Decomposition, Soil & Nutrient Availability — Practice · Grade 7 Science

Chapter: Ecosystems, Matter & Energy

What this practice covers

By the end of Decomposition, Soil & Nutrient Availability you should be able to:

8 questions, written answer, multiple choice and numeric, each with a worked explanation.

Vocabulary used: soil, nutrient, decomposition, pore space, dry mass.

Problem 1

Why are spaces in soil relevant?
Answer and explanation

They can hold air and water and affect movement and root conditions, rather than being biologically meaningless empty areas.

Problem 2

Do decomposers return usable energy as sunlight?
Answer and explanation

No. They change matter pathways while energy transfers through metabolism and heat; continued energy input is still needed.

Problem 3

Why compare dry masses rather than wet masses alone?
Answer and explanation

Changing water content can alter wet mass without an equivalent change in the organic material being studied.

Problem 4

Why is “more water always improves soil conditions” unsupported?
Answer and explanation

Water can occupy air spaces and interact with other conditions; the effect depends on the organism and setting.

Problem 5

Illustrative dry-material study over the same interval: matched samples each start at 20 g; A ends at 14 g and B at 18 g. Moisture treatment differs; other conditions are stated matched. Calculate mass losses and explain the supported pattern with a boundary limitation.
Answer and explanation

A shows 6 g loss and B 2 g, so A lost 4 g more in this illustrative comparison. This is consistent with a treatment-related decomposition question under the stated matching. It does not identify every organism or destination of matter. Check replication, dry-mass method, and material transfers before generalizing to every soil.

A loss: 20−14=6 g.

B loss: 20−18=2 g.

Compare dry-mass method and missing destinations.

Response criteria

  • Explain the relevant biological process accurately.
  • Use the supplied evidence and show calculation steps with units where needed.
  • Connect a labeled model or mechanism with the claim.
  • State an assumption, evidence limitation, or useful next observation.

Problem 6

A leaf bag is weighed wetter at the end and gains mass. Does that prove organic material was created?
Answer and explanation

No. Added water or other material can change the measured mass. Define the boundary and compare dry material if that is the intended quantity.

Response criteria

  • Explain the relevant biological process accurately.
  • Use the supplied evidence and show calculation steps with units where needed.
  • Connect a labeled model or mechanism with the claim.
  • State an assumption, evidence limitation, or useful next observation.

Problem 7

Which could confound wet-mass decomposition measurements?
  1. Changed water content
  2. Correct units alone
  3. A stated interval alone
  4. A labeled table alone
Answer and explanation

Changed water content

Water differences affect wet mass independently of dry organic material loss.

Problem 8

Dry material changes from 25 g to 19 g. How much dry mass is lost?
Answer and explanation

6 g

25−19=6 g, without identifying every destination.

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