A winter coat for the soil
337 words over 29 numbered lines, then 6 parts worth 13 marks (Modular Unit 2 (4WBI2) style, 12–13 marks). Read the article, answer each part, then open its mark scheme and set your mark.
Illustrative article written for Scholar Lab, not a Pearson text. The biology is real; the studies, people and figures are invented or rounded for practice unless a source is named.
A winter coat for the soil
After the wheat harvest in late summer, many fields are left as bare
soil until the next crop is sown. Over the winter, rain washes nitrate
ions out of the bare soil and into streams and rivers. The farmer
loses a mineral that crops need, and the river can be harmed.
On one farm, the farmer now sows clover straight after the harvest.
Clover is a legume. Its roots have small swellings called root
nodules, which contain nitrogen-fixing bacteria. These bacteria
convert nitrogen gas from the air in the soil into ammonium ions. The
clover uses the nitrogen to make amino acids and proteins, and in
return it supplies the bacteria with sugars.
In spring the clover is cut and ploughed into the soil before the next
wheat crop is sown. Decomposers such as bacteria and fungi break down
the proteins in the dead clover and release ammonium ions. Nitrifying
bacteria then convert the ammonium ions into nitrate ions, which the
roots of the wheat absorb. Plants use nitrate ions to make proteins,
so a lack of nitrate leads to poor growth and yellow leaves.
The clover also protects the soil. Its roots hold the soil together,
and the plants take up nitrate ions that would otherwise be lost. In
waterlogged soil, denitrifying bacteria convert nitrate ions back into
nitrogen gas, so soil that drains well keeps more of its nitrate.
An agricultural college compared the yield of wheat grown in three
groups of plots. The plots were the same size and the same variety of
wheat was sown in each one. The first group had bare soil over winter
and no fertiliser. The second group had clover over winter and no
fertiliser. The third group had bare soil over winter and the usual
amount of fertiliser in spring. The results are shown in Figure 1.
Artificial fertiliser is made using a large amount of energy from
fossil fuels. If too much fertiliser is spread, some of it is washed
into rivers, where it can cause eutrophication.
Figure 1 Mean yield of wheat grown after three different treatments
Read the article and study the figure. Use the information in the article and your own knowledge to answer the questions. The lines in brackets show where to look; select them to highlight the text.
Use the information in the article and your own knowledge to answer the questions. The lines in brackets show you where to look; select them to highlight the text.
State what is meant by the term nitrogen fixation.
(1)Describe how the nitrogen in the clover plants becomes available to the wheat plants.
(3)Use Figure 1 to calculate the percentage increase in mean wheat yield when clover was grown over winter, compared with bare soil with no fertiliser.
(2)Give two control variables, other than the size of the plots and the variety of wheat, that the college should have kept the same.
(2)Explain how fertiliser washed into a river can cause the death of fish.
(3)Suggest two reasons why a farmer might choose to grow clover over winter rather than spread more artificial fertiliser.
(2)Total for this question: 13 marks
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