Physical Properties of Soils
I. Topic

This lecture deals with physical properties of soils and relates directly to Lecture 2. A significant portion of soil taxonomy deals with soil physical properties. These properties in turn have a significant impact on plant growth and productivity.

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II. Learning Objectives
Understand the role of soil texture and structure in crop growth 
sunflower Be able to relate texture and structure to soil aeration and water infiltration 
sunflower Be aware of influence of soil temperature on seedling development and crop growth and development
sunflower Understand the role of tillage in ameliorating problems caused by poor structure and drainage 
sunflower Understand the positive and negative relationships between tillage practices and water and wind erosion

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III. Overview

Soil physical properties have a significant influence on crop plant growth and development. Soil texture and structure are the most important of these properties. Soil texture is most important in the areas of water holding, nutrient supply, and on ease of tillage. Soil structure has the greatest influence in the areas of soil aeration, water infiltration, and soil temperature.

The primary method of addressing many of these soil physical properties has been the use of tillage. Tillage can change the soil structure, improve moisture intake and storage, improve aeration and fertility. However, tillage can also impact on soil loss from both soil water and wind erosion.

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Two basic physical properties of soils are texture and structure

Soil texture and structure have strong influence on: Soil Texture
Clays have greater water-holding capacity than silts and water movement is slower 
Sands are called "light" soils 
  • They do not become sticky and hard to till
Nutrient holding ability varies with the textural class
Textural class (sand, silt or clay) has a lot to do with planning a cropping system.
There are 3 clay minerals found in soils
Plasticity more predominant as montmorillonite content increases

Soil Structure
Principle forms generally recognized are: 
  • Platy 
  • Prismatic 
  • Columnar 
  • Blocky 
Soil structure influences water and air movement in soils.

Soil structure is markedly affected by tillage operations
Measurement of soil structure Pore space  
Bulk density 
  • Weight for a given volume of soil 
  • Can use to evaluate structure 
  • More compacted/less pore space 
  • Subsoil layers generally have higher bulk densities/less O. M./are less well aggregated/compacted by heavy equipment
Compacted soils limit root growth and development.
Soil Aeration
Ability to provide air of suitable composition to plant roots and to organisms growing in the soil 
If aeration impeded, soil air higher in CO2, lower in 02 
Diffusion of air through soils is directly dependent on volume of air-filled pores
Infiltration Rate
Is the rate which rain or irrigation water enters the soil 
Depends on both surface soil and subsurface horizons 
If water runs off and not into soil, water is not available for plants and contributes to erosion.

Rate is at maximum value soon after water application begins 
  • Then falls to a lower value 
    Condition of soil surface has significant influence on infiltration rates of soil 
    O. M. and soil texture also have influence 

    Success of a cropping system may be greatly influenced by the ability to capture and hold limited rainfall
    Soil Temperature
    Affect the biological processes in the soil 
    • Micro biological activity 
    • Seed germination 
    • Root growth
    Temperature influences microorganisms. In hot climates, organic matter consumed at a much faster rate than in cold climates.
    Soil temperature may vary appreciably with depth below the surface
    Surface temperatures may undergo a wide fluctuation
    Dark colored soils absorb a higher proportion of the radiant energy
    Angle of incidence makes difference

    Fluctuations in soil temperature are influenced by both heat capacity and thermal conductivity of the soil 
    • Thermal conductivity of water higher than soil and air 
    • Rate of heat flow increases with increasing moisture content

    If surface layer is wet, soil warms slower than dry soil because heat is conducted to lower layers and it takes more heat to change the temperature
    Cool wet soil delay planting. It can also be critical in areas with short frost free periods.

    Variation in temperature much greater in bare soil compared to one with vegetable cover 
    • Bare soil reaches high temp. Greater than air temp. 
    • At night, temperature falls below air temperature

    Shading effect beneficial in avoiding excessive soil temperatures 
    • Inter seeding crop avoids excessive soil temperature in tropical area 
    • Seed new crop in crop residue 
      • Both methods conserve soil moisture and reduce maximum soil temperatures

    Germination of seeds affected more by soil temperature than any other factor except moisture 
    • In temperate climates - temperature too low for germination 
    • In tropical climates - may be too high
    Roots are more susceptible to injury by high temperatures
    Tillage Practices
    Soils are tilled for three general reasons 
    • Change soil structure 
    • Manage crop residues 

    • Control weeds
    Tillage has both positive and negative influences on crop and soil management.
    Changes in soil structure Weed control is needed to minimize competition from weeds for A soil should be in good tilth (describe)

    Tendency is to reduce number and intensity of tillage operations

    In 1875, it took 150 man hours of labor to produce 270 kg of corn. By 1975, it took 4 man hours.

    Tillage can be done closer to optimum time 
    • Most midwestern farmers can plant their entire acreage in one week
    Soil Erosion

    Undesirable tilth usually leads to soil erosion
    Tillage when too dry or too wet can result in excessive cloddiness
    Erosion is a major environmental problem in the area of tillage.
    Water erosion   Wind erosion
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    V. Summary

    As long as the majority of our food crops are grown in the soil, a basic understanding of soils will continue to be essential to crop production. This section discussed two of the most important physical properties of soils, that of texture and structure. Soils that are too sandy (course textured) leach quickly and do not hold water well. This can influence both fertility and the environment, as well as limit the plant's ability to obtain water for turgor, photosynthesis, and nutrient uptake. Soil structure has a primary impact on soil porosity and aeration. If water does not move through soils, air is excluded and plant roots die. If water does not enter the soil (infiltration) and instead runs off, the water is lost to the plant and erosion often occurs. A secondary property, but none-the-less important, is soil color. This effects temperature, how slowly or quickly a soil dries, and has a direct influence on seeding date and germination. Finally, mankind uses tillage to effect the soil physical properties. This can be both beneficial and harmful (water and wind erosion potential of tilled land).

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    VI. Self Assessment
    sunflower Know the difference between soil texture and structure.
    sunflower Be able to explain how texture and structure influence crop plant growth.
    sunflower Know the relationship between pore space and plant root growth and development.
    sunflower How does soil temperature effect crop plant growth? 
    sunflower What is tillage and how does it influence soil physical properties? 
    sunflower What is tillage and how does it influence soil physical properties? 
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     Last updated January 23, 2008