Page 106 - Calculating Agriculture Cover 20191124 STUDENT - A

9-18 Land, Soils and Fertilizers CH 9]

How is soil porosity determined? There are two equations used to calculate soil porosity. The
first equation uses the total volume and the volume of the void.

Porosity = (Volume of Voids / Total Volume) x 100%.

The second equation uses the total volume and the volume of the solid.

Porosity = [ ( Total Volume — Volume of the Solid ) / Total Volume ] x 100%.

Which soil has maximum pore space and moisture levels?
Clay soils have the highest porosity, and sand the lowest. For a given volume of soil,
sand weighs more and clay less, even though particle density is about the same. The
difference is the size of the pores. Because sand particle size is larger, so are the pores.
Density of Soil: Bulk Density and Particle Density. Soil bulk density is the mass of
dry soil per unit of bulk volume, including the air space. Soil bulk density can vary
substantially among different soil types and is affected by management practices (e.g.
tillage, livestock grazing, timber harvesting). Bulk density is the weight of soil in a given
volume.
The particle density (packed density) or true density of a particulate solid, a grain of
sand, silt or clay, is the density of the particles that make up the particulate solid, in
contrast to the bulk density, which measures the average density of a large volume of the
particulate in a specific medium.

Example 1 What is the percent pore space when bulk density is 1.3 and the particle density is 2.6?

Solution:
bulk density
1.00 − —————— x 100 % = % pore space
particle density

1.3
1.00 − —— x 100 % = % pore space
2.6

1.00 − 0.50 x 100 % = % pore space
0.50 x 100 % = % pore space

50.0% = % pore space

Example 2 What is the percent solid soil in Example 1?
When evaluating soils, it is the practice to
Solution:
% solid soil = 100 % − % pore space dry the soil (evaporate the moisture) by
oven drying the soil; bake the soil in an
= 100% − 50 % oven to evaporate the moisture.
= 50% solid soil

Example 3 A cubic foot of soil holds 62.4 pounds of water. With a soil density of 1.32, how much
does a cubic foot of oven-dry soil weigh?

Solution:
62.4 lbs./cu. ft. of water x 1.32 = 82.4 lbs./cu. ft. of soil

82.4 lbs — 62.4 lbs = 20 lbs/cuft for oven-dry soil The weight of 1 cuft of oven-dry soil.
Example 4 In the preceding example, what is the soil particle density if the pore percent is 50
percent?

Solution:
pd particle density 100% pore space
—— = ——
1.32 bulk density 50% pore space

pd 1.32 100% 1.32
—— x —— = —— x ——
1.32 1 50% 1
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