Examkin

Question

Difficulty: HardEcological Factors and Their Measurement

A biology student investigating edaphic factors collected 100 cm3100\text{ cm}^3 of an undisturbed garden soil sample in a graduated cylinder and added 100 cm3100\text{ cm}^3 of water. After stirring thoroughly and allowing all trapped air bubbles to escape, the final settled volume of the soil-water mixture measured 160 cm3160\text{ cm}^3. What is the percentage porosity of this soil sample?

  1. 40%40\%Answer
  2. B
    60%60\%
  3. C
    25%25\%
  4. D
    16%16\%

Answer

The percentage porosity of the garden soil sample is 40%40\%.
Soil porosity represents the percentage of pore space (voids filled with air or water) within a given volume of soil. When 100 cm3100\text{ cm}^3 of water is added to 100 cm3100\text{ cm}^3 of dry soil, the expected combined volume without pores would be 200 cm3200\text{ cm}^3. The actual mixture volume of 160 cm3160\text{ cm}^3 indicates that 40 cm340\text{ cm}^3 of air was displaced by water. Expressed as a percentage of the original 100 cm3100\text{ cm}^3 soil volume, the porosity is 40 cm3100 cm3×100%=40%\frac{40\text{ cm}^3}{100\text{ cm}^3} \times 100\% = 40\%.

Step-by-Step Solution

1
Calculate the theoretical total volume if no air spaces were present.
Theoretical volume = Volume of soil+Volume of water=100 cm3+100 cm3=200 cm3\text{Volume of soil} + \text{Volume of water} = 100\text{ cm}^3 + 100\text{ cm}^3 = 200\text{ cm}^3.
Water fills the pore spaces previously occupied by air in the soil sample.
2
Determine the volume of air spaces (pores) in the soil sample.
Volume of air spaces = Theoretical volume−Actual mixture volume=200 cm3−160 cm3=40 cm3\text{Theoretical volume} - \text{Actual mixture volume} = 200\text{ cm}^3 - 160\text{ cm}^3 = 40\text{ cm}^3.
The reduction in expected total volume corresponds directly to the volume of displaced air.
3
Calculate the percentage porosity using the ratio of pore space volume to initial soil volume.
Percentage Porosity=(Volume of air spacesInitial volume of soil)×100%=(40 cm3100 cm3)×100%=40%\text{Percentage Porosity} = \left(\frac{\text{Volume of air spaces}}{\text{Initial volume of soil}}\right) \times 100\% = \left(\frac{40\text{ cm}^3}{100\text{ cm}^3}\right) \times 100\% = 40\%.
Porosity measures the percentage of total soil volume occupied by pore spaces.

Key Concept

Soil Porosity Measurement (Edaphic Factor Analysis)
Rate this question

Topics

Ecological Factors and Their Measurement

More questions from this topic