Re: Which heats up faster, soil or sand?

First of all, sand is a type of soil so you might want to frame the question 
as sand versus clay or sand versus loam. Check a soil textural 
triangle to see the different names for soil types. 

The answer will depend on several factors including the types of soil and soil 
colors. An especially important factor is if the soil samples are wet or dry.

Let's assume you have a clay and a white sand. Start with air dry samples of 
equal volumes for both. The bulk density (mass of dry soil per unit volume) of 
the clay is about 1.05 grams per ml, and the sand is about 1.55 grams per ml 
(Hausenbuiller 1972). Therefore, there is much more mineral mass to heat up 
with the sand. Plus, sands are often lighter in color than clays so reflect 
more heat than clay.

The reason why sand has a greater bulk density is that the particles are 
larger than in the clay so there is less pore space or porosity. Sand 
particles range from 2 mm to 0.05 mm in diameter. Clay particles are less than 
0.002 mm in diameter. The sand has 42% porosity by volume. The clay has 60% 
porosity (Hausenbuiller 1972)

It becomes much more complicated if the samples are moist because you also 
have to heat the water the samples contain. Let's assume the samples are at 
field capacity, which is the water content on a volume basis after the soil 
has been thoroughly irrigated and drained. Sand has a field capacity of about 
7% and a clay soil has a field capacity of about 40% by volume. That would be 
0.07 grams of water per ml of sand and 0.4 grams of water per ml of clay. The 
density of water is 1 gram per ml. (Hausenbuiller 1972)

Add the bulk density and the water content and you get:

1.55 + 0.07 = 1.62 g/ml for sand
1.05 + 0.4 = 1.45 g/ml for clay

The sand still has a bigger mass to heat up. However, you have to consider 
that it takes a lot more energy to heat a gram of water than to heat a gram of 
soil minerals. The specific heat is the calories it takes to raise the 
temperature of 1 gram of a substance 1 degree Celsius. The specific heat of 
water is one of the highest at 1 calorie per gram. Soil minerals have a 
specific heat of about 0.2 calories per gram (Brady 1984). Taking the 
different specific heat values into account. 

1.55 x 0.2 + 0.07 x 1  = 0.38 calories/ml for sand
1.05 x 0.2 + 0.4 x 1   = 0.61 calories/ml for clay

Moist clay requires about 60% more energy to heat than moist sand. Even if the 
sand reflects more heat than the clay, the moist clay will still probably heat 
up more slowly. 

You can measure bulk density of soil samples for yourself using a graduated 
cylinder and a balance. You can air dry the soil or dry it in an oven. Be 
aware that drying soil in an oven may create odors! 

If you have a flood lamp as a heat source and a thermometer, you can measure 
the differences in heating time between soil types and water. 

References

Brady, N.C. 1984. The Nature and Properties of Soil. New York: Macmillan.

Hausenbuiller, R.L. 1972. Soil Science: Principles and Practices. Dubuque, 
Iowa: Wm. C. Brown


Which Stores Heat Energy Better, Water or Sand?


Studying the Insulating Properties of Sand, Soil and Water