Physical Properties of Soil

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• Physical properties of soil include color, texture, structure, porosity, density, consistence, temperature, and air.

• Soil texture, structure, porosity, density, and consistence are related to types of soil particles and their arrangement.

• Primary particles include sand, silt, and clay, categorized based on their effective diameter.

• Soil texture is the relative proportion of sand, silt, and clay in a soil.

• There are 12 textural classes ranging from sand to clay.

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• Soil structure is the arrangement of soil particles into different geometric patterns.

• Soil structure is classified based on shape, size, and stability.

• Soil texture and structure regulate porosity, density, water and air movement in soil.

• Soil temperature is slightly higher than air temperature and influences soil biota and plant processes.

• Soil air contains higher carbon dioxide and moisture and lower oxygen concentration than atmospheric air.

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• Soil physical properties and management practices affect the availability of water, air, and nutrients for plant growth.

• Determining the physical properties of soils helps in effective soil management.

Soil Texture

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• Soil texture is determined by the relative proportions of sand, silt, and clay in the soil.

• Soil texture influences properties such as porosity, permeability, water-holding capacity, and susceptibility to erosion.

• Loam soil with a small amount of organic material is considered ideal for conventional agriculture.

• Soil texture affects soil behavior, nutrient retention capacity, and water retention.

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• Sand resists compaction and increases soil porosity.

• Silt is more chemically and physically active than sand.

• Clay has a high retention capacity for water and nutrients and resists erosion.

• Clay soils bond tightly and have a strong mitigation effect of organic matter.

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• Soil components larger than 2.0 mm are classified as rock and gravel.

• Organic soils have a substantial organic component.

• Organic soils are classified based on the percentage of clay and organic matter.

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• Soil texture is classified based on the diameter range of soil particles.

• Sand is coarse and gritty, silt is smooth and powdery, and clay is sticky and plastic.

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• Soil texture influences properties such as water-holding capacity, aeration, drainage rate, organic matter level, decomposition of organic matter, compactability, erosion susceptibility, shrink/swell potential, sealing ability, tillage suitability, pollutant leaching potential, nutrient storage ability, and resistance to pH change.

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• Soil textural triangle is used to determine soil classes based on physical texture.

• Soil texture can be determined qualitatively or quantitatively.

• Soil texture has agricultural applications and helps determine crop suitability and soil response to environmental and management conditions.

• Different classification systems use different textural classes based on the percentages of sand, silt, and clay.

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• Twelve major soil texture classifications defined by the USDA

• Classifications named after primary constituent particle size or combination

• Loam used to describe equal properties of sand, silt, and clay

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• Soil texture quantitatively defined as relative proportions of sand, silt, and clay

• Soil texture triangle used to represent different soil textures

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• Soils classified into three textural groups and twelve textural classes

• Arranged in increasing fineness

• Loam is a soil texture with equal proportions of sand, silt, and clay

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• Soil texture influences various soil properties such as porosity, water holding capacity, aeration, etc.

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• Soil structure formed by clumping of sand, silt, and clay

• Aggregates form peds, which shape soil into distinct geometric forms

• Soil clods result from mechanical disturbance of the soil

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• Different soil structural classes: Granular, Prismatic, Blocky, Single Grained, Platy

• Each class has distinct characteristics and is found in different soil conditions

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• Soil structure affects various soil properties and is influenced by water and farming practices

• Soil structure provides clues about soil characteristics and past soil evolution

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• Soil structural classes based on the degree of development or cementation within peds

• Weak, Moderate, Strong, Structureless are the different classes

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• Soil structure shaped by swelling, shrinkage, and mechanical processes

• Plant roots, organisms, and soil chemistry also contribute to soil structure formation

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• Soil aggregation continues through the actions of bacteria, fungi, and organic matter

• Soil chemistry affects the aggregation or dispersal of soil particles

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• Soil particle density and bulk density are important indicators of soil compaction

• Soil particle density is unchanging, while bulk density includes air space and organic materials

Page 39: Porosity

• Pore space is the part of the soil volume not occupied by mineral or organic matter

• Pore size varies, with the smallest pores holding water too tightly for plant roots

• Plant-available water is held in ultramicropores, micropores, and mesopores

• Macropores are generally air-filled when the soil is at field capacity

• Soil texture determines the total volume of the smallest pores

• Soil structure influences larger pores that affect soil aeration, water infiltration, and drainage

• Tillage temporarily increases the number of pores of largest size, but they can be degraded by the destruction of soil aggregation

Page 40: Porosity

• Pore size distribution affects the ability of plants and organisms to access water and oxygen

• Large, continuous pores allow rapid transmission of air, water, and nutrients

• Small pores store water between rainfall or irrigation events

• Pore size variation compartmentalizes the soil pore space, allowing different organisms to coexist

Page 43: Consistency

• Consistency is the ability of soil to stick to itself or other objects and resist deformation and rupture

• Measured at three moisture conditions: air-dry, moist, and wet

• Consistency quality depends on the clay content

• Resistance to fragmentation and crumbling assessed in the dry state

• Resistance to shearing forces assessed in the moist state

Page 46: Consistency

• Terms used to describe soil consistency in different moisture states

• Dry soil: loose, soft, slightly hard, hard, very hard, extremely hard

• Moist soil: loose, very friable, friable, firm, very firm, extremely firm

• Wet soil: nonsticky, slightly sticky, sticky, very sticky; nonplastic, slightly plastic, plastic, very plastic

• Cemented soil: weakly cemented, strongly cemented, indurated

Page 50: Soil Temperature

• Soil temperature depends on the ratio of energy absorbed to energy lost

• Soil temperature range: -20 to 60 °C, mean annual temperature varies according to biomes

• Soil temperature regulates seed germination, plant growth, root growth, and nutrient availability

• Seasonal, monthly, and daily variations in soil temperature

• Heavy mulching can slow soil warming in summer and reduce surface temperature fluctuations

Page 51: Soil Temperature

• Agricultural activities must adapt to soil temperatures

• Timing of planting maximizes germination and growth

• Anhydrous ammonia application optimized below 10 °C

• Preventing damage to crops from frosts and freezing of saturated soils

• Soil temperature can be raised by drying soils or using clear plastic mulches

Page 52: Soil Temperature

• Factors affecting soil temperature: water content, soil color, relief, and soil cover

• Ground cover color and insulating properties influence soil temperature

• Whiter soil tends to have lower soil temperatures due to higher albedo

• Soil temperature affects seedling survival, root systems, and physical, chemical, and biological processes

Page 53: Soil Temperature

• Soil temperatures are increasing worldwide due to global climate warming

• Opposing views on expected effects on carbon capture and storage

• Threats include permafrost thawing, carbon destocking, and ecosystem collapse

Page 54: Soil Water (retention)

• Soils can process and hold a considerable amount of water

• Pores provide passage and retention of gases and moisture

• Soil's ability to retain water is related to particle size, clay soils retain more water

• Sands provide easier passage of water through the profile

• Clay type, organic content, and soil structure also influence soil water retention

Page 55: Soil Water (retention)

• Field capacity is the maximum amount of water a soil can retain

• Wilting point is when plants cannot liberate remaining moisture from soil particles

• Available water is the range between field capacity and wilting point

• Soil water retention is essential for plant growth and survival

• Soil moisture affects thermal properties, seed germination, flowering, and faunal activity

Page 56: Soil Water (retention)

• Percolation is the process of soil absorbing water and draining downwards

• Soil water retention provides an ongoing supply of water to plants

• Soil moisture affects soil temperature and biological triggers

• Water has roughly double the heat capacity of soil, affecting temperature gain or loss