Grade 10 learners investigated the effect of nitrogen-fixing bacteria on plant growth in soil with low nitrogen levels.

The independent variable is the factor that is intentionally changed or manipulated by the experimenter.
3.2.1 The independent variable is the presence or absence of nitrogen-fixing bacteria.

Nitrogen-fixing bacteria convert atmospheric nitrogen gas into forms usable by plants, such as ammonia or ammonium. This process is called nitrogen fixation.
3.2.2 Nitrogen-fixing bacteria convert atmospheric nitrogen gas ($\text{N}_2$) into ammonia ($\text{NH}_3$) or ammonium ($\text{NH}_4^+$), making nitrogen available for plants to absorb and use for growth.

Validity in an experiment means that the results are reliable and accurately measure what they are intended to measure. This is achieved by controlling all other variables.
3.2.3 One way to ensure validity is to keep all other conditions constant for both pots, such as the amount of water, light, temperature, type of soil, or number of seeds.

In soil with low nitrogen, plants cannot get enough nitrogen for essential processes like protein synthesis and photosynthesis. Nitrogen-fixing bacteria provide this crucial nutrient.
3.2.4 In soil with low nitrogen levels, nitrogen-fixing bacteria convert atmospheric nitrogen into a usable form for plants. This increases the availability of nitrogen, which is essential for plant growth, leading to better overall development and height.

Set B (Pot B) did not receive nitrogen-fixing bacteria, while set A did. This allows for a comparison.
3.2.5 Set B serves as the control group in this investigation. It provides a baseline for comparison to determine the effect of the nitrogen-fixing bacteria.

Compare the growth in Pot A (with bacteria) and Pot B (without bacteria).
Pot A: Final height 28 cm - Initial height 5 cm = 23 cm growth.
Pot B: Final height 12 cm - Initial height 5 cm = 7 cm growth.
3.2.6 The conclusion is that nitrogen-fixing bacteria significantly enhance plant growth in soil with low nitrogen levels.