Soil Salinization Lab

One of the most common (and important!) labs in AP Environmental Science is the Soil Salinization Lab.

There are many versions of the lab that are great. This is mine.  It has been altered and changed with ideas from lots of teachers over the year.

(Side note:  I am a HUGE fan of inquiry, but I do not do an inquiry lab for this topic for a couple of reasons.  I want kids to do a proper controlled experiment before they do experimental design later on. They can refer to this lab when they are designing their own later in the year. This is part of scaffolding. With each controlled experiment (such as Ocean Acidification), I remove more instructions and parameters. Another reason is that I also want the kids to test specific concentrations of salt, because we use the results for an LD-50 lab later in the year.)

Materials

Supplies needed for the Soil Salinization Lab. These are provided for each group of 4 kids.
  1. Seeds
  2. Graduated cylinders with optional pipets to help kids with precise measurements. Mark one graduated cylinder for the 5% solution and one for the distilled water. Same with pipets. This prevents cross-contamination. Train the kids to be careful and prevent cross-contamination. Even a couple of drops of salt left over in a graduated cylinder can influence results.
  3. Overhead markers
  4. Scissors to cut paper towels
  5. 5% salt solution (mix yourself)
  6. Distilled water
  7. Beakers
  8. Petri dishes or ziploc bags. Petri dishes can be reused for an air pollution lab-Airborne Particulates.

Prepare a 5% salt solution by mixing 5g of NaCl (Table salt) with 95ml distilled water. Make enough for your classes.  I make about a liter for my 5 classes of APES.

I like pea seeds and prepare these containers for the groups. Tell the kids to keep covered to prevent fungi contamination. Fungi does ruin the soil salinization lab on occasion. I’ve had it happen once in 12 years. Use very clean petri dishes and keep beakers covered when not in use to prevent this.

Cover the seeds to prevent fungi spores from contaminating the lab

What seeds to use?

I’ve used peas, mung beans and radishes for soil salinization over the years. My favorite is peas, but the others work as well. Radishes tend to have lower tolerance to salt. You can even have students test different seeds in different groups. Mung beans sprout quickly and have a nice tolerance to salt as well.

Radish germinate quickly, but have a lower tolerance to salt
Mung beans are nice, but hard to find sometimes. They need longer than 2 days to get good results.
My favorite is pea seeds. This is after two days, but we usually take data after 3-4 days so the kids can measure a longer sprout.
This is a bit longer than I like to leave the seeds. They begin to degrade and smell. The best time is 3-4 days.

Using Ziploc Bags

I use snack-sized Ziploc bags with my online APES class that I meet with on Saturday mornings a few times a year. The bags are convenient as I don’t need them back when the lab is over and they’re inexpensive.  In my regular classes, I prefer petri dishes as they can be reused from year to year and are easier for the kids to work with the seeds.

The write up using ziploc bags can be found here.  I also use a Flipgrid conclusion so they can film their results and show me at home.

Use a sharpie to mark the bags and create a paper towel “taco”. Wet the paper towels before putting the seeds in or they roll around too much.

Day 1 of the Lab

Go over the first page of the lab together with students.  They have difficulty with the terms (control, variables, constants) if they’re out of practice. Also, teach the students to use words such as “increasing” or “decreasing” in their hypothesis as this is good practice for FRQs.  Its also more of a “collegiate” and precise hypothesis.

Students need to do the math for serial dilution before beginning the lab. I usually go how to do the math and then they can easily solve.

After doing the math, they can prepare the soil salinization dishes.

Make sure they use separate graduated cylinders for 5% NaCl solution and distilled water. Have a “mixing” beaker to mix the solution before pouring over the seeds. This makes sure that the solution is evenly mixed.

Students preparing petri dishes

Best way to prepare the dishes

Make a paper towel taco and place seeds inside
Pour solution over the seeds
Cover with the top of the taco
Tuck in the sides. If towels stick out, they will wick moisture out of the dish and the dish will be ruined. Label with an overhead marker so the ink can be washed off when finished.
Stacks of completed dishes. 9 stacks of 6 dishes for one period.

Day 2 the lab (3-4 days later is optimal)

Students will take data on the # and % of seeds that germinated and measure the sprout length. I tell students to measure the longest sprout in each dish.

Measuring the sprout in mm
Students sometimes have trouble knowing if a seed germinated so I have this drawing to help.

Results take about 15 minutes. Then students wash and dry dishes. I store and use the dishes again for an air pollution lab and again next year.

 

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