Geoscience Reference
In-Depth Information
Chapter 8
Soil Is Never Without Water
Regardless of the circumstances, it is undeniable that some amount of water resides
within all soil pores across the natural landscape. The amount can be easily deter-
mined by merely weighing a small soil sample, drying it at temperature 105 °C for
about 6 h, and weighing it again. The difference of the two weights - those before
and after drying - is the weight of all water initially present in the soil. When we
divide this value by the weight of the oven-dried soil and multiply by 100, we obtain
the weight percentage of the soil water content. This value lacks the information
needed to ascertain which portion of the soil pores or what fraction of the total
porosity was fi lled by water. To obtain this important information, we have to divide
the volume of water expelled from the soil as a result of drying it at 105 °C by the
initial volume of the soil sample before it was dried. Inasmuch as the density of soil
water is nearly identical to 1, i.e., its weight in grams is numerically equal to its
volume in cm 3 , we obtain the volumetric fraction of soil water content by dividing
the weight of water initially present in the soil by the initial volume of the soil
sample. Multiplying this fraction by 100, we obtain the volumetric percentage of
soil water content. And from this calculated value, we recognize immediately which
portion of the soil pores or what fraction of the total porosity was fi lled by water.
Practical example: because soil is frequently sampled with cylinders having a
volume of 100 cm 3 , we pushed one of them into the soil. Upon removing it from the
fi eld, we cut away the soil in excess above and below the edges of the cylinder. After
weighing it and subtracting the previously measured weight of the empty cylinder,
we obtain 164 g - the net weight of the fresh soil sample. After thoroughly drying
and weighing the sample within the cylinder and again subtracting the weight of the
empty cylinder, we obtain 138 g - the net weight of the dry soil having a volume of
100 cm 3 . Subtracting the net weights of the fresh and dried sample (164-138 g), we
learn that 26 g of water or a volume of 26 cm 3 was expelled from the fresh soil.
Hence, the volumetric soil water content of the fresh soil was 26 %. If the total
porosity of the soil was 48 %, then about 54 % of the pore space in the fresh soil was
fi lled by water.
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