Task 4: Soil Salinization Lab
Abstract:
Introduction:
Salinization refers to a buildup of salts in soil, eventually to toxic levels for plants (3,000-6,000 ppm salt
results in trouble for most plants). Salt in soils decrease the osmotic potential of the soil so that plants
can’t take up water from it. When soils are salty, the soil has greater concentrations of solute that does
the root, so plants can’t get water from soil. The salts can also be directly toxic, but plant troubles usually
result primarily from inability to take up water from salty soils. Problems with salinization are most
commonly associated with excessive water application, rather than with too little.
All irrigation water contains dissolved salts derived as it passed over and through the land, and rain water
also contains some salts. These salts are generally in very low concentrations in the water itself.
However, evaporation of water from the dry surface of the soil leaves the salts behind. Sometimes,
whitish salt crust can result on the surface of soils. In extreme cases, when the salt crust is too thick, it
can’t be flushed, as water just runs off the salty surface.
Salinization is especially likely to become a problem on poorly drained soils when the groundwater is
within 3m or less of the surface (depending on soil type). In such cases, water rises to the surface by
capillary action, rather than percolating down through the entire soil profile, and then evaporates from the
soil surface.
Salinization is a worldwide problem, particularly acute in semi-arid areas which use lots of irrigation water,
are poorly drained, and never get well flushed. These conditions are found in parts of the Mideast, in
China’s North Plain, in Soviet Central Asia, in the San Joaquin valley of CA, and in the Colorado River
Basin; all areas where the soil profile never (or rarely) gets well flushed. Globally, something on the order
of 20% of the world’s irrigated acreage is estimated to be affected by salinization, with salt concentrations
high enough in about 10% of irrigated acreage to decrease yields significantly.
Salinization obviously reduces crop productivity. In the US, salinization may be lowering crop yields on
as much as 25-30% of the nation’s irrigated lands. In Mexico, salinization is estimated to be reducing
grain yields by about 1 million tons per year, or enough to feed nearly million people. In extreme cases,
land is actually being abandoned because it is too salty to farm profitably. Salt buildup is an existing or
potential hazard on almost all of the 42 million acres of irrigated farmland in the United States.
Salt kills germinating seedlings by removing the water from their cells (plasmolysis, exomosis). Several
salts and their ions are responsible: NaCl, CaCl, KCl, MgCl, MgSO4, HCO3 - CO3 --MgCO3, NO3-, NH4+, K+.
In this investigation we will be investigating the effects of just one salt; NaCl will affect germinating
seeds in a similar way, as would other salts or combinations of salt. NaCl has a solubility of 35.7 grams
per 100ml of H20.It is known that ocean water has a 3.5% concentration (3.5 grams per 100ml oh H20).
Freshwater has a concentration of 0.005% salt (0.005 grams per 100ml of H20). We will attempt to
investigate how increasing the concentration of NaCl will affect the growth of radish plants, measured
by shoot length and germination rate. We hypothesize
that…________________________________________________________________________________
______________________________________.
Materials
Ziploc Bags (8)
Paper Towels (8)
Radish Seeds (40)
Sharpie
Procedure:
1.
2.
3.
4.
5.
6.
7.
8.
9.
Read over the introduction and answer the prelab questions.
Gather materials.
Count out 8 sets of 5 radish seeds.
Label the outside of each bag with the NaCl concentration used and all of your group members’
names. (use a Sharpie)
Soak one paper towel in the first salt solution. Do not wring it out too much – it should be
somewhat drippy.
Scatter the seeds on the paper towel… be sure they’re not touching each other.
Fold the paper towel over to make an envelope to hold your seeds in place.
Place the folded paper towel with seeds into the Ziploc bag, and seal the bag. Leave a small
amount of air inside the bag (seeds need oxygen to germinate!)
Repeat steps 5-8 for each salt concentration.
Prelab:
1.
Explain the process of osmosis. Show how a seed surrounded by salty water would dehydrate.
Osmosis is the diffusion of water across a selectively permeable membrane. Water moves to where there is a
higher solute concentration. For example, if a seed is in a solution with a high salt concentration, then the water
inside the cells of the seed will leave the cell because the concentration of salt is higher outside the seed.
2.
Describe how to create a 0.5% NaCl solution by mass. (Hint: Density of H 2O is 1 g/mL)
0.5 grams of NaCl/100 mL of H2O
3.
Formulate your hypothesis. What are you changing and what are the two variables that will change as a
result of the independent variable you are changing? Make sure you quantify your hypothesis.
If ___I increase the amount of NaCl____, then __the number of seeds that will fail to germinate will be
great and the shoot length will be shorter. ______.
4.
Identify all the experimental variables.
Independent: Concentration of NaCl
Dependent: the number of seeds that fail to germinate and the shoot length
5. What will your control be?
Control: the 0% salt concentration solution.
Data:
Concentration
of NaCl
0%: 0 g/100mL H2O (control)
1%: 1 g/100 ml H2O
2%: 2 g/100 ml H2O
3%: 3 g/100 ml H2O
Control
0%
1%
2%
3%
4%
Dilutions of salt Concentrations
4%: 4 g/100 ml H2O
5%: 5 g/100 ml H2O
6%: 6 g/100 ml H2O
7%: 7 g/100 ml H2O
GROUP DATA
# Seeds Dead
(not germinated)
% Seeds
Dead
(not germinated)
CLASS AVERAGES
Avg. Shoot
Length
(mm)
# Seeds Dead
(not germinated)
% Seeds
Dead
(not germinated)
Avg. Shoot
Length
(mm)
5%
6%
7%
Results:
Create 2 graphs (one for average shoot length and one for # of seeds that germinated) with your independent
variable on the X-axis and your dependent variables on the Y axis.
Figure 1: This graph shows the relationship between NaCl concentration added and average shoot length.
Figure 2: This graph shows the relationship between NaCl concentration and # of seeds that did not germinate
(died).
1. Create a Dose-Response Curve for % NaCl vs. Class Average % Seeds Dead. Label axes
properly!
Then, answer the following questions:
A) What is the LC50 of NaCl on radish seeds? Label it on
your graph. Explain how you found this.
B) How is this data relevant to agricultural practices?
Figure 3: Dose-Response curve indicates the % of NaCl necessary to kill 50% of the seeds, or prevent them from
germinating.
Discussion:
Answer the following questions in paragraph form, on a separate sheet of paper. If you do any additional
research, make sure you cite sources in APA format.
2. What is the relationship between the % of seeds that died and the concentration of the salt water.
BE SPECIFIC/QUANTIFY YOUR DATA
3. What is the relationship between the average length of the shoots and the concentration of the
salt water.
BE SPECIFIC, QUANTIFY YOUR DATA
4. Did your experiment support your hypothesis? Why or why not?
5. List 3 errors that may have occurred to invalidate the data & how they may be corrected.
6. RESEARCH: Explain why increasing levels of salt concentrations affect seed growth and why
irrigation seems to be the main cause of this.
7. Suggest one way that you might be able to prevent NaCl from reducing your crop yield (plants
you harvest) on the farm you designed for task 3. Could you test this before implementing your
plan? How?