THE INVESTIGATION BEGINS!
– Each team will create 4 water and
soil mixtures.
– The water in the mixtures will
dissolve the chemicals responsible
for the pH of the soil.
A farmer needs good land to successfully
raise crops. What is the probability of finding
fertile soil on our watery planet?
71% of the Earth is water and 29% is land.
29% of land
Of that 29%, only 11% of the land is
capable of growing crops.
11 % of land
Of that 11%, 30 to 40% of the land is too
acidic for most crops.
Remaining fertile land
The world’s population is projected to increase
from 7 billion to 9 billion people by 2050.
• Can global food production be increased to
feed so many more people?
• Can technology improve crop production
on land that is less than ideal—land that is
too acidic or too basic?
What is soil?
• Soil is one of the three major natural resources (along with air
and water).
• It is the upper layer of earth in which plants grow.
• It is made up of a mixture of organic remains (like decayed
leaves, wood, and dead organisms), clay, and rock particles.
• It also contains water, air, countless living organisms, and
minerals from the breakdown of rock particles.
“To be a successful farmer, one must first
know the nature of the soil.” Xenophon
Why is soil important?
• Soil is the “skin of the earth” and is essential for life on Earth.
 It provides a place to grow for all kinds of plants.
 It takes in and gives off gases like carbon dioxide,
methane, and water vapor.
 It provides homes for animals, microorganisms, and fungi.
 It absorbs, holds, releases, and purifies the water that
runs through it.
Plants get important nutrients from soil, just as you get
nutrients from food and vitamins.
Calcium
Iron
Phosphorus
Sulfur
Magnesium
Zinc
Potassium
Nitrogen
Soils found around the world are very different
from each other.
• Their characteristics are dependent on the
type of rock from which they are formed
and the climate in which they are located.
• Different soils have unique physical and
chemical properties.
Physical properties of soil include layering, color,
and texture.
Chemical properties of soil include cation
exchange capacity and soil pH.
The focus of this activity is the
identification of a soil’s pH level.
The pH level in soil is a measure of how basic
or acidic it is.
• pH is determined by the concentration of the hydrogen ion,
the term pH meaning “power of hydrogen”.
• Acids have a pH of 0 - 6 because they have excess hydrogen
(H+) molecules, and bases have a pH of 8 -14 because they
have excess hydroxide (OH-) molecules.
• Pure water is considered neutral with a pH of 7 because the
concentrations of H+ and OH- ions are equal.
A pH scale provides a visual way of
measuring acids and bases.
Indicators are used to measure pH because
they change color in the presence of acids and
bases.
Some plant pigments are natural pH
indicators. Red grapes, hibiscus flowers, and
red cabbage contain such pigments.
Red cabbage juice is the indicator in this
activity.
• Anthocyanin found in red cabbage reacts with both acids and
bases.
• Strong acidic solutions turn the anthocyanin red and strong
bases turn it greenish-yellow.
• Weaker acids create a pinkish-purple color, and weaker bases
create a deep blue color.
Standard pH scale
Red cabbage pH
scale
Plants are unable to grow
successfully outside of a
preferred pH range.
• Acidic soils (low pH), dissolve
aluminum and manganese in the
soil.
 The dissolved metals bind with critical plant
nutrients such as calcium and phosphate
making them unavailable to plants’ root
systems.
• Basic soils (high pH) prevent vital
nutrients from dissolving enough
for plants’ roots to fully absorb
them.
In the U.S., very acid soils can be found in the Blue
Ridge Mountains in the Appalachians.
pH levels there can be as low as 3.5.
That is equivalent to orange juice.
Extremely alkaline soils can be found in the western
United States.
Soils in Utah can reach an alkalinity level
of 8.0, nearly that of baking soda.
Acid soils are an increasingly serious global
threat to successful crop production.
• Acid soils with a pH of less than 5.5
limit crop production on 30 to 40% of
the world’s potentially farmable land.
• Aluminum toxicity is the single most
important factor and leads to stunted
root growth.
The application of agricultural lime can help
raise soil pH
• The addition of lime increases root cell growth and
lowers the absorption of aluminum.
• However, this is not a practical solution for all parts of
the world due to the expense of application and
erosion on hilly terrain.
Scientists are investigating ways to increase
crop production in areas where lime
applications are impractical.
• Through selection and breeding strategies,
aluminum-tolerant varieties of major crops are
being developed.
• The resulting higher crop yields will help to feed
the world’s rapidly growing population.
INVESTIGATION WRAP-UP
• A very small percentage of Earth’s land is fertile
enough to raise crops.
• Appropriate soil pH levels are critical for successful
crop growth.
• Aluminum toxicity due to acidic soils is an increasing
problem.
• Farmers routinely test their soil for pH levels.
• Knowing the pH levels helps farmers make intelligent
decisions for achieving the best crop production.
• Due to the world’s growing population, maximizing
crop production is critical.
“We can grow more food, but we can’t grow
more soil.”
• Many companies are committed
to maximizing farmers’ corn
yields.
– One program integrates 20
different data sets (including
water and soil pH) to identify
which seeds best fit a field’s
soil condition.
– Farmers receive prescriptions
unique for each of their fields.