Chapter 4

Physical properties of Soil

(book excerpts)

Soil is comprised of minerals, soil organic matter, water, and air (Figure 4.1). The composition and proportion of these components greatly influence soil physical properties which have a profound effect on how soils influence soil quality and productivity. Physical properties of soil include color, texture, structure, density, consistence, temperature, and porosity, the fraction of pore space in a soil. In turn, these properties affect air and water movement in the soil, and thus the soil’s ability to function. Colors of soils vary widely and indicate such important properties as organic matter, water, and redox conditions. Soil texture, structure, density, porosity, and consistence are related with types of soil particles and their arrangement. The mineral fraction of the soil—sand, silt, and/or clay—makes up about 45 percent of the soil volume. Their relative proportion in a soil is called soil texture. Soil structure is the arrangement of soil particles into different geometric patterns. Soil texture and structure together regulate porosity, density, compactness, retention, and movement of water and air in soil. Particle and bulk density are measurable soil properties used to express soil density. Soil porosity refers to the amount of pores, or open space, between soil particles. Soil consistence is a measure of the response of soil to mechanical stresses or manipulations at various moisture contents. Soil temperature influences life processes of soil biota including plants. Soil color as other physical properties including texture, structure, and consistence are used to distinguish and identify soil horizons (layers) and to group soils according to the soil classification system called Soil Taxonomy. Pore space is about 50 percent, which is very important for water holding and air exchange. Organic matter makes up roughly 1 to 5 percent of the volume. Most agricultural soils are about 2 to 5 percent organic matter. Soil air is retained in soil pores plays role in respiration of plant roots and microorganisms and transformation of mineral and organic matter. Thus, soil physical properties are a key consideration for productive and sustainable agricultural management, land development, and environmental stewardship.

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Topics Within This Chapter:

  • Soil Texture
  • Soil Textural Classes
  • Groupings of Soil Texture Classes
  • Nature of Soil Separates
  • Sand
  • Silt
  • Clay
  • Soil Texture Affect on Soil Behavior
  • Air-Water Relationships
  • Soil Fertility
  • Determining Soil Texture
  • Soil Structure
  • Soil Structure Dynamics
  • Structural Ped Characteristics
  • Soil Structure Shapes
  • Soil Structure Size
  • Soil Structure Grade
  • Aggregate Stability
  • Slake Test
  • Soil Structure Affect on Water Infiltration
  • Soil Structure Degradation
  • Causes of Structural Degradation
  • Soil Density
  • Particle Density
  • Importance of Particle Density
  • Bulk Density
  • Soil Bulk Density Affect on Plant Growth
  • Soil Properties Affect on Bulk Density
  • Measuring Soil Bulk Density
  • Identifying Compacted Soil
  • Soil Porosity
  • Pore Size
  • Soil Conditions Affecting Soil Porosity
  • Factors Affecting Soil Porosity
  • Irrigation
  • Tillage
  • Rain
  • Determining Soil Porosity
  • Soil Consistence
  • Moisture Content Affect on Consistence
  • Soil Temperature
  • Conditions Affecting Soil Temperature
  • Soil Color
  • Munsell Color Chart
  • Factors Affecting Soil Color