Writing prompt Grade 6 Science
The White Crust at Tessendale Farm: Where the Salt Came From
A grade 6 science argument prompt: students use water, soil, and rainfall records from a fictional farm to explain why salt is building up in an irrigated field.
Objectives
- Students will make a claim about why salt is building up in an irrigated field.
- Students will support that claim with at least three measurements from the farm records.
- Students will explain why evaporation leaves dissolved salt behind in the soil.
Vocabulary
- Evaporation
- Evaporation happens when liquid water, often warmed by the sun, turns into an invisible gas called water vapor that rises into the air. It is how water leaves oceans, lakes, and puddles.
- Recycled water
- Wastewater that has been cleaned and used again, often to water parks, golf courses, and crops instead of being thrown away. Purple pipes usually carry it.
- 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: dissolved solids, tile drain, salinity
Teacher overview
Students write a short scientific argument explaining why a pale crust has spread across part of a fictional irrigated field, using water quality, soil, and rainfall records collected over five years. The task rests on one idea that students can reason their way to: water can leave a field as vapor, but the salt dissolved in that water cannot. A common misconception, worth naming before students write, is that evaporation removes whatever was in the water, the way a drain does. Evaporation separates rather than removes, so every gallon that evaporates from a field leaves its dissolved salt in the soil. Students who miss that distinction will blame the crust on the crop, on the weather alone, or on something spilled, and their evidence will not line up.
Prompt
Background
Tessendale Farm lies just outside the fictional town of Hartsmere. Everything here about Tessendale and Hartsmere was written for this assignment.
The farm grows beans on 60 acres. For many years it watered the field with river water. Then a long drought arrived. The town of Hartsmere grew, its own demand rose, and the state cut the farm's river share. Starting five years ago, the farm began watering with recycled water piped from the Hartsmere water reclamation plant.
Recycled water is treated wastewater. Treatment removes solids, germs, and many nutrients. It does not remove most of the salts that are dissolved in the water, because those salts stay in solution all the way through the plant.
In the last two summers a pale crust has appeared on the soil. It shows up on the high, dry end of the field and at the far ends of the furrow rows. The low corner near the tile drain, which stays wet, has no crust. Bean plants near the crust have brown, dried edges on their leaves, and yields on that end of the field have fallen.
Table 1. Water the farm applied (fictional farm records)
| Source | Dissolved solids | Years used |
|---|---|---|
| Ellinghurst River water | 180 mg/L | until five years ago |
| Hartsmere recycled water | 720 mg/L | the past five summers |
| Water leaving the tile drain, last August | 2,100 mg/L | measured once |
Table 2. Field records (fictional farm records)
| Measurement | Five years ago | Last year |
|---|---|---|
| Salt in the top foot of soil, high end of field | 0.6 grams per kilogram | 2.4 grams per kilogram |
| Salt in the top foot of soil, low corner by the drain | 0.5 grams per kilogram | 0.8 grams per kilogram |
| Winter rainfall | 17 inches | 7 inches |
| Bean yield, high end of field | full crop | about half a crop |
The farm's normal winter rainfall is about 18 inches. The last four winters brought 11, 9, 8, and 7 inches.
Your task
Write a scientific argument of four or five paragraphs. Answer this question: why is salt building up in the soil at Tessendale Farm?
State a clear claim. Support it with at least three specific measurements from Table 1 or Table 2. For each measurement, explain what it shows. Use the word evaporation correctly at least once. Finish by naming one measurement the farm should take next to test your explanation.
Planning Organizer
Fill in every row before you draft. Each row matches one part of the rubric.
| Part of the argument | What to write | My notes |
|---|---|---|
| Claim | One sentence naming why salt is building up in the field. | |
| Evidence 1 | A pair of numbers from Table 1 that compares the two water sources. | |
| Reasoning 1 | What does that difference mean for how much salt arrives each summer? | |
| Evidence 2 | A soil number from Table 2, with the place it was measured. | |
| Reasoning 2 | Why would the high, dry end hold more salt than the wet corner by the drain? | |
| Evidence 3 | A rainfall number, or the number measured in the drain water. | |
| Reasoning 3 | What does rain normally do to salt in soil, and what happened when the rain stopped? | |
| Conclusion | Why the crust appeared in the last two summers, and one measurement the farm should take next. |
From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.
Exemplar & Rubric
Exemplar response (written at grade 6 level)
Salt is building up at Tessendale Farm because the farm now applies saltier water, and the water keeps leaving the field as vapor while the salt stays in the soil.
The first evidence is the change in water source. River water carried 180 mg/L of dissolved solids. The recycled water carries 720 mg/L, which is four times as much. If the farm applies about the same number of gallons each summer, about four times as much salt now arrives with every watering.
The second piece of evidence is where the crust shows up. The high, dry end of the field went from 0.6 grams of salt per kilogram of soil to 2.4 grams. The low corner next to the tile drain went only from 0.5 to 0.8. That pattern fits evaporation. On the dry end, water soaks in and then evaporates from the surface or is taken up by the plants, and both paths leave the salt behind. In the wet corner, extra water keeps moving down and carries salt with it into the drain.
The third piece of evidence is the rain. The farm normally gets about 18 inches of winter rain, but the last four winters brought 11, 9, 8, and 7 inches. Winter rain used to soak past the roots and wash the summer's salt down and out. Dry winters stopped doing that job, so each summer's salt was still there when the next summer's salt arrived.
The drain sample supports the same idea from the other direction. The water leaving the drain last August read 2,100 mg/L, far saltier than the 720 mg/L that went on the field. Water left the field as vapor and the salt that stayed behind made the leftover water saltier. The crust appeared in the last two summers because five years of added salt met the two driest winters. To test this explanation, the farm should measure the soil salt again after applying one extra deep watering in winter. If the explanation is right, the salt in the top foot of the high end should drop.
Rubric
| Criterion | Emerging | Developing | Proficient | Advanced |
|---|---|---|---|---|
| Claim | No claim, or the claim restates the question | Names a cause but leaves it vague | States one clear cause of the salt build-up | States a clear cause that names both the saltier water and the missing flush |
| Evidence | No measurements from the records | Uses one measurement, or gives numbers with no units or places | Uses at least three specific measurements with units and locations | Uses three or more measurements chosen to test the explanation, including the drain sample or the rainfall record |
| Reasoning | Restates the number without explaining it | Explains one piece of evidence | Explains what each measurement shows about salt in the field | Explains each measurement and states clearly that evaporation removes water but not dissolved salt |
| Organization and conventions | Hard to follow; frequent errors interrupt meaning | Some order; errors distract in places | Clear paragraphs, correct use of evaporation, few errors | Clear paragraphs, precise vocabulary, a closing next measurement, and clean conventions |
Differentiation
Support
Set up a simple demonstration first: put a spoonful of salt water in a shallow dish on a sunny sill in the morning and look at the dish after lunch. Give these students a copy of Table 1 with the two water numbers circled, and sentence frames such as "The recycled water carries ___ mg/L, which is ___ than the river water carried." Allow a three-paragraph response and accept a labeled drawing of the field, with arrows for water in, vapor out, and salt staying, in place of the final paragraph.
Extension
Ask students to write the short memo the fictional farm would send to the Hartsmere plant, explaining what dissolved solids have to do with the crop and asking one specific question about the recycled water. A second option is to predict which end of the field would recover first if the farm returned to river water, and to give the reasoning behind the prediction.
ELL supports
Pre-teach "dissolved," "evaporation," "crust," "drain," and "recycled water" with a labeled two-panel drawing: a dish of salt water in the sun, and the same dish later with a white ring. Provide a bilingual glossary card. Let students talk through the organizer with a partner before drafting, and accept a first draft that uses the sentence frames word for word.
Extensions
Have students find out whether any water utility near them sells recycled water for irrigation, and report what the utility says the water may and may not be used for. A second option is to design a simple test of the explanation in class: two cups of the same soil, one watered with plain water and one with salty water, both left to dry between waterings, with a plan for what to measure and when.