Soil Properties Lecture Notes PDF

Physical Properties of Soil Chapter 4  Objectives After completing this chapter, you should be able to: Describe the concept of soil texture and its importance Identify the texture of a sample of soil Describe soil permeability and related properties Descr...

Physical Properties of Soil Chapter 4  Objectives After completing this chapter, you should be able to: Describe the concept of soil texture and its importance Identify the texture of a sample of soil Describe soil permeability and related properties Describe structure and its formation and importance Explain other physical properties Discuss soil compaction and tilth Physical Properties of Soil Soil characteristics: grower can see or feel Neither chemical nor biological, but both affect them Greatly affect how soils are used to grow plants or other activities  Soil Texture Affects soil traits Water-holding capacity aeration Most fundamental soil property Determined by the proportion of soil particles Sand (large) Silt (medium) Clay (small) Soil Particle Size Effects of Particle Size Affects two important features Specific surface area Soil pores: Number and Size Macropores (aeration pores): large Micropores: small  Specific Surface Area Is the total surface area of all the soil particles in a volume of soil. Why important? Reactions on particle surfaces Increased surface area, more nutrients Water is held as film around soil particles Increased surface area, more total water films Surface Area Surface Area  Soil Pores Macropores (aeration pores): large Twisted nature slows movement of water and air As water drains, pulls air with it, occupying macropores Micropores: small Retains some water after draining Water films merge, excluding oxygen Difficult pathway for water movement Soil Pores Soil particle size affects pore size Small particles create many pores Large particles create fewer but larger pores Micropores usually hold water Macropores usually hold air  Soil Separates Division of mineral particles Used by soil scientists Consists of three broad classes Sand (divided into four subcategories) Silt Clay Makes up fine earth fraction portion of the mineral particles of soil smaller than 2 millimeters. Determines soil texture  Sand Characteristics Largest of the soil separates Composed mainly of weathered grains of quartz or other minerals Particles range in size Enough sand in a soil creates large pores, so sand improves water infiltration (rate at which water enters the soil) and aeration Large amounts of sand lower the ability of the soil to retain water and nutrients  Silt Characteristics Medium-sized soil separate Silt particles are silky or powdery to the touch, like talc Best ability to hold large amounts of water in a form plants can use Erodes readily in moving water and wind  Clay Characteristics Smallest of the soil separates Consists of tiny, sheet-like crystals Results from chemical reactions between weathered minerals to form tiny particles of new minerals USDA System of Soil Separates Comparison of Soil Separate Size  What about the other “big stuff”? Large particles (larger than 2 mm i.e. gravel) not part of soil texture Add little specific surface area and no pores Reduces water- and nutrient-holding capacity Especially if >15% of soil volume  Textural Classification Soil usually consists of more than one soil separate All three separates are found in most soils Actual % is called soil texture 12 textural classes shown in the soil triangle Determining soil texture Amount of sand, silt, and clay in a soil can be measured by mechanical analysis  Soil Triangle  Textural Classification Largest class: Clay Why? As little as 40% clay Loam Mischaracterized as equal sand, silt, and clay But portions are close in % Sand Particle size greatly influences soil properties Note dominant sand fraction (coarse sandy loam, loamy fine sand)  Textural Classification Soils can generally be classified as fine, medium, or coarse  Characteristics of Textural Classes – Indicative of several soil properties Infiltration Downward entry of water into the soil Percolation Downward movement of water through the soil profile Water-holding capacity Example: Fine soils retain nutrients better Rapid percolation in coarse soils leaches nutrients Clay particles retain nutrient chemicals  Soil Density and Permeability Particle density (PD) Density with no pore spaces Varies due to mineral type and organic matter Average 2.65 grams per cubic centimeter, low variation Bulk density (BD) Actual density of a soil is less than the PD (includes pore space) More pore space – lower BD Variation due to volume of pore space Indicator of soil quality – higher or lower better? BD = weighted dry soil/volume dry soil = g/cm3  Porosity Soil porosity Measure of the soil volume that holds air and water Total pore space – measure of the soil volume that holds air and water Porosity = (wet weight (g) – dry weight (g)) X 100 soil volume Porosity from BD and PD: Porosity = BD/PD X100 Which is more porous – sand or clay? Texture Affects Soil Pores  Permeability Permeability Ease with which air, water, and roots move through the soil Depends on number of pores, but mostly size and continuity of pores Descriptive term with no numerical value – can’t be directly measured Movement of water can be measured – hydraulic conductivity “perc test”  Soil Structure How soil clumps together to form soil aggregates Peds: naturally occurring aggregates Sand grain to several inches more aggregated soil – better air and water flow Clods: clumps of soil caused by tillage Soil aggregation improves soil fertility and quality of its functions Classification traits Grade: ped distinctiveness and strength Class: ped size Type: shape Soil Structure: Class Soil Structure: Type Soil Structure: Type Example: Strong fine granular structure  Structureless Soil Single-grain Sandy soils and soils without structure Massive soils Fine textured soils that lack structure or function as a soil mass Lack permeability  Types of Soil Structure Granular structure Commonly found in A horizons Platy structure Usually found in E horizons Blocky structure Typical of many B horizons Prismatic structures Tend to occupy lower B and C horizons  Formation of Soil Structure Two-step process Soil creates a loose ped and the second cements it Weak aggregates are cemented to make them distinct and strong Large aggregates often contain smaller units Smaller units held together by calcium ions that bridge adjacent soil particles (flocculation) When sodium displaces calcium (deflocculation), structure degrades Structure is not a permanent soil feature Can be degraded by mismanagement  Soil Consistence Behavior of soil when pressure is applied Relates to the degree that soil particles stick to one another or other objects Depends on how moist a soil is Soils types and characteristics Wet soil: stickiness and plasticity Moist soil: loose, friable, and firm Dry soil: determined by trying to crush an air-dried mass of soil in the hand moisture levels. Consistence Terms for Soil Wet Stickiness Plasticity Moist Dry Nonsticky Nonplastic Loose Loose Slightly sticky Slightly plastic Very friable Soft Sticky Plastic Friable Slightly hard Very sticky Very plastic Firm Moderately hard Very firm Hard Extremely firm Very hard  Soil Tilth Physical condition of tilled soil Suggests how easy the soil is to till How easily a good seedbed can be made How easily seedlings can come up Ease of root growth Combination of physical properties: texture, structure, permeability, and consistence Tillage Improves soil tilth for a time, improving soil air–water relations for new seedling Possible negative impacts?  Injuries to Soil Tilth: Compaction Results when pressure is applied to the soil surface – Primarily alters soil traits related to pores and soil strength (degree that a soil can resist movement owing to pressure) Can create: – Reduced porosity and permeability – Reduced air exchange and potassium uptake – Decreased infiltration rates – Increased erosion and evolution of nitrous oxide – Reduced percolation and oxygen availability – Tillage pan – compacted layers just below depth of tillage – If soil particles cement together - hardpan  Soil Compaction Measurement Compare bulk density with nearby unaffected soil BD Most accurate Cone penetrometer  Injuries to Soil Tilth: Direct Aggregation Destruction Plowing tends to create large aggregates Rototiller crushes soil aggregates Favored by gardeners for workability Structure degraded over time How tillage destroys aggregates Stirring in oxygen – increase organic matter oxidation Decreasing organic glue holding peds together Smashes non-weakened peds  Injuries to Soil Tilth: Puddling and Clods Working wet soil – bad…especially soils high in clay When pressure is applied to wet soil aggregates, they fall apart Puddling – conversion of aggregates soil into massive soil Decreased water movement Puddling on purpose? Rice paddies, canals, and reservoirs Reduces water leaking away  Injuries to Soil Tilth: Surface Crusts Raindrops hit bare soil and destroy aggregates Small particles fill pores between large particles Dries into hardened crust Higher clay content worsens effect Irrigation can increase crust formation Result in: Reduced water infiltration and soil aeration Reduced seed emergence Irrigation must be applied slower  Improving Tilth Based on texture, structure, permeability, and consistence Texture and consistence usually can’t be changed Improving structure and avoiding compaction  Improving Tilth: few BMPs Never work wet or very dry soil Avoid unnecessary traffic Reduce number of tillage operations Use modern tillage practices Deep plowing or subsoiling  Physical Property Management: Overview Considerations – Avoid aggregate destruction – Avoid puddling and clods – Minimize surface crusting – Improve tilth  Soil Channels and Pans Channels – Large, continuous pores extending from the surface and leading deeper into the soil – Channels created by soil life - biopores Any hardened layer of soil is a pan  Types of Pans – Claypans Occur where extreme illuviation has caused a very high clay content in a subsoil layer – Fragipans Like claypans, result from clay accumulation. Clay combines into hard, brittle layer – Plinthite Layers cemented by special clay common to tropics Hardens to brick-like substance and cannot be reversed by wetting – Caliche and duripans Layers of soils in which chemicals cement soil particles together Lime cements caliche – American Southwest common issue  Soil Temperature Critical for plant growth and development 3 Important factors – Sunlight and air energy inputs – Absorption and conductance of heat – Loss of heat at the surface Temperature effects – seed germination – root growth – water and nutrient availability – biological activity  Managing Soil Temperature Natural conditions with no control – Light intensity and air temperature Watering and soil manipulations – Watering can cool – Plowing in fall can raise spring soil temperatures – Mulching most effective Fire and Soil Temperature – After a fire, vegetation is gone (allow light penetration) – Black ash absorbs more heat – Mineral resources recycled back to soil  Soil Color Indicates: – Nutrient composition – Organic matter – Drainage of soils Color as a guide to soil use – Used to classify soils according to color Describing Soil Color – Munsell color system – matches color chips Hue – color (i.e. red or yellow) Value – lightness or darkness of color Chroma – purity of a dominant color – denoted as a number Munsell Color System