The purpose of this resource is to measure the pH of a soil horizon. Students mix dried and sieved soil samples with distilled water. The mixture is allowed to settle until a relatively clear layer is formed. Students use a pH pen, pH meter, or pH paper to determine the pH of the sample. The procedure is done three times for each horizon. Educational levels: Primary elementary, Intermediate elementary, Middle school, High school

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GLOBE® 2005 Soil pH Protocol - 1 Soil
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Introduction
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Soil pH Protocol
Purpose
To measure the pH of a soil horizon
Overview
Students mix dried and sieved soil samples 
with distilled water. The mixture is allowed 
to settle until a relatively clear layer, called the 
supernatant, is formed. Students use a pH pen, 
pH meter, or pH paper to determine the pH of 
the sample. The procedure is done three times 
for each horizon.
Student Outcomes
Students will be able to apply laboratory tests 
for pH to soil samples. Students will be able 
to relate pH to the physical and chemical 
properties of a soil sample.
Science Concepts
Earth and Space Sciences
Soils have properties of color,  
 texture, structure, consistence,            
 density, pH, fertility; they support   
 the growth of many types of plants.
The surface of Earth changes.     
Water circulates through soil  
changing the properties of both the 
soil and the water. 
Physical Sciences
Objects have observable properties.
Chemical reactions take place in every 
part of the environment.
Scientific Inquiry Abilities
Identify answerable questions.          
Design and conduct an investigation.          
Use appropriate tools and   
 techniques including mathematics            
 to gather, analyze, and interpret data.
Develop descriptions and explanations, 
predictions and models using evidence.
Communicate procedures and 
explanations.           
Time
 One 45-minute class period
Level
All
Frequency
Once for each soil horizon
Materials and Tools
Oven-dried, sieved soil
Distilled water
Pencil or pen
100-mL graduated cylinder
Glass stirring rod or other stirring device
100 mL beaker
pH meter, pH pen, or pH paper
Balance (accurate to 0.1 g)
Soil pH Data Sheet
Preparation
Collect required soil samples.
Review the pH Protocol in the Hydrology 
Investigation
Dry and sieve soil samples, and store them in 
a sealed container.
Calibrate the balance to 0.1 g.
Calibrate the pH pen or meter. (See procedure 
for calibration in the pH Protocol in the 
Hydrology Investigation.)
Prerequisites
Soil Characterization Protocol
GLOBE® 2005 Soil pH Protocol - 2 Soil
Soil pH Protocol –  
Introduction
The pH, or the amount of hydrogen ions in a 
sample, is an important consideration when 
studying soil. As in the study of hydrology, the pH 
scale is used as an indication of the concentration 
of hydrogen ions in the soil. Dry, sieved soil is 
dissolved in a specified volume of water with a 
known pH. The extent to which the dissolved 
soil changes the pH of the water is an indicator 
of the number of hydrogen ions contained in the 
soil. pH is measured on a logarithmic scale and 
represents the negative logarithm of the hydrogen 
ion concentration in moles/L. For example, a pH 
of 2 represents a concentration of 1x10-2 moles/L 
(0.01) hydrogen ions, and pH 8 represents a 
concentration of 1x10-8 moles/L (0.00000001) 
hydrogen ions. 
Figure SO-PH-1
141312111098765
Very
strong
Very
strongStrong Slight Slight Strong
Basic
Neutral
Possible pH Ranges Under Natural Soil
Acidic
Apple: 5.0
Spinach: 6.0
Tomato: 5.5
Wheat: 5.5
Cucumber: 5.5
Carrot: 5.5
White Pine 4.5
Cranberry: 4.2
Moderate
Most 
desirable
Most agricultural
Extreme pH range for most mineral
Moderate
3 421
When soil contains a high concentration of 
hydrogen ions, it is considered to be acidic and 
when it has a low number of hydrogen ions, it is 
considered to be basic. pH 7 is considered to be 
“neutral” (neither acidic nor basic). The pH scale 
ranges from 1-14 with pH 1 being extremely acidic 
(1x10-1 or 0.1 moles of hydrogen ions per liter), 
and pH 14 being extremely basic (1x10-14 moles 
of hydrogen ions per liter or 0.00000000000001 
moles/L). 
Soil pH is an indication of the soil’s chemistry and 
fertility. The pH affects the chemical activity of the 
elements in the soil, as well as many of the soil 
properties. Different plants grow best at different 
pH values. See Figure SO-PH-1. Farmers and 
gardeners may add materials to their soil to change 
its pH depending upon the type of plants they want 
to grow. The pH of the soil may also affect the pH 
of ground water or of a nearby water body, such 
as a stream or lake.
GLOBE® 2005 Soil pH Protocol - 3 Soil
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The pH of soil controls many of the chemical 
and biological activities that take place in the soil 
and also indicates something about the climate, 
vegetation, and hydrologic conditions under 
which the soil formed. The pH of a soil horizon 
(how acidic or basic the soil is) is affected by the 
parent material, the chemical nature of the rain and 
other water entering the soil, land management 
practices, and the activities of organisms (plants, 
animals, and microorganisms) living in the soil. 
For example, needles from pine trees are high in 
acids, and as they decay over time, they can lower 
the pH of the soil.
Teacher Support
Preparation
Have students review the pH Protocol in the 
Hydrology Investigation and practice using the pH 
equipment by testing the pH of different liquids 
at different pH levels.
Measurement Procedures
To measure pH, students mix dry soil samples 
with distilled water until the soil and liquid are in 
equilibrium and provide an accurate measurement 
of the soil pH. A 1:1 soil/water solution is used 
for this protocol because it is similar to a standard 
method for professional soil pH measurements.
Managing Materials
If students are going to use a pH meter to 
measure pH, while performing their soil pH 
measurements, students should be sure that 
the pH meter is working properly. They should 
calibrate it according to the directions given in the 
pH Protocol in the Hydrology Investigation and, if 
necessary, replace weak or dead batteries.
For some soil samples, especially those high 
in clay, the soil in the water will not settle after 
mixing and will not form a supernatant. In this 
case, after thorough mixing according to the 
protocol, place the pH meter or paper into the top 
of the soil/liquid suspension and take a reading.
Managing Students
If a single team of students is measuring all three 
samples of a horizon, have them process the 
samples in parallel, not in sequence. This will 
allow the protocol to be completed in less than 
45 minutes. 
Questions for Further Investigation
What natural changes could alter the pH of a 
horizon?
How does the pH of the rain affect the pH of a 
soil horizon?
How does the pH of the soil affect the pH of local 
water bodies?
How does climate affect the pH of a horizon?
How do slope and aspect affect the pH of a 
horizon?
How does the type of vegetation growing on the 
soil affect the pH of the soil?
GLOBE® 2005 Soil pH Protocol - 4 Soil
Soil pH Protocol 
Lab Guide
Task
To obtain three pH readings for a soil horizon
What You Need
❑ Dried sieved soil        ❑ pH Data Sheet
❑ Distilled water   ❑ Pencil or pen
❑ 100-mL graduated cylinder ❑ Glass stirring rod or other stirring device
❑ Four 100-mL containers ❑ pH meter or pH paper
❑ Balance (accurate to 0.1 g)
In the Lab
1. In a cup or beaker, mix 40 g of dried and sieved soil with  
40 mL of distilled water (or other amount in a 1:1 soil to water 
ratio) using a spoon or other utensil to transfer the soil. 
2. Stir the soil/water mixture with a spoon or other stirrer until 
it is thoroughly mixed. Stir the soil/water mixture for 30 
seconds and then wait for three minutes for a total of five 
stirring/waiting cycles. Then, allow the mixture to settle until 
a supernatant (clearer liquid above the settled soil) forms 
(about 5 minutes).
GLOBE® 2005 Soil pH Protocol - 5 Soil
3. Measure the pH of the supernatant using the pH paper or meter. 
Dip the pH paper or calibrated pH meter in the supernatant. 
Record the pH value on the Soil pH Data Sheet. If pH meter 
requires calibration, gloves should be worn.
4. Repeat steps 1-3 for two more samples from the same horizon. 
Soil pH Protocol Lab Guide - Page 2
GLOBE® 2005 Soil pH Protocol - 6 Soil
Soil pH Protocol– 
Looking at the Data
Are the data reasonable?
A soil’s parent material, the climate under which it 
formed, the vegetation it supports and the amount 
of time it has had to develop, determines its pH. In 
general, soil pH values range from 4.0 for acidic, 
organic rich soils, to 8.5 for soils with a high 
number of free carbonates. Occasionally, the pH 
can go as low as 3.5 or as high as 10. 
Generally, the pH will not vary much from horizon 
to horizon in a soil profile. This is because the 
pH scale is a base 10 logarithmic scale, so the 
differences of 1 unit in pH mean there are 10 
times more hydrogen ions, or 10 times more 
acidity. In some cases, there might be a drastic 
change in parent material causing a very 
different pH between horizons. For example, 
materials may be deposited onto a horizon or 
some human intervention has occurred such 
as addition of limestone. Drastic changes in soil 
pH between soil horizons may help students to 
uncover clues about the story of the soil at that 
location. Students should expect some change 
in pH from the top to the bottom of the profile, 
depending on the amount of organic matter, free 
carbonates, and weathering of the soil. If there 
is a lot of organic matter at the surface, and no 
limestone has been added, the pH in the upper 
horizons may be lower than those below. Where 
carbonates are present, horizons tend to have 
high pH values.
What do scientists look for in the 
data?
Many different scientists are interested in soil pH 
data. Soil pH measurements, in combination with 
soil characterization measurements and other 
GLOBE measurements, provide scientists a great 
deal of information about the environment. For 
example, soil pH helps scientists understand the 
chemical properties of the soil and the potential for 
certain nutrients to be stored or released. With this 
information, scientists determine the suitability of 
a soil for plant growth. 
Scientists also predict the movement of materials 
into the hydrologic system. They consider 
rainfall chemistry when they make predictions 
of changes in soil chemistry and soil pH over 
time. 
Soil pH helps scientists to reconstruct the 
development of a soil and predict the nature of 
the soil in the future. 
An Example of a Student Research 
Project
Middle school students from Keflavik, Iceland 
collected soil samples while carrying out soil 
characterization measurements on a soil pit. They 
dried and sieved three samples for each horizon 
in their sample site and performed the Soil pH 
Protocol on each sample. 
In order to analyze their data, they decided to 
graph the pH measurements they collected. 
The graph plotted the pH measurements for 
each horizon at the mid-point depth of each soil 
horizon. The students calculated the mid-point 
depth according to the following equation:
(Bottom Depth – Top Depth)/2 + Top Depth
Horizon 1: (10-0)/2 + 0 = 5.0 cm
Horizon 2: (23-10)/2 + 10 = 16.5 cm
Horizon 3: (44-23)/2 + 23 = 33.5 cm
Horizon 4: (65-44)/2 + 44 = 54.5 cm
GLOBE® 2005 Soil pH Protocol - 7 Soil
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The results of their measurements are in the table below.
 Horizon Top Depth Bottom Depth Mid-Point Depth pH  
     (mean of 3 replicates)
 1 0.0 10.0 5.0 5.5 
 2 10.0 23.0 16.5 5.6 
 3 23.0 44.0 33.5 5.8 
 4 44.0 65.0 54.5 5.9 
Using the data in the table, the students plotted the mean pH at the mid-point depth of each horizon 
as shown in the graph below.
The students noticed that the pH was lowest at the top of the soil profile and increased with depth. 
They hypothesized that weathering of the soil at the surface and inputs of rain or organic matter 
caused the low pH at the top of the profile.
The students were interested in knowing whether the trend in pH they observed was typical of soils 
in other parts of the world with different climates and vegetation. According to the MUC classification 
guide, the land cover type at their site was dwarf-shrub/moss tundra. They used the GLOBE data 
archive to search for other schools that had made soil pH measurements, and found two schools 
in areas different from their own.
0
–10
–20
–30
–40
–50
–60
pH
pH vs. Depth
5.4 5.5 5.6 5.7 5.8 5.9 6.0
GLOBE® 2005 Soil pH Protocol - 8 Soil
One school was a secondary School in Deir Allah, Jordan. The students at this school reported their 
vegetation type as row crop or pasture. The pH data reported by this school are given in the table 
below. 
Deir Allah, Jordan
 Horizon Top Depth Bottom Depth Mid-Point Depth pH  
     (mean of 3 replicates)
 1 0.0 20.0 10.0 8.0 
 2 20.0 33.0 26.5 8.2 
 3 33.0 44.0 38.5 8.5 
 4 44.0 100.0 72.0 8.5 
The other school they chose was a middle school in New York, USA. The students at this school 
reported their vegetation type as evergreen-needled trees. The pH data reported by this school 
are given in the table below.
New York, USA
 Horizon Top Depth Bottom Depth Mid-Point Depth pH  
     (mean of 3 replicates)
 1 0.0 13.0 6.5 2.9 
 2 13.0 23.0 18.0 3.4 
 3 23.0 35.0 29.0 3.4 
 4 44.0 60.0 52.0 4.0 
The students then plotted the pH values at the midpoints for each of the three schools on one 
graph as shown below.
0
–10
–20
–30
–40
–50
–60
–70
–80
pH
D
ep
th
Comparison of Soil pH Measurements
3 42 5 6 7 8 9
USA
Iceland
Jordan
GLOBE® 2005 Soil pH Protocol - 9 Soil
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The students noticed considerable differences in the pH values at each of these locations. The soil 
in Jordan had much higher pH values than the soil in Iceland, while the New York school had much 
lower pH values. They noticed a trend of increasing pH with increasing depth at all three of schools. 
The students concluded that deeper soils have a higher pH in many different kinds of soils.
The students realized that more information about each location would help them better understand 
the pH differences at the sites. They decided that in the future, they would contact students at each 
school using GLOBE mail to find out more about their locations. They also planned to download 
precipitation and temperature data from the GLOBE data archive to see whether the differences in 
the amount and pH of annual rainfall and mean annual temperature at these schools would give 
an indication of why the pH values were so different in the soils.


Soil pH Protocol

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Soil pH Protocol

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