Soil Salinization Lab: An Investigation of Range of Tolerance
OVERVIEW
Over 15% of the earth’s agricultural land is irrigated in order to maximize crop yields.
Irrigation water contains dissolved salts such as CaCO3, MgCO3, NaCl, CaCl2, KCl, and
MgSO4 (to name a few). These salts are leached from soil and rocks during percolation
or runoff surface water. Groundwater can have significant amounts of salts, especially
CaCO3 and MgCO3 leached from sedimentary rocks comprising the aquifer. Following
excessive irrigation, the remaining surface water evaporates and leaves a layer of
dissolved salts behind. The process is repeated and the accumulation of these salts causes
salinization of the soil. Soil salinity can affect crop germination levels and yields. In this
lab you will investigate the effect of different levels of soil salinity on the germination of
mung bean seeds.
1. You will first use the serial dilution technique to prepare the various aqueous
salt (NaCl) solutions. These solutions will be used to simulate different levels
of soil salinity. They will have salt concentrations varying from 0.219% to
3.50%, the latter solution being similar to the salt content of ocean water.
Distilled water will be used as a salt-free (0%) solution.
2. Next, you will plant the mung bean seeds in a petri dish with paper towel
“soil” moistened with one of the aqueous salt solutions or distilled water.
There will be a total of six petri dishes, one for each solution. As the
experiment progresses, it may be necessary for you to add additional
amounts of a solution to a petri dish to prevent the paper towel and seeds
from drying out.
3. For the next three days, you will record the number of seeds that have
germinated in each dish. After recording the number of seed germinations on
the final day, you will properly dispose of the lab materials and cleanup the
lab.
TIME REQUIREMENTS
This lab activity requires 4 consecutive days. It is best to start this lab on a Monday
or Tuesday so that data collection can be finished before the weekend. Each part of the
laboratory procedure requires the following amount of time:
Part 1: This activity requires about 30 minutes of the class period.
Part 2: This activity requires about 10 minutes per class period for the next two days.
Part 3: This activity requires about 30 minutes per class period on the last day.
MATERIALS
Balance - 1
Beakers (250 mL) - 6
Petri dishes with lids – 6
Stirring rod – 1
Mung seeds – 90
NaCl – 4 g
Paper towels – 3
Graph Paper sheets – 1
Plastic wrap (15 cm X 15 cm) – 6 pieces
Graduated cylinder – 1
Distilled water – 500 mL
Labels/Tape – 6 pieces
Rubber bands - 6
PART 1: Prepare the Aqueous Salt Solutions:
1. Mass 3.5 g of NaCl on the balance. Add this to a 250 mL beaker. Add 100 mL of
distilled water to this beaker. Stir well. Label the beaker as having a 3.50% (w/v) salt
solution.
2. Pour half of this solution (50 mL) into the second beaker (use a graduated cylinder)
and add an additional 50 mL of distilled water to this solution. Stir well. This beaker
now contains a 1.75% (w/v) solution. Label this beaker as having a 1.75% (w/v) salt
solution.
3. Remove half of the 1.75% solution (50 mL) from the second beaker and pour it into a
third beaker. Add an additional 50 mL of distilled water. Stir well. The third beaker
now has a 0.875% (w/v) solution. Label this beaker as having a 0.875% (w/v)
solution.
4. Remove half (50 mL) of the 0.875% solution in this third beaker and pour it into the
fourth beaker. Add an additional 50 mL of distilled water to create a 0.438% (w/v)
solution. Label this beaker as having a 0.438% (w/v) salt solution.
5. Remove 50 mL of the 0.438% solution from the fourth beaker, pour it into the fifth
beaker and add an additional 50 mL of distilled water. Stir well. This is now a
0.219% (w/v) solution. Label this beaker as having a 0.219% (w/v) salt solution.
6. In the sixth beaker, add 50-100 mL of pure distilled water. Label this beaker as pure
water (0%).
7. Cover all six beakers with a square of plastic wrap and secure it with a rubber band.
Be sure to keep the beakers covered with the plastic wrap when they are not in use in
order to minimize evaporation of the solution.
8. Wash the graduated cylinder and return it to the equipment table.
Prepare the Petri Dishes:
1. Cut out paper towel circles that are the size of the bottom of the 6 petri dishes. Cut
out two paper towels for each dish. Place the paper towels in the bottom of each petri
dish.
2. On the side (or top) of each petri dish, label the concentration of aqueous salt solution
that will be added to that dish. You should have 6 labels (3.50%, 1.75%, 0.875%,
0.438%, 0.219%, and 0%). Try to place the label in a position on the petri dish that is
easy to read.
Plant the Mung Seeds:
1. Place 15 mung bean seeds on top of the paper towel, in the bottom of each petri dish.
2. Seeds need water in order to germinate. Initially moisten the paper towel on the
bottom of the petri dish with the aqueous salt solution that corresponds to that label.
Be sure to moisten the petri dish labeled 3.50% solution with the aqueous salt solution
from the 3.50% beaker, etc. The seeds should not be floating in solution, but must be
be very moist.
3. Place the lids on the petri dishes and the plastic wrap on the beakers. Place the petri
dishes in a protected area (the mung seeds do not need light to germinate, but will
require some indirect light to grow).
Make a Hypothesis:
Problem – Which salt solution will have the highest number of germinated seeds?
1. Devise a hypothesis which will answer your problem. Be sure to include an
explanation.
2. Identify the Independent Variable, the Dependent Variable, and five Controlled
(Constants) Variables in your experiment.
PART 2: Record Germination Observations:
1. Check your petri dishes to determine the number of seed germinations after one, two,
and three days.
2. Record the data on your data table.
Check Liquid Levels in Petri Dishes:
1. Check to see if your paper towel and seeds are drying out. Remember, the seeds need
water for germination.
2. If necessary, add a small amount of the solution corresponding to the label on the petri
dish to keep the paper towel and seeds moist.
3. Be sure to cover the petri dishes again when you are finished.
PART 3: Record Final Germination Observations:
1. Check your petri dishes to determine the number of seed germinations on the final day
of the lab.
2. Record the data in your data table.
ANALYSIS QUESTIONS:
1. What can be the effect of salinized agricultural land on developing crops?
2. What are the sources of the salts on the salinized land?
3. Why was it necessary to keep the beakers covered in the experiment?
4. Why was it necessary to stir the beakers well when making the serial dilutions?
5. Why was it necessary to have some seeds in distilled water (0.0%)?
6. Make a graph showing the relationship (for each solution) between the number of
seeds germinated and the salt concentration. Be accurate and label your axes.
7. What are possible sources of error in this experiment?
8. Discuss three specific ways that farmers can remediate the salinization of their
cropland.
9. Looking at your answers to number 8, give one negative aspect to each of the
remediation techniques listed above.
10. Using your graph and data, fully discuss the results of your experiment. Formulate a
conclusion based on your hypothesis and relate these results in your discussion.