2Unit 2 Objectives Differences in sand, silt, clay & soil textures Understand soil structural classesImportance of soil porosity & aerationKnowledge of soil color and its importance
3Soil TextureSoil Separates – particle size groups of sand, silt, and clayProportion of each determines the soil textureTexture affects water intake rates, water storage, soil tilth, aeration, fertility
4Soil Texture Soil Textural Classes Clay – soils that are more than 60% claySilt – soils with high silt contentSand – soils with highest content of sandSoils that don’t exhibit a dominant area in any of the three called loamSoil Textural TriangleOrganic matter content has no bearing on these values
8Soil Texture Particle Size Analysis How to determine soil textural classificationStoke’s LawSettling rates of each of the soil separates based upon its buoyancy, gravity, and resistance to water frictionPlacing a soil sample into proper solution, then allowing each soil separate to settle will help determine soil texture
9Rock FragmentsParticles >2 mm diameter called rock fragments & can be classified by shapeHave no bearing on soil textureRounded fragmentsGravel, cobble, stone, boulderFlat fragmentsChanner (smallest), flagstone, stone, boulder
10Rock Fragments% of rock fragments in a soil may be used to help describe a soil texture<15% by volume: no mention15 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)
11Soil StructureSoil Structure – arrangement of particles into aggregatesAggregates – 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)
12Soil Structure Fragment – pieces of broken peds Concretion/Shot – mass of precipitation of certain chemical dissolved in percolating watersSoil Structural ClassesPeds described by three characteristicsType (shape)Class (size)Grade (strength of cohesion)
13Soil Structure Types Blocky (angular or subangular) Columnar Granular PlatyPrismatic
15Soil Structure Classes Grades Structureless Soils: no noticeable peds Very fine, fine, medium, coarse, very coarseGradesEvaluated by distinctness, stability, & strength of the pedsStructureless Soils: no noticeable pedsNoncoherent mass of sand (single grain)Cohesive mass such as clay soils around here (massive)Especially found in lowland wet soils
16Soil Structure Structured soils Weak: peds can barely be distinguishedModerate: peds visible, most can be handled without breakingStrong: very visible peds, easily handled without breakingStructure is very important influence on soil propertiesWhat affect might different structures have on soil?Infiltration of air, fertilizers, & water?
17Soil Structure Genesis of Soil Structure Peds form due to shrink/swell of soil & adhesive materialsMostly 5/6 sided shapesPrismatic structure tends to develop early in the genesis of soil w/ vertical crackingMore blocky structure will develop as the soil matures (especially in clay soils) due to horizontal cracking
18Soil Structure Granular peds Platy structure Tends to be influenced by: tillage, rodents, worms, frost actionHeld together by organic matterMostly round shapesLimited to surface horizonPlaty structureRequires force: water, equipment, livestock
19Soil Structure Deterioration of Aggregates Increasing Na+ as exchangeable ions speeds deterioration of soil structureDisperses ions in the soil, therefore, breaking natural soil bondsOften forms when water has high salt content, and improper drainage
20Soil Porosity & Permeability Pore spaces – portion of the soil not occupied by mineral or organic solidsOften referred to as the soil matrixTypically occupied by: air, water, living rootsIrregular shape, size, & direction to poresWhich soil has the largest/smallest pores?How does that affect the soil & crops?
21Soil Porosity & Permeability Pore sizes are more important than total pore spaceRelative amounts of air & water in pores fluctuatesRainDeep percolationTranspirationEvaporation
22Soil Air Free oxygen must be available Required for root growth (respiration) and by soil microbes for organic matter decompositionWell-aerated soil is best, w/ rapid, continuous gaseous exchangeFactors affecting gas exchange ratesPore sizesPore continuityTemperature
23Soil Air Composition of Soil Air Atmospheric air Depth in the soil Wetting/dryingCoverings on the soil surfaceComposition of Soil AirAtmospheric airN2 = 79%O2 = 20.9%CO2 = .038%
24Soil Air Rates of O2 Exchange Soil air Some O2 used, much CO2 producedSoil air CO2 may be 10%Range of O2 values from 10% to virtually noneWhat type of soil would be on each end of the range?Rates of O2 ExchangeOxygen diffusion rate (ODR) – rate at which gases in the soil exchange w/ O2 in the atmosphere
25Soil Air Factors affecting ODR Pore sizeWater filled poresDiffusion of CO2 gas through water is 10,000x slower through water than airDepth in the soilAt ~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?
26Soil Air Oxidation-Reduction Potential (Eh or Redox) Describes tendency for chemicals in the soil or water to be oxidizedA measure of the availability of O2 in the soilHigh redox = O2 is present, low redox = O2 absentMost plants must have O2 in the soil at root growthGive an example of a plant that doesn’t
27Soil Air Most plants grow best in an oxidized (aerated) soil Free oxygen is the primary acceptor of electrons in the soilWhat does this mean?More soil nutrients stay/converted soil plant available formsN is not lost to the atmosphere as muchPlant roots are able to respire
28Soil 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
29Soil Air Anaerobic glycolysis Results in much less energy availabilityDecomposition of organic matter is much slowerHow do deficient O2 concentrations occur?WaterloggingCompactionHigh clay soils what pinch pores when wetO2 consuming organic matter decomposersWhat can we do as managers of the soil to improve O2 concentrations?
30Consistence (Strength) Consistence – soil’s response to mechanical forcesResistance to ruptureSoft/hard when dryFriable (crumbly), firm, rigid when wetPlasticityTolerate considerable deformation w/out breakingStickinessEase w/ which the soil is manipulated, or even walked on
31Soil 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 warmerDepends on soil moisture as well
32Soil Color Soil Color vs. Soil Properties White colors – common w/ salts or lime deposits are presentMottles (rust colors) – soil may have periods of inadequate aerationGleying (bluish, grayish, greenish) – subsoils, prolonged periods of waterloggingDarker colors – higher levels of organic matter
33Soil Color Munsell Color Charts Chart used to help ID soil color accuratelyHue: dominant spectral or rainbow colorValue: relative blackness or whitenessChroma: purity of the color (as chroma increases, the color is more brilliant)
34Soil Temperature Relation of Soil & Air Temp Net heat absorbed by the Earth = heat lost in form of longwave radiationPhotoperiod – affected by latitudeSoil temp can change by soil depth & time of dayTakes significant air temp changes to change soil temp deeper than 12” (& more than just daily range)
35Soil Temperature Factors Affecting Soil Temp Avg. summer & winter soil 3’ rarely differ by more than 9° FFactors Affecting Soil TempHow much heat reaches the soil surfaceSoil coveringsPlastic mulchesSun angleSlope faceSoil
36Soil Temperature What happens to the heat in the soil (dissipation) Amount of heat needed to change soil temp = heat capacityGreatly affected by soil water contentHow?Thermal conductivity – increases w/ soil-water content increasing, decreases as air-filled pores increaseMoist soils resist temp change, but conduct heat readilyDry soils change temp faster, but conduct heat poorlyWhat does this mean for the soil, which is better?
37Soil Temperature Living w/ Existing Temps Maximizing seed germination & growthWheat – 40 to 50° FCorn – 50 to 85° FWhen using anhydrousApply when soil 4” is 50° F or lessReduces N lossesFreeze/thawMay cause heaving – resulting in death of shallow rooted crops
38Soil Temperature Modifying Temp Effects Responsible for bringing stones to the surface in fieldsModifying Temp EffectsIf you have crops that are feasible/profitable to do soClear plastic surface coversIncreases soil temp fasterClear plastic mulchesCan speed growth & maturity of sweet corn & strawberries
39Soil Physical Properties & Engineering AASHTO & Unified Engineering Soil Classification SystemUsed by engineers to classify soils based on particle size to determine construction limitationsAtterberg LimitsLiquid limit – relates to the amount of water a soil can retain & not breakPlastic limit – the water content at which a thread of soil can no longer hold together
40Soil Physical Properties & Engineering Plasticity Index – difference between liquid limit & plastic limitImportant measures for engineers to be able to understand what the soil will do under various conditionsHelps then understand what moisture needs to be present for effective compacting (make a solid base for roadways, buildings, etc.)