Demonstration of Factors Affecting Soil Temperature
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Objectives:
Materials:
I used a fixture purchased at the local hardware store for about $5 and a 150 watt light bulb as the heat source.  The heat source was placed approximately 30 cm (12 inches) above the soil surface.		 Other Materials:
thermometers
various soil samples, including a sieved sand
straw or hay to use as a mulch
water
plastic (clear, white, or black) or paper mulches 
Modes of heat transfer:
conduction (objects in contact), convection (through air currents), radiation (heat transfer over a distance without contact or air flow)		 Conduction is the primary mode of heat transfer in soil.

Radiative heating is affected by the amount of radiation reflected or absorbed.
Water:
Hypothesis: 
  • Have students guess which soil will warm more quickly when heated.
  • After lights are turned off, have students guess which soil will cool more rapidly when heat source (lights) are removed.
The sieved sand was placed in the container to a depth of approximately 4 cm (1.75 inches).  Water was poured on the sand on left side of the container.  You can observe the darker color in the picture.  Wet soils are darker than dry soils.

After about 30 minutes, the dry soil temperature had increased about 10o Celsius, while the wet soil temperature had increased about 1o Celsius.

Why it happens: The specific heat of water is 1.0 calorie per gram while the specific heat of soil is about 0.2 calories per gram.  This means it takes five times more energy to raise the water temperature 1o than the soil temperature.  Or, with the same energy input, the soil temperature will increase five times more than the water temperature.

This is the reason you walk on wet sand at the beach in the summer.

Farmers in the northern states use tillage to dry and warm soils in the early spring so they can plant earlier and benefit from a longer growing season.

Farmers in hot regions sometimes use irrigation to cool the soil and especially the crop.
Color:
Use soils of three different colors: dark, medium, and light.
Hypothesis:
  • Have students guess which soil will warm more quickly when heated.
If you want the results to be what you expect (dark soils absorb more energy and have a higher temperature, while light soils reflect more energy and will be cooler), sieve the soils first so that the aggregate (clod) size is uniform.

If your students are more advanced and can think through other results:
The example on the left began with the same initial soil temperature.  After 30 minutes under the heat lamp, the dark soil temperature had increased 12o C, the red (middle) soil had increased 8o C, and the light soil temperature had increased 6o C. 

Why?  In these samples, the dark soil had many large aggregates, and thus many large pores (voids, empty spaces).  Since most heat transfer in soil occurs through conduction, there were fewer surfaces in contact to allow heat transfer.
Mulch:
Hypothesis:
  • Have students guess whether the mulched or unmulched soil will warm more quickly when heated.
  • Have students guess which condition will cool more quickly after heat source (lights) is removed.
The temperature on the bare side increased 12o C in 30 minutes while the temperature under the straw mulch only increased 1o C.  The mulch increases the amount of incoming energy (radiation) reflected, and it increases the amount of air space, insulating the soil surface.  Remember, if particles are not in contact, heat cannot be conducted.

Mulches are used to keep soils warmer once temperatures begin to drop in the fall.  In the spring, a mulched soil will warm more slowly, but will have less day-night temperature variation.  Mulched soils also tend to have a higher water content, contributing to slower warming.

Mulches are effective erosion control practices, protecting soil surfaces from raindrop impact or abrading wind-blown particles.  They also slow water and wind movement at the surface.  But in the northern states, sometimes farmers will remove the mulch from the seed zone to warm the soil so they can plant earlier.
Surface configuration:
Hypothesis:
  • Have students guess which surface position (ridge/bed or interrow/furrow) will have the higher temperature after heating.
  • Have students guess whether the deep temperature will be higher or lower than the surface temperature.
The soil is arranged in a bed and furrow (ridge and interrow).  Four thermometers were placed in the soil: shallow in the bed (ridge), deep in the bed (ridge), shallow in the furrow (interrow), and deep in the furrow (interrow).

The shallow ridge temperature increased 15o C, the deep ridge increased 7o C, the shallow furrow increased 5o C, and the deep furrow increased 1o C. 

The ridge (bed) temperature increased more because it has more surface area to absorb energy.

Farmers in northern states also use this method to warm their soils and lengthen their growing season.
Copyright 2005. Clay Robinson, Ph.D., as to all resources: Materials may not be reproduced without Dr. Robinson's written consent. Students are prohibited from selling (or being paid for taking) notes or webpages during this course to or by any person or commercial firm without the express written permission of the developer of these pages.
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