Range & Forest Ecology: Soils What is soil? (…from the latin solum meaning floor) An independent 3-dimensional natural body occupying the earth’s surface and capable of supporting plant growth. Its properties result from the integrated effect of climate and living organisms acting upon parent material as conditioned by relief over periods of time.

Range & Forest Ecology: Soils 3 Viewpoints Pedological – soil is considered a “natural body”; emphasis is on geologic history & soil formation process (time scale of 10 3 to 10 7 years), with minor emphasis on practical utilization. Edaphological – consideration of soil from the standpoint of higher plants; emphasis is on properties of soil as they relate to plant production, e.g nutrient availability, slope, aspect, moisture, texture (time scale of 10 0 to 10 2 years). Ecological – consideration of soil as a component of an ecosystem; emphasis on flows of energy, nutrient cycling dynamics, microbial activity & decomposition (time scale of 10 1 to 10 2 years).

Range & Forest Ecology: Soils Components of Soil (plant’s perspective) 1)Mineral grains – provides anchorage, pore space (H 2 O, air), & nutrients on exchange basis. 2)Organic matter – plant & animal residues in various stages of decomposition, soil organism exudates; can be source of plant nutrients through a microbially-mediated cycling process; enhances water storage & improves soil structure. 3)Soil water – solvent medium for nutrients required by plants. 4)Soil air – provides oxygen for cellular function & atmospheric nitrogen for N-fixing organisms (atmosphere 78% N 2 ).

Range & Forest Ecology: Soils The development of soil can be thought of as occurring in two phases… 1)Soil Genesis – the weathering of rock substrates by: Mechanical forces – expansion & contraction due to thermal flux; erosional forces by wind & water; plant roots can exert sufficient forces to cleave & exfoliate rock fragments. Chemical reactions – many; principal agent is percolating rain water charged with CO 2 (weak acid solution) which affects carbonates (dissolution), feldspars & micas (hydrolyze to clays & release cations); biological processes form organic acids.

Range & Forest Ecology: Soils Continued… 2)Soil Formation – Hans Jenny (1941) characterized soil formation (S) as a function of five independent variables: climate (cl), organisms (o), topography (r), parent material (p), & time (t). S = ƒ(cl, o, r, p, t) Organism include such elements as the soil microbial community, litter inputs, vegetation type. Parent material largely determines chemical characteristics of the derived soils. → The interaction of organisms & parent material with climate produce a soil with characteristic features. Jenny, H The factors of soil formation. New York: McGraw-Hill.

Range & Forest Ecology: Soils Continued… Parent material is “the state of the soil system at time zero for a particular set of soil-forming environmental factors.” Rocks & Minerals 1)In Place 2)Transported Agent Gravity Water Ice Wind Parent Materials Residual – Less than ½ of earth’s surface Colluvial – Gravity-induced Alluvial – Flood plain Marine – Fluctuating sea level Lacustrine – Lake deposit Glacial – Till, outwash Aeolian - Loess

Range & Forest Ecology: Soils Soil Profile Development – Soil formation is largely a biochemical process, whereby: Organic material (plant roots, litter inputs, soil organisms) mixes with inorganic mineral fractions. Microbial activity accelerates chemical weathering. Primary/original minerals are transformed into secondary minerals. Resulting in development of horizons with characteristic color, textural & structural qualities.

Range & Forest Ecology: Soils Regolith – Unconsolidated layer above hard, unweathered, bedrock Solum – Upper portion of the regolith that has been altered through biochemical and physical processes. The material between the solum and bedrock is referred to as the C horizon. It is slowly changing into solum. Pedon – A 3-D sampling that displays the full range of properties that are characteristic of a soil (1-10 m 2 ). Soil Profile – One vertical face of a pedon. Soil Horizons – Horizontal layers, differentiated by color or texture, described within a profile Soil Profile Development – continued…

Range & Forest Ecology: Soils Soil Profile Development – typical horizons: Master Horizons O – Organic horizon (>20% OM by weight); partly or mostly decomposed OM; found in wetlands, forest litter layers; provides nutrients (nitrogen, potassium, etc.), aids soil structure (acts to bind particles), enhances soil moisture retention. A – Mineral horizon with accumulated OM (<20%); typically darker than other horizons. E – Zone of maximum leaching (eluviation); loss of clays, soluble OM, Fe oxides; common in forest soils; light colored. B – Mineral horizon with accumulation ( illuviation ) from above; usually contains most clays and fines ; usually a more dense layer. C – Soil parent material, either in situ or transported; minimal weathering and biologic activity. R – Hard, unweathered bedrock.

Range & Forest Ecology: Soils Soil Profile Development – continued… All master horizons will not necessarily occur in a single soil. When they do they are normally found in this order Master horizons can be subdivided using numbers, ex. A1- A2-A3

Range & Forest Ecology: Soils O Horizon 1.Organic materials above mineral soil (litter layer) 2.Oi - slight decomposition 3.Oe - medium decomposition 4.Oa - highly decomposed

Range & Forest Ecology: Soils A Horizon 1.Humus accumulation in mineral soil 2.Darkened color

Range & Forest Ecology: Soils E Horizon 1.Zone of maximum eluviation; loss of organic matter, clays, and/or Fe-oxides 2.No humus accumulation = lighter color

Range & Forest Ecology: Soils 1. Illuviated - accumulation of clays (silicate, Fe & Al oxides), O.M., CaCO 3,... - usually, from above (A or E) some from sides or below 2. Complex in highly developed soils - numerous subdivisions, e.g. color, lime, structure, etc. B Horizon

Range & Forest Ecology: Soils 1. Unconsolidated material 2. Little affected by weathering processes C Horizon

Range & Forest Ecology: Soils Physical Properties – Texture & Structure 1) Texture – Characterization of the solid inorganic phase of soils & refers to the relative proportions of sand, silt & clay; called the soil separates & constitute the “fine earth” fraction (diameter < 2 mm) of the soil. Very Coarse Sand Coarse Sand Medium Sand Fine Sand Very Fine Sand Diameter (mm) 2.0 – – – – – – less than Soil Separate Sand Silt Clay

Range & Forest Ecology: Soils Physical Properties – Texture & Structure 1) Texture – Continued… Rock fragments (diameter > 2 mm) are described separately on a pedon description; modifiers are used when rock fragments occupy more than 15 percent by volume, ex. gravelly sandy loam. Organic horizons (Oi, Oe, Oa) are described based on the degree of decomposition (fibric, hemic, sapric, etc.).

1) Texture – Continued… Particle Size Analysis 1. In the laboratory: mechanical analysis - Sands are determined by sieving - Silt and clay is determined based on settling time using Stoke's Law

Texture – Continued… 2) Steps to determine soil texture in the field by “feel” have been developed; useful for a quick & dirty estimate; requires practice (see handout).

Range & Forest Ecology: Soils Physical Properties – Texture & Structure 2) Structure – Soil structure is the way soil particles aggregate together into what are called peds. Peds come in a variety of shapes depending on the texture, composition, and environment.

Range & Forest Ecology: Soils Physical Properties 2) Structure – Continued… Agents that bind aggregates & promote stability: fungal mycelia, microbial exudates, H & Ca ions, & clays. Structural stability is the capacity for the peds to retain their shape, i.e. absorb water & not disintegrate. Soil aeration, water movement, & plant root penetration are greatly enhanced by good soil structure.

Range & Forest Ecology: Soils Soil Chemistry: Important aspect is the relationship between the ions (i.e. nutrients) (a) in soil solution, (b) adsorbed on charged particles, & (c) as constituents of mineral & organic fractions, which tend towards equilibrium; dynamic interaction.

Range & Forest Ecology: Soils Organic Matter & Biota Exchangeable ions Surface adsorption Solid phases & Minerals Nutrient Uptake by Plants Soil Solution Soil Air Rainfall, Evaporation, Drainage, Addition of Fertilizer

Range & Forest Ecology: Soils Soil Chemistry The availability of plant nutrients are most directly influenced by (1) soil pH, and (2) cation exchange capacity (CEC). 1) Soil pH – –Strictly, measure of the hydrogen ion concentration in solution: - log 10 [H + ]; [H + ] = concentration (activity) of H + –Neutrality is pH = 7 (H + = OH - ) –As acidity , pH 

Range & Forest Ecology: Soils pH range in soils

pH vs Availability of Nutrients Strongly AcidStrongly Alkaline Med. Acid Slightly. Acid Very Slightly Acid Very Slightly Alkaline Slightly Alkaline Med. Alkaline Phosphorus Potassium Calcium Magnesium Sulfur Iron Manganese Boron Copper and Zinc Molybdenum Nitrogen

Range & Forest Ecology: Soils Soil Chemistry 2) Cation Exchnage Capacity (CEC) – Measure of the number of negatively charged sites on charged particles that attract exchangeable cations (via electrostatic charge). Ca ++ Mg ++ K+K+ H+H+ Al +++ NH 4 + Ca ++ Al +++ Mg ++ NH 4 + K+K+ K+K+ H+H+ H+H+ Mineral particle OM fraction (Humus)

Range & Forest Ecology: Soils Important CEC properties: Size of CEC determined by soil properties (permanent & pH-dependent charge) Large quantity of nutrients can be held on CEC (particularly clay) Provides rapid buffering for most cations Exchangeable ions held against leaching Exchangeable ions readily available through cation exchange

Range & Forest Ecology: Soils Summary Components of soil are mineral grains (anchorage & nutrients), organic matter (nutrient cycling), water (solvent medium), & air (oxygen & nitrogen). Soil formation is the product of climate, organisms, topography, parent material, & time. Soil profile development encompasses the interaction of the above factors to develop characteristic soil horizons. Soil texture refers to content of sand, silt, & clay particles. The nature & arrangements of peds & aggregate stability are the most significant characteristics of soil structure. The central concepts in soil chemistry are based on the equilibrium of ions in solution. CEC is a measure of exchangeable cations on negatively charged sites in the soil, & is influenced by the presence & kinds of clay minerals & humus.