Physical Properties of Soils

Theoretical Soil Gases (O2, CO2, H2O) (25%) Water (H2O) (25%) Minerals Organic Matter (1 - 5%) Soil is a porous medium consisting of minerals, OM, water and gases. Minerals (45 - 49%)

What is Soil? Soil is a dynamic natural body having properties derived from the combined effects of climate and biotic activities, as modified by topography, acting on parent materials over time.

Soil-Forming Factors Parent Materials Climate Biota Topography Time Minerals and organic materials present during it's formation. Climate Weathering coupled with average temperature and moisture of a region. Biota Plants and animals help to create a soil. Topography influence of moisture and erosion Time It may take hundreds of years to form one inch of soil from parent material. Parent Materials: Materials from volcanoes, sediment transported by wind, water or glaciers or minerals left behind by drying lakes are good examples. Climate: cycles of freezing and thawing, wetting and drying, and their frequency coupled with average temperature and moisture of a region. Biota: As they die, organic matter incorporates with the weathered parent material and becomes part of the soil. Living animals such as moles, earthworms, bacteria, fungi and nematodes are all busy moving through or digesting food found in the soil. All of these actions mix and enrich the soil. Topography: Topography is the hilliness, flatness, or amount of slope of the land. Soils vary with topography primarily because of the influence of moisture and erosion. In many areas, moist, poorly drained soils are located in low areas, and depressions of the land. In contract, soils in sloping areas can be drier and well drained. These soils tend to be moderately and well developed. Erosion can remove all or part of the topsoil and subsoil, leaving weakly developed soil. Time: It may take hundreds of years to form one inch of soil from parent material. Only the top few inches are productive in the sense of being able to sustain plant growth. This is why soil conservation is so important.

The C horizon is a transition area between soil and parent material. The organic horizon or O horizon consists of detritus, leaf litter and other organic material on the surface of the soil. This layer is dark because of decomposition. The A horizon or topsoil is usually darker than lower layers, loose and crumbly with varying amounts of organic matter. Usually the most productive layer of the soil. The B horizon or subsoil is usually lighter in color, dense and low in organic matter. Most of the materials leached from the A horizon stops in this zone. The C horizon is a transition area between soil and parent material. At some point the C horizon will give up to the final horizon, bedrock. These horizons collectively are known as a soil profile. The thickness varies with location, and under disturbed conditions: heavy agriculture, building sites or severe erosion for example, not all horizons will be present. As water moves down through the topsoil, many soluble minerals and nutrients dissolve. The dissolved materials leach downward into lower horizons.

Parent Material Consolidated Unconsolidated Rocks and minerals Residual parent material Formed in place Consolidated Unconsolidated Deposited by gravity Deposited by wind Deposited by water Deposited by ice Deposited by streams Deposited in lakes Wind transported Water transported Ice transported Gravity transported Lacustrine Alluvium Marine Glacial Till Outwash Ice contact Dunes (sand) Loess (silt) Colluvium

Mobility zone-all components Intensified mobility zone-all components Tundra (treeless plain, permafrost) Taiga-podzol zone (cold, swampy, forested area) Steppes (plains) Semidesert and desert Savannas (open prairie, scattered trees) Tropical forest Mobility zone-all components Intensified mobility zone-all components Precipitation 600-700 150-200 Vegetation Accumulation Evaporation t °C 25 100 2 1500 3000 Depth of weathering (m) Annual Precipitation (mm) Effects of climate on depth of soil weathered. The warm and wet conditions of the tropical forest result in the most deeply weathered soils. This is reduced by drier conditions in the semi-arid regions and by both precipitation and colder temperatures in the boreal forest and tundra zones. Strakhov (1967)

Soil Profile - Forest Oi – Organic, tree leaves, slightly decomposed Oe Oa A E Bhs Bs C R Oi – Organic, tree leaves, slightly decomposed Oe – Organic, moderately decomposed Oa – Organic, highly decomposed A – Horizon of mineral soil mixed with OM Eluviation, most fine roots E – Zone of maximum eluviation, light color Decreased roots, decreased pH Bhs – Horizon with accumulation of Humus, Fe and Al oxides, and clay. Bs – Horizon of accumulation of Fe and Al oxides C – Horizon of parent material. Presence of Ca and Mg carbonates in some soils R – Bedrock

Soil Texture Sand: 0.02 to 2.00 mm Silt: 0.002 to 0.02 mm Clay: < 0.002 mm Primary minerals: sand, silt Secondary minerals: clay Primary minerals – similar to rock from which they formed, round or irregular Secondary minerals – weathering of primary minerals, plate-like micelles are formed) lots of surface area (important for soil chemistry) Based on %content of sand, silt, clay, soils are grouped into textural classes

Soil Textural Triangle

Soil Structure Formation of secondary structures called aggregates from sand, silt, clay Held together by Organic secretions by plants Microbial gums Soil wetting/drying Good structure improves soil air & water relations for roots & microbes

Soil Water Water in the soil moves along a potential gradient from wet to dry, just like during transpiration

O- H+ Cohesion/Adhesion Because of its structure, water is attracted to both the soil (adhesion) and to itself (cohesion), reducing its potential

Water Potential ()  = m + g + o For pure liquid water,  = 0 For almost anything else,  < 0  = m + g + o m = matric potential; function of adhesion/cohesion g = gravitational potential; How much force is gravity using to “pull” the water down? o = osmotic potential, caused by the presence of dissolve ions in water, often much smaller than the others All three of these potentials help to determine how much work it will take the roots to get to the water, and thus regulates water supply.

Matric Potential (m)

Matric Potential (m)

Osmotic Potential (o) Salt (NaCl) Na+ Cl- + - The beaker contains distilled water and is separated into two compartments by a semi-permeable membrane (b) Sodium chloride is added to the right compartment and dissociates into ions that create a negative osmotic potential, pulling water across the membrane (c) Water moves across the membrane until the increase in pressure in the right compartment equals the osmotic potential

Important Water Potentials Field Capacity, fc = -0.01 – 0.033 MPa Permanent Wilting Point, wp = -1.5 MPa, on average Air-Dry Soil, ad = -3.0 MPa

Available Water 0.08 0.16 0.24 0.32 0.40

Available Water Sand Silt Clay 50 40 10 25 49 1 45 5 30 20 Pore space at field capacity (% volume) Pore space at wilting point (% volume) Total Water Unavailable Water 9% Available Water 20% Available Water