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Lesson plan Grade 11 Science

Bramblewood Creek Farms: Cutting Nitrogen Runoff to the Stream

A grade 11 lesson where students calculate a fictional farm's nitrate reduction from cover crops and design an added practice to cut nitrogen runoff to a stream.

Objectives

  1. Students will trace the pathways unused fertilizer nitrogen can take off a farm field.
  2. Students will calculate a percent reduction in stream nitrate concentration from before-and-after monitoring data.
  3. Students will design a nutrient-management practice and explain whether it targets surface runoff or groundwater leaching.

Vocabulary

Leaching
When water slowly pulls a substance out of the material it touches. Corrosive water can leach lead from old pipes into drinking water.

Also: nitrogen cycle, runoff, buffer strip, cover crop

Teacher overview

Agriculture accounts for roughly 70 percent of global freshwater withdrawals, and farms shape water quality as well as water quantity: nitrogen fertilizer that a crop does not take up can run off across the surface or leach down into groundwater, eventually reaching a stream. This lesson uses Bramblewood Creek Farms (a fictional 500-acre farm created for this lesson) to walk students through a before-and-after nitrogen-reduction program and have them design an additional practice of their own. A common misconception this lesson addresses directly: students often think "using fertilizer" and "polluting water" are the same choice, when the real variable farms can control is how much of the applied nitrogen actually reaches the crop versus how much leaves the field unused.

Procedure

Hook (5 min): Ask students what happens to fertilizer that a crop's roots do not absorb. Where might that leftover nitrogen go?

Direct instruction (15 min): Explain the relevant part of the nitrogen cycle for a farm field: nitrogen is applied as fertilizer, taken up by the crop for growth, and any nitrogen the crop does not use can either run off across the surface during rain or leach downward through the soil into groundwater. Both pathways can eventually deliver nitrogen to a stream, where it can fuel algae growth. Introduce two management practices: buffer strips (vegetation left along a stream that intercepts surface runoff before it reaches the water) and cover crops (planted between growing seasons specifically to take up leftover soil nitrogen that would otherwise leach away).

Guided practice (15 min): Present Bramblewood Creek Farms' monitoring data: before adopting cover crops and buffer strips, nitrate-nitrogen in the adjacent stream measured 12 mg/L; after the farm adopted both practices, the stream measured 5 mg/L. Calculate the percent reduction together as a class, and discuss which practice most plausibly addressed which pathway (buffer strips intercept surface runoff; cover crops reduce what is available to leach).

Independent practice (10 min): Students choose one additional nutrient-management practice from a provided list (reduced tillage, precision fertilizer timing matched to crop uptake, or a constructed wetland at the field's edge) and write two to three sentences explaining whether it primarily targets surface runoff or groundwater leaching, and why.

Closure / exit ticket (5 min): Students answer one question: "Name one downstream ecosystem effect that reducing Bramblewood Creek Farms' nitrate runoff, as this program did, would help prevent."

From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.

Answer Key
  1. Answer: 58.3 percent (guided practice). (12 − 5) ÷ 12 × 100 = 7 ÷ 12 × 100 = 58.33%, which rounds to 58.3%.
  2. Independent practice answer: Answers will vary. Students should name a specific practice and correctly match it to a mechanism: precision fertilizer timing mainly reduces leaching by keeping nitrogen available closer to when the crop can use it; reduced tillage mainly reduces surface runoff and erosion because crop residue left on the field slows water moving across the surface; a constructed wetland mainly intercepts runoff before it reaches the stream, similar to a buffer strip.
  3. Exit ticket answer: Excess nitrate reaching a stream can fuel algae growth; when that algae later dies and decomposes, the decomposition process consumes dissolved oxygen, which can leave too little oxygen for fish and other aquatic life.

Differentiation

Support (below grade level)

Provide the percent-reduction formula already set up with Bramblewood Creek Farms' two numbers filled in, so students only need to complete the arithmetic. Give a simple two-column reference sheet listing which practices target runoff and which target leaching for independent practice.

Extension (above grade level)

Ask students to calculate what stream nitrate concentration Bramblewood Creek Farms would need to reach a 75 percent reduction from its original 12 mg/L reading, and to name one additional practice, beyond cover crops and buffer strips, that could plausibly help close that gap.

ELL supports

Pre-teach "runoff," "leaching," and "buffer strip" using a simple diagram showing rain moving across a field's surface (runoff) versus rain soaking down through the soil (leaching), with arrows pointing to a stream in both cases. Allow independent practice to be answered with a short phrase plus a labeled arrow diagram instead of full sentences.