Testing Garden Soil (NPK)

Published October 24, 2012 | Updated January 9, 2016
Subject: Science
Season: Winter, Spring, Summer, Fall
Place of Learning: Garden
Resource Type: Lessons
Grade Level: Grade 5, Grade 6, Grade 7, Grade 8
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In this lesson students will learn how to test their garden soil for nitrogen, phosphorous, and potash (potassium).



After this lesson, students will be able to:

  • Understand how to read the N-P-K number on a bag of fertilizer
  • Explain that nitrogen, phosphorous, and potash are important compounds for plant and fruit growth
  • Demonstrate how to use a home N-P-K test kit
  • Soil sample from garden
  • Home soil test kits (can be purchased inexpensively online)
  • Fertilizer samples that have an N-P-K number
  • Have examples of fertilizers with N-P-K numbers displayed
  • Have soil sample ready or have students collect soil samples
  • Familiarize yourself with your particular soil test kit

At the Opening Circle

  1. Show students the N-P-K number on a bag of fertilizer and explain that these numbers stand for the percentages of nitrogen, phosphorous, and potash.
  2. Explain to students the function of N-P-K (nitrogen helps plant foliage, phosphorous helps roots and flowers grow, potash is important for plant health.
  3. Show students test kits and explain how to use them.
  4. Divide students into three groups, each group will test either nitrogen, phosphorous, or potash.

At the Work Stations

  1. Have students do soil test according to directions of your particular test kit.

At the Closing Circle

  1. Have students share their results with the rest of the class.
  2. Discuss methods to amend the soil naturally. Explain that bags of chemical fertilizers often add these elements without contributing to overall soil tilth and that there are other important elements in soil. Explain that compost, worm castings, certain manures, bonemeal, etc.. are natural healthy ways to amend your soil.

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