Which Soil Holds Water Best?
Three learners test three soils from around their school. They put 100 g of each soil into a funnel lined with filter paper, pour 50 ml of water on top, and collect what drains out after 10 minutes.
- Sandy soil: 42 ml collected
- Clay soil: 9 ml collected
- Loam soil: 25 ml collected
The teacher asks them to judge which soil is best for growing vegetables in the school garden. The garden is on a slope and the rainy season brings heavy afternoon storms. Learners must also explain one weakness of their test.
1. Calculate how much water each soil retained, and use the results to judge which soil is best for the garden. Explain your reasoning.
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Retained water = 50 ml − water collected.
- Sandy: 50 − 42 = 8 ml retained
- Clay: 50 − 9 = 41 ml retained
- Loam: 50 − 25 = 25 ml retained
Judgement: loam is best for the garden. Sandy soil drains too fast and holds little water, so plants on a slope would dry out between rains. Clay holds the most water, but on a slope during heavy storms it drains very slowly; water sits on the surface, roots get no air, and the soil can wash away as mud. Loam holds a moderate amount: enough for roots between rains, but it still lets excess water drain. The data support loam, and the slope and heavy storms make loam's balance the deciding factor.
2. Give one weakness of this test and suggest one improvement that would make the judgement more reliable.
Read the worked answer
Weakness: the test measures drainage over only 10 minutes after one watering, which does not show how the soil behaves over a whole rainy season or how it supports plant roots. It also uses the same 50 ml for all soils, which may not reflect real rainfall, and the soils may have been packed differently in each funnel, changing the flow.
Improvement: repeat the test three times per soil and take an average, pack each soil to the same depth and firmness, and record drainage at several times (for example 1, 5 and 10 minutes). A further improvement is to grow the same seedlings in each soil for three weeks and compare growth and health, because water retention alone does not prove a soil is best for vegetables.