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Unit 2: Soil Physical Properties Chapter 2. Unit 2 Objectives  Differences in sand, silt, clay & soil textures  Understand soil structural classes 

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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:

1 Unit 2: Soil Physical Properties Chapter 2

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

3 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

4 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




8 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

9 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

10 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)

11 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)

12 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)

13 Soil Structure  Types  Blocky (angular or subangular)  Columnar  Granular  Platy  Prismatic


15 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

16 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?

17 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

18 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

19 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

20 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?

21 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

22 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

23 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%

24 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

25 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?

26 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

27 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

28 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

29 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?

30 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

31 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

32 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

33 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)

34 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)

35 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

36 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?

37 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

38 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

39 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

40 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.)

41 Assignment  Assignment 2.1 on WebCT

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