The purpose of this resource is to measure the amounts of nitrogen, phosphorous and potassium in each horizon in a soil profile. Using a NPK test kit, students mix a dry, sieved soil sample into a solution and chemically extract the N, P, and K as nitrate, phosphate, and potassium. The N, P, and K amounts in the sample are determined by comparing the solution to a color chart. Students describe the N, P, K amounts as high, medium, low, or none. These measurements are conducted three times for each horizon. Educational levels: Middle school, High school

The GLOBE Program, University Corporation for Atmospheric Research (UCAR)

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Digital Library for Earth System Education

GLOBE® 2005 Soil Fertility Protocol - 1 Soil
Learning A
ctivities
A
ppendix
W
elcom
e
Introduction
Protocols
Soil Fertility Protocol
Purpose
To measure the amounts of nitrogen (N), 
phosphorus (P), and potassium (K) in each 
horizon in a soil profile
Overview
Using a NPK test kit, students mix a dry, sieved 
soil sample into a solution and chemically 
extract the N, P, and K as nitrate, phosphate, 
and potassium. The N, P, and K amounts in 
the sample are determined by comparing the 
solution to a color chart. Students describe the 
N, P, K amounts as high, medium, low, or none. 
These measurements are conducted three times 
for each horizon.
Student Outcomes
Students will be able to measure the nitrate 
nitrogen, phosphate phosphorus and potassium 
contents of soils.
Students will be able to relate soil fertility to the 
physical and chemical properties of the soil.
Science Concepts
Earth and Space Sciences
Soils store nutrients in different amounts 
depending on the type of nutrient and 
the soil properties.
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. 
Soils are often found in layers, with 
each having a different chemical 
composition and texture.
Soils consist of minerals (less than 2 mm), 
organic material, air and water.
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.
Life Sciences
Atoms and molecules cycle among the 
living and nonliving components of the 
ecosystem.
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 for three groups of 
students to analyze one horizon.
Level
Middle and Secondary
Frequency
Once for each soil profile 
Materials and Tools
Oven-dried, sieved soil (sieved with a 
Number 10 sieve with 2 mm mesh 
attached to a frame)
GLOBE (or equivalent) NPK kit
Distilled water
Beaker
Teaspoon
Clock or stopwatch
Soil Fertility Data Sheet (more than one Data 
Sheet may be needed for each profile)
Latex gloves
Goggles
Preparation
Obtain dry, sieved soil samples.
Collect required equipment.
Spread out newspapers or other cover on table 
to keep area clean.
Prerequisites
Soil Characterization Protocol 
GLOBE® 2005 Soil Fertility Protocol - 2 Soil
Soil Fertility Protocol – 
Introduction
In order to grow, plants require sunlight, water, 
air, heat, and nutrients. Table SO-FE-1 lists 
the macronutrients (nutrients required in large 
amounts) and micronutrients (nutrients required 
in smaller quantities) required for plant growth. 
The fertility of a soil indicates the availability of 
these nutrients for plants to grow.
 Macronutrients Micronutrients
 Nitrogen (N) Iron (Fe) 
 Phosphorus (P) Zinc (Zn) 
 Potassium (K) Manganese (Mn)
 Sulfur (S) Copper (Cu) 
 Calcium (Ca) Boron (B) 
 Magnesium (Mg) Molybdenum (Mo)
  Chlorine (Cl)
Some soil nutrients are positively charged, while 
other nutrients are negatively charged. Nutrients 
that are positively charged, such as potassium, 
calcium, and magnesium, are held by negatively 
charged soil particles. These nutrients are removed 
from the soil by plants or by weathering processes. 
Negatively charged particles, such as nitrogen 
(in the common form of nitrate), phosphorus 
(in the common form of phosphate) and sulfur 
(in the common form of sulfate) are not held by 
negatively charged soil particles. These nutrients 
are more easily leached and removed from the soil. 
When a soil loses its nutrients or does not have 
the nutrients needed for plant growth, farmers and 
gardeners replenish nutrients by adding fertilizers 
to soils. 
The Soil Fertility Protocol measures the abundance 
of three nutrients: nitrate (nitrogen), phosphate 
(phosphorus), and potassium, in each horizon of 
a soil profile to determine if the soil is fertile for 
plant growth.
Nitrogen (N) is an element found in high 
concentrations in the atmosphere, but relatively low 
concentrations in soil. For nitrogen to be used by 
most living things, the N
2
 molecules must be broken 
apart. In soil and water, this usable nitrogen takes the 
form of nitrate (NO
3
), nitrite (NO
2
) and ammonium 
(NH
4
), with nitrate being the most common. In 
general, these forms of nitrogen are rapidly taken 
up by plants and are an important component 
of plant proteins. Nitrate (NO
3
-), because of its 
negative charge, is not held by negatively charged 
soil particles and is easily removed (leached) from 
the soil as water passes through it. Nitrate can also 
be converted to nitrogen gas (N
2
) or ammonia 
(NH
3
) and be volatilized (evaporated or removed as a 
gas) out of the soil. Therefore, it is important that 
farmers and gardeners add nitrogen fertilizers when 
plants need the nutrient the most and can absorb 
it before it is removed from the soil by leaching. 
When nitrogen is added to the soil in the form of 
organic matter, it is stored longer because it becomes 
available to plants slowly, as the organic matter 
decomposes.
Phosphorus (P) is used as part of the energy 
pathway in the plant. Plants use phosphorus in the 
form of phosphate (PO
4
 -3). Because of its negative 
charge, phosphate is easily leached from the soil. 
Plants are only able to take up phosphate when soils 
are at neutral soil pH values of 5.0-8.0. At low pH 
values (<5.0), phosphate combines with iron (Fe) 
and aluminum (Al) to form phosphates that are not 
soluble and cannot be taken up by plants. At high 
pH values (>8.0), phosphate combines with calcium 
Figure SO-FE-1
Table SO-FE-1
Soil in low pH (acidic) conditions
Clay Particle
Soil in high pH (basic) conditions
K+
K+
AIPO4
PO4
–3
NO3
–
NO3
–
––
––
––
––
–
–
Clay Particle
K+
K+
CaHPO4
PO4
–3
NO3
–
NO3
–
––
––
––
––
–
–
GLOBE® 2005 Soil Fertility Protocol - 3 Soil
Learning A
ctivities
A
ppendix
W
elcom
e
Introduction
Protocols
(Ca) to form calcium phosphate, which is neither 
soluble nor available for plants to take up from 
the soil. When phosphate occurs in one of these 
insoluble compounds, it becomes easy to remove 
from the soil when the soil particles are eroded. 
Like nitrogen, phosphorus is also more slowly and 
easily available to plants when it is added as part 
of decomposing organic matter.
Potassium (K) plays the role of activating cell 
enzymes in plants. It is readily available to plants in 
its elemental state (K+), and because of its positive 
charge, is easily stored on negatively charged soil 
particles. The largest source of potassium is from 
the decomposition of potassium containing 
minerals, such as mica.
Teacher Support
Preparation
Make sure students understand the importance 
of each nutrient they are measuring before they 
conduct the Soil Fertility Protocol.
Have students review the NPK kit instructions 
before conducting the Soil Fertility Protocol. 
Managing Materials
To measure soil fertility, students may use a 
GLOBE soil test kit or an equivalent product that 
produces a relative value for nitrate, phosphate, 
and potassium. 
Measurement Procedures
The basic method for measuring soil fertility consists 
of mixing a soil sample with water and chemically 
extracting the N, P, and K as nitrate, phosphate, 
and potassium. The N, P, and K amounts in the 
sample are determined by comparing the solution 
to a color chart. 
Using the GLOBE soil test kit to determine nitrogen 
(N), students compare the extraction solution with 
a pink color chart. To determine phosphorus (P), 
students compare the extraction with a blue color 
chart. To determine potassium (K), the students 
place the tube containing the extraction solution 
over a column of black boxes, and the amount of 
the blackness that can be observed through the 
cloudiness of the solution is compared with the 
column next to the tube.
Students should wait no more than 10 minutes 
to read the color change in each tube. Waiting 
longer may give erroneous results.
Based on previous experience with the GLOBE 
soil test kit, almost all readings for nitrate 
nitrogen result in values of none or low.  This 
may be due to the rapid removal of nitrate 
from the soil once it has been added (either by 
plant uptake or leaching) or due to the lack of 
sensitivity of the kit to measure nitrate.
With some soil samples, especially those high in 
clay, students will need to repeat the extraction 
procedure more than once to obtain enough 
solution for the N, P, and K analysis.
Managing Students
In order to complete the analysis within a class 
period, have different students do the analysis 
for N, P, and K simultaneously after the extraction 
solution has been made.
Safety Procedures
1. Students should wear gloves when 
handling chemicals and the water sample.
2. Students should wear goggles when 
working with chemicals. They should 
also wear surgical masks when opening 
powdered reagents.
3. School authorities should be consulted on 
the proper disposal of used chemicals.
Questions for Further Investigation
How might natural changes affect the fertility of 
a horizon?
What differences between locations could affect 
the fertility of a horizon?
How does the soil fertility affect the types of 
vegetation that can grow on a soil?
How does the soil particle size distribution affect 
the nutrient content of a horizon?
How does climate affect the nutrient content of 
a horizon?
How does the type of vegetation growing on the 
soil affect the nutrient content of the soil?
GLOBE® 2005 Soil Fertility Protocol- 4 Soil
Soil Fertility Protocol
Lab Guide
Task
To obtain three soil fertility readings for every horizon in a soil profile
What You Need
❑ Dried sieved soil
❑ Soil Fertility Data Sheet
❑ Plastic teaspoon
❑ GLOBE NPK test kit or equivalent kit (that  
measures the relative amount of nitrate,      
phosphate, and potassium in a soil sample)
 
30 mL
Part 1. Nutrient Extraction: 
1. Fill the extraction tube from your 
Soil Test Kit to the 30 mL line with 
distilled water.
2. Add 2 Floc-Ex tablets. Cap the tube 
and mix well until both tablets have 
disintegrated.
3. Remove the cap and add one 
heaping spoonful of dry, sieved soil.
4. Cap the tube and shake for one 
minute.
5. Let the tube stand until the soil settles 
out (usually about 5 minutes). The 
clear solution above the soil will be 
used for the nitrate nitrogen (N), 
phosphate phosphorus (P), and 
potassium (K) tests.
 Note: For some soils, especially those 
high in clay, there may not be enough 
clear solution extracted. If more clear 
solution is needed, repeat steps 1-5.
❑ Distilled water
❑ Pencil or pen
❑ Latex gloves
❑ Goggles
GLOBE® 2005 Soil Fertility Protocol - 5 Soil
Part 2. Testing for Nitrate Nitrogen:
Use the pipette to transfer the clear solution above the soil to one of 
the test tubes in the Soil Test Kit until the tube is filled to the shoulder. 
(If more solution is needed, repeat Part 1).
1. Add one Nitrate WR CTA Tablet. Be sure that all of the pieces 
of the tablet are added to the test tube and try not to touch 
the tablet as you place it into the tube.
2. Cap and mix until the tablet disintegrates.
3. Rest the test tube in a cup or beaker. Wait 5 minutes for color 
to develop. (Do not wait longer than 10 minutes).
4. Compare the pink color of the solution to the Nitrogen Color 
Chart in the Soil Test Kit.
5. Record your results (High, Medium, Low, or None) on the Soil 
Fertility Data Sheet.
6. Discard the solution and wash the tube and the pipette with 
distilled water.
7. Repeat this procedure with the liquid from each of the soil 
samples. Be sure to rinse the pipette and tube with distilled 
water after they are used.
Soil Fertility Lab Guide - Page 2
GLOBE® 2005 Soil Fertility Protocol- 6 Soil
Part 3. Testing for Phosphate Phosphorus: 
1. Use the clean pipette to transfer 25 drops of the clear 
solution above the soil to a clean test tube. (If more 
solution is needed, repeat Part 1).
2. Fill the tube to the shoulder with distilled water.
3. Add one Phosphorus Tablet to the tube and cap it. Be 
sure that all the pieces of the tablet are added to the test 
tube.
4. Mix until the tablet disintegrates.
5. Rest the test tube in a cup or beaker. Wait 5 minutes (but 
no more than 10 minutes) for color to develop.
6. Compare the blue color of the solution to Phosphorus 
on the color chart in the Soil Test Kit.
7. Record your results (High, Medium, Low, or None) on the 
Soil Fertility Data Sheet
8. Discard the solution and wash the tube and the pipette 
with distilled water.
9. Repeat this procedure with the liquid from each of the 
soil samples. Be sure to rinse the pipette and tube with 
distilled water after they are used.
Soil Fertility Lab Guide – Page 3
GLOBE® 2005 Soil Fertility Protocol - 7 Soil
Part 4. Testing for Potassium: 
1. Use the clean pipette to transfer the clear solution above the 
soil to a clean test tube until it is filled to the shoulder. If more 
solution is needed, repeat Part 1.
2. Add one Potassium Soil Tablet to the tube. Be sure that all 
the pieces of the tablet are added to the test tube. Cap and 
mix until the tablet disintegrates.
3. Hold the tube over the black boxes in the left column of the 
K portion of the color chart. Look through the “cloudiness” 
of the solution in the test tube and compare it to the 
shaded boxes in the right column. Record your results (High, 
Medium, Low, or None) on the Soil Fertility Data Sheet.
4. Discard the solution and wash the tube and the pipette with 
distilled water.
5. Repeat this procedure with the liquid from each of the soil 
samples. Be sure to rinse the pipette and tube with distilled 
water after they are used.
 
 
Soil Fertility Lab Guide – Page 4
GLOBE® 2005 Soil Fertility Protocol - 8 Soil
Soil Fertility Protocol – 
Looking at the Data
Are the data reasonable?
Nitrogen (N):
The soil test kit used in GLOBE measures nitrogen 
in the form of nitrate (NO
3
). Because nitrate 
has a negative charge, it is not attracted to the 
negatively charged surfaces of the soil. As a 
result, any nitrate added to the soil is quickly 
taken up by plants, washed out with water 
passing through the soil, or removed (volatilized) 
as nitrogen gas. Therefore, nitrate values may be 
low or none for most soil samples. If the soil has 
recently been fertilized, or if there is a steadily 
available source of nitrogen, such as from the 
addition of organic material from compost or 
manure, the levels of nitrogen may be higher.
Phosphorus (P):
GLOBE soil test kits measure phosphate (PO
4
-3), 
the form of phosphorus that is most easily taken 
up by plants. Soil test kit readings of phosphate 
should be low if the soil pH is less than 5.0 or 
greater than 8.0. This is because at low and high 
pH levels, phosphate forms compounds with 
other elements in the soil making it difficult for 
plants to use. For example, when the soil pH is 
low and iron is present (making the soil look 
red), iron phosphate is formed which holds 
the phosphate very tightly, not freeing it for 
plants to use. At neutral pH levels (around pH 7), 
phosphate is more easily taken up by plants and 
usually shows up as a medium or high phosphate 
reading with the soil test kit.
Potassium (K):
The amount of potassium present in the soil 
depends on the availability of potassium minerals 
in the parent material of the soil. The largest 
natural sources of potassium are potassium rich 
minerals such as micas, which release potassium 
into the soil through weathering. Potassium 
can also be added to the soil as a fertilizer. 
Since potassium is a positively charged ion, it is 
attracted and held to the negatively charged soil 
surface. The fertility test kits will show medium 
or high readings for potassium for many soils. A 
low potassium reading may be an indication of 
an extremely weathered soil.
What do people look for in these 
data?
Knowing the relative amounts of nitrogen, 
phosphorus, and potassium in the soil helps 
scientists to recommend the type and amount of 
fertilizers or other nutrients farmers and gardeners 
should add to their soils for plant growth. For 
example, they may recommend adding fertilizers, 
composts or manure to make a soil more fertile. 
N, P, K measurements also help scientists to better 
understand other soil properties, such as the 
number of negatively charged soil surfaces, the 
amount of iron and organic matter in the soil and 
the degree to which a soil has been weathered. 
N, P, K measurements can also help scientists 
determine the type of parent material from which 
the soil formed. 


Soil Fertility Protocol

http://www.globe.gov/documents/352961/353769/npk.pdf

Soil Fertility Protocol

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