An experiment was set up to investigate how quickly water drains through two different types of soil: loam soil and clay soil. Each funnel contained 250 g of dry soil. First, water was added to make the soil completely wet. Then, 120 cm3 of water was poured onto the wet soil in each funnel and allowed to drain into a measuring cylinder. The volume of water collected in the cylinder was recorded every 3 minutes until all 120 cm3 had been collected. The results are shown below:
Time (minutes)Volume of water collected (loam soil, cm3)Volume of water collected (clay soil, cm3)3802561104891207012120921512011018120120 \begin{array}{|c|c|c|} \hline \text{Time (minutes)} & \text{Volume of water collected (loam soil, } \text{cm}^3\text{)} & \text{Volume of water collected (clay soil, } \text{cm}^3\text{)} \\ \hline 3 & 80 & 25 \\ \hline 6 & 110 & 48 \\ \hline 9 & 120 & 70 \\ \hline 12 & 120 & 92 \\ \hline 15 & 120 & 110 \\ \hline 18 & 120 & 120 \\ \hline \end{array} Time (minutes)369121518Volume of water collected (loam soil, cm3)80110120120120120Volume of water collected (clay soil, cm3)25487092110120(i) Name the independent variable in this investigation.
(ii) State what is meant by the term dependent variable.
(iii) Identify which of the following is a key controlled variable in this investigation:
(iv) Calculate the average rate in cm3\text{cm}^3cm3 per minute at which water drained through the clay soil over the entire duration of the drainage (18 minutes). Give your answer to 3 significant figures and show your working.
(v) The student has a choice of two measuring cylinders to measure the drainage volume: Cylinder X has division marks every 1 cm3, while Cylinder Y has division marks every 2 cm3. State which of these two measuring cylinders would give a more precise reading and explain why.
(i) Soils that drain very rapidly, like sandy soil, have large air spaces and retain very little water. Suggest why plants find it difficult to grow in such soils.
(ii) Clay soil has very small air spaces that easily fill with water, displacing air. Explain why plants find it difficult to absorb mineral ions and grow in waterlogged clay soil.