Fair Test with Bean Seedlings and a Waterlogged Tomato Plant
At Nkonyeni Primary School, Grade 7 pupils set up a fair test to find out whether bean seedlings need light. They used four identical tins. Tin A was placed on a window sill and watered 50 ml each morning. Tin B was placed in a dark cupboard and watered 50 ml each morning. Tin C was placed on the window sill but not watered. Tin D was placed in the cupboard and given 100 ml each morning. After ten days, the seedlings in A were green and 9 cm tall; those in B were yellow, thin and 12 cm tall; C had died; D had mould on the soil. The pupils concluded that light is not needed because B was tallest. In the same week, a tomato plant in the school garden, which receives full sun, had yellow lower leaves and its soil was always wet because a tap dripped nearby. A learner wants to add more fertiliser.
1. Identify the mistake in Tin D that spoils the fair test and explain how it affects the pupils' conclusion.
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Tin D is not a fair comparison because two things changed: it was in the dark like B, and it received 100 ml of water instead of 50 ml. A fair test changes only one variable. Because D had extra water, mould grew, and the result cannot tell us whether light or extra water caused the difference. To fix it, D should be in the dark and watered 50 ml, or D should be on the window sill and watered 100 ml if water amount is being tested separately. The pupils should compare A and B only for light, keeping the same soil, same tin, same amount of water and same type of seed. Without this control, any conclusion about light is unsafe.
2. The pupils' conclusion that light is not needed is not supported by the evidence. Explain what the results actually show about light and healthy growth.
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The conclusion 'light is not needed' is not supported. B was taller, but it was yellow and thin. Height alone is not a measure of healthy growth. In darkness the seedling uses stored food and grows a long, weak stem towards the light; it cannot make chlorophyll, so it is yellow. A was green and 9 cm tall, showing normal growth. To conclude about light, the pupils need healthy measures such as green leaf colour, leaf number and dry mass, not just height. The experiment should also be repeated. The evidence shows that light is needed for green, strong growth, not that it is unnecessary.
3. For the tomato plant with yellow lower leaves in wet soil, is adding more fertiliser the best action? Explain using evidence about water and roots.
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Adding fertiliser is not the first action. The tomato plant's soil is always wet because of the dripping tap. Roots need air spaces in the soil for respiration; waterlogged soil fills those spaces, so roots cannot take up water and minerals well, and lower leaves turn yellow. Extra fertiliser can make this worse: it adds salts, can burn roots and may be washed away. First, repair or divert the tap and improve drainage. Then check whether the yellowing is due to waterlogging or lack of nutrients. Only after the soil is well drained should a small amount of fertiliser be considered, applied according to instructions.