Know Your Nitrogen
9 - 12
In this lesson, students will test for plant-available soil nitrogen and learn how farmers use this test to precisely match fertilizer application to meet crop needs and reduce the amount of nitrogen left in the soil. Grades 9-12
Two 60-minute sessions
For the class:
- Two kitchen sponges
- Two clear bowls
- Two clear cups
- Food coloring, red
- Measuring cup
- Soil core sampling tubes (or use shovels to dig uniform cores from the first 6" of soil)
- A place to collect soil core samples
- Electronic scale
- Food or aquarium grade calcium chloride
- One gallon of distilled water
- (add 6 grams of calcium chloride to 1 gallon of distilled water to make up the 0.01 M calcium chloride solution)
For each group:
- One, 10-500 ppm nitrate test strip
- 50 ml centrifuge tube
- Quart-sized resealable bag
- Waterproof marker
- Nitrate test strip color chart
For each student:
conservation: the wise use of resources, to conserve them for use by present and future generations
crop rotation: the successive planting of different crops in the same field over a period of years to maintain or improve soil quality and reduce pest problems
fertilizer analysis: the actual composition of a fertilizer as determined in a chemical laboratory using standard methods
leaching: downward movement of materials in solution through the soil
water holding capacity: the amount of water that a soil can hold before nutrients begin to leach out
Background Agricultural Connections
This lesson is part of the series called, Fertilizers, Chemistry, and the Environment. These lessons introduce students to chemistry and environmental science concepts. Activities are modeled after real-life challenges that modern farmers face while producing our food, fiber, and fuel. Labs are inquiry based and promote critical thinking skills. Other related lessons include:
Whether we are talking about your lawn, flower garden, or the wheat in your pasta dinner, plants need nutrients to grow. When crops are harvested the nutrients they utilized from the soil are removed with them. These nutrients must be replaced back into the soil so it remains fertile and can continue to grow crops in future years. Farmers and home gardeners use a variety of fertilizers to provide plants with essential nutrients. There are many different types of fertilizers that fit the needs of different crops and growing conditions. You may be familiar with fertilizers if you have applied compost, fish emulsion, animal manure, or other pre-packaged fertilizer to your lawn or garden. These fertilizers play an important role in growing beautiful flower gardens, green lawns, and providing affordable and nutritious food.
The use of any type of fertilizer comes with the responsibility to precisely follow instructions for storage, preparation, and application. From farms to home gardens and golf courses, we all must ensure that fertilizers are used correctly. Strict regulations, scientific research, and developments in new technology ensure that modern farming methods involve the selection of the appropriate fertilizer, application method, application timing, and monitoring of nutrients to provide crops with enough nutrients for optimum yield, while also protecting the environment. Applying too much fertilizer to crops can have adverse effects on the environment as well as the crop, and is an unnecessary expense. Replacing nutrients with fertilizer at the right time and in the right amount helps farmers grow enough food for an increasing world population as the amount of available farmland decreases. Achieving optimum yields without applying excessive nutrients is a goal of all farmers.
This lesson provides students with insight into one method farmers use to determine the amount of nitrogen fertilizer their crop may need. Of the primary nutrients, farmers are especially interested in nitrogen for several reasons. Nitrogen is required for plants to form proteins which make up much of their tissues, and nitrogen availability is often limited in the soil. While 78% of our atmosphere is nitrogen gas, most plants cannot use this form of nitrogen. Nitrogen is most available to plants when it is in the nitrate (NO3-) form or when it is in the ammonium (NH4+) form. Nitrogen can be made available in plant-usable forms of nitrogen through the action of nitrogen-fixing microbes found in the soil and the root nodules of some types of plants, such as legumes. These forms of nitrogen can also be supplied to plants through the use of various types of fertilizers.
Before applying fertilizers, farmers test their soil to determine factors such as texture, pH, porosity, organic matter, nutrient content, and more. Farmers also test the tissues of the crops they are growing to compare with the nutrient components from the soil analysis. These factors are important in determining which nutrients to use, how much and when to apply, and how to irrigate. These considerations ensure the best utilization by plants for healthy and efficient crop production.
Throughout the nation, many organizations fund research on new techniques for managing agricultural nitrogen and develops workshops and training programs for farmers and ranchers. State and regional water boards regulate, monitor, and provide financial assistance to protect water resources and achieve water quality objectives. For more information see Answers to Commonly Asked Questions.
- Prior to the lesson, place a slightly damp, but not saturated, kitchen sponge above a clear bowl. The bowl should be small enough so that the sides of the sponge suspend it slightly above the bottom of the bowl. Repeat with a second sponge and clear bowl. Prepare two separate cups of water, one containing ½ cup of water and the other containing one cup of water. Also have one bottle of red food coloring on hand.
- Tell students that in this example the sponge represents an agricultural field. Just like a sponge, soil can only hold a certain amount of water. Explain that in this demonstration the red food coloring represents a fertilizer solution.
- Ask for two volunteers to come to the front of the room. The volunteers represent farmers who irrigate their crops differently. Instruct each student to place two drops of food coloring in the center of their sponge. Now instruct the student with ½ cup of water to slowly pour the ½ cup of water onto the center of one sponge. Instruct the other student to slowly apply one cup of water to the center of the other sponge. Have students record their visual observations about the process, and the amount of leachate that accumulates under the sponge, in a science journal or notebook. A class discussion should bring up the following points:
- The sponge, like soil, can only hold a certain amount of water. The phrase “water holding capacity” describes the ability of a particular type of soil to hold water against the force of gravity. Soil can only hold a limited amount of water.
- After the sponge reached its water holding capacity, the water moved through the sponge carrying the fertilizer with it. Today's farmers and scientists work together to implement nutrient management practices that protect the environment and apply the precise amount of nutrients and irrigation needed by the crop.
- Since water carries the fertilizer to the plant roots, it is important to understand how water travels through soil. Regulatory agencies, farmers, ranchers, and educational organizations are constantly working on research and monitoring projects to utilize the best nutrient management practices possible while growing healthy crops.
- Explain that in this lab, students will carry out the same nitrate soil test used by farmers to measure soil nitrate levels in order to match fertilizer application rates to the needs of their crops. Students will:
- test soil samples for available nitrogen; and
- learn about best management practices to maximize nutrient utilization and environmental stewardship.
Explore and Explain
- As a class, read the Summary of 2008-09 Large Scale Irrigation and Nitrogen Fertilizer Management Trials in Lettuce by Michael Cahn and Richard Smith, Farm Advisors, Monterey County (attached in Essential Files). This is a technical document and students may have difficulty understanding the terminology. The reason for including this reading in the lesson is to help students understand that fertilizer application is complex and involves a great deal of science.
- As a class, discuss the article. Were students surprised by the precision involved in modern agriculture? Make a concept map on the board of all the things these farmers needed to know about their crop and their soil. What skills would students need to run a lettuce farm like the one described in the article? Add responses to the concept map on the board.
- Tell students that they have earned a spot as an intern for the UC Cooperative Extension farm advisor in your county: "Your boss has given you an assignment to collect soil core samples from a local lettuce farm. You will need to collect numerous samples from the field and you have decided to enlist your classmates to help collect soil samples. Once you have collected your soil samples, you will return with them to the lab and will follow lab procedures that your boss has printed out for you."
- Distribute the handouts for the Know Your Nitrogen lab and Nitrate Quick Test Procedure. Go over lab safety procedures and remind students to carefully read the instructions and gather the necessary materials before starting the lab. During the first day of the lab, demonstrate the correct method for soil sampling and draw a grid on the board to show where each group will sample the field. This will help ensure that a representative sample is collected. Lead students to the designated location to collect soil sample cores.
- In addition to collecting soil samples, we are also interested in soil drainage. Soil texture, shallow soil with fractured bedrock, and soils with a water table close to the surface all affect soil drainage. There are precise laboratory methods for determining soil porosity and soil drainage, however, we can do a quick assessment with a shovel and some water.
- Ask two students to volunteer to dig a hole that is one square foot wide by 12 inches deep, level on the sides and bottom.
- Once the hole is dug, students should fill it with water and let it soak in for an hour or so. When all the water has drained, the hole should be refilled with water and students should note the amount of time it takes for the water to soak in. If the water drains faster than 4 inches per hour, the soil is highly porous. Soil with low porosity will drain less than one inch per hour.
- Soil porosity is the amount of pore space occupied by water and gases in the soil. Ask students why farmers would be interested in soil porosity and soil drainage.
- On the second day of the lab, students will follow the lab procedures for carrying out the Nitrate Quick Test on their soil samples.
- After students have completed the nitrate soil test, ask them to write their nitrate level on the board in their assigned soil grid space. Discuss the class findings and provide any guidance the students may need before moving on to the lab questions.
- Experiment with drainage in different soil types. Have students collect soil samples and determine the soil texture using a field test, laboratory analysis, or soil map. Allow students to use different soil types in their experimentation. Discuss the role of percolation and the soil's water holding capacity in drainage.
- Write down key terms so students can see them and connect them to the spoken word. If appropriate, connect a visual to each term introduced.
- As a class, create a flow chart to illustrate the procedure for the Know Your Nitrogen lab. Address questions that come up during the illustration process and prior to starting the lab.
Have students research best management practices used by farmers to protect water quality, including buffer zones, denitrification beds, and conservation tillage. Have student groups make a poster about individual best management practices and present it to the class.
Introduce students to the 4Rs of nutrient stewardship—the right source, right rate, right time, and right place. “The Right Way to Grow: 4R Nutrient Stewardship” is an 11-minute video which gives an overview of the 4Rs and explains how stewardship applies to large-scale agriculture producers as well as small farms. The video is divided into Part 1 and Part 2.
After conducting these activities, review and summarize the following key concepts:
- Nitrogen is a macronutrient for the growth of plants. It is required in large quantities for healthy plant growth.
- Growing plants removes nitrogen and other nutrients from the soil.
- Nitrogen and other nutrients can be replaced in the soil through organic or inorganic fertilizers.
This lesson was funded in 2011 by the California Department of Food and Agriculture’s (CDFA) Fertilizer Research and Education Program (FREP). Chemistry, Fertilizer, and the Environment was designed to reinforce chemistry and environmental science concepts while educating students about the relationships between food, plant nutrients, farmers and the environment.
Executive Director: Judy Culbertson
Illustrator: Erik Davison
Layout and Design: Nina Danner
Recommended Companion Resources
Mandi Bottoms and Shaney Emerson
California Foundation for Agriculture in the Classroom