Soil Structure - Physical Properties

Soil structure is the second most influential characteristic, after texture, in determining the behavior or any given soil. Soils with similar characteristics (vegetation, climate, texture, and depth) but different structure will react differently under similar conditions. Structure influences water infiltration, building site development and growth of plants. When combined with soil texture, structure influences the distribution of soil solids and pore space (called the soil bulk density).

Soil structure is defined as the grouping or arrangement of primary particles (sand, silt, clay and organic matter) into larger, secondary particles called aggregates or peds. These aggregates can be described in terms of shape, size, and grade (distinctness), as you will learn a little later.

In this section the physical, chemical and biological factors which influence the formation of soil structure are discussed. The different shapes or types of structure are presented, and you will discover how those shapes can affect air and water movement. The effects (both positive and negative) of certain human activities on soil structure are considered.

Importance of Soil Structure

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Structure is important in that it can modify the influence of soil texture. For example, a (structureless) soil high in clay will have very fine pores because of the higher packing ratio of small particles. Without the ameliorating influence of soil structure, air, water and plant roots would move through the soil with great difficulty. Structure provides larger spaces between aggregates to facilitate movement. Air, water and plant roots can penetrate deeper in the soil; this can be important to plant survival during times of drought. The larger voids serve as short-term storage space for water, easily accessed by plants.

Formation of Soil Structure

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Both biological and physical-chemical processes are involved in formation of soil structure. Physical-chemical processes are important in flocculation (or "bringing together") of soil particles into aggregates, and in swelling and shrinking of clay masses. Biological processes aid in stabilization of those aggregates through physical action of burrowing animals, the binding nature of plant roots, and production of organic glues by microorganisms.

Physical-Chemical Processes

Positively charged ions such as calcium (Ca2+), magnesium (Mg2+) and aluminum (Al3+), (which are known as polyvalent cations because they have more than one positive charge), are key in initiating the formation of soil structure. Aggregation begins with flocculation of clay particles (platelets) into microscopic clumps called floccules; the cations that are caught between two platelets attract the negative charges on both platelets, binding them together. Look at the mineral smectite and find the polyvalent cations in the structure. Note that sodium (Na+) is not polyvalent, but monovalent (one positive charge); its effect is quite different and will be discussed below. The polyvalent cations (including Ca2+, Fe3+ and Al3+) may also attract and bind with hydrophobic (water repelling) humus molecules allowing them to bind to clay surfaces. These clay-humus particles bind with each other and with grains of silt to form the smallest of the primary aggregates, perhaps as small as 0.01 mm. These small particles aid greatly in stabilizing the slightly larger (<0.25 mm) microaggregates which consist of fine or very fine sand grains, smaller clumps of silt grains, clay and organic debris all bound together by root hairs, organic root exudates, and fungal threads.

Do you remember that monovalent cations like sodium, Na+, have a different effect on soil aggregation? The single positive charge on sodium (combined with a relatively large ionic radius) means that sodium is not very efficient at neutralizing negative charges on clay and on organic matter; the attractive forces between the cation and the negatively charged colloids are not great enough to overcome the natural repulsion of one negatively charged clay platelet by another. The clay is not able to flocculate, and the result is a layer of nearly structureless soil. Soils in arid and semi-arid climates are often high in sodium, and exhibit a characteristic structure close to the surface called columnar structure, which severely limits air, water and root penetration.

As a soil dries out, the clay platelets move closer together and cause shrinking in soil volume. Cracks will form along tiny zones of weakness, and over the course of several wet/dry cycles this network of cracks becomes better defined. Plant roots, as they repeatedly remove water from the same vicinity, reinforce a drying pattern and contribute to physical aggregation of the soil. The process of freezing and thawing in the soil also contributes to the drying process as ice crystals form. And shrinking and swelling that results from wet-dry and freeze-thaw cycles creates tiny cracks or fissures (shrinking) and pressure (swelling) that break apart structureless masses of clay to eventually form soil peds or aggregates.

Biological Processes

The most prominent of the biological processes are burrowing activities of soil animals; the binding activity of fine roots and fungal hyphae; and the production of organic “glues” by microorganisms like bacteria and fungi. Soil animals such as earthworms move soil particles as they burrow through the ground; plant roots will also do this. Particles which come in close proximity to one another are more likely to form aggregates; channels created by plant roots or burrowing activity act like large pores, breaking up clods and helping to define larger structural units. Plant roots and fungal hyphae exude sticky organic substances (called polysaccharides) which physically cement soil particles together. And as bacteria decompose organic material they contribute their share of polysaccharides and other organic glues. Consider the fact that a single gram of surface soil (about one teaspoonful) contains 109-1010 bacteria , and you can see how these sticky by-products might affect soil aggregation! We mentioned the role or organic matter above in the physical-chemical discussion, but organic matter contributes in one other way. As a general rule, the more organic matter the soil contains, the greater the populations of microorganisms and other decomposers.

Describing Soil Structure

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Many types or shapes of structure occur in soils. Other soils have no true structure and are called structureless. Certain deposits, for example sands in a sand dune, are called single grain because there is little to no attraction between sand grains. On the other textural extreme, some clay soils occur as large cohesive masses and are termed massive in structure. Many soils, however, will exhibit definite and repeatable shapes that we can describe with four general categories.

Granular structure is generally spherical in shape, and sometimes resembles BBs. The aggregates may be separated easily from one another, and the outer surfaces do not fit well together (not like jigsaw puzzle pieces). Aggregates can be <1 mm to perhaps 10 mm in diameter. Granular structure is most commonly found in surface horizons, especially those enriched with organic matter (an A horizon). Grassland vegetation and earthworm activity encourage granular structure.

Block-like structure is similar in shape to a cube: all dimensions are of nearly equal length, and typically range from <5 mm to over 50 mm in diameter. They are not formed individually, but take their shape from surrounding peds. Angular blocky peds have sharp, well-defined edges and their rectangular faces are distinct. When most of the edges are somewhat rounded, the structure is described as subangular blocky. Block-like structures most often occur in B horizons (or the subsoil).

Prism-like structures are those that are longer than they are wide. They are variable in height from horizon to horizon and from soil to soil; diameters (width) may range from <10 mm to over 100 mm. Similar to the block-like structures, prism-like structures take their shape from surrounding peds. In fact, they are often associated with swelling types of clays and are commonly found in subsurface B horizons. If the tops of the prisms are horizontally flat and angular, the structure is described as prismatic. In certain soils, the prisms have rounded tops somewhat like a biscuit; this is called columnar structure. Columnar structure is associated with soils high in sodium, common in arid and semi-arid regions (remember the discussion above on Na+ and structure?).

The final structure shape is platy. Platy structure is characterized by relatively thin (<1 mm to about 10 mm) horizontally oriented peds that look like plates stacked one on top of another. It may occur in surface or subsurface horizons as a natural product of soil formation or development. Unlike other shapes, it may be inherited from a soil’s parent material especially if it was deposited by water (alluvium, flowing water; or lacustrine, lake water) or ice (glacial).

Grade describes the distinctness of the structure, and is combined with the cohesion of the soil within units compared to the adhesion between individual units. Terms that are used for grade are weak, moderate and strong. If the structural grade is weak, aggregates are barely observable in the soil profile. When peds are gently disturbed (for example, shaking them gently between your hands) the material parts into a mixture of whole and broken units. Weak structure may be easily compromised by management activities.

With moderate grade the structural units are well formed and easily distinguished in the soil profile. When disturbed, the aggregates part into a mixture of mostly whole units, some broken units, and some material that is not in structural units. Individual peds will part from adjoining peds somewhat cleanly. When grade is described as strong the structural units are clearly seen in the profile and shape is easily identified. Peds separate cleanly from other peds and retain their shape when disturbed by shaking.

Structural class is the description of the size of the units. Verbal descriptions have size ranges as seen below. The size limits refer to the smallest dimension of any given structural unit, for example the width of a prism or the thickness of a plate.

Shapes of Structures
Size Classes platy
(mm)
prismatic and columnar
(mm)
blocky
(mm)
granular
(mm)
Very fine <1 <10 <5 <1
Fine 1 - 2 10 - 20 5 - 10 1 - 2
Medium 2 - 5 20 - 50 10 - 20 2 - 5
Coarse 5 - 10 50 - 100 20 - 50 5 - 10
Very coarse >10 >100 >50 >10

Impacts of Soil Structure

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Soil pores are dependant on the size and shape of soil structure. Pores are multi-access highways in the soil for air, water and plant roots. They are defined and controlled by soil structure. The larger and straighter the pores are, the more efficient they are at moving air and water through the soil. Water movement is of course important for plant growth, but also plays a role in the movement of nutrients and other fine particulates around in the soil (called translocation). Air movement through, into and out of the soil is also crucial for both plants and soil animals. Metabolic activities (respiration) of all living creatures below the surface create CO2 gas, but they need to consume oxygen. Plants can obtain oxygen from the above-ground environment, but soil microorganisms are dependant on the soil environment; oxygen must be available below the surface for aerobic organisms to survive. Soils which are poorly aerated can experience a build-up of toxic gases like methane (CH4) and ethylene (C2H4); if gaseous exchange between the atmosphere and the soil atmosphere occurs readily, favorable environments can be maintained.

Granular structure occurs most often in surface soils and is at that atmosphere/soil atmosphere interface. Fortunately, granular structures offer the most pore space and some of the largest pores of any structure. This is important for gas exchange, water infiltration, and seedling root penetration. Subangular blocky structure, usually found in subsoils but which sometimes occurs at the surface, is somewhat similar to granular. Angular blocky structure tends to pack closer to adjoining peds than does subangular blocky, so consequently is more limiting to air and water movement. Prismatic is similar to angular blocky, although the length of the flow path (along the long vertical sides of the prisms) is greater. Columnar structure has more limitations. As mentioned in discussions above, columnar structure occurs in soils higher in sodium and is often near the surface. The rounded tops of the structure are related to the dispersing effects of the monovalent Na+ which prevents clays from flocculating. This effectively seals the soil to air and water movement in either direction, up or down. Arid and semi-arid climates experience high volume storm events, so the infiltration capabilities of the soil are critical. Platy structure has the least amount of pore space with the highest degree of tortuosity.

Effects of Management Techniques on Structure

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Humans impact the soil in many different ways. Additions of chemicals in the form of fertilizer or waste material, removal of vegetation, agriculture, construction, and recreation all leave a mark on the soil resource. Many of these activities impact the structure of the soil, sometimes in positive ways, sometimes negatively.

Additions of fertilizer to agricultural land can have a positive effect on soil structure. By increasing plant growth and quality, roots help with stability of soil aggregates. Applications of liming material (high in calcium, a key player in flocculation) encourage better structure and tilth. Organic materials in the form of plant residue or animal manure quickly decompose and participate in the development of soil aggregates, and also provide favorable conditions for microorganisms.

Agricultural practices such as tillage introduce air into the soil and physically break up the soil. By aerating the soil some additional pores space is temporarily created, and organic residues decompose at a faster rate. Under proper soil moisture conditions, breaking up clods of soil with poor or weak structure will increase the surface area and facilitate aggregation. Conservation tillage practices have greater benefits to the soil than conventional tillage. Under conservation practices, the need for tillage is minimized and plant residues are left on or near the soil surface. Conventional tillage requires more frequent tilling. A primary pass is made to turn plant residue several inches below the surface. This is followed by secondary tillage operations such as harrowing, which kills weeds and breaks up clods prior to planting. After planting, the soil may again be tilled for weed control and to break up any crusting of the surface soil. These multiple passes can compact the soil and result in the formation of a “plow pan” and platy structure. The amount and size of pores will decrease in this zone with concomitant air and water movement. With decreased rates of infiltration, surface runoff and soil erosion become issues. Plant roots have greater difficulty penetrating the platy structure and compacted soil, and limited rooting depth can affect plant survival. Irrigation, if not properly applied, can compound this problem by breaking up aggregates, increasing sodium content, and leaching clay.

Recreational activities also have the potential for compacting soil. Hiking trails which concentrate foot traffic frequently exhibit platy structure near the surface. Once this condition has developed, it is probably best to limit trail use to the same area.

Construction affects soil structure by removing surface layers of soil, perhaps to depths beyond the zone of weathering where the soil is massive. Soil is intentionally compacted for road beds or buildings; depending on soil moisture conditions, structure may be altered or completely destroyed.

Summary

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Structure is one of the most important physical characteristics of the soil. It influences water and air movement into and through the soil. It provides some physical stability and helps with plant support. The pores that result from soil structure act as a storehouse for water and soluble plant nutrients.

Soil structure forms through the processes of flocculation and stabilization. Polyvalent cations such as Ca2+ in combination with organic molecules promote flocculation and formation of tiny aggregates. Roots, fungal hyphae and sticky organic glues help to stabilize the microaggregates and assist in formation of larger aggregates.

Soil structure is described in terms of grade, class and shape, for example: moderate fine angular blocky. Different structure shapes have different pore characteristics which influence the movement of air and water. Movement in platy structure is tortuous and slow, while in granular structures it can be quite rapid.

There are many management activities practiced by humans which influence structure. Some activities have positive effects on soil structure, such as additions of fertilizers and conservation tillage. Other activities lead to reduction in the quality of structure and pore space.

Activities

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  1. Obtain a sample of soil with a high amount of clay. Divide the sample in two and crush or grind fairly well. Add one sample to a plastic bottle with water only. Add the other sample to a bottle with water plus 5 grams of Calgon™. The chemical name for Calgon is sodium hexametaphosphate, which can be used to disperse clays for particle size analysis. Shake both samples, and allow the samples to settle. Which one will take the longest? Think about the affect that sodium has on flocculation; which sample will have smaller aggregates or particles? Remember from the texture exercises that smaller particles take longer to settle than larger particles.

  2. Find a hiking trail or any area around the school where foot traffic has worn a bare path. Dig a small, shallow hole in the path, and another adjacent to the path. Do you see any differences in structure in the soil that is in the path? Is it platy? Compaction of the soil as you might get with a foot path often results in platy structure. (You can add another segment to this exercise by measuring the bulk density of the soil, as we will discuss in the next section.)