Growing the same crop on a field every year can gradually reduce the fertility of the soil and allow pests and diseases to build up. To avoid these problems...
Question 1 Report
Growing the same crop on a field every year can gradually reduce the fertility of the soil and allow pests and diseases to build up. To avoid these problems, some farmers change the crop grown on each field from one year to the next in a planned sequence. A farmer in England follows this practice, growing four different crops in turn over a four-year period before returning to the first crop again.
One of the crops in the sequence is a legume, a bean plant whose roots carry small swellings called root nodules. Fig 3.1 shows the four-year rotation, setting out which crop is grown on the land in each year of the cycle. The diagram represents the order in which the crops follow one another around the fields. Using the diagram and your knowledge of soil, nutrients and nitrogen, answer questions about the problem caused by growing one crop repeatedly, how changing the crop helps the soil, the special role of the bean crop, the effect on pests, a drawback of rotation, and other ways to keep yields high.
Fig. 3.1
Answer Details
(a) Growing the same crop every year means the plants remove the same mineral ions from the soil each year [1]; so the soil fertility (ion content) falls and the crop yield drops. [1]
(b) Changing the crop helps because different crops need and remove different mineral ions [1]; so the soil is not depleted of any one ion and has time to recover its levels. [1]
(c) The bean crop is special because its roots carry nodules containing nitrogen-fixing bacteria [1]; these bacteria convert nitrogen gas from the air into nitrates / nitrogen compounds [1]; this adds nitrate to the soil, increasing fertility and reducing the need for nitrogen fertiliser for the next crop [1].
(d) Effect on pests (max [3]): pests and diseases specific to one crop cannot build up [1]; because their host crop is not present in the field the following year [1]; so their numbers fall and their life cycle is broken [1].
(e) One drawback (any one): rotation is more complex to manage / gives a lower yield of the most profitable cash crop / may need different equipment for each crop. [1]
(f) Other ways to keep yields high (any two): use fertilisers [1]; use insecticides / herbicides / selectively bred high-yield varieties / machinery / greenhouses [1].
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