Soil Management: Soil Physical Properties

Understanding the Basics: Soil Physical Properties

The term physical properties includes:

• soil texture (sand, silt, clay)
• soil structure
  • structural form
  • structural stability and strength
  • porosity
  • bulk density
• organic matter
• water and air
• temperature.

A good understanding of what these components do and how they interact can help you better appreciate their considerable effect on crop production.

Soil Texture

Texture refers to:

• the mixture of different-sized mineral particles in a soil
  
  • soil particles range in size from gravel and stones to very fine clay particles
  • the percentage of sand, silt, and clay.

Sand has the largest particles; silt are smaller, and clay are the smallest. The texture of your soil influences all other soil physical properties, including drainage, water-holding capacity, soil temperature, aeration, and structure.

Soil texture can be considered an inherent soil property that you can't affect easily. However, you should know your soil texture and be aware of the limitations of that soil. (See the remainder of this book for more information on managing specific soil types.)

There are two ways to determine soil texture: a field method using your hands, and a laboratory method using a hydrometer.

Hand texturing is used in the field to identify soil textures quickly. The first step is to determine the sand content. Rub a small amount of soil in the palm of your hand - is it under or over 50% sand?		If the sand content is less than 50%, add water to create a soil that is wet enough to roll.
Squeeze the soil roll between your thumb and forefinger to make the longest possible ribbon. A loam soil will form only a short ribbon.		Clay soils will form a much longer ribbon.

The laboratory method relies on the fact that heavier particles such as sand drop out of suspension more quickly. Here are some sample figures from a Soil Analysis Report:

Sand 18.2%    Silt 44.7%    Clay 38.0%

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Soil Structure

Soil structure refers to how textural particles (sand, silt, and clay) are arranged into clumps or aggregates. The aggregates are bound together by clay and organic matter.

Soil structure can be considered in terms of form, stability, and strength.

Structure affects:

• drainage
• root growth
• infiltration
• germination
• aeration.

Soil Organic Matter

Of all the components that make up our soil, organic matter is the most important.
Organic matter:

• plays a major role in moisture retention, helping crops withstand drought
• contributes to the chemical and biological properties of the soil
  • is a source of and exchange site for nutrients
  • affects the fate of applied pesticide
• contributes to the physical properties of the soil
  • organic matter provides glue-like substances that act to stick individual particles together to form stable aggregates and good soil structure.

Dense, fibrous root systems encourage the development of stable, granular aggregates. These help form the type of seedbed that's resistant to crusting. Try to include grasses and forages in your rotation.

Soil aggregate stability and porosity are directly affected by soil organic matter content.
This is then observed as less crusting, better water infiltration and drainage, reduced compaction and erodibility, and an improved water-holding capacity.

Crops and other plants vary in their ability to influence aggregate formation and stability:

• long-term crops with dense fibrous root systems help to form water-stable aggregates, e.g. forage grasses and legumes
• row crops such as corn, soybeans, or vegetables have relatively sparse root systems.

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Building Organic Matter (A Calculation)

Soil organic matter is measured in the top 15 centimetres or to plow depth. This "hectare/furrow slice" weighs about 2,000,000 kilograms. Thus, 1% organic matter equates to 20,000 kilograms.

* dry matter production depends on a number of factors related to plant growth

In a best case scenario, only 20% of any residue returned to the soil will make it to the organic matter pool. The remaining 80% becomes part of living organisms, is released as gases during digestion, or has not become part of the organic matter flow.

It takes 5 kilograms of residue to make 1 kilogram of organic matter.

20,000 kg O.M. x (5 kg residue/1 kg O.M.) = 100,000 kg residue (1% O.M. increase)

Thus, it requires 100,000 kilograms of crop residue to raise the soil organic matter 1%. Assuming an average residue return of 5,000 kilograms from the above table:

100,000 kg residue/5,000 kg residue/y = 20 yrs

It would take 20 years to build the organic matter by 1% (provided that the soil was never worked to speed up decomposition). But don't despair! It may be a slow process, but it's possible to improve over time. Cover crops and manure certainly help.

Work to either improve or at least maintain organic matter. If you do nothing and continue cropping, your organic matter levels will continue to drop.

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Soil Temperature

The temperature of the soil follows the temperature of the air, but with a time lag. As you go deeper in the soil, air temperature has less effect on soil temperature.

Although air temperature has a great influence on soil temperature, there are other factors at play.

Water content affects the rate of temperature change. More heat is needed to warm a wet soil than a dry one. Evaporation is occurring simultaneously, absorbing heat and keeping the soil cool.

Sunshine also affects soil temperature. Any shading, i.e. clouds, weeds, or residue, will reduce the transfer of energy to and from soil.

Frost occurs when the temperature at the soil surface drops below the freezing point. Most spring frosts are associated with rapid cooling of the soil under very clear, still conditions. The temperature at the soil surface can be 4-5° C cooler than the air 1.5 metres above. The amount of cooling at the surface under these conditions depends on how warm the soil is to start with, and how quickly heat can move out of the soil.

Dark soils absorb more heat; light-coloured residues tend to reflect heat, causing soils to warm more slowly.

We often see crop damage from frost in fields that have been freshly cultivated, because cultivation creates an insulating zone of fluffy, dry soil at the soil surface. This zone blocks the movement of heat out of the soil, allowing temperatures to drop low enough to cause crop damage, while adjacent areas that weren't cultivated are not damaged.

Heavy crop residue can also increase crop damage from frost by insulating the soil and preventing the release of stored heat. Sunshine and soil water content play major roles in determining the amount of heat stored.

Available in Published Version of Soil Management

• Soil Structure
  • Structural Form
  • Structural Stability and Strength
  • Factors affecting soil aggregate formation and stability - Chart
  • Factors affecting soil strength - Chart
  • Soil Porosity
  • Bulk Density
• Soil Organic Matter
  • Effect of Soil Organic Matter on Soil Types and Structural Problems - Chart
• Soil, Water and Air
  • Soil Water
  • Water Availability for different soil types - Chart
  • Soil Air

| Introduction | Physical Properties | Chemical Properties | Biological Properties |
| Information & Interpretations | Soil Structure | Erosion | Other Soil Management Problems |
| Best Management Practices for Soil | Table of Contents |

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