Investigating the Effects of Plant Density on Growth Rate Essay Sample

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1. Design – Part A

1.1 Defining the Problem

Focus / Research Question

How does plant density affect the growth of radish seeds?


If the population of plants in soil increases then the growth of the radish seeds will be reduced.

Background Information / Theory

Germination is the first stage of plant growth, resulting in a seedling. Seeds need soil, heat, oxygen and water to germinate. Initially the food required for the seed to germinate is stored in the seed (Wikipedia 2012, Factors Affecting Seed Germination). Once the seed has germinated, the roots will start to grow towards the soil and the sprout will grow upwards. As the first tiny leaves appear, photosynthesis is started.

In the next stage of growth the plants start to compete for water and nutrients in the soil and also for sunlight and carbon dioxide above. Seeds that are planted too close to one another are known to grow taller initially to compete for sunlight ( 2012, Etiolation). However this growth will not last long because as they grow bigger, the plants will have to compete for other resources in the soil like space for roots, water and nutrients (Wikipedia 2012, Factors Affecting Growth).

In most cases, a high plant density will cause the plants to experience malnutrition, have stunted growth and eventually die. In this specific situation, excessive plant densities are likely to produce irregular-sized, misshapen roots (Doug Hocking 2007, Plant Spacing).

Investigation Variables

Table 1 – Practical Investigation Variables

Type of Variable

Identified Variable

Independent Variable

Density of seeds

– 1 seed (control)

– 2 seeds

– 4 seeds

– 8 seeds

– 12 seeds

– 16 seeds

Dependent Variable

Growth Rate of Plants

Controlled Variables

– Amount of soil in each sample

– Type of soil

– Size of plant pots

– Type of seeds used

– Amount of seeds used in each trial

– Time between sample measurements

– Time between sample watering

Uncontrolled Variables

-How accurate measurements are

1.2 Controlling Variables

Treatment of Control Variables

Table 2 – Control Treatment of Variables


Control Treatment

Amount of soil

400g of soil in each pot

Type of soil

Same soil used in each pot

Size of plant pots

500g pot used in each sample

Type of seeds used

Only radish seeds used

Amount of seeds used in each trial

Same amount of seeds used in each trial type

Time between sample watering

Watering – twice a day – 5:00am & 5:00pm

Amount of water used on each sample

12cm³ each watering

Control Experiment

In this investigation the independent variable is the density of seeds planted within the soil, as this is the one variable that is manipulated and tested throughout the experiment. When picking the control variable it was decided to use 1 radish seed because it holds the lowest density that could be observed in the investigation. All other aspects of the experiment remain the same to ensure a fair test. By doing so, there is a fair basis for making observations about the growth rate of the plants (dependent variable) for each density of plants.


1. Collect all materials

2. Divide 25 plant pots into 5 groups of 5 and label plant pots 1, 2, 3, 4, 5 into rows of A, B, C, D and E

1. Use 5 plant pots for the control, label them CA, CB, CC, CD, CE (C for Control) and place it next to E

2. Place 500cm³ beaker on the electronic scale and press tare button to bring the weight down to 0.00g

3. Measure out 500g in beaker, adjusting when needed and insert the soil into a plant pot

4. Repeat steps 4 – 5 for all plant pots

5. Line a pencil up with a ruler and mark the pencil at 2cm from the bottom

6. Using the previously marked pencil, skewer 2 holes to the mark into every plant pot labeled 1, in the row of plant pots labeled A, B, C, D and E

7. Using the previously marked pencil, skewer 4 holes to the mark into every plant pot labeled 2, in the row of plant pots labeled A, B, C, D and E

8. Using the previously marked pencil, skewer 8 holes to the mark into every plant pot labeled 3, in the row of plant pots labeled A, B, C, D and E

9. Using the previously marked pencil, skewer 12 holes to the mark into every plant pot labeled 4, in the row of plant pots labeled A, B, C, D and E

10. Using the previously marked pencil, skewer 16 holes to the mark into every plant pot labeled 5, in the row of plant pots labeled A, B, C, D and E

11. Using the previously marked pencil, skewer 1 hole to the mark in the control plant pot labeled C

12. Place one seed in every hole skewered in soil in all plant pots and cover seed again with soil in plant pot

13. Wait until 5:00am and 5:00pm every day to water each pot plant with 200cm³ of tap water

14. After watering all pot plants at 5:00opm every day, using a ruler, measure the height of all seedlings in each plant pot

15. Record averaged results for each plant pot on a table of results

16. Repeat steps 15 – 17 for all pot plants for 10 days

2. Data Collection and Processing

2.1 Recording Raw Data

Quantitative Data

Table 5 – Initial Results

Plant Density (# of seeds)

Trial #

Hieght (cm) +/- o.5cm

3.1 Conclusion

Conclusion Statement

The growth of Raphanus Sativus (radishes) consistently decreased as the population density of seeds planted increased.

Conclusion Explanation

In this investigation, the results collected prove the hypothesis statement to be correct as the growth of the seeds decreased, while the plant density increased. The initial results that led to this conclusion can be viewed in Table 5 of this report. There are slight fluctuations and differences among these initial results within and between the different plant density trials. However, as seen in Table 6, through statistical analysis the initial results are averaged and the outcome presents the investigation with evidence to prove the hypothesis correct. The final results that support this hypothesis are graphically represented in Graph 1 as the steady decrease of growth between sample types can be viewed in this statistical representation.

3.2 Evaluating Procedures


The reliability of this experiment was ensured through a range of controlled variables and different factors to produce a fair test. There were a large range of variables that were kept controlled, to ensure reliability, can be viewed in Table 2. For each type of sample tested, five trials were conducted, thus enabling the investigation to gain further accurate results. As seen in Table 5 of the initial results these repeat measurements for each sample type do support each other. Most uncertainties within this investigation were not large enough to have a significant effect on the results. The graphical representation of the final results (Graph 1) matches the predicted hypothesis significantly. Through statistical analysis the standard deviations for the results were calculated. None of the standard deviations within the results exceeded 1 unit, thus proving this test to have high precision and accurate results.

Limitations / Weaknesses / Errors in Laboratory Investigations

As the investigation was quite thoroughly controlled, few limitations were observed. However, although the temperature was controlled, it may have been beneficial to grow the plants in a further controlled environment such as a laboratory. Human error may also have contributed to the small lack of precision within the results. Through the excessive control applied to the experiment, small amounts of ecological validity may have been lost during the investigation.

Significance of weaknesses on experimental results

Through the identification and alteration of these limiting factors this investigation may gain more accuracy.

3.3 Improving the Investigation

Modifications to experiment

Few modifications to the experiment may be made to enable further accuracy within the results. The limiting factors mentioned in 3.3 could be altered or modified, thus producing more accuracy.

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