, which behave in different ways. Worm number is greatest in clayey soils of humid regions and it is virtually zero in sandy soils in arid regions. At the Waite Institute in Australia, 250 to 750 worms per square meter were observed in old pastures, compared with only about 20 worms per square meter in a wheat-fallow rotation . Tillage exerts a rather drastic effect on earthworm number, reducing population density to about 15% of its value in nontilled soils . Worms can move around in the soil either by pushing soil aside or by ingesting the soil, having the overall effect of making tunnels into the soil mass. McKenzie and Dexter [48, 49] have reported that earthworms can exert a mean maximum axial pressure of 73 kPa and a mean radial pressure of 230 kPa. Ingested soil is molded in the guts of earthworms at pressures of about 260 Pa and cast, when excreted, at much lower bulk density (around 1.15t/m3) in comparison withe the soil in which the worms live (1.5-1.6 t/m3). Earthworm burrows provide pathways of reduced mechanical impedence to root penetration, so that extensive rooting in earthworm tunnels often is observed [50, 51].
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