Soil Structure

Typically, the individual mineral particles in surface soils are coated and glued together with colloidal organic matter and encrusted with inorganic cements forming spatial clusters within the matrix known as aggregates or peds. In 1982, two Australian soil scientists, Drs. Judy Tisdall and Malcolm Oades, presented a conceptual model of the aggregated, hierarchical nature of the soil system and described the linkages between the architecture of the soil habitat and the role of microbial activity in its genesis. If soil was but a single ped, scientists could describe its physical and chemical properties in sufficient detail to understand how the nature and activity of soil organisms are controlled. But soils are composed of highly variable peds, derived from a wide range of parent materials that exist within innumerable landscapes, and exposed to diverse climates. And they have formed in concert with the development of the complex communities of living organisms that make up the biosphere.

Overall, the shape, size, and arrangement of the aggregates within the soil profile describe soil structure. Three assemblages of aggregates are recognized with diameter classes of 0.002-0.020, 0.020-0.250, and >0.250 mm and are referred to as microaggregates, mesoaggregates, and macroaggregates, respectively.

Microaggregates are formed by flocculation of fine silt and clay particles, amorphous minerals (composed of oxides and hydroxides of aluminum, silicon, iron, and manganese and silicates of aluminum and iron), and nonhumic and humic substances, largely dominated by electrostatic and van der Waal forces. Polyvalent cations such as Al3+, Fe3+, Ca2+, and Mg2+ adsorbing onto their surfaces and reacting with exposed functional groups promote these flocculation reactions. Sticky polysaccharides and proteins derived from plant and animal tissues, microbial cells, and exudates from roots, hyphae, and bacteria further enhance these stabilization reactions. In particular, soil microorganisms produce extensive exopolysaccharides, which they use to adhere to individual soil particles.

The core of mesoaggregates is usually the residual debris left from the decay of plant and microbial tissues. Bits of decaying particulate organic matter and their colonizing microbial biofilms become encrusted with fine mineral particles and they act as nuclei for the formation of aggregates and contribute to their resilience. Microaggregates can form throughout the surface soil matrix wherever intense microbial activity associated with organic matter decomposition occurs. The larger macroaggregates are formed only where a network of living and decaying plant roots, fungal hyphae, and microbial filaments physically enmesh clusters of micro- and mesoaggregates for a period sufficient for them to be chemically linked.

In the rhizosphere, hyphae of arbuscular mycorrhizal fungi contribute to the aggregation effect as they grow into small pores and bind soil particles together. Although macroaggregates comprise microaggregates, not all soil microaggregates exist as macroaggregates. Macroaggregates can contain soil primary particles that may eventually go on to form microaggregates. This hierarchical organization of soil structure and aggregation, i.e., large aggregates being composed of smaller aggregates, which in turn are composed of even smaller aggregates, is characteristic of most soils (Fig. 2.4).

Micro- and mesoaggregates tend to be especially resistant to mechanical breakdown, for example, from the impact of rainfall or from slaking—rapid rewetting of dry soil—or from freezing and thawing. The restricted size of the pores within these aggregates (<0.01 mm diameter), also referred to as intraaggregate pore space, can limit the interactions of soil organisms. The pore space surrounding microaggregates and contained within macroaggregates is collectively referred to as the soil interaggregate pore space. It contains plant roots and is usually rich in fresh inputs of particulate macroorganic matter (>0.05 mm diameter). Macroaggregates usually remain intact as long as the soil is not disturbed, for example, by earthworm and other faunal activity or by disturbance such as heavy rains. Macroaggregation is important for controlling microbial activity and soil organic matter turnover in surface soils because it gives fine-textured, clayey and loamy-textured soils pore space characteristics similar to those of sandy soils. The physical properties affected by macroaggregation include pore size distribution, pore continuity and tortuosity (irregular, twisted pores), aeration, drainage, and tilth.

soil habitat scale and observation

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