Presentation on theme: "Unit 2: Soil Physical Properties Chapter 2. Unit 2 Objectives Differences in sand, silt, clay & soil textures Understand soil structural classes "— Presentation transcript:
Unit 2: Soil Physical Properties Chapter 2
Unit 2 Objectives Differences in sand, silt, clay & soil textures Understand soil structural classes Importance of soil porosity & aeration Knowledge of soil color and its importance
Soil Texture Soil Separates – particle size groups of sand, silt, and clay Proportion of each determines the soil texture Texture affects water intake rates, water storage, soil tilth, aeration, fertility
Soil Texture Soil Textural Classes Clay – soils that are more than 60% clay Silt – soils with high silt content Sand – soils with highest content of sand Soils that don’t exhibit a dominant area in any of the three called loam Soil Textural Triangle Organic matter content has no bearing on these values
Soil Texture Particle Size Analysis How to determine soil textural classification Stoke’s Law Settling rates of each of the soil separates based upon its buoyancy, gravity, and resistance to water friction Placing a soil sample into proper solution, then allowing each soil separate to settle will help determine soil texture
Rock Fragments Particles >2 mm diameter called rock fragments & can be classified by shape Have no bearing on soil texture Rounded fragments Gravel, cobble, stone, boulder Flat fragments Channer (smallest), flagstone, stone, boulder
Rock Fragments % of rock fragments in a soil may be used to help describe a soil texture <15% by volume: no mention 15 to 35% by volume: name the dominant kind of rock fragment (ex. Stony loam) 35 to 60% by volume: add very to the description (ex. Very Stony loam) >60% by volume: substitute extremely into description (ex. Extremely Stony loam)
Soil Structure Soil Structure – arrangement of particles into aggregates Aggregates – secondary units composed of many soil particles held together by organic matter, iron oxides, carbonates, clays, etc. Peds – natural aggregates, vary in water stability (clod is used if soil is broken by artificial means)
Soil Structure Fragment – pieces of broken peds Concretion/Shot – mass of precipitation of certain chemical dissolved in percolating waters Soil Structural Classes Peds described by three characteristics Type (shape) Class (size) Grade (strength of cohesion)
Soil Structure Classes Very fine, fine, medium, coarse, very coarse Grades Evaluated by distinctness, stability, & strength of the peds Structureless Soils: no noticeable peds Noncoherent mass of sand (single grain) Cohesive mass such as clay soils around here (massive) Especially found in lowland wet soils
Soil Structure Structured soils Weak: peds can barely be distinguished Moderate: peds visible, most can be handled without breaking Strong: very visible peds, easily handled without breaking Structure is very important influence on soil properties What affect might different structures have on soil? Infiltration of air, fertilizers, & water?
Soil Structure Genesis of Soil Structure Peds form due to shrink/swell of soil & adhesive materials Mostly 5/6 sided shapes Prismatic structure tends to develop early in the genesis of soil w/ vertical cracking More blocky structure will develop as the soil matures (especially in clay soils) due to horizontal cracking
Soil Structure Granular peds Tends to be influenced by: tillage, rodents, worms, frost action Held together by organic matter Mostly round shapes Limited to surface horizon Platy structure Requires force: water, equipment, livestock
Soil Structure Deterioration of Aggregates Increasing Na+ as exchangeable ions speeds deterioration of soil structure Disperses ions in the soil, therefore, breaking natural soil bonds Often forms when water has high salt content, and improper drainage
Soil Porosity & Permeability Pore spaces – portion of the soil not occupied by mineral or organic solids Often referred to as the soil matrix Typically occupied by: air, water, living roots Irregular shape, size, & direction to pores Which soil has the largest/smallest pores? How does that affect the soil & crops?
Soil Porosity & Permeability Pore sizes are more important than total pore space Relative amounts of air & water in pores fluctuates Rain Deep percolation Transpiration Evaporation
Soil Air Free oxygen must be available Required for root growth (respiration) and by soil microbes for organic matter decomposition Well-aerated soil is best, w/ rapid, continuous gaseous exchange Factors affecting gas exchange rates Pore sizes Pore continuity Temperature
Soil Air Depth in the soil Wetting/drying Coverings on the soil surface Composition of Soil Air Atmospheric air N 2 = 79% O 2 = 20.9% CO 2 =.038%
Soil Air Soil air Some O 2 used, much CO 2 produced Soil air CO 2 may be 10% Range of O 2 values from 10% to virtually none What type of soil would be on each end of the range? Rates of O 2 Exchange Oxygen diffusion rate (ODR) – rate at which gases in the soil exchange w/ O 2 in the atmosphere
Soil Air Factors affecting ODR Pore size Water filled pores Diffusion of CO 2 gas through water is 10,000x slower through water than air Depth in the soil At ~3’ depth, ODR is ½ to ¼ rate of top few in. So, how does this affect our high-clay soils? What does is affect? What makes the problems worse? What might improve ODR?
Soil Air Oxidation-Reduction Potential (Eh or Redox) Describes tendency for chemicals in the soil or water to be oxidized A measure of the availability of O 2 in the soil High redox = O 2 is present, low redox = O 2 absent Most plants must have O 2 in the soil at root growth Give an example of a plant that doesn’t
Soil Air Most plants grow best in an oxidized (aerated) soil Free oxygen is the primary acceptor of electrons in the soil What does this mean? More soil nutrients stay/converted soil plant available forms N is not lost to the atmosphere as much Plant roots are able to respire
Soil Air Aeration & Energy for Plant Growth Energy obtained from sun Stored in chemical bonds (photosynthesis) Energy released by breaking the bonds (respiration) w/ O 2, aerobic glycolysis plus respiration makes much more energy available to the plant ~19x more than anaerobic glycolysis
Soil Air Anaerobic glycolysis Results in much less energy availability Decomposition of organic matter is much slower How do deficient O 2 concentrations occur? Waterlogging Compaction High clay soils what pinch pores when wet O 2 consuming organic matter decomposers What can we do as managers of the soil to improve O 2 concentrations?
Consistence (Strength) Consistence – soil’s response to mechanical forces Resistance to rupture Soft/hard when dry Friable (crumbly), firm, rigid when wet Plasticity Tolerate considerable deformation w/out breaking Stickiness Ease w/ which the soil is manipulated, or even walked on
Soil Color Dark soils absorb more heat than light colored soils Do you think this helps explain some planting date differences? Just because they’re dark doesn’t mean they’re warmer Depends on soil moisture as well
Soil Color Soil Color vs. Soil Properties White colors – common w/ salts or lime deposits are present Mottles (rust colors) – soil may have periods of inadequate aeration Gleying (bluish, grayish, greenish) – subsoils, prolonged periods of waterlogging Darker colors – higher levels of organic matter
Soil Color Munsell Color Charts Chart used to help ID soil color accurately Hue: dominant spectral or rainbow color Value: relative blackness or whiteness Chroma: purity of the color (as chroma increases, the color is more brilliant)
Soil Temperature Relation of Soil & Air Temp Net heat absorbed by the Earth = heat lost in form of longwave radiation Photoperiod – affected by latitude Soil temp can change by soil depth & time of day Takes significant air temp changes to change soil temp deeper than 12” (& more than just daily range)
Soil Temperature Avg. summer & winter soil 3’ rarely differ by more than 9° F Factors Affecting Soil Temp How much heat reaches the soil surface Soil coverings Plastic mulches Sun angle Slope face Soil
Soil Temperature What happens to the heat in the soil (dissipation) Amount of heat needed to change soil temp = heat capacity Greatly affected by soil water content How? Thermal conductivity – increases w/ soil-water content increasing, decreases as air-filled pores increase Moist soils resist temp change, but conduct heat readily Dry soils change temp faster, but conduct heat poorly What does this mean for the soil, which is better?
Soil Temperature Living w/ Existing Temps Maximizing seed germination & growth Wheat – 40 to 50° F Corn – 50 to 85° F When using anhydrous Apply when soil 4” is 50° F or less Reduces N losses Freeze/thaw May cause heaving – resulting in death of shallow rooted crops
Soil Temperature Responsible for bringing stones to the surface in fields Modifying Temp Effects If you have crops that are feasible/profitable to do so Clear plastic surface covers Increases soil temp faster Clear plastic mulches Can speed growth & maturity of sweet corn & strawberries
Soil Physical Properties & Engineering AASHTO & Unified Engineering Soil Classification System Used by engineers to classify soils based on particle size to determine construction limitations Atterberg Limits Liquid limit – relates to the amount of water a soil can retain & not break Plastic limit – the water content at which a thread of soil can no longer hold together
Soil Physical Properties & Engineering Plasticity Index – difference between liquid limit & plastic limit Important measures for engineers to be able to understand what the soil will do under various conditions Helps then understand what moisture needs to be present for effective compacting (make a solid base for roadways, buildings, etc.)