A farmer carried out a trial to find out how the mass of nitrate fertiliser added to a field affected the grain yield of a wheat crop. Different parts of th...
Question 1 Report
A farmer carried out a trial to find out how the mass of nitrate fertiliser added to a field affected the grain yield of a wheat crop. Different parts of the field were given different amounts of nitrate fertiliser, and the grain yield from each part was measured at harvest. The results of the trial are shown in the graph in Fig. 7.1.
(a) Using Fig. 7.1, state the grain yield that was obtained when no nitrate fertiliser was added. [1]
(b) Using Fig. 7.1, calculate the increase in yield when the mass of nitrate added rose from 0 to 100 kg per hectare. [1]
(c) Explain why adding nitrate ions to the soil increases the growth of the wheat plants. [3]
(d) Explain why adding more than 100 kg per hectare of nitrate gives very little extra yield. [2]
(e) Some of the excess nitrate drains from the field into a nearby river. Explain how this can lead to the death of fish. [3]
Answer Details
This question tests reading a yield graph and understanding why nitrate fertiliser increases plant growth, plus its pollution effects.
(a) With no nitrate added, read the y-value where nitrate = 0: the grain yield is 3.0 tonnes per hectare. [1]
(b) The yield at 100 kg per hectare is 7.0 t/ha, so the increase from 0 to 100 kg is \( 7.0 - 3.0 = 4.0 \) tonnes per hectare. [1]
(c) Why nitrate increases growth, [3]: nitrate ions supply nitrogen [1]; the plant uses this nitrogen to make amino acids and hence proteins [1]; proteins are needed for growth, for making enzymes and for building new cells [1].
(d) Why more than 100 kg gives very little extra yield, [2]: another factor has become limiting [1]; for example light, water, temperature, or another mineral now limits growth, so extra nitrate cannot raise the yield further [1].
(e) How excess nitrate draining into a river kills fish (eutrophication), [3]: the nitrate causes rapid growth of algae and water plants [1]; when these die, decomposer bacteria multiply and use up the dissolved oxygen in the water during respiration [1]; the fish then die from lack of oxygen [1].
Exam tip: in eutrophication it is the decomposers respiring, not the algae, that remove the oxygen the fish need.
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