Lesson 8

Overview

Biological and agricultural concepts

The Salty Solution A study of the effects of salinization on plant growth and development

In many areas of the world, including the United States, salinization of the soil, or the accumulation of salts, is an important concern. A salin­ization problem can vary in magnitude from small, isolated cases, as when road salt accumulates after snow melts, to larger, more serious soil problems in regions where plants are heavily irrigated or ocean tides salinize soils and groundwater.

This experiment investigates the effects of different salt solutions on the germination and growth of brassica rapa seeds. Students can use this experiment to study the effects of salt on seed germination, and, if time permits, can continue looking at the growth and development of plants in a saline environment.

Students will have an opportunity to generate alternative hypotheses, and collect, record, compare and interpret quantitative data. After completing this investigation, students will have a better understand­ing of how varying amounts of salt affect the growth and development of plants.

Plant germination, growth and development Osmosis Ecology Water quality Soil science Scientific method

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Lesson 8

The teachable moment

Background

8-2 I Salinization

Teacher material

Both the biology and agriculture teacher can use this investigation to illustrate a plant or crop science unit. The biology teacher may want to use the material when addressing environmental issues or the affect of an abiotic factor (salt) on a biotic system (plants). In the agriculture classroom, the material can illustrate topics such as irrigation, land­scape management and water quality.

Though all soils contain some level of soluble salts, soils containing enough water-soluble salts to affect the germination and growth of plants are considered saline or salty soils. Saline soil conditions reduce the agricultural value and productivity of the land. More than one­fourth of the irrigated farmland in the United States is affected to some extent by soil salinity (Donahue et al. 1990).

Saline soils result when salts dissolved in soil moisture move to the surface of the soil where they are left as a crust in high concentration when the water evaporates. Repeated irrigation with low-salt water, which often occurs in areas of the Southwest, can cause salinization of the soil. Saline soils are also common in low-lying areas where drain­age is poor and in areas where groundwater seeps into surface soil. Salts accumulate at the soil surface as water evaporates or is used by plants.

Saline soils also result when salty wastewater from oil or gas drilling procedures dumped or any time soil is flooded with salty water. Soils near an ocean can become saline from salty ocean sprays just as soils near highways and roads become saline from road salt applied to melt ice and snow.

High concentrations of salt in the soil cause problems for plants. Con­centrated salt disrupts the balance of water (the physical process of osmosis) in the soil and in the plant tissue, causing the plant to pump water out of the cell and into the surrounding soil water. Less water is available to the plant, which can cause the plant to wilt. In addition, the sodium, carbonate and chloride ions found in saline soil are poten­tially toxic to plants.

In general, when salinization is a problem, a farmer will see uneven or irregular plant growth in a field, and a whitish crust on the soil surface from accumulated salt. Plants growing in a saline environment often have a bluish-green appearance. In addition, soil salinity tends to show up as more of a problem in dry years.

This investigation focuses on the effects of different salt solutions on

Lesson 8

Teacher Management Preparation

Activity time

Materials

Teacher material

plant germination, growth and development. Students hypothesize as to how plants will be affected when grown in a salty environment and test these ideas using bottle constructions.

Two hours for collection of materials and preparation of salt solutions. You should make up appropriate amounts of 0.5,1.0, 2.5, and 5. percent salt solutions according to the size of your class. Students should work in groups of two or four. One liter of one of the salt solutions per group will be more than adequate. Each group will test use a different salt concentration and compare the results with a distilled water con­trol. Each group makes two column-a control and an experimental.

Solution Grams of salt added to 1liter of water

5.0% 2.5% 1.0% 0.5%

50 25 10 5

Construction of bottle columns will take 1/2 to one full class period. Setting up the activity will take one class period (this includes filling columns with soil, planting seeds, watering, and setting up laboratory sheets).

For Option A (germination only), watering, taking measurements, and observing will take five to 15 minutes per class period for 5 days. An additional class period will be needed to discuss individual group results and any further extension experiments.

For Option B (growth and development), watering, taking measure­ments, and observing will take five to 15 minutes, three days/week for five weeks. (See Activity Timetable, page 0-14.)

• Four 1-liter labeled bottles per group of 4 students • Jiffy Mix or a 50/50 combination of peat moss and vermiculite • Liquid measuring devices, such as film canisters or beakers. • Twenty seeds per group. Fast Plants are recommended for

Option B, but almost any seed can be used for Option A. • Non-iodized pickling salt for making the saline solutions • One liter of each salt solution per group; 0.5, 1.0, 2.5, 5.0 percent. • Tap or distilled water • liquid fertilizer, such as Peter's (use standard 1 T I gal)

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Lesson 8

Tips and Safety

Key terms

References

8-4 I Salinization

Teacher material

If students are building bottle biology constructions for the first time, allow extra class time for setting up the initial activity.

Two-liter bottles may be substituted for 1-liter bottles, but all quanti­ties of water must be doubled.

For Option B, add wicks to the construction to make watering more consistent and maintenance free.

Use salt that is pure NaCl (pickling salt). Do not use salt that has aluminum silicate added to it.

Refer to the Wisconsin Fast Plants introductory section for growing instructions.

Leaching: when water passes through soil carrying soluble materials, such as nutrients, along with it.

Osmosis: the movement of a fluid (like water) through a membrane (like a cell wall) in order to create an equal concentration of dissolved solids (like salt) on either side of the membrane.

Salinization: the accumulation of soluble salts in soil, which can ad­versely affect plant growth.

Soluble: capable of being dissolved in a liquid

Donahue, R.L., R.H. Follett, and R.W. Tolloch. Our Soils and Their Management. Interstate Publishers, Inc. Danville, Illinois, 1990.

Harpstead, M.I., F. D. Hole and W.F. Bennett. Soil Science Simplified. Ames, Iowa. Iowa State Univ. Press, 1988. pp. 118-9.

Hausenbuilder, R.L. Soil Science. Dubuque, Iowa: Wm. C. Brown, 1985. pp. 466-89.

Lesson 8

Introduction

Student material

The Salty Solution A study of the effects of salinization on plant growth and development

Just as humans can't survive by drinking salty ocean water, plants can't grow well in the presence of too much salt either. How does soil get too salty? And how much salt is too much?

Accumulation of salt in water or soil is called salinization. This is a problem in many areas of the world, including the United States. Saline soil conditions reduce the agricultural value and productivity of the land. More than one-fourth of the irrigated farmland in the United States is affected to some extent by soil salinity.

Saline soils result when salts dissolved in soil moisture move to the surface of the soil, where they are left as a crust in high concentration when the water evaporates. Repeated irrigation of plants with low-salt water, which often occurs in areas of the Southwest from Texas to California, can cause salinization of the soil. Saline soils also com­monly occur in low-lying areas where water drainage is poor and in areas where groundwater seeps into surface soil. Salts accumulate at the soil surface as water evaporates or is used by plants.

Saline soils also result when salty wastewater from oil or gas drilling procedures is dumped or any time soil is flooded with salty waters. Soils along the ocean can become saline from salty ocean sprays just as soils near highways and roads become saline from road salt applied to melt ice and snow.

High concentrations of salt in the soil cause problems for plants. Con­centrated salt disrupts the balance of water (the physical process of osmosis) in the soil and in the plant tissue causing the plant to pump water out of the plant cells and into the surrounding soil water. Less water is available to the plant, which can cause the plant to wilt. In addition, the sodium, carbonate and chloride ions found in saline soil are potentially toxic to plants.

In this experiment, you will hypothesize about what concentration of salt will affect a plant's growth. You will test your ideas using plastic soda bottles, soil and seeds. If you live in an area where crops are

Salinization I 8-5

Lesson 8

Materials

Procedure Option A

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Student material

irrigated, you may want to get try and obtain some of that water and experiment with it.

• four 1-liter plastic bottles • a pair of scissors • soil or Jiffy mix • a marking pen • two film canisters • a plastic ruler • two beakers • tap or distilled water • 20 seeds • forceps or tweezers • salt water (0.5, 1.0, 2.5 or 5.0 percent)

Option A investigates the effect of salt on plant germination only.

1. Prior to setting up your lab, complete student worksheet, Option A, numbers 1-4.

2. Take two one-liter bottles and remove the labels.

3. Cut the bottles eight em below the neck. (See directions for the terraqua column in the Bottle Biology basic introductory section.)

4. Remove the bottle caps and punch a drainage hole in the center of each cap. Replace the bottle caps.

5. Label one column with the percent salt solution that your group has been assigned and the other column with "control." Write your names on your columns.

6. Measure out 350 mls of soil into each beaker.

7. Dampen each portion of soil with 50 mls of water.

8. Place the damp soil into the upper unit of the column.

9. Pour 100 mls of the salt solution on the column you have labeled as the salt column. Pour 100 ml of distilled water on the control column.

Lesson 8

Option B

Student material

10. Place 10 seeds on the surface of the soil in each column.

11. Cover the seeds with 30 mls of additional dry soil.

12. Water the column with distilled water until water begins to drain into the lower reservoir.

13. Discard the water that has drained through and fill the bottom reservoir with distilled water.

14. Place the columns under a light source (3 to 4 em from the light).

15. Water the columns with an additional30 mls of the appropriate solution in two days.

16. Check for germinating seeds daily. Count the total number of seedlings that are healthy and fully emerged from the soil for five days.

17. In Table B, place a check mark for each seed that has germinated. To determine the percentage of seeds that have germinated, multiply the number of seedlings germinated by 10.

18. Be prepared to discuss your results in a class discussion and complete Table C.

19. Complete the discussion questions.

In Option B you will witness a complete 40-day Fast Plant life cycle. The rate at which your plants develop will vary depending on the amount of light and the temperature of your classroom. We suggest that you build your bottle columns on a Monday and plant your seeds on Tuesday. (See timetable on page 8-15)

Prior to setting up the lab, complete student worksheet, Option B, numbers 1 to 3.

1. Complete steps 1 through 16 as in Option A (preferably on Monday and Tuesday), except add a Pelion wick running from the upper reservoir into the lower chamber in the column.

2. On the third day after planting (Friday), count and record in Table B the number of seedlings that are healthy and fully emerged from the soil. Add 30 mls of salt solution to the salt

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Lesson 8

8-8 I Salinization

Student material

column and 30 mls of distilled water to the control. Add 5 mls of fertilizer to each column. Choose and number your three healthiest plants and remove the other by cutting their stem right at the point where they come out of the soil.

3. On day 6, record any additional plants that might have germinated. Thin plants leaving three plants.

4. On day 8, measure plant height and fertilize each column with 12 mls fertilizer.

5. Measure the height of the plants on days 6, 8, 10, 13 and 15 (see data sheet). Measure from the soil to where the new leaves are forming (the apex of the plant). Calculate and record the average height of your plants.

6. On days 15, 17, and 20 count and record the number of flowers that are fully opened. Calculate and record the average number of flowers on your three plants. Pollinate your flowers on each of these days. On day 15 fertilize each column with 12 mls of fertilizer.

7. On day 20, punch off all new flower buds leaving the ones that you pollinated. Continue to do this for the rest of the life cycle.

8. On days 27, 29 and 30, count and record the number of pods found on your plants. Compute and record the average number of pods per plant.

9. Remove your plants from water on day 36.

10. When pods are dry, around day 41, harvest them and count the number of seeds produced by each plant. Compute and record the average number of seeds per plant.

11. Be prepared to discuss your results with the class. You will record the class results in Table C.

12. Answer the discussion questions.

Lesson 8 Data sheets: Option A

The Salty Solution

Date seeds were planted. __ _

Salt solution used %

1. What is your hypothesis?

2. Fill in Table A before setting up your experiment.

Table A - The same materials should go into each of your columns. Record the following:

Amount of soil: _______ _

Type of soil: _______ _

Type of water: _______ _

Amount of water: -------

Number of seeds: _______ _

3. What is the only difference between the designs of your control and test?

4. What do you think will be the major effect of salt on your plants?

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Lesson 8

Results and discussion Option A

8-10 I Salinization

Data sheets: Option A

Table B -Number of germinated plants

Salt Concentration ( %)

Day Date # plants germinated

control experimental

control experimental

3

6

Total control experimental

Table C- Class Data

Salt Concentration

Data Day Control 0.5% 1.0%

Total number 1 of seedlings 2

3 4 5

2.5% 5.0%

1. What is the relationship between salt concentration and seed germination?

2. At what concentration of salt do plants not germinate? Why do you think this is?

3. Are there any salt concentrations, besides your control, that seem to have no effect on seed germination?

4. Why is it important to compare your results to the control (0% salt)?

Lesson 8 Data sheets: Option A

5. If your plants were allowed to grow and develop, what do you think they would look like? How would the plants in the control column compare to the plants growing in the salty environment?

6a. Did groups experimenting with the same salt solution obtain similar results?

b. If not, describe some variables that may have caused some of these differences.

c. Choose one variable and describe how you could control this "variable" in an experiment.

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Lesson 8

Data sheets Option B

8-12 I Salinization

Data sheets: Option B

1. What is your hypothesis?

2. Fill in Table A before setting up your experiment.

Table A- Experimental Design

Variables Control Test

amount of soil

type of soil

type of water

amount of water

number of seeds

3. What do you think will be the major effect of salt on your plants?

Lesson 8

D ay D t a a

Number of 3 seeds germinated

Height: Plant 1

3 2 3

average

6 Number of additional seeds germinated

Height: Plant 1

8 2 3

average

Height: Plant 1 10 2

3 average

Height: Plant 1 2

13 3 average

15 Number of flowers: Plant 1

17 2 3

20 average

27 Number of pods: Plant 1

29 2 3

31 average

Number of seeds: Plant 1

41 2 3

average

Table B- Data C trlCl on o oumn

15 17 20

27 29 31

Data sheets: Option B

S ltC 1 a oumn ( o/c) ()

15 17 20

27 29 31

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Lesson 8

Results and discussion Option B

8-14 I Salinization

Data sheets: Option B

Table C- Class Data

Salt concentration

Data Control 0.5% 1.0% 2.5%

total% germination

average height

average number of flowers

average number of pods

average number of seeds

1. Describe the effects of higher salt concentrations on: a. germination

b. plant height

c. flower and pod production

d. seed number

5.0%

2. At what stage of development do plants seem to be the most sensitive to high salt concentrations?

3. Are there any salt concentrations, besides your control, that seem to have no effect on seed germination?

4. Why is it important to compare your results to the control (0% salt)?

Sa. Did all groups with the same salt solutions obtain similar results?

b. If not, describe some variables which may have caused some of the differences.

c. Choose one variable and describe how you could control this "variable" in an experiment.

Lesson 8 Data sheets: Option B

Activity Timetable - Option B

Monday

• build columns

Day6 • record germin-

ation

Day13 • measure height

Day20 • count and

pollinate flowers • terminate buds

Day27 • count pods

Day41 • harvest seeds

Extensions

Tuesday Wednsday Thursday Friday

Day3 • plant seeds Dayl • record germination • add 130 mls • fertilize

salt sol'n • add 30 rnls salt sol'n • thin to 3 plants

DayS DaylO • fertilize • measure height • measure height

DaylS Day17 • measure height • count and • count and pollinate flowers pollinate flowers • measure height

Day29 Day31 • count pods • count pods

Day36 • remove plants from water

1. Does leaching the soil with tap water decrease the effects of salinization?

2. Do different plants react to salty water the same?

3. How do other environmental factors affect the growth and development of plants?

4. What effects do different salt solutions have on plant embryos?

5. What is the lowest level of salt that affects plant development?

6. How much salt actually enters the roots of plants grown in salty water?

Salinization I 8-15