Soil Investigation
Temperature: Soil temperature affects climate, plant growth, the timing of budburst or
leaf fall, the decomposition rate of organic material, and other biological, chemical, and
physical processes that take place in the soil.
How do flowers and other plants know when to start growing in the spring? How do
farmers know when it is safe to plant their crops? Soil temperature plays an important
role in both of these decisions. Each spring, soil is heated from above by warmer air and
by solar radiation. Once the soil reaches a certain temperature, it is time to plant and
grow.
Salinity: salinity is a measure of the saltiness of the soil. Many plants have trouble
growing in soil that contains too much salt. High soil salinity makes it more difficult for
plants to get water from the soil and can interfere with their obtaining the proper
nutrients. The table below provides a
general idea of the effect salinity has on
plants.
Plant Response to Salinity
Soil can become saline by the natural
weathering of minerals, irrigation, or
run-off from salted roads. Poor drainage
and hot, dry weather also contribute to
the build-up of salt in the soil. Sodium
chloride, NaCl, is the most common
salt, but others such as calcium chloride,
CaCl2, and magnesium sulfate, MgSO4,
are often present as well.
Salinity
(dS/m)
Plant response
0–2
few problems
2–4
some sensitive plants have trouble
4–8
most plants have trouble
8 – 16
only some plants will survive
above 16
very few plants will survive
Soil salinity is determined by measuring
the electrical conductivity of a soil-water mixture. The higher the salinity of the soil, the
higher the conductivity of this mixture will be. Soil salinity is commonly reported in units
of dS/m, deciSiemens per meter.
Soil pH: When you think of pH, you probably think of acidic and basic solutions. But
soil can be acidic or basic, too. Soil pH, sometimes referred to as soil acidity, can be
expressed using the pH scale. The pH scale ranges from 0 to 14. Soils with pH above 7
are basic or sweet. Soils with pH below 7 are acidic or sour. A soil with a pH of 7 is
neither acidic nor basic, but is neutral.
The pH of soil is an important factor in determining which plants will grow because it
controls which nutrients are available for the plants to use. Three primary plant nutrients
– nitrogen, phosphorus, and potassium – are required for healthy plant growth. Because
plants need them in
large quantities, they are
called
macronutrients. They
are the main
ingredients of most
fertilizers that
farmers and gardeners
add to their soil.
Other nutrients such as
iron and manganese
are also needed by
plants, but only in
very small amounts.
These nutrients are
called micronutrients.
The availability of
these nutrients depends
not only on the
amount but also on the
form that is present,
on the rate they are
released from the
soil, and on the pH of
the soil. In general,
macronutrients are more
available in soil with high pH and micronutrients are more available in soil with low pH.
Figure 1 shows the effect of pH on the availability of nutrients in the soil.
Acid precipitation can be very harmful to the environment. It can kill fish by lowering the
pH of lakes and rivers. It can harm trees and plants by burning their leaves and depriving
them of nutrients. In addition, it can weather away stone buildings and monuments.
Carbon dioxide, CO2, is a gas found naturally in the air. When CO2 dissolves into water,
it produces a weak acid called carbonic acid, H2CO3. This makes precipitation slightly
acidic naturally. Precipitation of pH 5 to 6 is common and does not generally cause any
problems. Oxides of sulfur and nitrogen released into the air by fossil fuel burning power
plants, various industries, and automobiles dissolve into atmospheric water to form acids
such as H2SO3 (sulfurous acid), H2SO4 (sulfuric acid), HNO2 (nitrous acid), and HNO3
(nitric acid). The resulting precipitation can be as acidic as pH 4, precipitation with a pH
below 5.6 is generally considered to be “acid precipitation.” Most acidic precipitation
occurs over and downwind of heavily populated and industrialized areas.
Acid precipitation is not problematic in all locations. Some soils contain substances that
will help neutralize acid precipitation. These substances, called buffers, are commonly
composed of limestone, calcium carbonate, or calcium bicarbonate. The buffers help
stabilize soil pH and, because they dissolve into water runoff, they help stabilize the pH
values of surrounding lake, stream, and pond waters as well.
Moisture: Soil moisture is water that is held in the spaces between soil particles. Dry soil
is made up of minerals and air pockets, called pore spaces. A typical volumetric ratio
would be 55% minerals and 45% pore space. As water is added to the soil, the pore
spaces begin to fill with water. Soil that seems damp to the touch might now have 55%
minerals, 35% pore space and 10% water. This would be an example of 10% volumetric
water content. The maximum water content in this scenario is 45% because at that value,
all the available pore space has been filled with water. This soil is referred to as being
saturated, because at 45% volumetric water content, the soil can hold no more water.
Soils collect, store, and release water. Collection occurs as water enters the soil through
surface pores in a process called infiltration.When forces of retention within soil are
greater than removal forces water storage is possible. Water release takes place when
plant uptake, drying, or gravitational forces overcome retention.
Compost, aerobically decomposed remnants of organic materials, is commonly mixed
into soil to improve soil fertility and water holding capacity. Grass clippings, leaves,
sawdust, kitchen refuse, wood ashes, garden refuse, and shredded newspapers are just
some of the common materials that are composted.
Mulch, in contrast, is placed on the soil surface. Mulch affects soil moisture by slowing
evaporation, reducing weed transpiration, and reducing runoff. Grass clippings, leaves,
sawdust, wood chips, straw, shredded newspapers, and compost are common materials
used as mulch. Inorganic mulches, such as plastic sheeting, rocks, and gravel are also
widely used.
Commercial water absorbing polymers, such as Soil Moist®, Stockosorb®, and TerraSorb®, are water management tools that purportedly reduce evaporation, water runoff,
and soil erosion when mixed into soil.
Procedure:
I. Temperature
1. Connect a Temperature Probe and the data-collection interface.
2. Obtain a soil sample. Use a long nail, or similar tool, to make a vertical hole in the
soil that is 10 cm deep and which will firmly accommodate a Temperature Probe.
3. Insert the Temperature Probe into the soil to a depth of 10 cm.
4. When the temperature reading stabilizes record the displayed value (to the nearest
0.1ºC) as the soil temperature 10 cm below the sample surface.
II. Salinity
1. Prepare the water-soil mixture.
a. Place 50 g of soil into a 250 mL beaker.
b. Add 100 mL of distilled water and stir thoroughly.
c. Stir once every three minutes for 15 minutes. Continue with Steps 2–3 while
waiting.
2. Connect the Conductivity Probe and the data-collection interface. Set the switch on
the Conductivity Probe box to the 0–20000 µS/cm salinity range (equivalent to 0.20
dS/m).
Calibrate the sensor using the 2-point calibration option of the Vernier data-collection
program or manually enter the calibration values as directed by your instructor.
4. Collect salinity data.
a. Place the tip of the electrode into Sample A. The hole near the tip of the probe
should be completely covered by the water-soil mixture.
b. Start data collection.
c. Stop data collection after about 15 seconds.
d. Use the statistics option to determine the mean salinity value.
III. Soil pH
1. Prepare the water-soil mixture.
d.
e.
f.
g.
Place 50 g of soil into a 250 mL beaker.
Add 100 mL 0.01 M CaCl2 and stir thoroughly.
Stir once every three minutes for 15 minutes.
After the final stirring, let the mixture settle for about five minutes. This allows the
soil to settle out, leaving a layer of water on top for you
to take your pH measurement. Proceed with Step 2
while you are waiting.
2. Connect the pH Sensor and the data-collection interface.
Important: For this experiment, your teacher already has
the pH Sensor in pH soaking solution in a beaker; be
careful not to tip over the beaker when connecting the
sensor to the interface.
3. Rinse the pH Sensor with distilled water.
4. Measure the pH.
a. Carefully place the tip of the pH Sensor into the liquid
part of the beaker contents. Make sure the glass bulb at
the tip of the sensor is covered by the water. Stir gently.
b. Continue gentle stirring. Note and record the pH value
when the reading stabilizes.
5. Rinse the pH Sensor with distilled water and return it
to its storage container.
Moisture:
Connect a Soil Moisture Sensor and the data-collection interface.
2. Obtain a soil sample.
3. Position the Soil Moisture Sensor. Note: The long axis of
the sensor should be placed horizontally, with the short
axis or “blade” oriented vertically as shown in the figure.
h. Use a thin implement such as a flat-bladed trowel to cut
a slot in the soil.
i. Place the sensor into the hole, making sure the entire
length of the sensor is covered.
j. Press down on the soil along either side of the sensor with your fingers. Continue
to compact the soil around the sensor by pressing down on the soil with your
fingers until you have made at least five passes along the sensor. This step is
important, as the soil adjacent to the sensor surface has the strongest influence on
the sensor reading.
4. Start data collection. When the soil moisture reading stabilizes, record the displayed
value (in %).
5. When removing the sensor from the soil, do not pull it out of the soil by the cable!
Doing so may break internal connections and make the sensor unusable.
QUESTIONS
1.
2.
3.
4.
What was the temperature 10 cm below the surface of the soil sample you tested?
How is soil temperature important to plants?
List three factors that influence soil temperature.
What is the salinity (in dS/m) of the soil sample you tested in the Preliminary
Activity?
5. According to the table on the previous page, how would plants respond to the soil
sample you tested?
6. Describe two ways in which soil can become saline.
7. What is the pH of the soil sample you tested? Is the soil classified as sweet, sour, or
neutral?
8. How is soil pH important to plants?
9. How is soil pH important in a discussion of the acid deposition and its influence on
plant growth?
10. What was the soil moisture value (in %) for the soil sample you tested in the
Preliminary Activity?
11. How is soil moisture important?
12. Some material used for making compost, such as grass clippings, leaves, sawdust, and
coffee grounds, are found nearly everywhere. Other desirable composting materials,
such as grapevine waste and seaweed, are not available in all locations. List three
materials used for making compost in your area that are not available in all locations.
13. Materials used as soil mulch vary from place to place. List three mulching materials
commonly used in your area.