Soil

The Development of Soil Structure

(1) those responsible for formation of structure and aggregates. (2) those responsible for the stability of aggregates once they have formed. The factors discussed here are: ִa) wetting and drying; -Wetting and drying cause swelling and shrinking leading to the formation of cracks in the soil and the formation of aggregates between these cracks. -Rapid drying generally produces small peds, while slow drying often produces large peds. -Therefore we often get small peds in topsoils, larger peds in subsoils. -Plant roots also form peds by drying soil around them through water uptake. ִb) freezing and thawing; -Alternate freezing and thawing causes a granulating action often producing platy peds. ִc) root activity; -Roots may form cracks both indirectly, due to the uptake of water and directly, due to their growth through the soil. - Smaller plant roots have a greater tendency to be deflected by pre-existing aggregates. -Soil strength may reduce the both the numbers of plant roots which penetrate the soil, and also the rate at which those roots will grow. -The death and decomposition of old roots leaves channels which are available for the movement of both water and gases, and for further root growth. ִd) faunal activity; -Soil fauna are active agents in the formation of soil structure. Both by the production of faecal pellets and by their burrowing activities. • Earthworms- form channels. • Other Invertebrates- form channels and chambers. • Vertebrates- form burrows. ִe) parent material -For certain soils it has been shown that at least some of their structural characteristics have been inherited from the parent material. ִf) cultivation. -Cultivation has been used by humans for millennia to produce seedbeds suitable for crop growth. -Soil factors such as the moisture content, the texture, the organic matter content, as well as the intensity of cultivation, all affect the degree of aggregation produced.

Porosity

- Amount of pore space. - Calculated approximately from bulk density and particle density (2.65 g/cm^3)

Pore Space

- Portion of the soil occupied by air and water; the spaces between primary soil particles (sand, silt and clay) and soil aggregates. Types: • Inter-aggregate pores • Intra-aggregate pores

Stone Content

-ִless rooting space (less soil in effect), -ִreduced water holding capacity, ִ-higher soil temperatures, -ִdamage to ploughs, and -ִdamage to root crops at harvest ִ-Excessive drainage (and leaching).

Type

Shape of structure) may be one of four primary types: ִ1) plate-like, ִ2) prism-like, ִ3) blocky and ִ4) spheroidal -includes all rounded aggregates, most being under 1cm in diameter. Subangular blocky: - irregular cuboidal aggregates with the three dimensions. -Penetrated by roots but less readily than crumbs or granules. -Water availability depends on texture - good in loams, moderate in heavy soils (clays). Angular blocky: - Aggregates with sharp, angular edges. Little root penetration of structures therefore water availability is low. Columnar peds - Prism like with rounded tops are columnar applies. Prism like with level and clean cut tops are prismatic. Prismatic peds: - Fine plant roots cannot usually penetrate these structures but do grow between them during dry periods. Plate-like: -Relatively thin, horizontal aggregates. -Can occur in the surface layers of native soils and subsoil horizons but often an indicator of structural degradation. Platy Peds: -Usually very compact, impeding root and water movement. Organic matter levels are frequently low.

Class

Size of structure: -Fine -Medium -Large

Stokes Law

V=kD^2 V = settling velocity k = constant (dependent on particle density and water temperature) D = particle diameter.

Grade

Weak (<1/3 of the soil material exists as aggregates). Moderate (1/3 to 2/3 of the soil material exists as aggregates). Strong ( > 2/3 of the soil material exists as aggregates).

Bulk Density vs Particle Density

• A useful measure, which integrates a range of soil properties (pore space, organic matter content, compaction, ease of rooting and ease of cultivation) is bulk density. 1) Extract a known volume of soil from a profile. 2)Dry it. 3) Weigh it. 4)Express as dry soil weight/ volume (g/cm^3) = Bulk Density. -Dependent more on pore space. -Low bulk density due to organic matter having low particle density, less pore space due to organic matter. - High bulk density due to lack of organic matter. • Influences on Bulk Density: ִ-Compaction. ִ-Loss of structure. -Loss of organic matter. • Used to predict and measure soil structure, compaction and likely root-growth conditions.

Stability

• Factors which allow the soil to retain any structural modifications. -Organic matter(food source in soil) l) a transient effect (mainly polysaccharides)(days and weeks). 2) a temporary effect (e.g. roots and hyphae)(weeks and months). 3) a persistent effect (ie. resistant organic polymers in conjunction with basic cations)(years). • Stable soils with low organic matter is due to either iron/aluminium oxides, carbonates, clay or silica. -Sesquioxides. -Clay particles. -Silicates. -Organo-clay complexes. -The nature of the exchangeable cations on the clays. -The electrolyte concentration of the soil solution. • Clay-sized Fraction- the amount and nature of the clay-sized fraction have been implicated in the stabilization of soil structure (as clay content increases from low to %40, aggregate stability increases). • Polyvalent Cations (Ca2+, Fe2+, Al3+) promotes flocculation (particles come together, settle out and form an aggregate).

Structures

• Single-grained structure implies a sandy textured soil with no cohesion between particles (e.g. a sand dune). • Massive structure is a continuous, sometimes cemented soil mass with very low organic matter. • The size of the aggregates usually increases down the profile. • Structure is much easier to observe when the soil is dry and the pores between peds are wide.

Soil Structure

• The spatial distribution and total organisation of the soil system as expressed by the degree and type of aggregation and the nature and distribution of pores and pore space. • Soil aggregates result from a tendency of the finer soil particles, especially clay and humus, to stick together. • The solid components cluster together forming larger units (called aggregates or peds) which are separated from each other by either pores (that are either air or waterfilled) or by planes of weakness. • Structure is critically important in soils with clay contents > ~20%. In these clayey soils, structure controls water movement and availability, heat transfer, aeration, bulk density, and porosity. • Affect root growth and ease of cultivation. • If clay contents < ~20 % the packing pores between sand grains are generally large and can provide adequate water movement and aeration. Field descriptions of soil structure usually give:- 1) Grade, the degree of development and distinction of the peds. 2) Class or size of peds. 3) Type or shape of peds.

Clay and Silt

• Clay has around 10,000 times the surface area of an equivalent weight of medium-sized sand. • The adsorption of water, nutrients, gas, and inter-particle attractions are all surface phenomena. • Clays tend be are highly cohesive and plastic when moist. • Silt particles can exhibit some plasticity and cohesion but to a much lesser extent than clay. • Soils high in silt and/or clay have a fine texture and a small pore size.

Sand

• Sand particles are not cohesive. • Sand particles maybe rounded or angular in shape. • The pores between sand grains are large, allowing rapid water and air movement.

Soil Texture

• The terms texture and structure are widely used when describing the physical nature of a soil. • Describes the sizes and shapes and the distribution of both soil aggregates and soil pores. Particle Size Analysis (Objective): • Clay < 2 micrometres • ִ Silt 2 - 20 micrometres (0.002-0.02mm) ִ• Sand (fine) 20 - 200 micrometres (coarse) 200 - 2,000 micrometres ִ • Gravel >2,000 micrometres 3 Stages- 1) Removal of gravel (and organic matter if excessive). 2) Destruction of structure and separation of aggregates into particles. 3)The proportion of sand, silt and clay is determined (eg by different rates of settling in a water column). Soil Textural Classes (Subjective): • Sands: soil dominated by sand, up to 70% or more sand by weight. • Loams: a diverse group containing sand, silt and clay separates in more or less equal quantities. • Clays: soil dominated by clay, >40% clay by weight. Both organic matter and gravel (soil solids > 2mm) are excluded from a determination of texture.