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 Types Blocky (angular or subangular) Columnar Granular Platy Prismatic

Soil Structure Classes Grades Structureless Soils: no noticeable peds 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 Platy structure 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 Composition of Soil Air Atmospheric air Depth in the soil Wetting/drying Coverings on the soil surface Composition of Soil Air Atmospheric air N2 = 79% O2 = 20.9% CO2 = .038%

Soil Air Rates of O2 Exchange Soil air Some O2 used, much CO2 produced Soil air CO2 may be 10% Range of O2 values from 10% to virtually none What type of soil would be on each end of the range? Rates of O2 Exchange Oxygen diffusion rate (ODR) – rate at which gases in the soil exchange w/ O2 in the atmosphere

Soil Air Factors affecting ODR Pore size Water filled pores Diffusion of CO2 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 O2 in the soil High redox = O2 is present, low redox = O2 absent Most plants must have O2 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/ O2, 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 O2 concentrations occur? Waterlogging Compaction High clay soils what pinch pores when wet O2 consuming organic matter decomposers What can we do as managers of the soil to improve O2 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 Factors Affecting Soil Temp Avg. summer & winter soil temps @ 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 temp @ 4” is 50° F or less Reduces N losses Freeze/thaw May cause heaving – resulting in death of shallow rooted crops

Soil Temperature Modifying Temp Effects 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.)

Assignment Assignment 2.1 on WebCT