Examkin

Question

Difficulty: Very hardEcological Factors and Their Measurement

An ecologist conducting an edaphic investigation needs to quantify the percentage of organic content (humus) in a soil sample without interference from soil moisture. Arrange the following procedural steps in the correct chronological sequence to complete this measurement:

  1. 1Heat the fresh soil sample in an drying oven maintained at 105∘C105^\circ\text{C} to constant mass to eliminate all soil water.
  2. 2Weigh the crucible containing the thoroughly dried soil to obtain its initial dry soil mass (m1m_1).
  3. 3Strongly heat the dried soil sample over a Bunsen burner until all organic components completely combust.
  4. 4Transfer the hot crucible containing the residue into a desiccator to cool to room temperature without absorbing atmospheric moisture.
  5. 5Reweigh the cooled crucible containing the remaining inorganic mineral residue to record its final mass (m2m_2).

Answer

The correct chronological sequence is: (1) Heat the fresh soil sample in a drying oven at 105°C to constant mass; (2) Weigh the crucible containing the thoroughly dried soil (m1m_1); (3) Strongly heat the dried soil sample over a Bunsen burner until all organic components combust; (4) Transfer the hot crucible into a desiccator to cool; (5) Reweigh the cooled crucible containing the remaining inorganic mineral residue (m2m_2).
The procedure relies on isolating water loss from organic mass loss. First, heating at 105°C drives off all soil moisture without burning organic material. Weighing the dried soil establishes the initial dry mass. High-temperature ignition burns off the organic matter completely. Placing the hot residue in a desiccator prevents atmospheric water absorption during cooling. Finally, reweighing the cooled residue provides the mass of mineral matter remaining, allowing calculation of humus content.

Step-by-Step Solution

1
Separate soil water measurement from organic matter combustion.
Oven drying at 105∘C105^\circ\text{C} evaporates capillary and hygroscopic water without scorching organic matter.
If soil is ignited directly without prior drying at 105∘C105^\circ\text{C}, the combined mass loss of water and organic matter will skew the humus calculation.
2
Measure baseline dry soil mass (m1m_1).
Establishes a precise starting mass consisting strictly of dry mineral matter + organic matter.
Accurate calculation of percentage loss requires knowing the exact initial dry mass of the soil sample.
3
Perform high-temperature ignition.
Organic matter (humus) is completely oxidized to carbon dioxide and water vapor, leaving inorganic ash.
Organic compounds break down completely only when subjected to direct strong heating with a Bunsen burner.
4
Cool the sample under dry conditions.
The crucible and mineral residue reach thermal equilibrium without re-absorbing atmospheric water vapor.
Weighing hot apparatus creates convection currents that alter balance readings, and exposed cooled ash rapidly absorbs atmospheric moisture.
5
Obtain final mass (m2m_2) and determine humus percentage.
Percentage of humus is calculated using m1−m2m1×100%\frac{m_1 - m_2}{m_1} \times 100\%.
The difference between initial dry mass and final burnt residue mass equals the total organic content present.

Key Concept

Determination of Edaphic Factors: Soil Organic Matter Content via Loss on Ignition
Rate this question

Topics

Ecological Factors and Their Measurement

More questions from this topic