Conduction is the transfer of heat through a solid, liquid or gas by direct contact.

I should already be familiar with: Temperature

How does this relate to agriculture?

Figure C: Soil Depth Temperatures During Summer and Winter

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Soil's ability to conduct heat depends on the material it is made of, how porous it is, and how much water it holds.  Air is a poor thermal conductor, so the more porous the soil, the less that it can conduct heat.  Because of air pockets trapped in the soil, heat and cold are transferred slowly to lower levels in the earth.  Temperature variations below the surface of the ground are less extreme than right at the surface and also tend to change more slowly with time than the surface temperatures.   The deepest layers in the soil lag the most from the surface temperatures.  This is important because soil temperature affects seed germination and plant growth.  Some crops are more sensitive to this than others. 

For example, cotton is very temperature dependent.  It will only germinate after the soil temperatures reach 68 degrees F at a two inch depth.  Planting cotton in cold soils increases the likelihood of chilling injury to seedlings, as well as weak and variable stands.  Replanting is expensive, and can complicate pesticide programs and delay maturity of the crop. 

Conduction is also important to understand if a grower intends to use plastic ground covers in crop production.  Because thermal conductivity of the soil is high relative to that of air, much of the energy  from sunlight that is absorbed by black plastic can be transferred to the soil by conduction if contact is good between the plastic mulch and the soil surface. Soil temperatures under black plastic mulch during the daytime are generally 5° F higher at a 2-inch depth and 3° F higher at a 4-inch depth compared to those of bare soil.

Soils with high water content conduct heat much more efficiently than dry soils because water is denser and makes better contact with the soil particles than the atmospheric gases do.

When bare skin is in direct contact with frigid air, the cold temperatures can chill the skin, leading to cell stress and eventually damage like frostbite.  This conduction of cold temperatures is enhanced if the skin is wet, since water is a better conductor of heat and cold than air is.  Cold winds blowing across skin enhance the chilling effect by convection, which removes the heat by the movement of air away from the body.  Wind chill is an index which is used to describe this effect of chilling due to cold, moving air over a heated body.  Field work should be avoided when wind chills are extremely low due to the potential for hypothermia and frostbite.

Activities to further explain this module: