Which nutrients do plants need, how do fertilisers supply them, and how does the nitrogen cycle keep soil fertile?
The roles of nitrogen, phosphorus and potassium in plant growth, how an NPK fertiliser bag is labelled, the nitrogen cycle including fixation, nitrification and denitrification, and the issue of keeping soil nutrient-rich under intensive arable farming.
A focused CCEA GCSE Agriculture and Land Use answer on soil nutrients, covering the roles of nitrogen, phosphorus and potassium, NPK fertiliser labelling, the nitrogen cycle with fixation, nitrification and denitrification, and maintaining fertility under intensive arable farming.
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What this dot point is asking
CCEA wants you to explain why plants need nitrogen, phosphorus and potassium, read an NPK fertiliser label, describe the nitrogen cycle (fixation, nitrification and denitrification, without chemical symbols), and discuss the problem of keeping soil fertile under intensive arable farming.
The three main plant nutrients (NPK)
A plant short of nitrogen looks yellow and stunted; one short of phosphorus has poor roots; one short of potassium has weak fruiting and is more prone to disease.
Reading a fertiliser label
A bag of artificial fertiliser carries three numbers, the NPK values, always in the order N : P : K. These give the relative proportions of nitrogen, phosphorus and potassium. For example, a 20:10:10 fertiliser has twice as much nitrogen as phosphorus or potassium, so it is a high-nitrogen feed suited to leafy crops such as grass. A farmer matches the NPK ratio to what the crop needs at that stage.
The nitrogen cycle
Nitrogen gas makes up most of the air, but plants and animals cannot use it directly. The nitrogen cycle moves nitrogen between the air, the soil and living things. CCEA does not require chemical symbols, but you must know the three named processes.
- Nitrogen fixation converts nitrogen gas into nitrates that plants can use. Nitrogen-fixing bacteria, including those living in the root nodules of legumes such as clover, do most of this; lightning also fixes some nitrogen.
- Decay and nitrification. When plants and animals die, or animals excrete waste, decomposers release ammonium compounds, and nitrifying bacteria convert these into nitrates, the form plant roots take up to make proteins.
- Denitrification is the loss step: in waterlogged soil, denitrifying bacteria convert nitrates back into nitrogen gas, which returns to the air.
Keeping soil fertile under intensive farming
In nature, nutrients return to the soil when plants and animals die and decompose. Intensive arable farming breaks this loop: the whole crop is harvested and removed, so the nutrients locked in it leave the field rather than rotting back in. Over time this lowers fertility, so farmers must replace nutrients by adding fertiliser, spreading manure or slurry, or growing legumes in a rotation.
Examples in context
Example 1. A high-nitrogen feed for grass. A dairy farmer wants a fast flush of grass for grazing, so they spread a high-nitrogen fertiliser such as 27:0:0 in spring. The nitrogen drives leafy green growth, giving more grass for the cows, but the farmer must time and limit the rate to avoid washing nitrates into waterways.
Example 2. Why waterlogged fields lose nitrogen. A poorly drained field stays wet for weeks. The waterlogging starves the soil of air, and in these conditions denitrifying bacteria convert nitrates back into nitrogen gas, so the field loses fertility. Improving drainage reduces this loss.
Try this
Q1. State one role each for nitrogen, phosphorus and potassium in plant growth. [3 marks]
- Cue. Nitrogen for leafy growth, phosphorus for roots, potassium for flowering, fruiting and disease resistance.
Q2. Name the process that converts nitrogen gas into nitrates in the soil. [1 mark]
- Cue. Nitrogen fixation.
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA Unit 1 style6 marksDescribe the nitrogen cycle, including the processes of nitrogen fixation, nitrification and denitrification, and explain why harvesting a crop affects it.Show worked answer →
Six marks need the main stores and the three named processes, plus the harvesting point.
Nitrogen gas makes up most of the air but plants cannot use it directly. Nitrogen fixation converts nitrogen gas into nitrogen compounds (nitrates) in the soil. This is done by nitrogen-fixing bacteria, including those in the root nodules of legumes such as clover, and also by lightning.
When plants and animals die, or animals produce waste, decomposers break down the proteins and release ammonium compounds. Nitrification is when nitrifying bacteria convert these ammonium compounds into nitrates, the form plants take up through their roots to make proteins.
Denitrification is when denitrifying bacteria in waterlogged soil convert nitrates back into nitrogen gas, removing nitrogen from the soil.
Harvesting removes the whole crop, so the nitrogen locked up in the plant is taken away instead of being returned to the soil when the plant dies and decomposes. This lowers soil fertility, which is why farmers add fertiliser or grow legumes to replace it. Chemical symbols are not required.
CCEA Unit 1 style3 marksA bag of fertiliser is labelled 20:10:10. Explain what this label tells a farmer.Show worked answer →
Three marks for naming the three nutrients and explaining the numbers.
The three numbers give the relative proportions of the three main nutrients in the fertiliser: nitrogen (N), phosphorus (P) and potassium (K), always in that order. So 20:10:10 means it contains twice as much nitrogen as phosphorus or potassium.
This tells the farmer the balance of nutrients they are applying. A high-nitrogen fertiliser like this would be chosen to promote leafy green growth, for example on grass, whereas a fertiliser with more phosphorus and potassium would suit root growth and flowering or fruiting.
Markers reward N, P and K identified in the right order plus the idea that the numbers are relative proportions a farmer uses to match the fertiliser to the crop's needs.
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