Lab / experiment Grade 7 Science
Stroudwell Ridge Soils: How Much Irrigation Water Does Soil Hold?
Students pour a measured 100 mL through sand, loam, and clay soil samples, calculate the percent each soil holds, and use the results to set an irrigation schedule.
Objectives
- Students will measure how much of a poured volume each soil holds and how much drains through.
- Students will calculate water-holding capacity as a percent of the water poured.
- Students will propose an irrigation schedule that fits the soil they tested.
Vocabulary
Key words: water-holding capacity, drainage, loam, irrigation, percent
Teacher overview
Farmers do not decide when to irrigate by the calendar alone, because the same amount of water lasts far longer in one soil than in another. This lab asks how much of the water poured onto a soil that soil holds and how much drains straight through. Students test sand, loam, and a clay-rich soil, then apply the results to the fictional Stroudwell Ridge Farm, which irrigates two fields with different soils. The Food and Agriculture Organization puts agriculture at about 70 percent of global freshwater withdrawals, so matching an irrigation schedule to the soil affects water use on a large scale.
Procedure
Build the three columns. Line each of the three funnels with a coffee filter and set each funnel over a graduated cylinder. Label them sand, loam, and clay.
Add equal amounts of soil. Measure 200 mL of dry sand into the first funnel, 200 mL of loam into the second, and 200 mL of clay-rich soil into the third. Level each one gently without packing it down, because pressing the soil changes the result.
Pour a measured volume. Pour exactly 100 mL of water slowly onto the sand, aiming for the center so the water does not run down the side of the filter. Start the timer.
Wait and read. After 10 minutes, read how many mL of water collected in the graduated cylinder below. Record that number as the water that drained through.
Repeat for the other two soils. Do the same for loam and then for clay, using a fresh 100 mL each time and waiting the same 10 minutes. Using the same volume and the same wait time is what makes the three results comparable.
Calculate what each soil held. Water held equals 100 mL poured minus the mL that drained through.
Safety note. Do not taste any soil or water, wash hands after handling soil, and wipe up spills right away so the floor does not become slippery. Keep the catch tray under the funnels the whole time.
Data & Analysis
Record your measurements in this table. The poured volume is 100 mL for every soil.
| Soil | Water poured (mL) | Water drained through (mL) | Water held (mL) | Percent held |
|---|---|---|---|---|
| Sand | 100 | |||
| Loam | 100 | |||
| Clay | 100 |
- In your trials, which soil let the most water drain straight through?
- Suppose 100 mL is poured onto a sand sample and 62 mL drains out. How many mL did the sand hold?
- What percent of the poured water did that sand sample hold?
- Suppose the loam sample drains 25 mL out of the same 100 mL. How many mL did the loam hold?
- What percent of the poured water did that loam sample hold?
- Using the amounts from items 2 and 4, how many times more water did the loam hold than the sand? Round to the nearest tenth.
- Stroudwell Ridge Farm gives its sandy field and its loam field the same amount of water on the same morning. Which field will need water again sooner, and why?
- Using your own results, name one change to the irrigation schedule for the sandy field that would waste less water, and explain what your data shows that supports it.
From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.
Answer Key
- Answer: The sand, in nearly every trial.
Accept the soil with the largest drained volume in the group's data. Sand has large, connected spaces between grains, so water passes through quickly instead of clinging to the particles. - Answer: 38 mL.
100 mL poured minus 62 mL drained equals 38 mL held. - Answer: 38 percent.
38 mL divided by 100 mL equals 0.38, which is 38 percent of the water poured. - Answer: 75 mL.
100 mL poured minus 25 mL drained equals 75 mL held. - Answer: 75 percent.
75 mL divided by 100 mL equals 0.75, which is 75 percent of the water poured. - Answer: 2.0 times.
75 mL divided by 38 mL equals 1.97, which rounds to 2.0 times as much water held. - Answer: The sandy field.
Sand holds much less of the water applied, and the rest drains below the roots, so the sandy field dries out first and needs water again sooner. - Answer: Accept any change supported by the group's data.
Strong answers water the sandy field more often with a smaller amount each time, since the soil cannot hold a large dose and the extra drains past the roots. Accept adding compost to raise the soil's holding capacity, or watering early in the morning, when students connect the choice to their measured percents.
Differentiation
Support
Pre-measure the soils into the funnels so groups begin at the pouring step, and give these students a table with the water held column already computed for one soil. Provide the percent formula with one worked example that uses neither sand nor loam.
Extension
Have groups test a fourth column of sand mixed with compost, predict where its percent held will fall compared with pure sand and loam, then check the prediction against the measured value and explain any difference.
ELL supports
Provide picture cards for funnel, filter, drained, and held, and a diagram showing water clinging between soil particles. Allow students to answer items 1 and 7 by pointing to the columns and using a sentence frame: "The ___ held ___ water, so that field needs water ___."
Extensions
Students can repeat the test with soil from the schoolyard, compare its percent held with the three prepared soils, and write a short recommendation for how often a garden in that soil would need watering during a dry week.